Solar Cell: From Research to Manufacture

Transcription

1 Solar Cell: From Research to Manufacture

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3 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society 1245-A01-01 Thin Film Silicon Photovoltaic Technology - From Innovation to Commercialization Subhendu Guha and Jeffrey Yang United Solar Ovonic LLC, 1100 W. Maple Road, Troy, MI ABSTRACT The last decade has witnessed tremendous progress in the science and technology of thin film silicon (amorphous and nanocrystalline) photovoltaic. The shipment of solar panels using this technology was about 200 MW in 2009; based on announcement of new or expanded production capacity, the shipment is projected to grow ten-times in the next 3-5 years. The key factor that will determine the wide-scale acceptance of the products will be the cost of solar electricity achieved using this technology. Efficiency of solar modules and throughput of production equipment will play a key role. In this paper, we discuss our roadmap to improve the product efficiency and machine throughput. INTRODUCTION The world market for photovoltaic (PV) has been growing at an annual rate of about 40% over the last five years. In spite of the economic downturn in the global economy, the shipment for 2009 exceeded 6,000 MW, a gain of 10% over The technology mix was dominated by single crystal and polycrystalline silicon with thin film silicon (both amorphous and nanocrystalline) accounting for about 3% of the total market [1]. The low material cost and ease of large-scale manufacturing have attracted many companies to thin film silicon, and over the last two years several companies have announced new or expanded production capacity. It has been projected that thin film silicon technology may capture 30% of the global PV market within the next five years. This will of course be determined by how competitive the thin film silicon products will be in comparison to other contenders. The key driver for large-scale deployment of PV is the levelized cost of electricity that depends on the energy yield in terms of kwh produced per kw installed and the installed system cost. Thin film silicon alloy solar cells are less sensitive to temperature than the crystalline counterpart, and it has been demonstrated that they produce more kwh/kw under real world conditions [2]. Flexible solar laminates have lower installation cost for rooftop application that reduces the overall system cost. In order to reduce module cost further, one must increase the efficiency and also improve production throughput so that the capital cost can be lowered. The goal is to offer solar electricity at grid parity. Innovation will play an important role in achieving this target. In this paper, we shall review the technology that we have been developing to address efficiency and throughput, and will discuss our roadmap to reduce cost further to reach grid parity.

4 UNITED SOLAR OVONIC (USO) TECHNOLOGY The three key components of USO technology are: 1. Roll-to-roll production, 2. Multijunction thin film silicon solar cell structure, and 3. Flexible solar laminates. As shown in Fig. 1, innovation has played a key role in taking the laboratory results to production. In 1981, we built our first roll-to-roll machine with the web transported through just a single chamber. A pilot plant producing same-gap amorphous silicon (a-si:h/a-sih) double-junction cells was installed in With advances in the laboratory demonstrating the advantages of the triple-junction cell technology, we built our first triple-junction processor with an annual capacity of 5 MW in It was at that time that we recognized the advantage of flexible products for the rooftop market and we introduced our first building-integrated photovoltaic (BIPV) product in With increasing acceptance of our products, we embarked on an aggressive expansion plan and today we have about 150 MW of annual capacity. The commercial laminate fabrication process consists of three basic steps [3, 4]. In the first, a proprietary roll-to-roll deposition technology is used to deposit an amorphous silicon/amorphous silicon-germanium/amorphous silicon-germanium (a-si:h/a-sige:h/a- SiGe:H) triple-junction solar cell on a flexible and lightweight stainless steel substrate using a radio frequency (rf) glow-discharge system. Six rolls of stainless steel, each 1.5 mile long, are loaded into the triple-junction processor and nine miles of solar cells are produced in 62 hours. The bottom sub-cell absorbs the red light, the middle cell the yellow/green light, and the top cell ^1981: Prototype machine 1986:0.5 MW machine riple-junction technology-nrel Collaborate 1996:5 MW machine 1997: Building-integrated BIPV) product 1994: NREL validation 1991:2 MW machine 2003: First Auburn Hills facility 2007: Greenville facility PV:BONUS-DOECollaboration Thin Film Partnership-SAI TODAY: Greenville campus expanded to 120 MWs Figure 1. From innovation to commercialization. The history of ECD-USO flexible product development.

