AN ABSTRACT OF THE THESIS OF. Electrical and Computer Engineering presented on November 1, John F. Wager

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2 AN ABSTRACT OF THE THESIS OF Jack Alexander Spies for the degree of Master of Science in Electrical and Computer Engineering presented on November 1, Title: Inorganic Thin-Film Solar Cells Abstract approved: John F. Wager The primary objective of this thesis is to explore new absorber and p-type window layer materials for thin-film solar cell applications. A new thin-film electron beam deposition system has been installed, is now operational, and has been used to deposit several types of solar cell absorber layers. Material investigations include iron silicon sulfide (Fe 2 SiS 4 ) and barium copper tin selenide (BaSn 2 SnSe 4 ) for absorber applications, and barium copper tellurium fluoride (BCTF) for p-type window layer applications. A key issue identified is related to difficulties associated with assessing new absorber materials without fabricating a complete, optimized solar cell. Two attempts to insert BCTFinto a thin-film solar cell were undertaken, involving copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe). The CIGS attempt was unsuccessful because of BCTF diffusion into CIGS during the CIGS deposition at C. BCTF insertion into a CdTe thin-film solar cell was partially successful, as it resulted in better performance than the control cell. However, these experiments were confounded by a time-dependant degradation of the quality of the CdTe back surface after the CdTe undergoes a CdCl 2 in oxygen post-deposition treatment, which is attributed to the effects of humidity at solar cell edges. Modern Schottky barrier and heterojunction theory is used in modeling thin-film solar cells. Analysis using this theory predicts that the CdTe/gold and CIGS/molybdenum (ignoring the possibility of MoSe 2 interfacial formation) interface should form poor quality, rectifying (nonohmic) contacts whereas the CdTe/BCTF/aluminium interface is predicted to function as a highquality (ohmic) p-type window layer in a CdTe thin-film solar cell.

3 c Copyright by Jack Alexander Spies November 1, 2007 All Rights Reserved

4 Inorganic Thin-Film Solar Cells by Jack Alexander Spies A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented November 1, 2007 Commencement June 2008

5 Master of Science thesis of Jack Alexander Spies presented on November 1, 2007 APPROVED: Major Professor, representing Electrical and Computer Engineering Director of the School of Electrical Engineering and Computer Science Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Jack Alexander Spies, Author

6 ACKNOWLEDGMENTS First and foremost, I would like to thank my family and friends for providing me with the support I needed to complete this work. Without their support, I would not have been successful in this endeavor. I would like to thank my major professor, Professor John F. Wager, for providing me with this opportunity to work in materials and devices research, and for providing me with funding and support to complete this work. His energy and insight will not be easily forgotten. A very special thank you to Chris Tasker. For all of his advice and assistance. His willingness to be more than just a colleague, but also a friend, has made this experience very enjoyable. I would also like to thank Peter Hersh, Heather Platt, Richard Schafer, Robert Kykyneshi, Professor Douglas A. Keszler, and Professor Janet Tate, for their assistance is this work. Additionally, I would like to thank Eric Sundholm, Rick Presley, and Manfred Dittrich for always being helpful. This work was funded by the National Renewable Energy Laboratory (NREL) subcontract number XAT

7 The length of this document defends it well against the risk of its being read. Winston Churchill

8 TABLE OF CONTENTS Page 1. INTRODUCTION LITERATURE REVIEW Device Operation Cadmium Telluride Material and Device Properties History of Cadmium Telluride Cadmium Telluride Device Structure Copper Indium Gallium Diselenide Material and Device Properties History of Copper Indium Gallium Diselenide Copper Indium Gallium Diselenide Deposition Conclusions EXPERIMENTAL TECHNIQUES Thin-film Deposition and Processing Thermal Deposition Thermal Evaporation Electron Beam Deposition Pulsed Laser Deposition Sputtering Multi-Compound Deposition Impingement Rate Post-Deposition Annealing Thin-Film Characterization Optical Characterization Absorption Coefficient Bandgap Transmission X-Ray Diffraction Electrical Characterization Conductivity Majority Carrier Type Contact Resistance Carrier Concentration

9 TABLE OF CONTENTS (Continued) Page Mobility Device Characterization Current-Voltage Conclusions HIGH-VACUUM ELECTRON BEAM DEPOSITION SYSTEM Electron Beam Deposition Design Substrate Stage E-Source Peripheral Systems Deposition Chamber Vacuum Pumps Control System Future Modifications Load-Lock Thermal Source Upgrade Sputter Gun Flange Deposition Processes Electron Beam Deposition Thermal Deposition Co-deposition Conclusion INORGANIC THIN-FILM SOLAR CELLS Material Development / Characterization Absorber Materials Iron Silicon Sulfide Barium Copper Tin Selenide Testing New Absorber Materials p-type Window Materials Barium Copper Tellurium Fluoride Copper Indium Gallium Diselenide

10 TABLE OF CONTENTS (Continued) Page Cadmium Telluride Schottky Barrier and Heterojunction Considerations Schottky Barrier Theory Heterojunction Theory Uncertainty Assessment Conclusion CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK Conclusions Recommendations for future work BIBLIOGRAPHY APPENDICES

11 LIST OF FIGURES Figure Page 2.1 Spectral irradiance of solar energy incident on the earth as a function of wavelength An idealized pin solar cell structure and energy band diagram Current-voltage characteristics of a solar cell Equivalent circuit of a solar cell, shown with a load resistance R L A cadmium telluride solar cell shown in the superstrate configuration Chemical structure of copper indium gallium diselenide Energy band diagram showing the valence band discontinuity caused by significant copper loss at grain surfaces of CIS grains Copper indium gallium diselenide solar cell shown in the substrate configuration The three-stage process for copper indium gallium diselenide deposition A homogenous solid under varied conditions of heating and vacuum A cross-sectional view of the Veeco ion beam sputter system An illustration of difficulties encountered in the deposition of a heterogenous material by thermal deposition to achieve a thin-film with proper stoichiometry Optical characterization of a semiconductor is accomplished by measuring the fraction of incident light that is reflected and transmitted as a function of wavelength Adjusted absorption curves representing the bandgap for direct allowed transitions, indirect allowed transitions, direct forbidden transitions, and indirect forbidden transitions X-ray diffraction pattern of crystalline silicon sulfide using the Bragg-Brentano method The transfer length method (TLM) structure used for contact resistance measurements A graph of measured resistance as a function of contact separation, as created from the TLM Mobility, majority carrier type, and carrier concentration of a material can be determined by using the Hall effect

12 LIST OF FIGURES (Continued) Figure Page 3.10 A superstrate solar cell structure with the test setup used to measure the currentvoltage characteristics of the device General deposition tool design flow chart The EBEAM substrate stage consists of the substrate plate and the Z-translation plate A cross-section of the E-source used in the EBEAM, showing two of the four pockets External view of the EBEAM A cross-sectional view of the EBEAM, showing the interior Vacuum state control diagram for the EBEAM The front-side of the EBEAM control rack The back-side of the EBEAM control rack External view of the EBEAM, showing the load-lock chamber A cross-sectional view of the EBEAM, showing the x-translator extended for loading and unloading of the substrate plate A cross-sectional view of a possible thermal source upgrade A design showing the atmosphere side and edge of the new sputter gun flange An idealized pin solar cell energy band diagram A chronological record of the reported maximum efficiencies for CIS/CIGS and CdTe solar cells from 1977 through X-ray diffraction spectrum of an iron silicon sulfide thin-film deposited by electron beam deposition X-ray diffraction spectrum of an iron silicon sulfide thin-film flash-annealed at C in hydrogen sulfide Optical assessment of a barium copper tin selenide thin film, showing the calculated absorption coefficient The test structure, estimated energy band diagram, and current-voltage characteristics of a barium copper tin selenide test structure

13 LIST OF FIGURES (Continued) Figure Page 5.7 The test structure, estimated energy band diagram, and current-voltage characteristics of a barium copper tin selenide test structure Integration of BCTF as a p-type window layer into a CIGS solar cell SEM images of the BCTF/ITO interface Two back-contact configurations used for contacting CdTe Three back-contact configurations used to complete the CdTe solar cell samples received from the University of Toledo Illuminated current-voltage curves for CdTe thin-film solar cells fabricated with different types of back-contacts Current-voltage curves for CdTe thin-film solar cells measured in the dark and under illumination Current-voltage curves for CdTe thin-film solar cells with back-contacts deposited at OSU three weeks after the CdTe solar cell samples were received at OSU, as measured in the dark and under illumination Energy band diagrams for a metal and a p-type semiconductor with Φ M < Φ S Energy band diagrams for a metal and a p-type semiconductor with Φ M < Φ S and Φ M < Φ CNL Energy band diagrams for a metal and a p-type semiconductor with Φ M < Φ S and Φ M > Φ CNL Energy band diagrams describing the electrical interaction between CdTe and gold Energy band diagrams describing the electrical interaction between CIGS and molybdenum Energy band diagrams describing the electrical interaction between BCTF and aluminium Energy band diagrams for two p-type semiconductors Energy band diagrams for two p-type semiconductors with Φ CNL1 < Φ CNL2 and Φ S1 > Φ S Energy band diagrams for two p-type semiconductors with Φ CNL1 > Φ CNL2 and Φ S1 > Φ S

14 Figure LIST OF FIGURES (Continued) 5.24 Energy band diagrams describing interface formation between CdTe and BCTF Page 5.25 Energy band diagrams describing interface formation between CdTe, BCTF, and aluminium

15 LIST OF TABLES Table Page 1.1 World wide production estimates for 2003 of current solar cell technologies The vacuum state table for the EBEAM Price and world-wide production of various metals and materials important to current and potential thin-film solar cells Potential absorber materials considered to date at OSU Melting points of various metals considered for use in new materials research at OSU Potential p-type window materials considered at OSU Solar cell performance summary for the current-voltage curves for various CIGS solar cells with back-contacts deposited at OSU and the CIGS, CdS, and topcontacts deposited at the University of Delaware Solar cell performance parameters for current-voltage curves of CdTe back-contacts measured and under illumination, i.e, copper/gold deposited at the University of Toledo, copper/gold deposited at OSU, BCTF/aluminium, and tellurium/bctf/aluminium Material properties for various semiconductors and metals relevant to the estimation of energy band diagrams for CdTe and CIGS Schottky barrier parameters for the CdTe/gold Schottky barrier, as estimated using ideal and non-ideal Schottky barrier theory Assessment of the charge neutrality level and the interface parameter for various semiconductors of interest to this analysis Schottky barrier parameters for the CIGS/molybdenum Schottky barrier, as determined by ideal and non-ideal Schottky barrier theory Schottky barrier parameters for the BCTF/aluminium Schottky barrier, as determined by ideal and non-ideal Schottky barrier theory Heterojunction parameters for a CdTe/BCTF heterojunction, as described by ideal and non-ideal heterojunction theory Schottky barrier and heterojunction parameters for a CdTe/BCTF/aluminium junction, as described by ideal and non-ideal theory

16 INORGANIC THIN-FILM SOLAR CELLS 1. INTRODUCTION Ninety-million miles away from Earth, two hydrogen atoms orbit as they approach each other. Intense gravitational forces overcome the repulsive columbic forces of each atom. Closer and closer they approach, the eventual outcome quite certain. In a show of intense light, heat, and sound, the two atoms fuse; two hydrogen atoms becoming one helium atom. Through conservation of energy, photons and other types of subatomic particles are released. Some of the photons reach Earth, covering the distance in just over eight minutes. Photons make up the rays of sun light, as they touch the globe. Some of these photons will be absorbed and re-emitted, and are seen as the visible colors of light. Others will be absorbed by plants, fueling the process of photosynthesis that sustains life on this planet. Some of these photons have wavelengths outside of the visible spectrum, and are thus invisible to a human observer. Each second 7.0 x J [1] of energy reaches the surface of the Earth from the sun. If just 2.14 x 10 3 % [2] of this incident energy from the sun could be converted into electrical energy, no other energy source would be needed to meet the world s power needs. This is the dream of photovoltaics [3]. The first solar cell was constructed in 1883 by Fritts, who coated selenium with gold and witnessed a photo response [4]. In 1954, silicon solar cells became a reality when Bell Labs reported on the sensitivity of doped silicon to light [5]. By the beginning of the next decade, solar cells were in space on the first communication satellite [6]. The current solar cell industry is comprised of several different types of material systems. Table 1 outlines present day solar cell conversion technologies. Crystalline silicon is the current technology leader, making up 89% of all solar cells on the market in 2003 [7]. This market dominance is attributed to advantages silicon solar cell technology has leveraged from the semiconductor and space industries. Thin-film solar cells have recently emerged as a competitor to crystalline silicon in the solar cell industry. Composed of amorphous or polycrystalline materials, the reduced capital cost of thin-film solar cells is their key advantage. Solar cell concentrators constitute the final photovoltaic technology, as described in Table 1. Solar cell concentrators consist of a small-area solar cell with a large-area optical assembly. Light is focused via optics onto the solar cell, resulting in an increased concentration of sunlight incident upon the cell. The key advantage of a concentrator system is the reduction in

17 2 solar cell material, which usually translates into a direct cost savings, as the solar cell material is often the most expensive constituent in a solar cell. However, concentrator systems require cooling, which causes a reduction in economic gain when factored into the overall cost of the complete solar system. Solar cells are currently not cost competitive with fossil fuels. The high cost of production, installation, and maintenance translate into solar energy costs of about 0.30 $/kwh, assuming an average 20 year lifetime of the solar cells. Compared to $/kwh for standard electricity rates based on fossil fuels, it is no wonder solar cell technology has had trouble attracting investors. To address this cost issue, several countries including Japan, Germany, and most recently the United States, have invested in solar cell subsidy programs, designed to cover a portion of the cost of solar cells, making them more attractive investments. Currently solar cells in Japan cost an average of 0.18 $/kwh due to government incentive programs. However, the industry cannot hope to rely on subsidy programs to survive in a competitive energy market; new strategies are required. This is where thin-film solar cells show promise. Of the three technologies, thin-film solar cells show the greatest potential for achieving low cost power, as they rely on small amounts of potentially cheap materials to manufacture. If the cost of thin-film solar cells could be reduced to 50% of their current cost, 0.15$/kWH, this would result in a revolution in the energy industry. Energy production would move from large-scale central power generation systems of today to distributed power generation systems relying on solar cell power generation spread throughout states, cities, and even neighborhoods. Thus, the objective of this thesis is to contribute to the development of new materials for thin-film solar cells, with special attention devoted to the use of low-cost, abundant, environmentally benign inorganic materials. Current state-of-the-art thin-film solar cells can be categorized into one of three separate materials groups, as shown in Table 1. Amorphous silicon solar cells, first proposed in 1974 [8], have had little commercial success in recent years. Proposed as an alternative to the high cost of crystalline silicon solar cells, the limited performance [9] and unstable nature [10, 11] of amorphous silicon is preventing this technology from becoming as dominate as its crystalline counterpart. Amorphous silicon is unique in its challenges and developments, and is not directly relevant to the research reported herein. Therefore, amorphous silicon is not be discussed further in this thesis. Cadmium telluride (CdTe) is currently under examination as a thin-film solar cell material. Considered to be a leading candidate for use in high-performance solar cells, this technology is examined in greater detail in the following chapter. Copper indium gallium diselenide (CIGS) is

18 Technology 2003 Production (MW) Proportion of Total Single and Polycrystalline Silicon % Thin-Films % Amorphous Silicon (a-si) % Cadmium Telluride (CdTe) % Copper Indium Gallium Diselenide (CIGS) % Solar Concentrators % Other % Total % Table 1.1: World wide production estimates for 2003 of current solar cell technologies [7]. Other technologies includes ribbon silicon and microcrystalline silicon, which do not fall into either of the listed technologies. 3 a recent addition to the thin-film solar cell material market, having been first reported 20 years ago [12]. This material system is currently the world leader in thin-film solar cell efficiency [13], a figure-of-merit which is explained in the following chapter. Details concerning CIGS as a solar cell absorber material are also explored in the following chapter. The structure of this thesis is as follows. Chapter 2 provides a technical background for thinfilm solar cell devices and technology, including common device structures, theory of operation, and relevant figures-of-merit. Also included in this chapter is a review of pertinent CdTe and CIGS literature. Chapter 3 describes procedures used for device fabrication, as well as characterization tools and techniques. Chapter 4 presents an in-depth look at the new electron beam deposition system constructed in the solid state processing laboratory at Oregon State University (OSU). Chapter 5 details new materials research, including novel work with BaCuTeF, Fe 2 SiS 4 and SnZrS 3. Chapter 6 contains conclusions and recommendations for future research.

19 2. LITERATURE REVIEW 4 Thin-film solar cells have been in development for approximately 30 years [8]. Always considered advantageous in terms of cost and production simplicity, the commerical emergence of cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) in recent years, are indicators of the potential of thin-film solar cells. To understand the significance of thin-film solar cells, a review of the current technology is required. This chapter includes a discussion of basic solar cell theory and operation, including relevant figures-of-merit, and contains a summary of pertinent CdTe and CIGS literature. 2.1 Device Operation Energy from the sun is incident upon the Earth over a wide range of wavelengths. Figure 2.1 shows spectral irradiance as a function of wavelength. Spectral irradiance is a measure of the power per unit area generated by photons of varying wavelength. Air mass zero, AM0, represents the spectral irradiance seen outside the Earth s atmosphere. The atmosphere of the Earth attenuates the intensity of solar energy before it reaches the surface, giving rise to AM1.5 [14]. For solar cells constructed for space applications, AM0 is the relevant spectral curve, while for terrestrial applications, AM1.5 is the relevant curve. The second x-axis on Fig.2.1 shows photon energy, in electron volts, corresponding to the wavelengths of incident photons. Any incident photons with energy in excess of the bandgap of a material can be absorbed by this material. Photons of energy in excess of the bandgap can generate electron-hole pairs, with any surplus in energy beyond that of the bandgap lost to the lattice as heat due to electron thermalization to the bottom of the conduction band or hole thermalization to the top of the valence band. Photons of energy less than the bandgap pass through the material without any carrier generation. Thus, the number of electron-hole pairs generated is maximized through the use of a small bandgap material. However, the amount of energy lost by carrier thermalization as heat is minimized by a large bandgap material. Therefore, an optimal bandgap is not at either extreme. For the simple case of a single layer absorber material under AM1.5 illumination conditions, the optimal bandgap is approximately 1.5 ev [3, 15]. The heart of a solar cell is the absorber material. The absorber layer is designed to be the region in which electron-hole pair generation occurs from the absorption of incident photons.

