International Journal On Engineering Technology and Sciences IJETS ISSN (P): , ISSN (O): Volume 1 Issue 7, November 2014

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1 IJETS ISSN (P): , ISSN (O): COMPARISON THE DEVELOPMENT OF POLYCRYSTALLINE THIN-FILM CU(IN,GA)SE2 SOLAR CELLS AND CDTE SOLAR CELLS Stephen Raja John Britto ID: ABSTRACT: -Solar energy is considered as the most promising alternative energy source to replace environmentally distractive fossil fuel. A solar cell, often called a photovoltaic (PV) cell, converts the energy in sunlight directly into electricity.. Photovoltaic, or PV, refers to the conversion of light energy into electricity using electronic devices called solar cells. Solar cell is Unlike silicon-wafer cells, which have light-absorbing layers that are traditionally 350 microns thick, thin-film solar cells have light-absorbing layers that are just one micron thick. A micron, for reference, is one-millionth of a meter (1/1,000,000 m or 1 µm). The most common thin-film semiconductor materials are cadmium telluride (), amorphous silicon (a-si), and alloys of copper indium gallium diselenide (). The semiconductor layer is typically deposited on a substrate or superstrate inside a vacuum chamber. A number of companies are pursuing lower-cost, non-vacuum approaches for manufacturing thin-film materials. This paper analysis both the features of and base on long-term stable performance and potential for low-cost production. Key words:,, Electro Deposition. I INTRODUCTION and are the two new kids on the block for thin-film solar. Both technologies promise rock-bottom manufacturing prices, and respectable efficiencies (10% or greater). First Solar may have all the manufacturing scale, but there is still a measure on which these technologies are still competing very closely: efficiency. Higher efficiencies are huge because the same amount of glass & aluminum provides more power and since modules are priced based on their power, that extra money goes straight to the bottom line of the manufacturer. II. OBJECTIVE Increasing module efficiency and cell efficiencies is one of the major strategies for reducing per-watt module price. Consistent improvements in PV cell efficiency have been realized for virtually every PV technology and module efficiency has followed this trend, albeit with a time and performance lag. This trend is projected to continue, owing to R&D improvements that produce higher best-cell efficiencies and manufacturing technology improvements that advance commercial modules toward best-cell efficiencies. As single-junction PV technologies approach the theoretical (Shockley-Queisser) efficiency limit for their respective semiconductor materials, the extent to which further cost reduction may be attributable to efficiency gains will be reduced, and more substantial cost reductions will need to be realized via other avenues. III. BAND GAP BACKGROUND To determine the efficiency the photons from the sun come in a wide range of wavelengths this is what gives us the color spectrum. 278

2 Figure 3 Wave length of color spectrum So the solar cells are tasked with absorbing that spectrum of light. Ideally, they would absorb the entire spectrum but in reality, there are a host of constraints (e.g. shading, reflection, and recombination) that reduce the amount that a cell can absorb. Figure 3.a and Band Gab Efficiency and have been selected because their band gap efficiency is very good, so a very thin layer can capture much of the sun s energy. This is where they get the name thin film. (One interesting thing in the chart below: notice how the thinner CdS layer (which is physically in front of the ) improves the cell efficiency. Table 3.1 and Efficiency The 10% efficiency means. It s actually way simpler than think. Full sunlight (noon on a summer day), means that a square meter of earth at sea level is hit with 1,000 watts of energy. So to put it mathematically, a full sun is equal to 1kW /. Efficiency is therefore a measure of the amount of sunlight that a module can absorb at full sun. A 10% efficient module that is exactly 1 square meter large would generate 100W in full sun (1,000W x 0.10). Using real numbers: a First Solar module is 1.2m x 0.6m (0.72 square meters). If it generates 80 watts at peak power, that module is 11.1% efficient (80W/(0.72 square meters*1000)). 3.1 The discuss on and The efficiency race between these two technologies has been close for decades). Both have a theoretically maximum efficiency of about 30%. 1. advocates brag that has the highest lab efficiency (19.9% efficient). However, it was done using an intensely complex process: quadruple sublimation (essentially, evaporating four different components at the same time, at very precise levels). 2. has a lower champion cell, but it is much simpler to manufacture. Its band gap (1.4 ev) is almost perfectly tuned to capture the most sun with the least material. 279