5 the blue light. An Al/ZnO back reflector at the bottom of the solar cell improves the reflectivity and texture of the substrate, which results in improved light trapping and enhancement in conversion efficiency. The second fabrication step consists of cutting the roll of solar cell into smaller pieces, and processing them for cell delineation, short passivation, and top and bottom current collection bus bar application. The third and final step consists of interconnecting the individual solar cells into a series string and encapsulating the string in UV-stabilized and weather-resistant polymers to form the final product. Figure 2 shows schematics of: (a) spectrum-splitting triple-junction solar cell structure, (b) cross section of the module, and (c) roll-to-roll a-si:h alloy processor and triple-junction structure formation. USO offers a unique and differentiated product (Fig. 3) compared to any of its competitors. The lightweight and flexible products come with an adhesive and a release paper at the back and can be easily integrated with the roof [4]. This reduces the installation cost significantly. The current laminate has a total-area efficiency of 6.7% and an aperture-area efficiency of 8.2%. USO has an aggressive plan to increase the aperture-area efficiency first to 10% and later to 12% within the next several years. In the next section, we shall discuss our approach to meet these goals. EFFICIENCY AND THROUGHPUT IMPROVEMENT We are using two parallel approaches to improve efficiency. One is replacing the Al/ZnO back reflector (BR) with a superior Ag/ZnO BR; the other is the use of an improved deposition process to develop high quality a-si:h, a-sige:h and nanocrystalline silicon (nc-si:h) alloys. The improved deposition process results in higher throughput from the deposition machine as well. Back reflector In the simplest case, the cell can be deposited on a specular surface resulting in just two optical passes in the cell. Multiple passes (Fig. 4) within the cell by the use of a textured BR increase the optical path and improve photon absorption, especially for the long-wavelength photons [5, 6]. Yablonovitch and Cody have shown that for random scattering, the path length can be increased by a factor of 4n 2, where n is the refractive index of the material [7, 8]. For a-si:h alloy, this amounts to about 50 passes or 25 reflections. Figure 5 shows the calculated value of the fraction of absorbed light as a function of wavelength for different number of internal reflections [5, 6]. The calculation assumes an a-si:h solar cell with an optical gap of 1.68 ev and /-layer thickness of 500 nm. The series of curves show the significant enhancement in photon absorption that can be obtained in the red/infrared part of the spectrum with an increasing number of reflections within the cell. This of course will lead to higher short-circuit current density. The enhancement in shortcircuit current density is larger when one uses a lower bandgap material.

6 Blue Cell Transparent Conductive Oxide Film Thickness of complete Multi-junction cell Back Reflector Flexible Stainless Steel Substrate (a) (C) Figure 2. Schematics of (a) triple-junction device structure, (b) cross section of module, and (c) roll-to-roll thin film silicon processor and triple-junction structure formation.

7 Conventional Solar Panels UNI-SOLAR* Laminates Figure 3. Comparison of (left) conventional rigid solar panels with (right) United Solar's flexible solar laminate. We have been using a bi-layer of Ag and ZnO in the laboratory to obtain efficient light trapping. The silver layer can be textured by sputtering at a high temperature. In some cases, ZnO is also textured either by depositing at a high temperature or by subsequent etching. Several tools are used to analyze the quality of the BR. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) are used routinely to evaluate the surface roughness. Increasing surface roughness leads to larger scattering, and this is analyzed by measuring the angular dependence of the reflected light when the light is incident normally. Losses at the reflecting surface are determined by measuring total and diffused reflection from the surface. Finally, the quantum efficiency of the finished cell determines the short-circuit current density. Figure 6 shows AFM and SEM photographs of transparent conductive oxides (TCO) and back reflectors made under various conditions and chemical treatments. It is clear that a wide INCIDENT LIGHT STAINLESS STEEL Figure 4. Schematic diagram of a textured back reflector WAVELENGTH (nm) Figure 5. Improvement in absorption with multiple trapping. 800