20 5 Spectral Irradiance W*m -2 *nm AM0 AM Wavelength (nm) Photon Energy (ev) Figure 2.1: Spectral irradiance is a measure of how much energy per unit area is incident upon the Earth as a function of wavelength. Air mass zero, AM0, is the full spectrum of energy as seen from outside the Earth s atmosphere. AM1.5 is the spectrum of energy as seen on the surface of the earth after atmospheric absorption [14]. However, an absorber material alone is not sufficient to constitute a solar cell; no carriers can be extracted without contacting materials. Therefore, other materials are required to create a functional solar cell. Figure 2.2(a) shows the basic structure of an idealized solar cell. The absorber is commonly p-type, such that the transport of minority carrier electrons is of primary importance in establishing the device operation, as a solar cell is a bipolar device. The superscript p indicates low doping of the absorber material, or almost intrinsic, allowing for large minority carrier lifetimes. The n + and p + layers allow for extraction of electrons and holes from the absorber material, as shown in Fig. 2.2(b). They are heavily doped to facilitate good ohmic contact with the top and bottom contact layers. The idealized solar cell shown in Fig. 2.2(a) is a homojuction, in which the n +, p, and p + layers are all constructed from a single type of material. If each of those three layers were to be constructed using different materials, this solar cell would constitute a double-heterojunction pin solar cell, which derives its name from the junctions between the n + and p regions and the p and p + regions. Both CdTe and CIGS are double-heterojunction solar cells, as discussed in this section. The n + and p + materials are often referred to as window layers, as they are chosen

21 6 n + p - Top Contact n-type Window Absorber hν Bottom Contact p + (a) p-type Window Bottom Contact Top Contact n + p - (b) p + Figure 2.2: A simple pin solar cell. (a) An idealized pin solar cell structure showing absorber, window layers, and contacts. (b) An energy band diagram of an idealized pin solar cell under short-circuit operation. The figure depicts carrier generation in the absorber material and carrier reflection at the n + - and p + -type window layers. to have large bandgaps relative to the absorber material. The large bandgaps allow the incident energy to reach the absorber material without being absorbed in the window layers. The large bandgaps also act to reflect carriers away from contact regions in which they would recombine in such a manner that they would not constitute power generation, as seen in Fig. 2.2(b) for the homojuction case. [3, 15]. Device operation of a pin solar cell, without any incident energy, is described by classical pin junction diode theory [3]. A representative current-voltage characteristic consistent with diode theory is shown in Fig. 2.3 as the dark curve. Under illumination, a light-induced current or photocurrent is generated, shifting the dark diode curve downward. The resultant curve is denoted as the illumination curve. Operation of the solar cell in the fourth quadrant of the current-voltage characteristics establishes that power generation is occurring. The illumination curve crosses the current axis and voltage axis, defining the short-circuit current, I SC, and open-circuit voltage, V OC, respectively. For a given solar cell, I SC and V OC define power generation extremes that can be attained by that device under illumination. Actual power generation under open-circuit and short-circuit conditions, however, is zero since power is defined as a product of the current through a device and the voltage across a device. Therefore, a maximum power point is also designated on the illumination curve. The maximum power point defines the operating point at which the maximum power can be generated by the given solar cell [3].

22 7 I Dark V M V OC V I L I M Illumination I SC Increasing FF Maximum Power Point Figure 2.3: Current-voltage characteristics for a solar cell. The dark curve represents the unilluminated diode characteristics of a solar cell. The illumination curve is a downward shift in the dark curve due photocurrent. Important solar cell parameters include the open-circuit voltage, V OC, the short-circuit current, I SC, the fill factor, FF, and the maximum power point represents the operating point corresponding to V M and I M. Two fundamental figures-of-merit exist for comparing solar cells. As evident from an assessment of Fig. 2.3, the fill factor is a measure of the ratio of the area defined by the maximum power point compared to the rectangle defined by a product of V OC and I SC, F F = V M I M V OC I SC. (2.1) The fill factor can take on values between 0 and 1, with 1 denoting a perfect match between V M and V OC, and I M and I SC. Often shown as a percentage, the leading CIGS device has a fill factor of or 78.1% [13]. The solar cell efficiency is a measure of the maximum power with respect to the total incident power, η = V M I M P IN. (2.2) The efficiency is also often reported as a percentage of the total input power. The leading CIGS device mentioned before has an efficiency of 19.2%, corresponding to a conversion of 19.2% of the incident energy into electrical energy [13]. Solar cells can be modeled by the equivalent circuit shown in Fig The diode represents the dark characteristics of the solar cell. The illumination current, I L, is represented by the current source. The opposite orientation of the current source compared to the direction of the forward current direction of the diode underscores that the diode and illumination currents flow in

23 8 r s I L I d r sh R L Figure 2.4: Equivalent circuit of a solar cell, shown with a load resistance R L. The diode, I d, represents the dark current, current through the device when no light is incident. Current source, I L, represents the illumination current which arises when light is incident on the solar cell. The shunt resistance, r sh, and series resistance, r s, of the device are represented as bulk resistors. opposite directions. This opposite direction of photocurrent flow is evident in the current-voltage characteristics of Fig. 2.3 as a negative shift in the diode curve under illumination. The two resistances in the equivalent circuit correspond to series and shunt resistances. Series resistance is a lumped parameter measure of the resistance seen by a carrier moving from one contact to the other contact under forward-bias. Ideally this resistance is zero, which would translate into an infinite slope in the forward-bias portion of the current-voltage characteristics shown in Fig Shunt resistance accounts for the possibility that, under reverse-bias conditions, carriers can travel from one contact to the other without transiting the barrier portion of the junction. Such a current flow path would be said to shunt the junction. Ideally the shunt resistance is infinite, corresponding to a zero slope in the current-voltage characteristics under a reverse-bias, as shown in Fig. 2.3 [16]. 2.2 Cadmium Telluride Cadmium telluride (CdTe) currently leads the thin-film absorber market in terms of manufacturing potential. Large volume deposition is possible with current technology, giving CdTe an advantage over its leading competitor, copper indium gallium diselenide, CIGS Material and Device Properties The only stable Cd-Te compound in the Cd-Te phase diagram, CdTe melts congruently, allowing for deposition of stoichiometric films using a variety of deposition technologies. The bandgap of CdTe is 1.48 ev [17], well suited for AM1.5 conditions of terrestrial solar cell applica-

24 9 tions. The absorption coefficient of CdTe is on the order of 10 5 cm 1 at wavelengths up to 500 nm [18], leading to a favorable quantum yield, a measurement of the number of carriers generated per incident photon. Chemically, cadmium telluride is characterized by tetrahedral atomic coordination, forming the binary structure known as zincblende [19]. Due to the binary nature of the material, doping CdTe is a tricky operation. Native defects cause self-compensation, rendering impurity dopants difficult to control. For this reason, post-deposition anneals are required for CdTe to have reproducible conductivity. This topic is discussed in detail History of Cadmium Telluride Cadmium telluride was first synthesized by Frerichs [20] in 1947 by reacting cadmium and telluride vapors in hydrogen gas. Jenny and Bube reported the semiconducting nature of the material in They showed that n- and p-type CdTe could be fabricated by doping CdTe with foreign impurities [21]. It was not until 1956 that Loferski proposed the use of CdTe as a photo-absorber material [17]. Rappaport demonstrated a single crystal homojunction CdTe solar cell in 1959 [22]. Constructed by diffusing indium into p-type CdTe, the resultant cell had a solar cell efficiency of 2%. In 1979, homojunction CdTe cells were demonstrated to have conversion efficiencies greater than 7% by the CNRS group in France [23, 24]. The cells were constructed by deposition of p-type CdTe films onto n-type CdTe crystals via close-spaced vapor transport. Heterojunction solar cells of n-type CdTe and p-type Cu 2 Te were investigated in the early 1960s [25, 26]. Although efficiencies on the order of 7% had been demonstrated with this concept, lack of a transparent p-type window material, as well as deposition and stability issues arising from the Cu 2 Te [27], caused an eventual shift away from n-type CdTe in favor of p-type conductivity. In the mid-1960s, Muller et al. demonstrated the first heterojunction of p-type CdTe and n-type cadmium sulfide (CdS) [28, 29], with efficiencies of less than 5%. In 1969, Adirovich et al. proposed the superstrate solar cell stack configuration [30]. In 1972, Bonnet and Rabenhorst revised the superstrate stack, adding a CdS buffer layer, resulting in the configuration that would become the industry standard for CdTe, shown in Fig. 2.5 [31]. Similar to the idealized pin solar cell structure shown in Fig 2.2(a), the key advantage of the superstrate structure is the fact that the solar cell is constructed with the n-type window layer contacting the glass substrate. This orientation allows for the CdS layer to be deposited prior to the CdTe absorber layer. This in turn allows for significantly improved junction quality between the CdS and the CdTe, as opposed to

25 10 Incident Light Substrate ITO ZnO CdS Top Contact n-type Window Layer CdTe Absorber Cu Te 2 Back Contact Figure 2.5: A cadmium telluride solar cell shown in the superstrate configuration. Note the lack of a p-type window layer, as expected for a pin solar cell. Also note that the incident light must pass through the substrate prior to reaching the top contact, potentially resulting in photon absorption losses. depositing CdS onto CdTe, as it allows for CdCl 2 treatment of the CdTe [32]. For this reason, the superstrate configuration is the standard orientation for modern CdTe solar cells. One thing to note is the fact that the incident light must pass through the substrate before reaching the n-type window layer. Photon losses, therefore, maybe incurred in the substrate, reducing the maximum efficiency of devices in the superstrate configuration. In 1977, Mitchell et al. reported the first use of indium tin oxide (ITO) as an n-type window layer for CdTe solar cells. The resultant device exhibited an efficiency of 9.7% [33] Cadmium Telluride Device Structure A current state-of-the-art cadmium telluride solar cell structure is shown in Fig Although some variations exist, all CdTe cells employ a similar structure. The first step in creating a cadmium telluride solar cell is deposition of the n-type window layer. Depending on the material, deposition techniques vary. Generally a transparent conductive oxide (TCO) is used as the n-type contact layer, and is deposited by sputtering. A highly resistive transparent (HRT) oxide layer is deposited on top, also by sputtering. The HRT layer prevents shunting through the CdS layer,

26 11 an unfavorable consequence of the thin CdS buffer layer required to minimize photon absorption. CdS completes the n-type window layer, and is usually deposited by chemical bath deposition. The absorber material is deposited on top of the CdS. Cadmium telluride thin-films can be deposited using a variety of techniques. Evaporation from elemental sources was the first technique developed, and remains a viable option due to the ease in deposition parameter control [34, 35]. Vapor transport of cadmium and tellurium species [36], and sputtering of CdTe or elemental targets [37, 38] have also been successfully used for thin-film CdTe deposition. Deposition of CdTe thin-films via evaporation is directly dependent on the process temperature. As the temperature is increased, the deposition rate increases and the thickness uniformity improves. Above C however, re-evaporation of cadmium and tellurium species limits the deposition rate. In order to achieve electrically optimal grain growth, a temperature in excess of C is required. Therefore, re-evaporation needs to be addressed. If the background pressure is increased above the vapor pressure of the cadmium and tellurium species, re-evaporation is greatly reduced. However, increased background pressure also reduces the mean-free-path of the cadmium and tellurium evaporates, thereby reducing mass transport over large distances. The solution is to place the substrate and source in close proximity. This type of evaporation is designated as close-spaced sublimation, and is a common deposition technique for thin-film CdTe [39, 40, 41]. Due to self-compensation of the CdTe, post-deposition anneal treatments are required to passivate and/or activate native defects [32]. Cadmium chloride (CdCl 2 ) is the most common treatment. Chlorine species are incorporated into the CdTe films through CdCl 2 precipitates [32, 42], CdCl 2 vapor [35], as well as other methods, followed by a thermal anneal. Electronically, CdCl 2 treatment causes the formation of an acceptor complex, V Cd + Cl T e. This acceptor complex creates an easily accessible shallow acceptor state at the back interface of the CdTe, creating a p-type layer [43]. The CdCl 2 treatment is also believed to promote recrystallization of the CdTe in a preferred (111) crystal orientation, and to enhance grain coalescence in the case of sub-micron grain films [44]. Following the post-deposition anneal, the back contact is deposited to finish off the cell. CdTe solar cells do not have a p-type window layer, as expected for pin solar cells. Difficulties with making contact to the CdTe have precluded incorporation of such a layer. Instead, electrical contact is made directly to the CdTe. The most common technique is to etch back some of the CdTe, leaving a tellurium-rich surface. Cu is then deposited to create a p-type Cu 2 Te layer thereby facilitating an ohmic contact to the CdTe. Metal or graphite contacts the Cu 2 Te layer, as shown

27 12 in Fig The back contact of a CdTe solar cell is a major area of research. Stability issues arising form the use of copper have prompted the need for an alternative back contact method. Potential alternative materials developed at Oregon State University are described in Chapter 5 [45]. 2.3 Copper Indium Gallium Diselenide Copper indium gallium diselenide (CIGS) currently holds the solar energy efficiency record for a single-layer thin-film absorber material, with an efficiency of 19.2% [13]. Poised for widespread use as a solar cell in both space and terrestrial applications, CIGS has also been shown to have high radiation tolerance [46, 47], and excellent long-term stability [48] Material and Device Properties CIGS is a substitutionally doped variant of copper indium diselenide (CIS), with gallium sitting on an indium site, Ga In. Depending on the gallium concentration, the bandgap varies from 1.02 ev for CIS up to 2.5 ev for a copper gallium diselenide (CGS). High-performance CIGS has a gallium concentration around [Ga] = , (2.3) [In] + [Ga] which corresponds to a bandgap of 1.2 ev. Considering the ideal bandgap for terrestrial AM1.5 conditions is about, 1.5 ev, CIGS cannot compete with cadmium telluride in absorption coefficient. The adsorption coefficient of CIGS is about 10 5 cm 1 for photon energies of 1.4 ev and above [49]. The chemical structure of CIGS is composed of interlocking CIS and CGS structures. Belonging to the chalcopyrite family, the basic structure of either CIS or CGS can be described as a diamond lattice, similar to sphalerite, with alternating copper and indium or gallium metals on each zinc site and selenium on the sulfur sites. An illustration of CIS can be seen in Fig. 2.6 [50]. Relative concentration of copper and indium/gallium in CIGS determines conductivity type. Copper-rich CIGS is always p-type, while indium/gallium-rich CIGS can be p-type or n-type, depending on the number of selenium vacancies. Indium/gallium-rich CIGS annealed in a selenium overpressure would result in p-type conductivity, while annealing in a reduced selenium atmosphere, the selenium vacancies would act as donors, resulting in n-type conductivity. High-performance CIGS is always indium/gallium-rich, with [Cu] = (2.4) [In] + [Ga]

28 13 Cu In Se Figure 2.6: Chemical structure of copper indium diselenide, which is in the chalcopyrite family and is characterized by a tetragonal structure containing copper or zinc in a two plus oxidation state. Gallium substitutional doping on the indium site would result in copper indium gallium diselenide (CIGS).