3 Figure 3.b and Efficiency Figure 3.1 Available datasheets to see the efficiencies promised Both efficiencies is almost same, that s why it s still an open debate. Physicists tend to predict that will eventually win the lab efficiency is so much higher. But Company Material Size Power Efficiency First Solar 0.72 Nanosolar 2.0 Wurth 0.73 Solyndra 1.97 Stion W 80W 160W 220W 70W 80W 150W 200W 110W 130W 9.7% 11.1% 8.0% 11.0% 9.6% 11.0% 7.6% 10.2% 10.0% 11.8% manufacturers have a healthy respect for the simplicity of ; First Solar may keep the thin-film efficiency crown for years to come, through their manufacturing prowess. Module efficiency is always lower often by 10-20%. (An analogy: cell efficiency is sort of like highway MPG, whereas module would be city MPG.) There are a bunch of reasons for this difference: resistance in the module, mismatch across cells, or shading from the front electrodes. Cell efficiency is still important but module efficiency is what you ll actually get in the real world. IV. COMPARISON OF AND CDTE Polycrystalline thin-film solar cells such as CuInSe2 (CIS), Cu(In,Ga)Se2 () and compound semiconductors are important for terrestrial applications because of their high efficiency, long-term stable performance and potential for low-cost production. Because of the high absorption coefficient(~ 105 cm 1) a thin layer of ~2 μm is sufficient to absorb the useful part of the spectrum. Highest record efficiencies of 19.2% for and 16.5% for have been achieved. Many groups across the world have developed solar cells with efficiencies in the range of 15 19%, depending on different growth procedures. Glass is the most commonly used substrate, but recently some effort has been made to develop flexible solar cells on polyimide and metal foils. Highest efficiencies of 12.8% and 17.6% have been reported for cells on polyimide and metal foils, respectively Similarly, solar cells in the efficiency range of 10 16%, depending on the deposition process, have been developed on glass substrates, while flexible cells with efficiency of 7.8% on metal, and 11% on polyimide have been achieved. The chalcopyrite semiconductor CuInSe2 and its alloy with Ga and/or S [Cu(InGa)Se2 or Cu(InGa)(Se,S)2], commonly referred as, have been leading thin-film material candidates for incorporation in highefficiency photovoltaic devices. CuInSe2- based solar cells have shown long-term stability and the highest conversion efficiencies among all thin-film solar cells, reaching 20%. A variety of methods have been reported to prepare thin film. Efficiency of solar cells depends upon the various deposition methods as they control 280

4 optoelectronic properties of the layers and interfaces. thin film grown on glass or flexible (metal foil, polyimide) substrates require p-type absorber layers of optimum optoelectronic properties and n-type wideband gap partner layers to form the p-n junction. Transparent conducting oxide and specific metal layers are used for front and back contacts. Progress made in the field of solar cell in recent years has been reviewed. has the highest stability under proton and electron irradiation compared to the other photovoltaic devices, which makes cells very interesting for space applications. High specific power is an important issue for space solar cells: if satellites are lighter they are easier and cheaper to launch in orbit. power (W/kg) and open numerous possibilities for a variety of applications. Thin-film solar cells based on or chalcopyrite absorbers can be grown in superstrate or substrate configurations. The superstrate configuration facilitates lowcost encapsulation of solar modules. This configuration is also important for the development of high-efficiency tandem solar cells, effectively utilizing the complete solar spectrum for photovoltaic power conversion. Back contact Absorber:, Buffer layer: CdS and others Front contact: CDOs Sub-strate : Class, plymide Figure 6.2 Cell Configuration Super- strate configuration Light Light Front contact:tcos Buffer layer: CdS and others Absorber:, Back contact Sub-strate : Class, plymide Sub-strate configuration power (W/kg) and open numerous possibilities for a variety of applications. As shown in Figure 1, thin-film solar cells based 281 on or chalcopyrite absorbers can be grown in superstrate or substrate configurations. The superstrate configuration facilitates low-cost encapsulation of solar modules. This configuration is also important for the development of high-efficiency tandem solar cells, effectively utilizing the complete solar spectrum for photovoltaic power conversion. 4.1 CELL CONFIGURATION solar cells Generally, solar cells are grown in a substrate configuration (Figure 4.1). This configuration gives the highest efficiency owing to favorable process conditions and material compatibility but requires an additional encapsulation layer and/or glass to protect the cell surface. This cover glass, in contrast, is not required for the cells grown in the superstrate configuration. CIS-based superstrate solar cells were investigated by Duchemin et al. using spray pyrolysis deposition, but efficiencies did not exceed 5%. The main reason for this low efficiency in CdS/ superstrate cells is the undesirable inter diffusion of Cd into CIS (or ) during the elevated temperatures required for absorber deposition on CdS buffer layers. The solar cells can be grown in both substrate and superstrate configurations ( Figure 4.1), but the highest efficiency is achieved in the superstrate configuration. The /CdS layers for superstrate cells are grown on transparent conducting oxide (TCO)-coated glass substrates. The glass substrate can be a low-cost soda-lime glass for growth process temperatures below 550C, or alkali-free glass for hightemperature processes ( C).Various kinds of back contacts can be applied, as they do not have to withstand the high