8 variety of textures can be obtained by changing deposition parameters or subsequent processing. Surface roughness ranging from 30 nm to 80 nm can easily be obtained. Typical angular scattering from various textured substrates is shown in Fig. 7 [9]. It is clear that scattering at an angle to the normal can be increased by increasing the texture of the metal or the ZnO layer. The largest scattering is seen when both the Ag and ZnO is textured. Theoretically, the perfect random surface for light scattering should produce a scattering light intensity proportional to cos(a), where a is the angle between the scattered light and the normal line to the surface (in Fig. 7 (b), 6 = 90 -a). The factor of cosine takes into account the effective area of the light spot on the surface to the viewer at a given angle. Figure 7 (b) plots the measured scattering light intensity divided by cos(a). It shows that the sample with textured Ag and thick ZnO layer has quite a flat response as a function of the viewing angle. Even though the scattering is adequate, one must make sure that both diffused and total reflections from the BR surface are adequate. Figure 8 (a) shows typical reflection plots for Al/ZnO and Ag/ZnO BR. It is clear from Fig. 8 (b) that higher reflection results in larger red response giving rise to higher short-circuit current density. Theoretically, the best back reflector is with flat Ag and textured ZnO, because the flat Ag reduces the plasmon loss at the Ag/ZnO interface and the textured ZnO provides the required light scattering [10]. Yan et ah have shown that it is true for ZnO thicker than 2 jum for a-sige:h solar cells [11]. With a thinner ZnO layer, a textured Ag is still necessary because the light scattering is not sufficient at the ZnO and a-si:h interface. We should mention that recent experiments on nc-si:h solar cells have shown that thinner ZnO gives rise to better cell performance [12]. This is attributed to poorer quality of nc-si:h material when grown on a highly textured substrate. Figure 6. AFM and SEM photographs of different textures of TCO and back reflectors.

9 A:Flat Ag,Thin ZnO B: Textured Ag, Thin ZnO C: Flat Ag, Thick ZnO D: textured Ag, Thick ZnO A: Flat Ag, Thin ZnO B: Textured Ag, Thin ZnO C: Hat Ag, Thick ZnO D: Textured Ag, Thick ZnO SO Angle(degree) Angle (degree) Figure 7. (a) Angular distribution of scattering light intensity 1(0) and (b) the scattering light intensity with correction of viewing area >) jm^. -^-onss -*-onag/zno A *\ ma/cm Wavelength (nm) 0.4 / \ mA/cta nc-si:h cells k \ Wavelength (nm) Figure 8. (a) Wavelength dependence of reflectance of Al/ZnO and Ag/ZnO BRs and (b) corresponding quantum efficiency plots of nc-si:h solar cells. Many new methods of efficient light trapping are now being explored. They include the use of optical confinement by a grating structure [13, 14] nano-particles [15] and photonic structures [16]. Comprehensive reviews can be found in the literature [17,18]. New Deposition methods Most of the reported glow-discharge systems use an //frequency of MHz. Very high frequency (vhf) has recently been used successfully to grow both a-si:h and nc-si:h materials. It was first shown by the Neuchatel group [19] that as the plasma excitation frequency

10 is increased, the deposition rate can be enhanced. The technique has now been used successfully to obtain high efficiency solar cells based on both a-si:h and nc-si:h materials. Yan et al. [20] measured the ionic energy distribution for //and vhf plasmas and found that the ionic energy distribution shifts to lower energy for vhf plasma. The ionic flux, on the other hand, is much higher. This explains the high deposition rate obtained using vhf and it is believed that intense low energy bombardment results in a more compact structure with high quality. We have been exploring the use of a modified vhf (MVHF) method to deposit a-si:h and a-sige:h solar cells. The modification involves innovative deposition parameters and novel cathode design. By optimizing the deposition parameters, we find that use of MVHF not only gives rise to a higher deposition rate, it can also result in higher efficiency, even if the deposition rate is increased by a factor of 2 [21]. Using a superior BR and MVHF deposition of a-si:ii and a-sige:h, we have demonstrated 9.5% stable large-area encapsulated cell efficiency using a double-junction cell [21]. The immediate impact of the introduction of this technology will not only be an increase in efficiency, but also a reduction in the footprint of the a-si:h processor (Fig. 9) leading to lower capital cost. The MVHF technique has also been applied successfully to make nc-si:h single- and multi-junction cells. Several barriers need to be crossed. The growth morphology of nc-si:h is complex and as the growth progresses, larger grains are obtained that can give rise to cracks and voids. Using a hydrogen dilution profiling technique [22] and the use of optimally chosen seed layers, we demonstrated initial cell efficiency of 15.4% [23]. The technique now has been expanded to grow both small-area [24] and large-area encapsulated cells [25] at higher deposition rates. The highest stable efficiency obtained from large-area encapsulated cells is 11.2% [25]. Current Manufacturing (O) Bottom 1 Middle z - UN «a! Top \!* U 50.3 I minutes Bottom Hz z E VHF: 4 min a-si:h/8 min a-sige:h minutes Top o I(O) Grid n Q_ VHFI3 VHF M Al/ZnQ. n Grki n RFi3 RFI2 RFM "SEZSI Figure 9. Reduction in footprint of deposition system with MVHF as compared to current rf manufacturing. 10