29 14 Grown in selenium excess conditions, high-performance CIGS has p-type conductivity, with carrier concentrations around cm 3 [51, 52]. The tolerance in relative concentrations of copper to indium and gallium is caused by copper vacancies and antisite defects leading to self-compensation within CIGS. Due to its low formation energy, the defect complex 2V Cu + In Cu readily forms, compensating for copper deficiencies. Vacancies and defects have no impact on carrier concentration, as they do not act as recombination centers [53], an important property discussed in this section. The compositional tolerance allows for significant variance in CIGS process conditions [54, 55], potentially reducing manufacturing costs. The surface of CIGS has been shown to be copper-poor in composition, using X-ray photoelectric spectroscopy. The compound CuIn 3 X Ga X Se 5 is often found, signifying copper loss into the bulk. The current understanding is that copper undergoes an electromigration into the bulk, facilitated by band-bending at the surface. As copper ions are removed, CuIn 3 X Ga X Se 5 begins to form. Once the entire surface is converted into CuIn 3 X Ga X Se 5, the reaction stops, as further loss of copper would result in an energetically unfavorable structural change [56]. Copper deficiencies cause a valence band discontinuity, as shown in Fig. 2.7, which is significant as discussed shortly in this section. When exposed to atmosphere, CuIn 3 X Ga X Se 5 does not form; instead the CIGS surface is oxidized. Oxidation of the CIGS surface is favorable, as the oxygen fills selenium vacancies, which act as recombination centers. For this reason, device quality CIGS is often post-deposition annealed in oxygen in order to passivate the surface [57]. When deposited as a thin-film, CIGS is polycrystalline, an undesirable microstructure for a solar cell absorber due to the potential for carrier losses in grain boundaries. The grain boundaries of CIGS, however, are inherently passivated as mentioned previously, thus preventing significant carrier losses [58]. This property allows for grain sizes of less then 1 µm to be used with some success. High-performance device-grade CIGS, however, is usually deposited to have grain sizes on the order of 1 µm [59, 60, 61]. In terms of material stability, CIGS exhibits significant resilience to lattice mismatch and defects [47]. This is believed to be a consequence of the polycrystalline nature of CIGS, as defects are gettered into grain boundaries, thereby allowing for the free flow of carriers throughout the grains. Due to significant copper loss at grain interfaces, a valence band discontinuity exists, preventing holes from reaching grain boundaries and recombining, as shown in Fig A lack of holes at grain boundaries prevents electrons from recombining, resulting in electrically passivated

30 15 In Cu Cu In V Cu 0/+ +/2+ 2-/- -/0 -/0 E C 1.02 ev E V Grain Boundary Figure 2.7: Valence band discontinuity caused by significant copper loss at grain surfaces of CIS grains. Holes cannot reach grain boundary recombination centers, as shown, thus preventing recombination of carriers. Addition of gallium, creating CIGS, would increase the bandgap and cause a shift of the donor levels deeper into the bandgap [62]. grain boundaries. With the gettering of the defects into the grain boundaries, the mobility of holes is improved, leading to the startling fact that solar cells made from crystalline CIGS cannot compete with the performance of polycrystalline CIGS thin-film solar cells [58] History of Copper Indium Gallium Diselenide Copper indium diselenide (CIS), the precursor to CIGS, was first synthesized by Hahn in 1953 [63]. In the early 1970s, Bell Laboratories developed the first solar cells with CIS [64], employing a junction created by evaporating n-type CdS onto a p-type single crystal of CIS. Crystalline CIS solar cells reached upward of 12% efficiency before being surpassed with thin-film variants [65]. Thin-film CIS was first fabricated in 1976 by Kazmerski et al. [66]. Constructed by evaporating CIS powder with excess selenium, efficiencies on the order of 4-5% were recorded. In 1981, when Boeing demonstrated a CIS thin-film solar cell with an efficiency of 9.4% [67], industry took notice. Electrochemical instabilities had recently been discovered in Cu 2 S solar cells, the leading copper-based thin-film absorber material, resulting in a dramatic shift of focus throughout the industry from Cu 2 S to CIS [68]. The Boeing cells were fabricated by depositing CIS by coevaporation, simultaneous evaporation using multiple elemental sources, onto ceramic substrates coated

31 16 with a bottom contact of molybdenum. CdS deposited by evaporation formed the junction, with CdS doped with zinc deposited on top to create a top contact [69]. CIS was first used to create a heterojunction in 1974, consisting of n-type CdS evaporated onto a p-type single crystal of CIS [64, 65]. The first thin-film heterojunction of CIS and CdS was reported by Kazmerski et al. [70] in In 1987, Chen et al. [12] added Ga to CIS as a dopant, creating CIGS. Increased bandgap and improved electrical conductivity were the motivations for this addition. The ceramic substrate became a focal point of research in 1993 [71]. Soda-lime-glass, chosen initially for its low cost, became the substrate of choice after research showed that it improved efficiencies of CIGS solar cells. Some improvement is attributed to the close thermal expansion coefficient match between the soda-lime-glass and the CIGS. Thermal expansion of the substrate relative to absorber could lead to film stress issues. If a significant mismatch exists between the film and the substrate, microcracks or adhesion failure may result [60]. Sodium diffusion from the soda-lime-glass into the CIGS is also believed to improve the CIGS cell performance. Wei et al. [72] show that for low concentrations of sodium, the electrical performance of CIGS improves; the sodium reduces the concentration of indium-copper antisite defects, In Cu, as the formation energy, H(Na InCu ) = 1.0eV, (2.5) is strongly exothermic. A reduction in In Cu, increases the hole concentration, removing carrier traps and lowering the Fermi energy level, thereby increasing the open-circuit voltage. If the concentration of sodium becomes too large, copper vacancies, V Cu, begin to fill with sodium reducing the hole concentration and the open-circuit voltage. Sodium incorporation has also been shown to lead to larger grain sizes in the CIGS, and higher degree of preferred orientation. The standard back contact for CIGS solar cells is molybdenum metal. Usually the molybdenum is deposited by sputtering to a thickness of 1 µm. During the CIGS deposition, a molybdenum diselenide, MoSe 2, layer forms at the interface [73]. The layer causes a conduction band discontinuity to form at the back contact, as shown in the ideal pin solar cell energy band diagram in Fig This discontinuity is beneficial, as it prevents electrons from reaching the back contact and recombining before they can be extracted. In 1986, the CdS layer used to create the heterojunction in CIS solar cells was reduced in thickness from 1 µm to 50 nm [74]. The method of deposition changed as well, with evaporation

32 17 replaced by chemical bath deposition. The reduction in CdS thickness was brought about to decrease photon absorption losses incurred in the CdS buffer layer. The current preferred method of CdS deposition is chemical bath deposition (CBD). Consisting of immersion of the CIGS film in a heated CdS solution, the solution etches away surface oxides and impurities, resulting in a dense homogenous film of CdS [75, 76]. TEM assessment of the CdS / CIGS interface shows that cadmium diffuses up to 10 nm into the copper-deficient region of the CIGS [77]. Current environmental policy has encouraged the removal of cadmium from CIGS solar cells due to its toxicity. Considering that cadmium only exists in the buffer layer of a CIGS solar cell, the options are to remove the buffer layer entirely, or replace it with a cadmium-free material. Buffer layers of ZnO [78], ZnS [79], and In 2 S 3 [80] have all been tried, with some success, and lead to an average reduction of efficiency of approximately 1%. Removal of the buffer completely [81] shows a reduction in efficiency on the order of 2-4%, which may indicate a need to passivate the CIGS surface prior to top contact deposition. The use of transparent conductive oxides for thin-film solar cell top contacts became common practice in the late 1980s. ZnO replaced (CdZn)S as the most common top contact material for CIGS thin-film solar cells [74]. Figure 2.8 shows a cross-section of a current state-of-the-art CIGS solar cell. The substrate is soda lime glass, which is sputter coated with Mo. Copper indium gallium diselenide is deposited on the Mo, followed by a layer of CdS. Highly-resistive ZnO is sputtered onto the CdS, followed by sputtering of conductive ZnO. The final step is the deposition of the Ni/Al grids via evaporation, used for current extraction from the top contact Copper Indium Gallium Diselenide Deposition Currently, two methods of CIGS deposition are commonly used in industry. Coevaporation, the simultaneous evaporation using multiple elemental sources, is the most common technique for research applications. The two-step process, first proposed by Grindle et al. [82] in 1979, is the alternative technique. It consists of deposition of metal layers followed selenium incorporation through annealing. Coevaporation is the dominant fabrication method employed, mostly due to its simplicity, usually implemented using separate Knudson-type effusion cells of Cu, In, Ga, and Se. Coevaporation relies on control of the evaporation fluxes, relative to each other, to achieve the desired film

33 18 Current collection grid Top Contact n-type Window HR ZnO / n + ZnO 0.5 µm CdS 0.05 µm Absorber Cu(InGa)Se 2 2 µm p-type Window Back Contact Substrate MoSe 2 Mo 0.5 µm Soda lime glass Figure 2.8: Copper indium gallium diselenide solar cell shown in the substrate configuration. Similar to a generic pin solar cell stack shown in Fig. 2.2, MoSe 2 forms the p-type window layer, while CdS and ZnO constitute the n-type window layer. stoichiometry. Some difficulties arise due to the low sticking coefficient of selenium, resulting in the incorporation of only a fraction of the evaporated selenium into the film. This issue is alleviated by selenium evaporated in excess during the CIGS deposition. [83]. In 1982, Boeing reported the development of a CIGS coevaporation process involving two separate stages. The first is a period of copper-rich CIGS deposition, required for large CIGS grain size formation [69]. Following the first stage, is a period of copper-poor CIGS deposition, required to achieve the composition for high-performance quality CIGS. In 1994 Kessler et al. [84] improved on the Boeing process by adding a second copper-poor CIGS deposition stage, later called the three-stage process. Figure 2.9 shows the deposition process as a function of time. The addition of the stage, prior to the copper-rich deposition step, improves the crystallinity of the result CIGS film [85], and improves the electrical properties, attributed to a Ga concentration gradient from the molybdenum back contact to the film s top surface [86]. Selenium is deposited throughout the three-stage process, due to its low sticking coefficient and a desire for minimal selenium vacancies. The three-stage process was used to fabricate the current solar efficiency record cell in 2003 [13].

34 19 T ss Relative Flux In Cu Temperature Ga 1 st Stage 2 nd Stage 3 rd Stage t Figure 2.9: The three-stage process for copper indium gallium diselenide deposition. The first stage involves deposition of indium and gallium, which is accomplished at a reduced substrate temperature. Copper is deposited in the second stage to achieve desired grain growth. The final stage is to deposit indium and gallium in order to obtain appropriate electrical characteristics, partially associated with bandgap engineering via compositional inhomogeneity. Due to the use of an elevated substrate temperature, significantly less indium and gallium are deposited in the third stage. A selenium overpressure exists throughout the process, resulting in a constant rate; a selenium overpressure is required in order to minimize selenium vacancy concentration.

35 20 The two-step process is the current alternative to coevaporation for CIS deposition. Devised for large-volume CIS processing, the highest efficiency solar cell reported by this technique is 16.2% [87]. The two-step process consists of the copper, indium, and gallium deposited in layers, usually by sputtering, followed by a selenium or a hydrogen selenide anneal. Gallium incorporation in the two-step process has been shown to improve adhesion to the molybdenum back contact. However the bandgap of the resultant film stays at 1.02 ev, making cells constructed using this technique a poor match for terrestrial AM1.5 applications. A post-processing anneal in an inert atmosphere for 60 minutes incorporates the gallium into the CIS, but this step is impractical for manufacturing [88, 89]. 2.4 Conclusions Thin-film solar cells represent a low-cost alternative to the existing solar cell technologies. In an industry where price is a key consideration, thin-film solar cells offer a realistic approach to low-cost solar generation power. In this chapter, the basic theory and operation of thin-film solar cells is presented, and relevant figures-of-merit are introduced. A brief review of pertinent CIGS and CdTe thin-film literature is also included.

36 3. EXPERIMENTAL TECHNIQUES 21 This chapter describes the experimental techniques used in the research leading to this thesis. Thin-film deposition and processing tools and methodology are discussed first. Several important thin-film optical and electrical characterization techniques are noted, with an explanation of the theoretical basis for each technique. 3.1 Thin-film Deposition and Processing This section examines the thin-film deposition techniques used for the research in this thesis. Thermal, electron beam, and pulsed laser deposition (PLD) are discussed, followed by a discussion of sputter deposition. Post-deposition annealing is also addressed Thermal Deposition All energetically stable materials, solid or liquid, exist in dynamic equilibrium, as depicted in Fig. 3.1(a) for a homogenous solid. Under dynamic equilibrium, the pressure exerted on a material and the vapor pressure of the material are equal, resulting in no net loss of molecules from the material. Pressure exerted on a material is a function of how many gas-phase molecules impact the surface of the material in a given amount of time. Vapor pressure of a material is a measure of the number of gas-phase species leaving the surface of the material. If a material is placed into a vacuum, as shown in Fig. 3.1(b), the reduction in pressure exerted on the surface of the material causes a net loss of molecules. The pressure exerted on the surface of the material is reduced, while the vapor pressure is the same, regardless of background pressure, leading to a migration of molecules from the solid or liquid phase into the gas phase. Application of heat to a material has a similar effect as vacuum, as shown in Fig. 3.1(c), with the average kinetic energy of the molecules increasing, leading to more excitation among the molecules, a larger vapor pressure, and a net loss of molecules. Unlike vacuum however, the pressure exerted on the surface of the material is not reduced from the equilibrium case. Instead, the net loss of molecules from a heated material is due to an increase in vapor pressure, resulting in more molecules leaving the surface of the material than are replaced by the pressure of the gas-phase molecules above the material.

37 22 If heat is applied to a material placed in vacuum, as shown in Fig. 3.1(d), both a reduction in pressure exerted on the surface by the gas-phase molecules and an increase in the vapor pressure of the material contribute to the net loss of molecules from the material. This combination of mechanisms leads to a larger net loss of molecules from the material than either individual mechanism. Thus, to a achieve high deposition rate, thermal deposition of a thin-film is often performed in vacuum. Once evaporated off of the target, the gas-phase molecules travel in random directions until they reach an area of low vapor pressure. If the concentration of gas-phase molecules is large, the pressure exerted by the incidence of these molecules will cause condensation at the surface. As the number of condensed molecules increases, the vapor pressure associated with this assembly of particles also increases. When the magnitude of the vapor pressure approaches the exerted pressure on the surface, dynamic equilibrium is reached and the number of condensed molecules will not increase. Described as rate limited deposition, this phenomenon rarely occurs unless the mass transfer of the gas-phase molecules is small. The high-vacuum pressure range often employed during an evaporation deposition both increases the evaporation rate, by reducing the local pressure on the target, and increases the deposition rate at the substrate, by removing almost all of the gas molecules present between the target and the substrate. Impurity incorporation into a growing film is also minimized by vacuum conditions, as the impurity flux decreases with improved vacuum pressure Thermal Evaporation Thermal evaporation is accomplished by passing a large current through a refractory metal boat or wire basket containing the evaporation source material. Limited to low melting-point materials, thermal evaporation is often restricted to metals. Deposition of metal contact layers is the most common application of thermal deposition in the research presented in this thesis Electron Beam Deposition Electron beam deposition employs a high-energy electron beam to heat the target, typically a pressed ceramic pellet. The electron beam is focused onto a small area of the pellet, providing the intense localized heating that allows electron beam deposition to deposit materials requiring much higher temperatures than are attainable using thermal evaporation. Often employed to

38 23 (a) Dynamic Equilibrium (b) Vacuum (c) Heating (d) Heating in Vacuum Figure 3.1: A homogenous solid under varied conditions of heating and vacuum. The number of molecules injected and ejected from the solid represent the variations in pressure and vapor pressure due to these conditions. (a) The pressure and vapor pressure under dynamic equilibrium conditions are equal, represented by a net loss of zero molecules. (b) The pressure is decreased below the vapor pressure under vacuum conditions, represented by a net loss of two molecules. (c) The vapor pressure is increased above the pressure under heating conditions, represented by a net loss of two molecules. (d) The combination of heating and vacuum conditions leads to a decrease in pressure, and an increase in vapor pressure, represented by a net loss of four molecules.

39 24 deposit refractory materials, electron beam deposition provides a method for depositing almost any material from the periodic table Pulsed Laser Deposition Pulsed laser deposition (PLD) employs a high-energy laser, activated in short duration pulses, to heat the target, typically a pressed ceramic. The laser is focused onto a small area, providing the intense localized heating necessary to deposit high melting-point materials. Some issues arise when using PLD for pure metallic targets, as the large thermal coefficients cause rapid diffusion of the heat generated by the pulses of light, preventing the intense localized heat required for evaporation. PLD is often employed to deposit multi-component materials, as the ablation caused by the individual pulses leads to improved stoichiometric transfer between the target and the substrate over other thermal deposition methods. The PLD system at OSU employs a krypton fluorine (KrF) laser with a wavelength of 248 nm. This choice of wavelength is advantageous, as the absorption of most materials is strong in the wavelength range of nm [90]. Each pulse of the KrF laser can deliver a set amount of energy. For depositing barium copper tellurium fluoride (BCTF), the PLD is set to deliver 116 mj per pulse, or 0.5 J/cm 2 normalized for the area of the laser. The frequency for BCTF is 10 Hz, corresponding to a energy flux of 5 J/cm 2 per second. The thickness of deposited films is often dictated by the number of pulses. For the PLD system at OSU, 6000 pulses deposits 70 nm of BCTF [91] Sputtering Thin-film deposition via sputtering involves the physical removal of atoms from a metallic or pressed ceramic target through bombardment with high energy ions. Sputtered atoms are ejected from the target and fly across the chamber where they condense onto the substrate. In the more common RF and DC sputtering methods, a voltage is applied between the target and the substrate to generate ions and accelerate them toward the target; RF sputtering applies an AC voltage at MHz, while DC sputtering applies a DC voltage. An ion beam sputtering processes does not rely on a substrate voltage to accelerate ions. Ion beam sputtering employs a collimated beam of energetic positive ions directed at the surface of the sputter target by the ion gun. As ions strike the target surface, a portion of their energy is transferred as kinetic energy to atoms in the sputter target, ejecting them from the

40 25 target surface. The process is carried out under medium vacuum, 10 4 Torr, so that ejected sputter target atoms are able to transit the target-to-substrate distance with a minimal number of gas-phase collisions. The molecules ejected from the target condense onto the substrate, forming a thin-film. The ion source on the Veeco ion sputter system, which is in the solid-state materials and devices processing lab at OSU, is a low-pressure DC discharge inside a cylinder mounted on the process chamber. A filament supplies electrons to the discharge, as shown in Fig Electrons are accelerated toward the inner can by an applied potential of 40 V. An electromagnet, shown as the field lines between the inner and outer can in Fig. 3.2, produces a magnetic field parallel to the axis of the cylinder. This magnetic field deflects electrons into circular trajectories, caused by the Lorentz force [3], F = qv B, (3.1) where q is the electron charge, v is the electron velocity, and B is the magnetic field strength. The elongated electron trajectories caused by the Lorentz force, increase the distance traveled by each electron between emission from the filament and collection on the inner can. This additional distance leads to an additional amount of time the electron exists in the can, increasing the probability that each electron will collide with a sputter gas molecule. The sputter gas is introduced into the discharge chamber through a precision leak valve at the top of the can. Once injected, sputter gas molecules collide with electrons, generating ions. Once generated, the ions move randomly in the cylinder until they approach the top grid, shown in Fig At the top grid, which is charged to a potential of +750 V, the ions see the ground potential on the middle grid and accelerate toward it. This collimated beam of ions accelerates through the middle grid and continues to accelerate through the bottom grid, which is held at approximately -212 V. Upon exiting the collection of grids, the ions are neutralized by electrons generated by the neutralizing filament. These electrons prevent both dispersion of the ion beam due to like-charge repulsion, and repulsion of the ion beam from the target due to electrical charge build-up. The bottom grid, with the negative potential, prevents the electrons generated at the neutralizing filament from interacting with the DC discharge. After passing by the neutralizing filament, the collimated beam of ions collides with the target, delivering large amounts of kinetic energy to the target species through inelastic collisions.

41 26 Inner Can (~40 V) Magnetic Coils Substrate Outer Can (>750 _ V) Top Grid (>750 _ V) - Gas Inlet Ionizing Filament Neutralizing Filament - Middle Grid (Grounded) Target Bottom Grid (-212 V) Figure 3.2: A cross-sectional view of the Veeco ion beam sputter system. The motion of a sputter gas molecule is illustrated, traveling from the gas inlet in the ion-source cylinder, through the three charged grids, to the target in the deposition chamber. This energy ejects the species into the gas-phase, creating the flux necessary to deposit a film onto the substrate. Sputtering was not used extensively for this research. Deposition of indium tin oxide (ITO) contacts constitute the only sputtering processing step.

42 Multi-Compound Deposition Deposition of binary and higher order compound films through evaporation generally requires the use of multiple evaporation sources, which is often referred to as co-deposition. Direct evaporation of a multi-component source material often results in a different stoichiometry in the film than in the source material, as each material has a different vapor pressure, leading to a different evaporation rate at a given temperature. Of the three thermal deposition methods shown in Fig. 3.3, PLD is the superior deposition method for depositing compound films. Electron beam evaporation is less successful than PLD, although superior to thermal evaporation. Sputtering, as shown in Fig. 3.3, provides good stoichiometric transfer between the target and the substrate due to the fact that the sputter yield is quite similar for most sputter species. The ejection of the target species caused by sputtering does not depend upon the individual vapor pressures of the target constituents, but primarily on the target stoichiometry. Sticking coefficient, however, can affect the stoichiometry of the sputter-deposited film. A material a with low sticking coefficient, e.g., most anions such as oxygen and sulfur, often are not efficiently transferred from the sputter target into the deposited film. Such deposited films are typically anion-deficient. Deposition or postdeposition processing in an anion overpressure is often employed to circumvent this anion-deficient nonstoichiometry problem Impingement Rate The impingement rate of a gas-phase material is the rate in which the molecules collide with a surface. Mathematically, impingement rate is determined by the RMS velocity of the gas-phase molecules, ῡ, and the density of the gas, n [92], φ = n ῡ. (3.2) 4 Assuming the gas is an ideal gas, its volume is directly related to temperature (Gay-Lussac s Law) and one mole of the gas always fills the same volume at the same temperature (Avogadro s hypothesis), the density can be determined by rearranging the ideal gas law [93], pv = NkT, (3.3) to get an equation in terms of deposition chamber pressure, p, Boltzmann s constant, k = J/K, and temperature, T [93], n = p kt. (3.4)

43 28 (a) Thermal Evap. (b) Electron Beam (c) Pulsed Laser (d) Sputtering Figure 3.3: An illustration of difficulties encountered in the deposition of a heterogenous material by thermal deposition to achieve a thin-film with proper stoichiometry. (a) Thermal evaporation has the most difficultly depositing a multi-component material, represented by the evaporation of one target specie. (b) Electron beam deposition only has difficulty depositing a multi-component material if two or more of the species have an extreme disparity in their melting points, represented by the evaporation of two target species. (c) Pulsed laser deposition (PLD) has minimal difficultly depositing multi-component materials, represented by an even stoichiometry between the target and evaporated species. (d) Although not a thermal deposition method, sputtering is included for comparison. Sputtering is similar to PLD, with minimal difficulty depositing multi-component materials, as represented by an even stoichiometry between the target and evaporated species. The RMS velocity of a gas is determined by the energy distribution of the gas phase species. As evaporated molecules have a Maxwell-Boltzmann distribution of energy [94], the RMS velocity for an evaporated species is, ῡ = 3kT m, (3.5) where m is the mass of one gas molecule. Substituting Eqns. 3.4 and 3.5 into Eqn. 3.2, impingement rate is redefined as, φ = 3 16 p 2 mkt. (3.6) Impingement rate is often quoted in units of nm/s. In order to express Eqn 3.6 in this form, the volume of a single gas molecule must be calculated, V = 4 3 πr3, (3.7) where r is the radius of the molecule. Multiplying Eqns. 3.6 and 3.7 gives an equation for impingement rate, φ = (p πr3 ) 2 3 mkt, (3.8)

44 29 dependent only on the radius, r, and mass, m, of the gas molecule, and the chamber temperature, T, and pressure, p. The impingement rate alone, however, does not define a the rate of deposition on a substrate. All molecules have a sticking coefficient, a number between zero and one that defines the number of molecules that collide and stick to a surface as a function of the total number of colliding molecules, 0 s 1. (3.9) In most cases, the sticking coefficient is a function of surface concentration; the first molecules to collide with a surface tend to have a sticking coefficient close to one; all subsequent molecules have dramatically decreasing sticking coefficients [95] Post-Deposition Annealing Post-deposition annealing of thin-films is an important step in thin-film processing. Although improved crystallinity can often be achieved by annealing, the temperature required to crystalize a thin-film is often prohibitive. Use of a lower-temperature anneal reduces the risk of damage to any previously deposited layer or the substrate, thus increasing the compatibility of the anneal with multi-component processes. Plastic and low-cost glass become feasible choices for substrate materials for anneals less than C. The increase in performance associated with a lower-temperature anneal is primarily attributed to increased short-range order at the atomic scale. A post-deposition anneal provides thermal energy to the constituent atoms, allowing them to rearrange or diffuse, thereby reaching more favorable locations and/or orientations. A low-temperature anneal typically involves amorphous or polycrystalline thin-films. If they possess adequate performance, in terms, for example, of mobility, amorphous thin-films are the most desirable microstructure due to the lack of grainboundaries in the film, which act as recombination centers for charge carriers, reducing the carrier concentration and associated mobility of the material. Rapid-thermal annealing, RTA, is the post-deposition annealing method used for this research. RTA provides rapid heating and cooling with typical temperature ramp times on the order of 30 seconds or less. The advantage of quick annealing is that it reduces the interdiffusion associated with a high temperature, due to the short duration of the anneal. Typically a nonreactive gas is used during the RTA to minimize oxygen incorporation into the film.

45 30 OSU employs an AET THERMAL RX SERIES rapid thermal processing system for RTA. The system utilizes halogen light bulbs to provide the rapid heating and cooling. The radiative heating, can be controlled almost instantaneously, allowing for the desired short temperature ramp times. Argon gas is the gas most often used in the RTA for this research. 3.2 Thin-Film Characterization Thin-film characterization can determine many material parameters required for device evaluation. Optical characterization assesses the photon interactions of a material. The absorption coefficient, optical bandgap, and transparency can all be determined using optical characterization. X-ray diffraction measurements can be used to measure the crystallinity and composition of a material. Electrical characterization determines the electrical properties of a material, allowing an assessment of the conductivity, majority carrier type, contact resistance, carrier concentration, and mobility. This section outlines the methodology and procedures involved in determining each parameter Optical Characterization Basic optical characterization consists of measuring the absorption coefficient, optical bandgap, and transparency of a semiconductor. This section outlines the testing methodology used to determine each of of these optical parameters. Optical characterization of a semiconductor thin-film is performed by shining a light onto the film and assessing the amount absorbed. Light of a known intensity and wavelength is incident on a thin-film, while sensors measure the intensity that is reflected and transmitted through the film. Figure 3.4 shows an example set of data, showing the intensity of reflected light intensity of transmitted light as a function of wavelength. Absorbtion is determined by normalizing the transmitted light via dividing it by the total amount of light that did not get reflected [96], T Norm = and subtracting the normalized transmission from one [91], T 1 R, (3.10) A = 1 T Norm. (3.11)

46 E+06 1.E+05 1.E+04 1.E+03 (cm -1 ) E+02 Wavelength (nm) Transmission Normalized Transmission Reflection Adsorption Coefficient Figure 3.4: Optical characterization of a semiconductor is accomplished by measuring the fraction of incident light that is reflected and transmitted as a function of wavelength. Normalized transmission, Eqn. 3.10, is used to determine the transparency of the film. The adsorption coefficient, Eqn. 3.13, is also a function of wavelength as shown Absorption Coefficient The adsorption coefficient is defined as the relative decrease in the intensity of light, L(hν), along the propagation path [96], α(hν) = d(l(hν)). (3.12) dx Extremely significant for solar absorber materials, the absorption coefficient dictates the required thickness necessary for a functional solar absorber material. Cadmium telluride, which has an absorption coefficient of 10 5 cm 1 at wavelengths up to 500 nm, must be at least 2 µm thick to absorb a majority of the solar spectrum, as described in Chapter 2. The adsorption coefficient of a thin-film is determined from the normalized transmission intensity [96], α = ln( T t 1 R ), (3.13) where t is the thickness of the thin-film. The adsorption coefficient is dependent on wavelength, as shown in Fig This dependence, often ignored in casual descriptions, defines the true absorption capabilities of a material; many materials have an absorption coefficient above 10 4, but only a few, such as CdTe, have an absorption above 10 4 for wavelengths above 700 nm [3]

47 32 Indirect Forbidden Direct Forbidden h ) Indirect Allowed Direct Allowed Energy (ev) Figure 3.5: The adsorption coefficient, α, multiplied by the photon energy, hν, and raised to the power γ, generates curves for estimation of the indirect forbidden (γ = 1 3 ), direct forbidden (γ = 1 2 ), indirect allowed (γ = 2 3 ), and direct allowed (γ = 2) bandgaps. Bandgaps are estimated by extrapolating the curves across the x-axis at the onset of absorption. For this examples: indirect forbidden 3.0 ev, direct forbidden 3.2 ev, indirect allowed 3.5 ev, direct allowed 3.9 ev Bandgap The bandgap of a semiconductor contains information about its electrical and optical properties. Based on possible electrical transitions, every semiconductor can have up to four different bandgaps; direct allowed, direct forbidden, indirect allowed, indirect forbidden. Figure 3.5 shows the adjusted absorption curves for each, with estimation of the bandgap for an example case. The absorption curves are adjusted by taking a product of the absorption coefficient, α, and the energy of a photon, hν, and raising it to the power γ, where γ is 1/3, 1/2, 2/3, and 2 for indirect forbidden, indirect allowed, direct forbidden, and direct allowed, respectively. In most circumstances, the only relevant bandgap is the smallest allowed bandgap, e.g., silicon has an indirect allowed bandgap of 1.12 ev. The notation of direct or indirect for a transition in a semiconductor is determined by the change in crystal momentum experienced by a carrier undergoing the transition. A direct transition is a transition in which the carrier experiences no change in crystal momentum, k=0. An indirect transition is any transition in which the carrier experiences a change in crystal momentum, k 0.

48 Allowed and forbidden transitions are denoted by a set of selection rules. If a transition adheres to the selection rules, it is an allowed transition, and will occur in high probability in the semiconductor. If a transition violates one or more selection rule, it is a forbidden transition. Forbidden transitions cannot occur at the Γ point in crystal momentum space, or k=0 in k- space. For k 0 however, the probability of a forbidden transition is not zero. The probability of a forbidden transition increases with a k 2 dependance, leading to some instances of forbidden transitions occurring in semiconductors. Selection rules for allowed and forbidden transitions are referenced to the matrix element, M ik, which defines the probability of an electron transition between two arbitrary energy levels i and k. Mathematically the matrix element is defined as [97], M ik = q 33 ψ i rψ k dτ, (3.14) where the wave functions ψ i and ψ k represent the electron distribution at the initial and final energy states, respectively; r is the radius vector of the electron from the origin to the final state k; q is the charge of an electron; and dτ represents an integral over the entire volume of the electron, leading to an integration over all space. The absolute square of the matrix element, M ik 2, is directly proportional to the probability of an electron transition from quantum state i to quantum state k. A non-zero matrix element corresponds to an allowed transition, while a zero matrix element corresponds to a forbidden transition. In order to assess the matrix element further, and define selection rules, the wave function must first be expanded. For simplicity, calculations are shown for a hydrogenic atom, an atom with only one electron. The selection rules derived for a hydrogenic atom apply to atoms with more than one electron with only some discrepancies. These inconsistencies are addressed in reference [97]. The expanded wave function of a hydrogenic atom is defined as, ψ n,l,ml = R n,l (r)y l,ml (ϑ, ϕ), (3.15) where R n,l (r) represents the radial component of the wave function, and Y l,ml (θ,φ) represents the angular component of the wave function. Expanding the angular wave function further [97], ψ n,l,ml = 1 2π R n,l (r)θ l,ml (ϑ)e im lϕ, (3.16)

49 separates all of the individual dimensions of the wave function. This is important as it allows for three independent integrals to define a hydrogenic wave function, as shown in Eqns. 3.17, 3.18, and To move from a ground state to an excited state, an electron must receive energy, most often in the form of an photon. For simplicity, assume a linearly polarized wave with an electric field vector of E = { 0, 0, E 0 }. In such an interaction, the matrix element would simplify, as only the z-component is relevant. The matrix element would become [97], (M ik ) z = 1 π 2π R i R k r 3 dr Θ lk,m 2π k Θ li,m i sinϑ cosϑ dϑ e i(m k m i )ϕ dϕ, (3.17) r=0 ϑ=0 ϕ=0 where each of the three integrals represents a summation over the entire space of the electron. If the light was not linearly polarized, but circularly polarized and traveling in the z-direction, only the x-component and y-component of the matrix element would be relevant in determining the probability of an electron transition. Therefore, the matrix element could be written as [97], (M ik ) x + i(m ik ) y = 1 π 2π R i R k r 3 dr Θ lk,m 2π k Θ li,m i sin 2 ϑ dϑ e i(m k m i+1)ϕ dϕ, r=0 ϑ=0 ϕ=0 (3.18) (M ik ) x i(m ik ) y = 1 π 2π R i R k r 3 dr Θ lk,m 2π k Θ li,m i sin 2 ϑ dϑ e i(m k m i 1)ϕ dϕ. r=0 ϑ=0 ϕ=0 (3.19) It is important to note, that while the ϕ integral is dependant on which form of polarized light is applied, the radial and ϑ integrals do not depend on the polarization of the light, and therefore hold true for all polarization states of light. Magnetic Quantum Number Selection Rule One of the selection rules corresponds to the magnetic quantum numbers, which are confined only to the ϕ integrals in Eqns. 3.17, 3.18, and The law of conservation of angular momentum, dictates that for a photon-atom interaction in which the absorbed (emitted) photon has a non-zero spin, the atom must increase (decrease) its z-component angular momentum by the same amount. Mathematically, this can be seen by considering the ϕ integrals in each of the expanded matrix elements. In order to quantify a change in the magnetic quantum number, m is defined as [97], 34 m = m i m k, (3.20) corresponding to the difference between the magnetic quantum number of each quantum state. For the ϕ integral in Eqn. 3.17, the only outcome that will not produce a zero, resulting in a probability of zero for the electrical transition, is if m is zero. This indicates that only absorption or emission

50 35 of linearly polarized light cannot change the angular momentum of the atom without breaking the conservation of angular momentum, and is therefore a forbidden transition. Therefore, the selection rule for linearly polarized light is [97], m = 0. (3.21) For circularly polarized light, defined by Eqns and 3.19, the ϕ integral in one of the two equations is non-zero only for values of m=±1. This indicates that absorption (emission) of circularly polarized light must increase (decrease) the angular momentum of the atom to conserve angular momentum. The selection rule for circularly polarized light is [97], m = ±1. (3.22) Parity Selection Rule The parity selection rule, also known as the Laporte Rule, corresponds to the orbital interaction between the initial and final quantum states. In order to understand this selection rule however, the mathematics must first be addressed. The integrand in Eqn. 3.14, must be an even function, or the value of the equation will approach zero as the integral is taken over the entire volume of space. The radial component, r is an odd function, requiring that the product of the wave functions, ψi ψ k, also be an odd function in order to make the product of the two an even function. The product of the wave functions is determined by the individual wave functions, in which the parity, or symmetry of electrical charge about the origin, reflects the sign of the each wave function. An even parity wave function is center-symmetrical in reference to the location of all positive and negative charges; an odd parity wave function is not center-symmetric in reference to the location of all positive and negative charges. Using this definition, all s and d orbitals are even parity, while all p and f orbitals are odd parity. In order to get an odd function for the product of the wave functions, transition from i to k must be between orbitals of opposition parity. In other words, transitions from s to p or s to f are allowed, while transitions from s to s or s to d are forbidden. Thus, the parity selection rule is [97], l = l i l k = ±1, (3.23) where s l = 1, p l = 0, d l = 1, and f l = 0. Spin Quantum Number Selection Rule

51 36 For a hydrogenic case, in which only one electron interaction exists, electron spin is constant for all wave functions and is therefore ignored. For non-hydrogenic atoms however, electron spin dictates a third selection rule, also known as the multiplicity rule. This rule dictates that transitions cannot change spin, corresponding to [97], S = S i S k = 0. (3.24) The spin quantum number selection rule is not as strict as the parity selection rule, as it only holds true for small atoms with minimal spin-orbit coupling. For heavy atoms with non-negligible spin-orbit coupling, this rule does not hold true. Other Selection Rules Selection rules corresponding to number of electrons [98], crystal electronic state [98], total angular momentum [97], orbital angular momentum [97], magnetic dipoles [97], and two-photontransitions [97] exist. However, the probability of such transitions are at least 2-3 orders of magnitude less than the transitions corresponding to the magnetic quantum number, parity, and spin quantum number selection rules, and only factor into calculations under extreme cases in which all other transitions are forbidden Transmission The transmission of a material is significant for any layer in a solar cell that does not function to absorb light. If photons are absorbed by any of the contact layers instead of the absorber layers, this energy is effectively lost, reducing the efficiency of the solar cell. The transmission of a material is always described in terms of the wavelengths of energy in which the material can transmit. The transmission of an example material is shown in Fig As explained in the absorption coefficient section, Section , the transmission, T, through a semiconductor is a function of the total intensity, L, of incident light minus the amount of light reflected, R, and absorbed, A,. Mathematically, this relationship is [96], T = L (R + A). (3.25) X-Ray Diffraction X-ray diffraction (XRD) is a non-destructive technique for measuring the level of crystallinity and/or the composition of a film. Often used in place of more expensive analytical techniques, XRD is best suited for comparisons between two materials. The XRD procedure compares a material of

52 37 unknown crystallinity and/or composition, to a material of known crystallinity and composition. Based on similarities between the diffraction patterns of the two materials, assumptions are made about similarities between the crystallinity and the composition of the two films. If no similarities exist in the diffraction patterns of the two materials, however, XRD only establishes the lack of correlation. For an amorphous material, compositional assessment is very difficult using XRD. There are number of XRD methods; Laue, Bragg-Brentano, Seeman-Bohlin, Berg-Barrett, Lang limited-projection, scanning-reflection and composition, and Laue backreflection [99]. In the course of this research, the Bragg-Brentano method is the preferred XRD method. The Bragg- Brentano method of XRD employs an incident beam of x-rays at an angle, α, above a sample material. An x-ray detector measures the intensity and angle of the reflected x-rays, which are emitted from the sample material at an angle of twice the Bragg angle, θ B, as defined by Bragg s law [100], 2d sin θ B = nλ, (3.26) where d is the lattice spacing of the sample crystal, λ is the wavelength of incident radiation, and n is the order number of the maximum [100]. The maxima represents the assorted crystal spacing of the sample material, as shown in Fig. 3.6 for crystalline silicon sulfide, SiS Electrical Characterization Basic electrical characterization consists of measuring the conductivity, carrier type, mobility, and carrier concentration of a semiconductor. This section outlines the testing methodology used to determine each electrical parameter Conductivity Conductivity is a measure of a materials ability to conduct electrical current. In the case of semiconductors, conductivity is related to carrier mobility and concentration according to [15], σ(x) = q[n(x)µ n (x) + p(x)µ p (x)], (3.27) where q is the electrical charge, µ n (µ p ) is the electron (hole) mobility and n (p) is the electron (hole) concentration per unit volume. In an strongly n-type (p-type) semiconductor, the term corresponding to holes (electrons) is negligible, yielding the more familiar simplified form of the equation [15], σ(x) n type qn(x)µ n (x), (3.28)

53 Intensity (degrees) Figure 3.6: X-ray diffraction pattern of crystalline silicon sulfide using the Bragg-Brentano method. Each peak represents a lattice spacing parameter of the silicon sulfide lattice, with the highest intensity representing the crystal spacing of the highest occurrence. σ(x) p type qn(x)µ n (x). (3.29) There are several methods commonly used to measure the conductivity of a thin-film. For first-cut analysis, placing the leads of a multi-meter onto opposite corners of an imaginary box drawn on the surface of a single layer film allows a measurement of the sheet resistance for the film. The inverse of conductivity, resistivity, can be determined by taking a derivative of sheet resistance in terms of distance [16], d(r sh (x)) dx = ρ(x) = 1 σ(x), (3.30) The error in estimating the conductivity with this method is large, however, due to the difficulty in estimating the voltage dropped across the probe, contact, and spreading resistances, as shown in Eqn [101], I measured = 2 Rprobe V probe + 2 Rcontact V contact + 2 Rspreading V spreading + R sheet V sheet. (3.31) To reduce this error, a 4-point measurement is often used, with two probes supplying the current, and two different probes measuring the voltage. This arrangement improves the accuracy of the

54 39 estimation of the sheet resistance, as a minimal amount of current traveling in the voltage probes results in smaller values for V probe, V contact, and V spreading. Although 4-point measurements are often accomplished using four probes in a collinear geometry, a common variation is the Van der Pauw resistance measurement, which relies on four small contacts placed on the edges of an arbitrary shape, assuming no isolated holes in the film. This type of measurement is often taken multiple times, using different contacts for the voltage measurement each time to approximate the conductivity of a film [16] Majority Carrier Type Majority carrier type is the majority carrier of a material. If a material has a majority of electrons, the material is n-type; if a material has a majority of holes, the material is p-type. The simplest method for determining majority carrier type is to use a hot-point probe and measure the Seebeck coefficient. A hot-point is nothing more than a multi-meter, with one terminal heated to an elevated temperature. When the probes are placed on a semiconducting material, the majority carriers diffuse away from the heat more readily, resulting in a build-up of voltage on the cold probe. If the voltage is negative the majority carriers are electrons and the material is n-type; if the voltage is positive the majority carriers are holes and the material is p-type. Known as the thermoelectric effect, or Seebeck effect after the German physicist Thomas Johann Seebeck who discovered it 1821 [102], the amount of voltage as a function of the applied heat measured by the cold probe is often referred to as the Seebeck coefficient, in units of µv/k. The magnitude of the Seebeck coefficient can give an estimate of the majority carrier density with reference to the minority carrier density. If the Seebeck coefficient is large, the majority carrier density is significantly larger than the minority carrier density; if the coefficient is small this is not the case. The Seebeck effect is not always an accurate measurement of majority carrier type, especially for materials with relatively small differences between electron and hole concentrations, less than In most cases, the results determined from a hot-point probe are only used as a base guess of the majority carrier type. If more exact measurements are needed, a Hall effect measurement is conducted Contact Resistance Contact resistance is a measure of voltage dropped across an interface of two dissimilar materials when a current is induced across the interface. Often denoted as a determination of

55 40 interface quality, contact resistance indicates the nature of the material interaction. As is the case for all semiconductor devices, a minimal contact resistance is desired for solar cells resulting in current research into contact materials for absorber layers, such as CdTe. Measurement of contact resistance can be achieved in a variety of ways, as explained in reference [16]. For this research, the transfer line method is the preferred method of determining contact resistance. Transfer Line Method The contact resistance between two materials can be determined using the transfer line method (TLM) [16]. The TLM relies on the structure, shown in Fig. 3.7, which measures resistance between contacts of varied separation. The resistance between each of the pads is plotted against the seperation distance, as shown in Fig Extrapolating the line of points back and finding the intersection with the Y-axis gives a estimate for twice the contact resistance between the pads and the underlying material. The slope of the line is a function of sheet resistance. The transfer length, L T, can be approximated using this technique by finding the intersection on the x-axis, as shown in Fig Carrier Concentration Carrier concentration is a reference to the number of free carriers per unit volume of one type in a semiconductor. Carrier concentrations usually falls between carriers/cm 3 and carriers/cm 3. In classical semiconductor engineering, dopants are added to semiconductors to increase or decrease the number of available carriers. In thin-film devices however, this type of engineering is sometimes impractical. Instead, heterojunctions of multiple materials are often used to create the desired device structures. Carrier concentration is important to solar cells, as a solar cell is a bipolar device; the device operation is dictated by the minority carriers. In this research, carrier concentration is measured using the Hall effect Mobility Mobility is a measure of how easily a carrier can move in a semiconductor. Defined in terms of drift velocity as a function of applied electric field [3], µ = v d E, (3.32)

56 41 L Z W d d d d Figure 3.7: Two materials, one represented by the white box and the other by the black boxes, are deposited in this structure to allow for resistance measurements over varying distance. The length and width of the pads, as well as the spacing between each, determine the size of the contact resistance that can be measured [16]. R T L T R C slope = RSH / Z 0 d Figure 3.8: A graph of measured resistance as a function of contact separation, as created from the TLM. Extrapolation of the line generated from the TLM data points crosses the y-axis at twice the contact resistance, R C. The slope of the line is a function of the pad width and the sheet resistance of the bottom material. The transfer length, L T, can be estimated by extrapolating the line across the x-axis as shown [16].

57 mobility is often expressed in the units of cm2 V s. Electrons tend to have larger mobilities than holes, as the effective mass of electrons is smaller in most semiconductors. extreme of 77,000 cm2 V s 42 Mobilities range from an cm2 for the electron mobility of indium antimonide (InSb)[16], to V s for electron mobilities of various organic polymers [103]. In solar cells, mobility is an important parameter. Related to the diffusivity of a material according to the Einstein relationship [15], D n = kt q µ n for n-type, (3.33a) D p = kt q µ p for p-type, (3.33b) mobility is therefore related to carrier diffusion length. The diffusion length of a material represents the distance a minority carrier can travel in the material before it recombines. Ideally the diffusion length of an absorber material is very large, decreasing the likelihood of minority carrier losses. Diffusion length is related to the diffusivity and mean-time between collisions (τ) [15], L n = D n τ n for n-type. (3.34a) L p = D p τ p for p-type. (3.34b) In order to maximize the diffusion length, therefore, absorber materials are often chosen to have large mobilities, to maximize diffusivity, and minimal dopants, to maximize mean-time between collisions [15]. Hall Effect The preferred method for determining the majority carrier type, carrier concentration, and mobility is the Hall effect. The Hall effect, discovered by Edwin Hall in 1879 [104], is the phenomenon whereby an electric field is created by applying a magnetic field perpendicular to the direction of current in a material, as shown in Fig. 3.9(b). The induced electric field is generated as a consequence of the motion of carriers in the material according to the Lorentz force [3], F = qv B, (3.35) where q is the electron charge, v is the carrier velocity, and B is the magnetic field strength. As carriers accumulate on one side of the material, as shown in Fig. 3.9(b), an electric field is

58 43 established, which can be measured as a Hall voltage, V H, which is defined as [3], V H = BI qdn V H = BI qdp for an n-type material, for a p-type material, (3.36a) (3.36b) where q is the magnitude of electron charge, n is the concentration of electrons, p is the concentration of holes, B is the magnitude of the magnetic field, I is the current flowing through the material, and d is the width of the material. Rearranging either equation gives [101], where R H is the Hall coefficient. V H = R H BI, (3.37) d Once the Hall voltage and Hall coefficient are determined, the carrier concentration can be calculated, as evident from Eqns and The majority carrier type is determined by the sign of the Hall voltage. If the Hall voltage is negative the material is n-type; if the Hall voltage is positive the material is p-type. The Hall mobility, µ H, is defined as [3], µ H = R H ρ, (3.38) where ρ is the resistivity of the material. The conductivity mobility for electrons (holes) in an n-type (a p-type) material is defined as [3], µ H = rµ n for n-type, (3.39a) µ H = rµ p for p-type, (3.39b) where µ n (µ p ) is the conductivity mobility, and r is the Hall scattering factor [3], where τ is the mean time between carrier scattering events. r = < τ 2 > < τ > 2, (3.40) The Hall effect of a semiconducting thin-film is measured at OSU using the symmetric Van der Pauw structure shown in Fig. 3.9(a). The magnetic field lines run perpendicular to the thin-film, and the current and Hall voltage are measured from the contacts as indicated Device Characterization A solar cell device is characterized by current-voltage characteristics taken both in the dark and under illumination. This section describes current-voltage measurement techniques used at OSU.

59 44 I I (a) V H I d I + - B (b) V H Figure 3.9: Mobility, majority carrier type, and carrier concentration of a material can be determined by using the Hall effect. (a) The symmetric Van der Pauw structure used at OSU for measuring the Hall effect of a semiconductor, shown with connections to each of the contacts. (b) The classic material block used to illustrate the Hall effect, showing the four contacts and path of magnetic field induced carrier motion required to determine the Hall voltage.

60 Current-Voltage 45 Electrical characterization of an solar cell consists of a DC current-voltage measurement. This measurement is used to determine the current-voltage relationship of the device, both in the dark and in the light, as explained in Chapter 2. The DC current-voltage measurements are obtained using a Hewlett Packard 4156B Precision Semiconductor Parameter Analyzer connected to a BNC-based Micromanipulators probe station. The probe station is located inside a dark box, allowing measurements to be performed without any photon interactions. A single-bulb light source consisting of a 250W halogen bulb suspended above the probe station provides light for the illuminated portions of the current-voltage characteristics. Mathematical scaling is required for determination of the open-circuit voltage, as the 250W bulb is not sufficient to deliver the 1000 W/m 2 power characteristic of solar illumination. Superstrate solar cells, which have require the light to be incident through the substrate, as shown in Fig. 3.10, require a special setup for measuring the current-voltage characteristics. As opposed to substrate solar cells, which can be placed with the substrate down, with both the light and the probes on the top surface, superstrate solar cells require that the light be on the opposite side of the probes, as shown in Fig With the Micromanipulators probe station, illumination from the bottom was not possible, requiring the superstrate solar cells to be inverted for characterization. 3.3 Conclusions This chapter outlines the fabrication and characterization methods used in this research. An in-depth look at bandgap characterization is also presented, including a discussion of forbidden and allowed bandgaps and the corresponding selection rules. Fabrication methods including thermal, electron beam, pulsed laser deposition (PLD), and sputter deposition are discussed in detail. Post-deposition annealing is also addressed. Electrical and optical characterization methods include measurements of film conductivity, majority carrier type, mobility, carrier concentration, absorption coefficient, optical bandgap, and transparency.

61 46 Thin-Film Layers Back Contact Top Contact Top Contact Probe Light Incident From Back Back Contact Probe Figure 3.10: A superstrate solar cell structure with the test setup used to measure the currentvoltage characteristics of the device. Indium metal is used on the corner of the structure to contact the bottom ITO layer and form a back contact. Sixteen dots of 1 8 diameter aluminium or gold are deposited on top to form the top contacts. Light is incident from the back during device characterization, as required for solar cells in the superstrate configuration.

62 4. HIGH-VACUUM ELECTRON BEAM DEPOSITION SYSTEM 47 This chapter describes a high-vacuum electron beam deposition system custom designed and constructed at OSU. Hereafter referred to as the EBEAM, the electron beam deposition system is the latest addition to the thin-film processing capabilities of the solid-state materials and devices processing lab at OSU. A system overview is presented, including a discussion of future modifications. An introduction to the types of deposition processes currently available with the EBEAM system is also included. 4.1 Electron Beam Deposition At OSU, rapid material development is an important strategy employed for new materials exploration. New materials exploration benefits significantly from an ability to quickly process and characterize a new material. Due to the low success rate associated with identifying and qualifying a new material possessing properties appropriate for a specific type of device or application, a method for processing materials quickly and cheaply is required in order to sift through a large number of candidate materials in a reasonable amount of time at an acceptable cost.electron beam deposition is capable of the cheap, rapid processing required for new materials exploration, making it a useful tool for an exploratory project. Electron beam deposition is advantageous for new materials exploration for a number of reasons. From a target material standpoint, electron beam deposition is unique in that any material from the periodic table, in any solid form that can fit into a crucible can be deposited with minimal difficulty. Through a combination of high-energy electrons which heat the material significantly, and a low background pressure, almost any material can be evaporated by electron beam deposition. Evaporation by electron beam deposition causes only minimal substrate damage from highenergy species; evaporated species have a Maxwell-Boltzmann distribution of energy dictated by the temperature [94]. Electron beam deposition, however, can create potentially harmful x-ray radiation due to the Bremsstrahlung phenomenon [105, 106]; photons of wavelengths in the x-ray band are generated by the rapid deceleration of a fast moving electron, characteristic of a collision with the nucleus of an atom [100]. The x-ray radiation has been shown to damage sensitive devices structures, and introduce traps into the growing film [105].

63 48 Electron beam deposition occurs under high-vacuum conditions, thereby minimizing the number of gas-phase collisions experienced by the evaporated species. Under high-vacuum, 10 6 Torr, the evaporated species have a larger likelihood of colliding with a surface of the deposition chamber than with another gas-phase molecule. The species are described as having a large meanfree-path. Due to the minimal number of gas-phase collisions, the high power-density of electron beam can generate large deposition rates. According to studies done by Airco Temescal, rates as high as 100 nm/s have been observed [95]. The average deposition rate in current electron beam processing is about 2 nm/s. A fast deposition rate translates into both a shorter deposition time and less contamination in the deposited film. Contamination arises from incorporation of background gas species in the vacuum chamber into the growing thin-film. For sulfur and selenium based thin-films, activated oxygen, O 2, is a contaminate which is always present in a deposition chamber. The impingement rate, derived in Chapter 3, (p πr3 ) φ = 2 3 mkt, (4.1) for activated oxygen with an ionic radius of nm [19] and an atomic mass of kg [19], is nm/s for a chamber pressure of 10 6 Torr and a temperature of 300 K. In order to minimize the amount of contamination in a thin-film, the deposition rate must be kept well above the estimated impingement rate of contamination. The faster deposition rate, therefore, provided by electron beam deposition minimized the amount of background gas incorporation. Given the advantages of electron beam deposition, and the lack of a modern electron beam deposition system in the solid state processing lab at OSU, the EBEAM was constructed over a period of three years, starting in Design The EBEAM was designed by Chris Tasker, with input from myself and other former and current members of the materials and devices research group at OSU. This section is an outline of that design process, highlighting the key features of the EBEAM design. When designing a new deposition tool, the first step is to understand the processing goals for the system. Types of materials, size and type of substrates, and deposition uniformity requirements are just some of the considerations that drive the initial design process. Design of a system is often from the inside out, as shown in Fig Processing goals factor into the design of the

64 49 Substrate Stage Deposition Source Power Supply Peripherals Deposition Chamber Vacuum Pumps Control Systems Figure 4.1: General deposition system design procedure. A useful rule-of-thumb is to design from the inside out, starting with the substrate stage, deposition source, and peripheral systems, and ending with the control systems. substrate stage, deposition source, and required peripheral systems for additional deposition or in situ analysis. The chamber size and layout are dictated by the design of the substrate stage, as well as the size and number of required subsystems. Once the chamber is designed, its volume and geometric flow paths are used to determine the pump speed and ultimate pressure required to create the necessary level of vacuum. In the case of electron beam deposition, high-vacuum pressures below 10 6 Torr are advised, although pressure up to 10 4 can be used [107]. The final design step for a deposition tool is to organize the control systems and interlocks required to operate the large ensemble of subsystems. The control systems of the EBEAM were designed simultaneously to the general system design, as explained in Section Substrate Stage The substrate stage of a deposition tool is the heart of the system. Required for all depositions, the substrate stage must have a clear, unobstructed view to all of the deposition sources.

65 50 Some design constraints for the substrate stage include; a method for loading and unloading of the substrates, a secure system for holding the substrates during deposition, and a technique for heating the substrates. Other design constraints could include additional substrate stage motion, use of voltage bias on the substrate stage, as well as methods for cooling the substrates, or in situ analysis of the growing films. When designing the EBEAM substrate stage, flexibility with regard to substrate size was a key consideration. Although it was anticipated that the most common substrate size would be 1 x 1, the EBEAM was designed to accommodate substrates ranging in size from 10 cm x 15 cm up to 6 inch diameter wafers. Thus, flexibility in the substrate size was factored into the design criteria of the substrate stage. The substrate stage of the EBEAM consists of two main pieces, as shown in Fig The Z-translation plate is suspended within the chamber, with some of the deposition utilities attached to it, including a deposition shutter and two thickness monitors. The substrate plate, which holds the individual substrates, fits into the Z-translation plate as shown, with the front plate aligning the substrates to the Z-translation plate. The substrate plate is removable, to allow for different front plates to be swapped in and out, depending on the substrate size required. The front plate shown in Fig. 4.2 is designed for commonly used 1 x 1 substrates. The name for the Z-translation plate comes from the fact that the plate is suspended from the Z-translator in the deposition chamber, as shown in Fig The translator allows for adjustable source-to-substrate distance, a desirable quality E-Source The Temescal STIH-270-2MB 4-pocket electron beam source was chosen for the EBEAM because it has been an industry standard for many years, with a proven track record and used units available at reduced cost. From a functionality standpoint, the STIH-270-2MB 4-pocket E-source is more than sufficient for the intended research applications, making it a logical choice for use in the EBEAM. With the 4-pocket Temescal STIH-270-2MB electron beam source, the EBEAM has four key processing advantages. The location of the filament on the E-source, used to boil off or source electrons, is below the body of the E-source, as shown in the cross-sectional view of the E-source in Fig This filament location minimizes coating of the fragile wire of the filament, thereby enhancing the lifetime of the filament dramatically. Water cooling protects the E-source

66 51 Back Plate 1 x 1 Substrates Substrate Plate Shutter Front Plate Base Plate Thickness Monitors Space for Front Plate Figure 4.2: The EBEAM substrate stage consists of the substrate plate and the Z-translation plate. The substrate plate can be removed, and holds individual substrates between the front plate and the back plate. The Z-translation plate is suspended within the deposition chamber and provides alignment between deposition sources and the substrate plate. from the heat generated by the electron beam, allowing for increased deposition temperatures and higher power density over a traditional thermal source. The STIH-270-2MB 4-pocket system of the E-source allows for up to four different materials to be deposited without exposing the deposition chamber to atmosphere between each run. The EBEAM can also raster the electron beam across the surface of the target material, using a Super Sweeper. Sweeping the electron beam improves the evaporation rate and target material utilization. Without the capability to raster the electron beam, the target material could only be deposited from one small spot, leading to a reduction in material efficiency. Overall, the STIH-270-2MB E-source provides an ideal deposition platform for use in the EBEAM. Additionally, the Temescal STIH-270-2MB 4-pocket electron beam source allows the use of crucible liners, constructed mainly out of graphite. Aside from protecting the source and allowing the removal of targets from the source, the liners provide an insulation layer between the target material and the water cooled source, allowing for deposition of material with high thermal conductivity. Without the crucible liner, the heat would diffuse to rapidly to form a localized melt [107].

67 52 Target Pellet Extra Pellet Water Cooling Filament Figure 4.3: A cross-section of the E-source used in the EBEAM, showing two of the four pockets. The electron beam is generated by electrons boiled off the filament and accelerated by a large electric field. Control of the electron beam is accomplished by a series of magnetic fields. Water cooling protects the E-source from the high temperatures generated by the electron beam. An electron beam power supply is required to generate the high voltages necessary to sustain the electron beam. As a research tool, the ability to deposit any conceivable material is highly desirable, leading to the purchase of the Temescal CV-8A electron beam power supply, which can supply voltages up to 10,000 volts. Currently, the power supply is set to supply 7,000 volts, as opposed to higher supply voltages, which leads to improved control for depositing low melting temperature materials Peripheral Systems Many different types of subsystems may be installed into a deposition tool. For an electron beam deposition system, a useful addition is a thermal source. Certain multi-component materials evaporate incongruently, i.e., atomic constituents do not evaporate at the same rate. Thus, stoichiometric thin-films cannot be obtained using a single non-continuous evaporation source. For target materials with extreme separation between melting temperatures, the higher temperature material may not deposit at all. To alleviate this problem, multiple sources are often employed to deposit the lower melting temperature materials simultaneously during electron beam deposition. This is referred to as co-deposition. The use of one or more thermal sources allows the E-source to be used to deposit the more refractory constituents.

68 53 Initial designs of the EBEAM included four thermal sources, intended for multi-source deposition. Currently however, only one thermal source is installed in the EBEAM and used for thermal deposition or for co-deposition in concert with the E-source. An ion source, for substrate cleaning and film densification is included in the design, shown in Fig. 4.4 and 4.5. A sputter flange, for up to three sputter guns, for sulfur and selenium depositions, is currently in the design stage. The sputter flange will be installed above the turbo pump, as indicated in Fig. 4.4, preventing the deposition flux from entering the pump directly. However, as seen in Fig. 4.5, the substrate stage will need to be modified to allow for the sputter guns to deposit onto the substrate Deposition Chamber A deposition chamber is designed so that it achieves a balance between cost and functionality. Each part of a deposition chamber can accommodate a flange of variable size, thereby providing the possibility of interacting inside and outside the chamber. A larger flange can accommodate a larger subsystem or multiple subsystems to be attached to the flange. Smaller flanges are more restrictive, usually allowing for only one small subsystem attachment. Maximizing the size and number of flanges on a deposition chamber, therefore, maximizes the functionality of the chamber. A larger flange is more costly however, leading to a trade-off between chamber cost and chamber functionality. A larger flange also increases the internal surface-area of the deposition chamber, which increases the required pump size and therefore the cost. When a deposition tool is designed for manufacturing, flange sizes are often minimized to keep the cost low, as a production tool is rarely used for anything outside the scope of the original design. For a versatile research tool however, more and larger flanges are desirable within given budget constraints. The EBEAM has a large, barrel-shaped chamber, dominated on one end by a large access door, as illustrated in Fig. 4.4 and Fig Designed to maximize the internal volume while minimizing the internal surface-area, the EBEAM chamber is designed to afford a spacious processing environment, without sacrificing pump-down time. Pump-down time is a function of the internal surface-area in the chamber, among other factors, with a large surface-area resulting in a long pump-down time, and a small surface area resulting in a short pump-down time. Tight-fitting deposition shielding lines the inside of the chamber, covering all chamber surfaces which would otherwise be coated during deposition. Due to an increase in the pump-down time associated with the removal of trapped gases within layers of deposited material that build up on the shielding over

69 54 Bellows Assembly Thermal Source Flange Z - Translator Location of Future Sputter Gun Flange Gate Valve Location of Future Ion Source Access Door Turbomolecular Pump Figure 4.4: External view of the EBEAM, showing the access door, Z-translator, gate valve, and turbomolecular pump. Future locations for an ion source and a sputter gun flange are also depicted. The bellows assembly provides a collapsible vacuum chamber, allowing for motion of the Z-translator. Substrate Stage Location of Future Sputter Gun Flange Location of Future Ion Source Thermal Source E - Source Figure 4.5: A cross-section of the EBEAM showing the relative location of the E-source, thermal source and substrate stage. Future locations for an ion source and a sputter gun flange are also depicted.

70 55 time, the shielding needs constant cleaning, leading to the incorporation of a large access door in the deposition chamber. The substrate stage and E-source are the central focus of the deposition chamber, with the thermal source off to one side, as shown in Fig Additional flanges cover all of the external surfaces of the chamber, as shown in Fig. 4.4 and Fig In addition to the access door, turbomolecular pump flange, z-translator flange, future sputter gun flange, future ion source flange, and thermal source flange, as shown in Fig. 4.4, two of the external flanges are currently unused. The 10 flange above the thermal source flange and the large flange on the bottom of the deposition chamber do not currently have an intended use. These flanges allow for future modifications, potentially allowing this system to stay current with state-of-the-art subsystems. The flexibility afforded by extra flanges on the deposition chamber was a key consideration in the design of the EBEAM system. Suspension of the substrate stage base plate, described in Section 4.2.1, is accomplished by four 3/8 steel rods connecting the base plate to the top flange on the deposition chamber, as shown in Fig The top flange of the deposition chamber is connected by a bellows assembly, which provides a collapsible vacuum chamber allowing motion of the top flange relative to the deposition chamber. Z-translation is accomplished by adjusting the distance between the top flange and the deposition chamber through the use of an AC motor assembly. The deposition source-to-substrate distance in the EBEAM can be adjusted from 9 to 15. The materials of a deposition chamber are usually chosen to have low gas permeation and high corrosion resistance, as well as other factors. Common materials include stainless steel, aluminium, and quartz/glass. The chamber for the EBEAM is constructed out of 304 series stainless steel. Which is chosen for corrosion resistance, low magnetic properties, high temperature range, and vacuum integrity. Material choices for vacuum seals are often based on processing compatibility, vacuum performance, and desired temperature range. In the case of the EBEAM, copper conflat R gaskets make up a majority of the seals, with the large access door, and assorted vacuum values, the only exceptions. The large access door and valves are sealed with Viton R elastomer (o-ring) seals, which provides a reusable vacuum seal, as is necessary for a door or application involving a moving seal. Copper conflat R gaskets, which provide lower gas permeation and higher temperature range than Viton R elastomer (o-ring) seals, can only be used once, making them ill suited for door sealing applications.

71 Vacuum Pumps As explained in Section 4.2.4, the internal surface-area of the chamber dictates the required pump size. Pumps are often specified with respect to a chamber volume, which provides a rough translation to the internal surface area. A common rule-of-thumb, is to use a pump rated for a pump speed of ten times the chamber volume per second. Thus, for the EBEAM with a chamber volume of 160 L, a pump speed of 1600 L/s is the recommended pump speed.[108]. The EBEAM uses a magnetically-levitated BOC STP-A2203 series turbomolecular pump, or turbo pump, to achieve high-vacuum. Turbo pumps consist of a series of fast spinning and stationary fan blades used to create a pressure differential across the length of the pump. The angular speed of the blades is determined by the rotational speed and the diameter of the pump, with larger pumps requiring lower rotational speed to achieve the same angular velocity at the tip of the fan blades. In order to pump out hydrogen molecules, which move at speeds on the order of 1900 m/s, the tip of the fan blades need to be moving at 400 m/s. In the case of the turbo pump used on the EBEAM, which is 10 in diameter, 27,000 rev/min is required to pump hydrogen. The turbo pump on the EBEAM has a listed pump speed of 2200 L/s for N 2, well above the 1600 L/s recommended for the deposition chamber size. This pump has an inlet pressure rated at 10 8 Torr. The additional pump speed in excess of the recommended value is important for electron beam deposition due to the high deposition rates, which have the potential of bogging down a high-vacuum pump during deposition. Backing the high-vacuum pump is a Leybold DRYVAC 100S foreline pump. Required to prevent a large pressure differential from building up across the turbo pump and causing pump failure, the foreline pump keeps the outlet of the turbo pump at pressures on the order of 10 3 Torr. The backing pump is a dry pump, with no oil exposed to the vacuum path. There are two disadvantages associated with using oil-based pumps. Oil-based pumps have the potential of back streaming oil up the fore-line, contaminating the turbo pump, deposition chamber, and substrates. Oil must also be changed on a regular basis in oil-based pumps, requiring increased maintenance and cost requirements. Use of a dry foreline pump makes the EBEAM an oil-free deposition system. The rough pump on the EBEAM is a Welch 1397 pump, capable of pumping the chamber down to 10 3 Torr in a matter of minutes. Although the Welch 1397 pump is an oil-based pump,

72 57 the rough pump is only used for a short duration, and never unattended. Careful use of this pump reduces the risk of oil back streaming up the roughline into the chamber. Under high-vacuum conditions, the motion of gas particles remaining in the chamber is governed by molecular flow. Characterized by a large mean-free-path between collisions, particles in molecular flow will only be pumped out of the chamber if they wander into the inlet of the pump. For this reason, the conductance of a high-vacuum pump to the chamber is very important. Conductance is defined as a measure of how easily a gas enters or flows through a pipe or conduit. In a deposition chamber, the concept of conductance can be illustrated by considering the crosssectional area of an opening, with a large opening corresponding to a large conductance and a small opening corresponding a small conductance. If a high-vacuum pump is connected to a small conductance opening on a chamber, the pumping speed of the pump is wasted and the pump down time will be conductance limited. Therefore, placement of the high-vacuum pump in a deposition chamber is crucial for achieving an optimal pump-down time. The turbo pump on the EBEAM is located on the lower side-wall of the deposition chamber, as seen in Fig This orientation provides a high conductance to the chamber and prevents objects and particles from falling into the inlet Control System The design of the EBEAM control system centers around a National Instruments CompactRIO 9102 controller, an embedded real-time controller chosen for its reconfigurability; RIO denotes reconfigurable input/output. The CompactRIO runs a LabVIEW R control code which interacts with a LabVIEW R front-end program running on a Dell Originally designed by Matt Spiegelberg, who designed the control system simultaneously to the general EBEAM system design, the programs control the vacuum states of the EBEAM, moving the system between three different vacuum states, as shown in Fig The system operator can call each of the five protocols shown in Fig. 4.6, depending on the machine state. Table 4.1 lists the gate valve and high-vacuum pump conditions that define each vacuum state. The final state on Table 4.1 is not possible due to the risk of high-vacuum pump damage if the gate valve is open and the high-vacuum pump is off. The CompactRIO interfaces with each of the EBEAM subsystems through a collection of DIN rail mounted connections, as shown in Fig Inputs to the CompactRIO include a series of interlocks, allowing the control code to determine the current vacuum state of the machine, and

73 58 run emergency shutdown protocols if a system interlock indicates potential harm to the EBEAM or operator. The interlocks rely on a 24 VDC signal provided by the power supply in Fig The outputs of the CompactRIO enable power to each of the EBEAM subsystems through a series of contactors in the power distribution box. These subsystems include all of the vacuum pumps and valves, as well as the E-source power supply. The 120 VAC bus provides power for all of the EBEAM control subsystems, shown in Fig Controls for all of the EBEAM subsystems are contained in one 19 rack, as shown in Fig The computer interface provides connection to the CompactRIO through a front-end LabVIEW R program contained on a Dell A simulation and program editor for the electron beam sweeper is also available in software. Directly above the computer, is the emergency off (EMO) button, placed for maximum visibility. The EMO button shuts off all power to the power distribution box, providing a safety outlet in case of an emergency. A power switch for the roughing pump is also close at hand, allowing the pump to be switched on only when needed, thereby conserving energy, and protecting the EBEAM from potential oil contamination. The turbo pump and vacuum gauge controller are included in the rack, as seen in Fig These subsystems display information about the turbo pump and chamber pressure, respectively, and are remotely controlled from the CompactRIO control code. Once a high-vacuum pressure is reached and the EBEAM is in the HIGH-VAC vacuum state, the E-source power supply is enabled. The E-source power supply controller, shown in Fig. 4.7, controls the filament current and bias-voltage of the E-source. Once the E-source is powered up, the electron beam sweeper takes over, running one of the programs uploaded from the computer, rastering the electron beam across the target surface. The thickness monitor controller interfaces with thickness monitors located inside of the deposition chamber. These thickness monitors provide an estimate of the deposition rate, based on input material density and acoustic impedance, compared against a measured oscillating frequency from the thickness monitor crystals. This information is used by the operator to control the deposition sources in order to achieve a desired film thickness. The substrate temperature controller and the z-translator controller, not shown in Fig. 4.7, are not connected to the CompactRIO directly, allowing them to be used independently of the EBEAM vacuum state. However, they do receive power from the 120 VAC bus connected to the power distribution box, and therefore will be shutoff if the EMO button is tripped.

74 59 Power Up Hi Vac OFF STANDBY HIGH-VAC Shutdown Standby Vent Figure 4.6: Vacuum state control diagram for the EBEAM. Each circle represents a vacuum state of the machine. Moving between each state is dictated by the operator, through a choice of five protocols illustrated by the arrows. Vacuum State Gate Valve High-Vacuum Pump OFF Closed Off STANDBY Closed On HIGH-VAC Open On - Open Off Table 4.1: The vacuum state table for the EBEAM, shown for each condition of the high-vacuum gate valve and the high-vacuum pump. The final state is not possible due to system interlocks protecting the high-vacuum pump.

75 60 Substrate Temperature Controller Vacuum Gauge Controller E-Source Power Supply Controller Electron Beam Sweeper Thickness Monitor Controller Turbo Pump Controller Rough Pump Power EMO Button Computer Interface Figure 4.7: The front-side of the EBEAM control rack. The computer interface provides interaction with the vacuum state control network. Each of the systems can be operated by the operator during some or all of the vacuum states. The emergency off (EMO) button, used to protect the operator or equipment, cuts power to all EBEAM subsystems.

76 61 24 VDC Power Supply CompactRIO CompactRIO Outputs CompactRIO Inputs DIN Rail Output Control Wires Input Control Wires Output to Systems 120 VAC Bus Input Power Power Distribution Box Figure 4.8: The back-side of the EBEAM control rack. All of the power required for operation of the EBEAM passes through the distribution box. The 120 VAC bus provides power for all of the control systems. The DIN rail connects the CompactRIO to all of the control wires. A 24 VDC power supply is used to supply the power for all the interlock systems.

77 Future Modifications Although construction of the EBEAM was only completed in the Fall of 2006, modifications and additions to the EBEAM are already underway. Currently, construction of a load-lock system is in the final stages. Design work on an upgrade to the thermal sources, and a sputter gun flange are also in various stages of progress Load-Lock Due to frequent depositions of sulfur, which readily absorbs water, and other material which have high rates of out-gassing, the time required to pump out the EBEAM is extremely long. A load-lock system is the obvious solution to this problem. A load-lock subsystem, shown in Fig. 4.9 and Fig. 4.10, is currently in the final stages of construction. The load-load was designed by me, with input from Chris Tasker, soon after completion of the EBEAM, as issues with long pump-down times in the deposition chamber became apparent. The idea behind a load-lock is to allow access to the deposition chamber for loading and unloading substrates, while the deposition chamber is under vacuum. Access to the chamber, without the requirement of venting the chamber to atmospheric pressure, potentially reduces the pump-down time associated with each deposition significantly. It also provides a more consistent deposition environment. The load-lock relies on an isolation gate valve, shown in Fig. 4.9 and Fig to separate the load-lock chamber from the deposition chamber when they are at different pressures. Operation of the valve will be controlled by the LabVIEW R program discussed in Section The standard operating procedure for the load-lock will be to open the load-lock door, shown in Fig. 4.9, and load the substrate plate, which is a separate piece of the substrate stage as described in Section 4.2.1, onto the X-translator. After closing the door a rough pump will rough out the load lock chamber through the small turbomolecular pump shown in Fig As the pressure of the chamber is reduced, the turbomolecular pump will be turned on, pumping the load lock chamber down to 10 5 Torr. At this point the gate valve separating the load lock chamber and the deposition chamber is opened, allowing the x-translator to load the substrate plate into the deposition chamber. The z-translator will provide the motion necessary to load and unload the substrate plate from the x-translator, as shown in Fig Once the substrate plate is loaded onto the substrate stage, the x-translator is retracted, allowing the gate valve to be closed. After deposition, the gate valve is opened and the x-translator is used to unload the substrate plate from the substrate stage. Once the x-translator is again retracted, bringing the substrate plate

78 63 into the load lock chamber, the gate valve is closed and the load lock chamber is vented. The deposition chamber will not need to be vented to atmospheric pressure to perform a deposition after the installation of the load-lock. Loading and unloading targets for the deposition sources, will still require use of the large access door, and therefore venting of the deposition chamber Thermal Source Upgrade Original plans for the deposition chamber of the EBEAM included four independent thermal sources in addition to the e-source, as described in Section Currently, a single thermal source resides inside the deposition chamber, providing the thermal deposition capability necessary for the current research. Figure 4.11 shows a cross-sectional view one possible thermal source configuration. The idea is to install a large plate inside the deposition chamber and bolt on four insulated posts. The posts will provide a connection for the hot terminal of each independent thermal source, while the plate will provide a common neutral connection Sputter Gun Flange Sputtering capability is a very desirable quantity and has motivated the design and construction of a sputter gun flange, shown in Fig Currently, the design involves two horizontally mounted sputter guns, as indicated. The location of the sputter flange, shown in Fig. 4.5 and Fig. 4.4, allows for the z-translator to shift the substrate stage up or down, depending on which sputter gun is employed for the deposition. Sputtering in the EBEAM will provide an opportunity for sulfur and selenium sputtering, which is not currently discouraged in the materials and devices solid state processing lab at OSU. 4.4 Deposition Processes Currently, there are three types of deposition processes available with the EBEAM; electron beam, thermal, and both. Each of these processes have been used extensively since the construction of the EBEAM. A brief discussion of each process with some examples is included here Electron Beam Deposition As the intended purpose of this tool, the EBEAM has been used for numerous electron beam depositions. Although limited by issues with stoichiometric transfer, as discussed in Chapter 3, electron beam deposition is capable of depositing almost any material, including highly refractory

79 64 Gate Valve Turbo Pump Z - Translator Access Door X - Translator Load-Lock Chamber E-Source Flange Deposition Chamber Figure 4.9: External view of the EBEAM, showing the load-lock chamber with assorted peripheral subsystems; turbo pump, gate valve, and x-translator. The e-source flange on the deposition chamber provides feedthroughs for the high-voltage lines, and requires a plexiglass case, as shown, to protect the operator. The z-translator and access door are also indicated. Substrate Stage Extended Translator E-Source Figure 4.10: A cross-sectional view of the EBEAM, showing the x-translator extended for loading and unloading of the substrate plate.

80 65 Individual Hot Connections Boat Insulator Common Neutral Connection Figure 4.11: A cross-sectional view of a possible thermal source upgrade, based on a drawing by Chris Tasker [108]. Individual boats will be connected to individual hot connections, with a common neutral connection. This design has the potential for up to four thermal sources Flanges for Sputter Guns Figure 4.12: A design showing the atmosphere side and edge of the new sputter gun flange, as designed by Chris Tasker [108]. The current design allows for two sputter guns to be installed in a vertical configuration.

81 materials. Some work has been done with the EBEAM, depositing refractory materials including molybdenum and tantalum with good success Thermal Deposition Depositions requiring a single thermal source are possible in EBEAM. Although multiple systems exist in the solid-state materials and devices processing lab at OSU for thermal deposition, the large substrate size, large maximum current limit, and acceptance of sulfur, selenium, and other materials restricted throughout the rest of the lab have led to the use of the EBEAM for thermal depositions. Gold, copper, tellurium, and aluminum have all been deposited successfully using the thermal source in the EBEAM Co-deposition Co-depositions, depositions involving both the thermal source and the electron beam source, are the largest percentage of work done in the EBEAM to date. Sulfur and selenium containing species, requiring a thermal deposition of elemental sulfur or selenium have both been carried out in the EBEAM successfully. Current research including Fe 2 GeS 4, BaCuSnS, and Fe 2 SiS 4 have all been deposited using the co-deposition method in the EBEAM. 4.5 Conclusion This chapter describes the new electron beam system designed and constructed in the solidstate materials and devices laboratory at OSU. Referred to as the EBEAM, the electron beam system has the capability to perform thermal, electron beam and co-depositions using both sources. Potential upgrades including a load-lock are discussed.

82 5. INORGANIC THIN-FILM SOLAR CELLS 67 This chapter is a summary of research results. The focus of this thesis is on material development for inorganic thin-film solar cell applications. Inorganic thin-film solar cells of interest can be described as pin heterojunctions, as shown in Fig. 5.1, consisting of two contact layers, an n-type and a p-type window layer, and an intrinsic absorber layer, as described in detail in Chapter 2. The focus of this research is on material development for the p-type window and absorber layers, both shown in bold in Fig This chapter is organized in semi-chronological order, presenting conclusions arrived at throughout the duration of this research project. A model, developed by Mönch for Schottkybarriers [109] and extended to heterojunctions by Wager [110], is employed to various back-contacts to assess the electrical interface properties. Work involving inorganic thin-film solar cells began at OSU with the implementation of barium copper tellurium fluoride (BCTF) as a p-type window layer into a CIGS solar cell. Issues with material compatibility between BCTF and CIGS resulted in a shift of research focus to new absorber materials, namely iron silicon sulfide and barium copper tin selenide (BCSS). Hampered by difficulties in testing new absorber materials, the focus then shifted back to BCTF, using BCTF as a p-type window material for CdTe solar cells. The material development work described in this chapter was undertaken in close collaboration with Peter Hersh, Heather Platt, Richard Schafer, Robert Kykyneshi, Professor Douglas A. Keszler, and Professor Janet Tate. 5.1 Material Development / Characterization The material development portion of this research was accomplished by the inorganic chemistry group at OSU, who has developed a reputation for expertise in identification and synthesis of new materials and compounds. Thin-film synthesis of the materials was preformed by the materials and devices and solidstate physics groups at OSU. Details of the thin-film deposition methods are included in Chapter 3. Characterization of the materials in powder, pellet, or thin-film form, through X-ray diffraction, and determination of Hall and Seebeck coefficients, was preformed as necessary by the

83 68 p-type Window Front Contact h Absorber Back Contact n-type Window Figure 5.1: An idealized pin solar cell energy band diagram under short-circuit current conditions. The five regions of an ideal thin-film solar cell are labeled, with the absorber and p-type window regions highlighted in bold as they are the focus this research.

84 inorganic chemistry and solid-state physics groups at OSU. Details of these various tests are included in Chapter Absorber Materials A solar cell is primarily defined by its absorber material. All materials in a solar cell must be compatible with the absorber layer. Ideally, two compatible materials would have a chemically neutral interface which is electrically conductive, allowing for carrier transport across the interface. In the most general sense, an absorber material is defined by a set of key material properties. An absorber material must have a large absorption coefficient, because it minimizes the thickness of the absorber layer; p-type conductivity is a requirement because solar cells are bipolar devices, with device operation determined by minority carriers, electrons in this case; a minimal dopant concentration is also crucial, as it maximizes the minority carrier lifetime of the photo-generated electrons. As explained in Chapter 3, an ideal absorber material is hypothesized to be p-type with an absorption coefficient on the order of 10 5 cm 1, for wavelengths up to 500 nm, and a carrier concentration of cm 3. In the twenty years since copper indium gallium diselenide (CIGS) was first developed by Chen et al. in 1987 [12], no thin-film absorber material has been implemented into a thin-film solar cell with an efficiency above 5%. In fact, very few absorber materials have even been proposed in that time, and none have provoked any serious attention in the thin-film solar cell market. A case is sometimes made for the continued commercial development of the current thin-film absorber materials at the expense of new absorber material development, in which it is claimed that current materials are sufficient for future applications and that new materials exploration requires too much investment in time and money. Given the potential and size of the world photovoltaic market, however, exclusive pursuit of this path seems imprudent. Although both cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) have been steadily improving with respect to solar cell efficiency since their inception, neither absorber material has realized a solar cell efficiency above 20 % [13]. According to Green [111], an efficiency of 31.1 % is required to compete with the costs associated with fossil fuel produced energy. Even with recent government subsidy programs designed to increase production and demand of solar cells by reducing the cost, CIGS and CdTe do not currently compete with any form of fossil fuel supplied energy on the basis of cost.

85 70 The first liability of current absorber materials is their cost and the availability of raw materials. Table 5.1 shows the price per metric ton and world-wide production in metric tons for various relevant metals and materials for the year The prices of indium and gallium are high, and have been steadily increasing, highlighted in bold in Table 5.1. Currently 65% of the indium consumed is for the production of indium tin oxide, used primarily in the flat-panel display industry [112]. Estimates for the future supply of indium appear reasonable; according to the US geological society [112] a shortage of indium is not expected in the near future, mostly due to large stock piles world wide. Due to the fact that indium is primarily recovered as a byproduct from zinc mining, however, the cost of indium is not expected to decrease, as the supply of zinc is decreasing while the demand for indium is increasing. For CIGS to compete economically with other absorber materials in cost, an alternative to indium is needed. If an alternative to indium is not found, the price of raw indium could make CIGS cost-prohibitive for most solar cell applications. Gallium, although cheaper than indium, is also not in an ideal situation, with only 60 metric tons produced in 2004 [113]. In an industry where price is a critical consideration, CIGS appears to have a difficult road fiscal ahead. Cadmium telluride is in a somewhat better position in terms of material cost, as evident from Table 5.1. Some issues may arise with respect to the cost of tellurium, as the world-wide production is low, as highlighted in bold in Table 5.1. Considering the current market, however, there appears to be a low likelihood of CdTe ever becoming cost-prohibitive due to the cost of raw materials. Ignoring raw material cost, environmental concerns have stymied the development of CdTe solar cells. Although some studies suggest minimal environmental harm associated with damage to residential installed CdTe solar cells from fire [114], and minimal environmental harm associated with a CdTe module sitting on a residential roof for years [115], factories constructing CdTe solar cells will be hazardous waste facilities, requiring elaborate worker protection and waste cleaning procedures. Cadmium is also controlled in the European Union [116] due to environmental concerns, eliminating a large potential market for CdTe solar cells. If these material and environmental concerns associated with CIGS and CdTe are momentarily ignored, the solar efficiencies associated with each system are not currently sufficient to compete with fossil fuel produced energy. In fact, according to some estimates, even if the raw material cost of the current solar cells can be reduced to zero, the current efficiencies at the module level are too low, requiring installation costs that translate into higher costs per unit energy than

86 71 Material Price ($/MT) World-Wide Production (MT) Barite (BaSO 4 ) ,240,000 Cadmium ,700 Copper ,600,000 Fluorspar (CaF 2 ) 157 5,060,000 Gallium 542, Germanium 592, Indium 592, Iron ,340,000,000 Selenium 54, Silicon ,900,000 Sulfur ,100,000 Tellurium 21, Tin 11, ,000 Table 5.1: Price and world-wide production of various metals and materials important to current and potential thin-film solar cells [113]. The raw material cost of indium and gallium, highlighted in bold, may limit the use of CIGS solar cells.

87 72 Efficiency (%) CIS/CIGS CdTe 19.2 % 16.5 % Figure 5.2: A chronological record of the reported maximum efficiencies for CIS/CIGS and CdTe solar cells from 1977 through 2007 [118]. No efficiency increase has been reported for CIGS or CdTe for at least 3 years. fossil fuel energy sources [117]. Figure 5.2 shows a chronological record of the reported maximum efficiency of each thin-film absorber material [118]. The trend shown is alarming, indicating an apparent saturation, suggesting minimal expected efficiency gain for any new developments involving CIGS and CdTe. If neither material can be improved significantly, reaching Green s theoretical efficiency of 31.1 % [111], these solar cells will face serious difficulties competing with current fossil fuel energy sources. Given these concerns regarding the commercial viability of CIGS and CdTe thin-film solar cells, it seems prudent to develop new absorber materials. Two promising absorber materials have been identified at OSU, iron silicon sulfide (Fe 2 SiS 4 ) and barium copper tin selenide (BaCu 2 SnSe 4 or BCSS). Variations of these two material systems are listed in Table 5.2, along with their approximate bandgap, as measured from diffuse reflectance of powder samples. In terms of cost, shown in Table 5.1, and environmental concerns, both Fe 2 SiS 4 and BCSS appear to be in a better position than the current absorber materials. The only question is what is the solar cell efficiency associated with each new material Iron Silicon Sulfide Iron sulfide (FeS 2 ), otherwise known as pyrite, has an absorption coefficient reported as high as cm 1, for wavelengths up to 1000 nm, and a bandgap of 0.95 ev [119], making it an attractive absorber material. Electrically, however, iron sulfide appears to be ill-suited for

88 73 Material Bandgap (ev) Fe 2 GeS SrCu 2 SnSe Fe 2 SiS BaCu 2 SnSe SrCu 2 SnS BaCu 2 SnS Table 5.2: Potential absorber materials considered to date at OSU. Iron silicon sulfide (Fe 2 SiS 4 ) and barium copper tin selenide (BaCu 2 SnSe 4 ) are currently in development. thin-film solar cell absorber applications, due to a phenomenon known as Fermi-level pinning [120]. Fermi-level pinning occurs when charged surface states at a semiconductor-contact interface effectively screen any additional applied voltage to the semiconductor, pinning the Fermi-level at a constant position within the bandgap, independent of the contact metal employed [121, 101]. In the context of solar cells, Fermi-level pinning limits the open-circuit voltage, reducing the solar conversion efficiency. In order to utilize the attractive absorption coefficient of iron sulfide while circumventing Fermi-level pinning problems, alloying of iron sulfide with another material is proposed. Germanium sulfide (GeS 2 ) was the first candidate considered for alloying. Early pellet work with iron germanium sulfide (Fe 2 GeS 4 ) showed promise; a bandgap of 1.5 ev, a conductivity of S/cm, and a Seebeck coefficient of +739 µv/k. Complications with depositing iron germanium sulfide thin-films, however, overshadowed its attractive properties. Thin-films of iron germanium sulfide deposited by electron beam deposition, from a pressed pellet of Fe 2 GeS 4, are believed to be almost entirely elemental germanium, as evident from XRD analysis and conductivity measurements. The cause of this poor stoichiometric transfer between the pellet and the thin-film is attributed to a result of a large disparity between the elemental melting points of iron and germanium, as shown in Table 5.3, and the low sticking coefficient of sulfur. As explained in Chapter 3, electron beam evaporation of two or more materials often results in a film of incongruent composition, with the lower melting point material being incorporated into the film at a higher percentage than the higher melting point material.

89 74 Material Melting Point ( 0 C) Barium 727 Copper Germanium Iron 1538 Silicon 1414 Tin Table 5.3: Melting points of various metals considered for use in new materials research at OSU [122]. In order to address deposition problems associated with germanium, silicon sulfide (SiS 2 ) was proposed as an alternative material for alloying with iron sulfide. The elemental melting points of iron and silicon are quite close, as shown in Table 5.3, increasing the likelihood of evaporating both metals in equal proportions during electron beam deposition. To compensate for the low sulfur content obtained in deposited iron germanium sulfide films, sulfur was not included in the pressed pellet. Instead, silicon-germanium pellets were used for electron beam deposition, with sulfur thermally deposited in a process known as co-deposition, as explained in Chapter 3. This deposition strategy was found to improve the pellet density, as both silicon and iron are metallic. Thin-film deposition of iron silicon sulfide via electron beam and thermal co-deposition shows promise. Figure 5.3 shows the x-ray diffraction (XRD) pattern of an iron silicon sulfide thin-film deposited by co-deposition, using an electron beam to deposit an iron-silicon pressed pellet, and a thermal source to deposit elemental sulfur. These XRD results indicate that the film is mostly amorphous, with three pronounced peaks. Identification of the compound or compounds associated with these peaks, however, has proved difficult, complicating determination of the film composition. Using a flash anneal of less than five minutes in a hydrogen sulfide (H 2 S) ambient at C, the deposited film was crystallized. The XRD pattern of the crystallized film can be seen in Fig. 5.4, compared to the crystal patterns of iron sulfide (FeS 2 ) and silicon sulfide (SiS 2 ). This comparison of peaks indicates a good likelihood that the crystalized film contains FeS 2, but a poor likelihood that the film contains SiS 2. The high vapor pressure of SiS 2 is believed to be the cause. These XRD results, however, only specify a lack of SiS 2 after the hydrogen sulfide anneal, as the

90 75 Intensity (degrees) Figure 5.3: X-ray diffraction spectrum of an iron silicon sulfide thin-film deposited by electron beam deposition. Aside from three noticeable peaks, the film appears to be amorphous. amorphous film XRD results cannot confirm or deny the presence of SiS 2. This lack of silicon sulfide does not necessarily indicate a complete lack of silicon in the film, elemental silicon and other silicon compounds may still exist. In the context of this research, the actual composition of the film is less relevant than the solar cell performance potential of the film. Assessing the solar cell performance potential of such a film, however, has proven quite difficult, as described in Section Barium Copper Tin Selenide Based on work with various barium-copper compounds at OSU, including BaCu 2 S 2 [123], BaCuSF [124], and current work with the p-type window layer BaCuTeF [125], barium copper tin selenide (BaCu 2 SnSe 4 or BCSS) was developed as a potential absorber material for use in thin-film solar cells. The use of tin and selenium provide a reduction in the bandgap, from the ev for BaCu 2 S 2, BaCuSF, and BaCuTeF, to a more appropriate 1.6 ev, as measured from a pressed pellet. Barium copper tin selenide was initially considered for electron beam deposition, however, with the large number of constituents and wide range of elemental melting temperatures, as shown in Table 5.3, pulsed laser deposition (PLD) was deemed a better deposition method.

91 Intensity θ (degrees) Deposited Film Iron Sulfide (FeS 2) Silicon Sulfide (SiS 2) Figure 5.4: X-ray diffraction spectrum of an iron silicon sulfide thin-film flash-annealed at C in hydrogen sulfide. X-ray diffraction spectra for crystalline iron sulfide and silicon sulfide are also included, indicating the presence of iron sulfide, but not silicon sulfide. The absorption coefficient of BCSS, 10 5 cm 1 for wavelengths up to 440 nm as shown in Fig. 5.5, is promising for thin-film solar cell absorber applications. Additionally, the hole concentration of cm 3 for PLD-deposited films BCSS is attractive for incorporation into a thin-film solar cell Testing New Absorber Materials In working with new absorber materials within the context of a rapid materials development strategy, one problem eclipses all others. As of yet, no simple test method for assessing the effectiveness of a new material for thin-film solar cell absorber applications has been developed at OSU. Realistic evaluation of an absorber layer appears to require the fabrication and testing of a complete solar cell and, furthermore, optimization of the solar cell. Appropriate solar cell characterization ideally involves illuminated and unilluminated current-voltage measurements to extract V OC, I SC, FF, and η, as described in Chapter 3. Additionally, solar cell lifetime assessment is of critical importance. Thus, solar cell absorber evaluation is currently the rate-limiting bottleneck in the rapid development of novel thin-film solar cells.

92 E+06 1.E+05 1.E+04 1.E+03 α (cm -1 ) E+02 Wavelength (nm) Transmission Reflection Normalized Transmission Absorption Coefficient Figure 5.5: Optical assessment of a barium copper tin selenide thin film, showing the calculated absorption coefficient. An absorption coefficient of greater than 10 5 cm 1 for wavelengths less than 440 nm is evident. An example of the problems associated with the current testing of absorber materials is illustrated in Fig Barium copper tin selenide, deposited in the stack shown in Fig. 5.6(a), with BCSS deposited onto an ITO-coated glass slide in which aluminium is used as a top contact material. An estimate of the corresponding energy band diagram, as shown in Fig. 5.6(b), possesses a noticeable barrier for hole transport at both interfaces. Due to the low doping of the BCSS layer, the width of the space charge layers are significant, reducing the likelihood of tunneling between the contact materials and the BCSS. When the device was tested, the resultant current-voltage curve showed no appreciable photo-generated response, as seen in Fig. 5.6(c) which shows both the illuminated and the unilluminated curves. This result, however, does not reveal anything about the absorber quality of the BCSS layer except that this test structure is probably dominated by its poor quality contacts. The lack of a photo-response could alternatively be attributed to this absence of carrier generation within the barium copper tin selenide, a short lifetime of photogenerated carriers, an excessive carrier concentration in the BCSS thereby masking the effect of photo-generated carriers, or shunting between the aluminium and ITO contacts, preventing a voltage from being dropped across the absorber. Unfortunately, almost nothing can be learned from a failed solar cell test of this type. A new testing methodology, therefore, is required if potential absorber layers are to be rapidly assessed for thin-film solar cell applications. Potential absorber materials have been iden-

93 78 Aluminium ITO BaCu SnSe 2 4 Al BaCu SnSe 2 4 (a) ITO Corning 1737 (Substrate) I E F 3 2 (b) V (c) Figure 5.6: The (a) test structure, (b) estimated energy band diagram, and (c) current-voltage characteristics of a barium copper tin selenide test structure. The lack of a photo-response in the current-voltage characteristics may be related to the presence of Schottky barriers at the top and/or bottom interface.

94 79 Contact Window Absorber Window Contact Substrate (a) (b) Figure 5.7: The (a) test structure, (b) estimated energy band diagram, and (c) current-voltage characteristics of a barium copper tin selenide test structure. The lack of a photo-response in the current-voltage characteristics may be related to the presence of Schottky barriers at the top and/or bottom interface. tified and thin-film fabrication has been accomplished, but nothing can be established for certain until a test can determine if the materials are everything they promise to be. The challenges in testing a new absorber material are typically associated with difficulties associated with contacting the absorber material. Obviously, high barrier metallic contacts are not able to facilitate carrier injection and extraction into or out of a lightly doped p-type material such as BCSS. One solution would be to create a standardized test structure, shown in Fig. 5.7(a), with a contact and window layer deposited in large quantities onto a standard substrate. An absorber material could than be deposited on top of the window layer, followed by deposition of another window layer and contact, as shown in Fig. 5.7(b). The advantage of this structure would be the minimization of control variables, assuming the same contact and window layers are used each time. Determination of the materials required for this structure would be difficult. Lack of a robust p-type window layer would currently limit the implementation of this idea.

95 5.3 p-type Window Materials A window material in a solar cell has two main purposes. 80 First, the material is chosen to have a bandgap of adequate width and an offset to provide a barrier to help suppress the recombination of minority carriers at the neighboring contact layer. Second, the material should be heavily doped to accommodate a high carrier concentration, facilitating the formation of an ohmic contact with the contact layer. Very few wide-bandgap, p-type semiconductors appropriate for use as p-type thin-film window layers are known to exist. A key motivation for OSU s entry into thin-film solar cell research was the fact that OSU is one of the few institutions in the world pursuing the development of wide-bandgap p-type semiconductors, originally for p-type transparent conductor applications. P-type window functionality is achieved for free in a standard CIGS solar cell, when an interfacial layer of MoSe 2 forms between the CIGS and the molybdenum back contact during CIGS deposition. A p-type window layer does not exist in the standard CdTe solar cell. Development of a new p-type window layer would allow a CIGS solar cell to be fabricated using different back contact materials, and would increase the overall efficiency of a CdTe solar cell. A desire to move to tandem or multi-junction cells, with more than one absorber layer stacked together in the same structure, has increased the demand for a new p-type window material. Currently, p-type window and back contact layers of CIGS and CdTe are not transparent, preventing the use of either technology in a tandem cell. All currently available transparent conductive materials are n-type. Therefore, a transparent contact layer for holes must involve an n-type contact material with an intermediate heavily doped p-type material in order to facilitate the formation of an ohmic contact via carrier tunneling. OSU has developed a number of p-type window layers for potential use in thin-film solar cells, as listed in Table 5.4 along with their expected bandgaps according to measurements performed on pressed pellets or thin-films. The most promising material to date is barium copper tellurium fluoride (BCTF) Barium Copper Tellurium Fluoride Barium copper tellurium fluoride (BCTF) was initially developed as a transparent p-type conductive material. With a wide-bandgap of 2.3 ev, a carrier concentration of cm 3, a mobility 1-5 cm 2 /V s, and % transparency in the visible portion of the electromagnetic

96 81 Material Bandgap (ev) Cu 3 TaS Cu 3 NbS Cu 3 TaSe BaCuTeF 2.3 Table 5.4: Potential p-type window materials considered at OSU. Barium copper tellurium fluoride (BCTF) is currently in development. spectrum, BCTF has the potential to function as an intermediate p-type layer which is necessary to create a transparent p-type contact Copper Indium Gallium Diselenide Initially BCTF was proposed as a p-type window layer for a CIGS solar cell. Considered to be a replacement for the MoSe 2 layer formed between the conventional molybdenum back contact and the CIGS during the CIGS deposition, BCTF is a good candidate for this application due to its wide bandgap, high carrier concentration, and p-type conductivity. Interest in an alternative p-type window layer for CIGS is not primarily motivated by the desire for a performance increase, however, as MoSe 2 is p-type window layer which is fortuitously created at the interface with the molybdenum back contact. However, a desire to remove molybdenum and replace it with a transparent material for multi-junction applications has led to an interest in p-type window layers for CIGS solar cell applications. In the case of a CIGS thin-film solar cell, the ideal back-contact needs to be a robust, widebandgap, p-type conductive material capable of withstanding all subsequent processing. Currently, an appropriate p-type contact material has not been identified. In fact, very few p-type widebandgap materials exist at all, and none have shown adequate conductivity to be used as a singlelayer back-contact solution. Instead, the transparent back-contact strategy being pursued involves the use of a robust wide-bandgap conductive n-type material, such as ITO, in conjunction with a thin p-type interface layer inserted between the CIGS and the ITO, thereby yielding a tunneling contact. This is the motivation for inserting BCTF into a CIGS solar cell. To test the use of BCTF as a p-type window layer in a CIGS solar cell requires actual fabrication of a CIGS solar cell with BCTF insertion. Due to the complexity of the processing required

97 82 for constructing a CIGS solar cell, which has taken years to perfect, and the fact that OSU does not currently have a system capable of depositing the four constituent materials simultaneously, as required for CIGS deposition, the logical path forward was to collaborate with a research group already versed in CIGS processing. Through the National Renewable Energy Lab (NREL), the funding source for this research, collaboration with the University of Delaware was established for procurement of CIGS solar cells. The plan was to deposit the back-contact materials, ITO and BCTF, and to then ship the samples to the University of Delaware for completion of the CIGS solar cell and characterization. The initial plan included four different back-contact configurations, as shown in Fig. 5.8(a). One significant point to notice about the structures shown in Fig. 5.8(a), is that the substrates are not the same for the ITO and molybdenum cells. This discrepancy is a consequence of material availability, with high-quality ITO only available at OSU on Corning-1737 glass, and molybdenum deposited by the University of Delaware onto soda-lime glass. The thermal expansion coefficient of soda-lime glass, which is the standard substrate for CIGS thin-film solar cells, is well matched to that of CIGS, minimizing film stress during CIGS deposition. The soda-lime glass also provides sodium ions to the CIGS, which has been shown to passivate the grain boundaries in the polycrystalline material, minimizing recombination at the interfaces. Corning-1737 glass, although usually considered a higher quality substrate for devices, due in part to its higher melting point, lacks the advantages enjoyed by the soda-lime glass in relation to better CIGS solar cell performance. The use of different substrates explains some of the differences observed in the current-voltage curves shown in Fig. 5.8(b). Electrical test results for each of the configurations shown in Fig. 5.8(a) are presented in Fig. 5.8(b), with relevant parameters, i.e., short-circuit current density, J SC, open-circuit voltage, V OC, fill factor, FF and solar conversion efficiency, η, included in Table 5.5. The current-voltage curves for the CIGS cells with molybdenum back-contacts are shown on the left axis in Fig. 5.8(b). Clearly, the standard Mo/MoSe 2 back contact, shown as the dashed curve on the left axis of Fig. 5.8(b), displays the best performance. Insertion of BCTF into the CIGS cell with a molybdenum back-contact degrades the solar cell performance somewhat, decreasing both V OC and FF while increasing I SC slightly, but the performance is still respectable. The noticeable degradation of the Mo/BCTF cell compared to that of the Mo/MoSe 2 is not entirely surprising, since MoSe 2 is known to function as an optimized p-type window layer, suggesting that the BCTF is either non-optimized or is superfluous.

98 83 Solar Cell J SC (ma/cm 2 ) V OC (V) FF η CIGS/MoSe 2 /Mo CIGS/BCTF/Mo CIGS/ITO CIGS/BCTF/ITO Table 5.5: Solar cell performance summary for the current-voltage curves for various CIGS solar cells shown in Fig. 5.8, with back-contacts deposited at OSU and the CIGS, CdS, and top-contacts deposited at the University of Delaware. All devices are tested at the University of Delaware. The current-voltage results of the ITO/BCTF cell and the ITO cell, as shown on the right axis in Fig. 5.8(b), are somewhat more complicated. Under illumination, the current-voltage curves in the fourth quadrant are nearly identical for both devices, suggesting minimal impact of the BCTF insertion layer with respect to power generation. However, the first quadrant dark currentvoltage curves are quite different. The ITO cell has significantly more hysteresis and also a larger turn-on voltage in the dark compared to that of the ITO/BCTF cell. The hysteresis, which is visible in both the light and dark curves for both ITO cells, is counter-clockwise hysteresis. Hysteresis present in current-voltage curves is usually attributed to ion migration or carrier trapping [126]. The hysteresis witnessed in the ITO cell current-voltage curves is tentatively attributed to carrier trapping, for the following reasons. First, the ITO cell substrate is Corning-1737 glass rather than soda lime glass; sodium is a likely source of ion migration. Second, since sodium appears to improve the CIGS crystalline quality and solar cell performance, CIGS processed using sodium would likely have a higher trapping density. Third, note a noticeable reduction in hysteresis in the current-voltage curves under illumination compared to the corresponding dark curves. This reduction could be due to trap filling via photo-generated carriers. Regardless of if carrier trapping is responsible for the hysteresis evident in Fig. 5.8(b), the magnitude of the hysteresis and the larger turn-on voltage of the ITO cell suggests that the ITO/BCTF cell is of a higher quality. Scanning electron microscope (SEM) images of the BCTF/ITO interface, as shown in Fig. 5.9, provide some insight into what maybe occuring in this BCTF insertion experiment. Figure 5.9 shows a BCTF/ITO interface after a C anneal in an argon ambient, the temperature at which CIGS is deposited. The BCTF layer can clearly be seen, indicating that the BCTF/ITO interface is compatible with an annealing temperature of C. In contrast, there is no evidence of the

99 84 CIGS CIGS CIGS CIGS Soda Lime Glass Corning 1737 Mo CdS BCTF ZnO ITO (a) I I V V Soda Lime Glass / Mo With BCTF (b) Corning 1737 / ITO Without BCTF Figure 5.8: Integration of BCTF as a p-type window layer into a CIGS solar cell. (a) The four different test structures for determining the effect of BCTF on the performance of a CIGS solar cell, including the standard Mo/MoSe 2 back-contact as a control. (b) Current-voltage characteristics under dark and illumination for each of the four test structures. Although some degradation is noticeable for the BCTF cases, the BCTF appears to only create minor changes in the currentvoltage characteristics. Table 5.5 lists the respective solar cell performance values for each curve.

100 85 BCTF layer in Fig. 5.9(b), suggesting that the BCTF constituents may have diffused into the CIGS film during deposition of the CIGS. In conclusion, it appears that BCTF cannot withstand the process conditions required for the deposition of CIGS. Use of BCTF as a p-type window layer must be attempted using a different thin-film solar cell system Cadmium Telluride The obvious alternative thin-film solar cell system for BCTF integration is CdTe. As the main competing solar cell system to CIGS, CdTe is constructed in the superstrate configuration, with the opaque contact on the top of the cell, making it a more appropriate test vehicle for the assessment of BCTF as a p-type window layer since the BCTF is inserted at the back-end of the process. The superstrate configuration is employed in the fabrication of a CdTe solar cell due to difficulties associated with making a p-type ohmic contact to CdTe. These ohmic contact formation challenges are relevant when attempting to insert BCTF as a p-type window in a CdTe thin-film solar cell. Two different methods are commonly used to form a back-contact to CdTe, copper diffusion [37] and cadmium etching [43]. In the case of copper diffusion, copper is applied in a paste or as a thin-film to the back of a CdTe solar cell and is then diffused into the CdTe, thereby creating a p + doped region in the CdTe, which facilitates the creation of a tunneling contact [37]. Figure 5.10(a) illustrates such a copper-diffused p + layer which is capped by a gold contact layer. The alternative method for forming a p-type ohmic contact to CdTe is to preferentially etch cadmium from the CdTe surface in order to create a tellurium-rich surface, which also results in the formation of a p + region near the CdTe surface [43]. Copper or an intermediate antimony telluride layer are sometimes used as a contact material to this tellurium-rich surface. Figure 5.10(b) shows a tellurium-rich etched surface which is capped with with an intermediate antimony telluride layer and a molybdenum contact layer. The antimony telluride is a common intermediate contact layer for CdTe because it is p-type with a high carrier concentration, and it can be easily sputter deposited [127]. For purposes of this discussion, copper doping is assumed to be the standard method for back-contact formation, and will be used as the control for the assessment of BCTF insertion into CdTe solar cells. The goal of BCTF insertion is to provide a p-type window layer for CdTe, blocking minority carrier electrons from recombination at the back contact and facilitating the formation of an ohmic contact for hole extraction at both the CdTe/BCTF interface as well as the BCTF/back-contact

101 86 ITO BCTF (a) ITO BCTF? CIGS (b) Figure 5.9: SEM images of the BCTF/ITO interface. (a) After a C anneal the BCTF is clearly visible on the ITO. (b) After CIGS processing at C, the BCTF layer is not visible. Possibly BCTF has diffused into the CIGS during the deposition process.