5 temperature of successive layer deposition. Cells in superstrate configuration have given the highest efficiency2 of up to 16.5% FRONT CONTACT solar cells During the early days of CIS and substrate cell development a bilayer of un doped and doped CdS served as a buffer and front contact, respectively.18,19 High conductivity in doped CdS was achieved either by controlling the density of donor type defects or by extrinsic doping with Al or In. Spectral absorption loss in the conducting CdS layer was reduced by increasing the band gap, alloying with ZnS or later replacing it with TCOs with band gaps of above 3 ev.18 Transmission spectra of various TCOs. or CDS BUFFER LAYERS Semiconductor compounds with n-type conductivity and band gaps between 2.0 and 3.6 ev have been applied as buffer for solar cells. However, CdS remains the most widely investigated buffer layer, as it has continuously yielded high-efficiency cells. CdS for high-efficiency cells is generally grown by a chemical bath deposition (CBD), which is a low-cost, largearea process. However, incompatibility with in-line vacuum-based production methods is a matter of concern. Physical vapor deposition (PVD)-grown CdS layers yield lower efficiency cells, as thin layers grown by PVD do not show uniform coverage of and are ineffective in chemically engineering the interface properties. The recent trend in buffer layers is to substitute CdS with Cd-free wide-band gap semiconductors and to replace the CBD technique with in-line-compatible processes. The first approach has been to omit CdS and form a direct junction between and ZnO, but the plasma (ions) during ZnO deposition by RF sputtering can damage the surface and enhance interface recombination. Possible solutions include ZnO deposited by metal organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD) or a novel technique, called ion layer gas reaction. High-vacuum evaporation (HVE)-grown CdS films exhibit47 sub-micrometer-sized, columnar grains that grow with preferred orientation parallel to the substrate. Recently, attempts have been made to enhance the crystal quality of CdS by the incorporation of O or CdCl2 as flux agent and post-deposition treatments in air and Ar. Impurities can compensate the doping in CdS and act as carrier traps, turning the CdS into a transport barrier modulated by light. This mechanism has already been investigated for cells, adapted models have been developed for cells.49 Hence, not only the direct influence of the front contact, but its indirect influence on the electrical properties of the CdS by inter diffusion of impurities across the TCO/CdS interface should be considered in terms of cell stability MATERIAL PROPERTIES OF THE ABSORBERS I III VI2 semiconductors, such as CIS or are often simply referred to as chalcopyrites because of their crystal structure. These materials are easily prepared in a wide range of compositions and the corresponding phase diagrams are well investigated For the preparation of solar cells only slightly Cu-deficient compositions of p-type conductivity are suited.53,54 Depending on the [Ga]/[InþGa] ratio, the bandgap of can be varied continuously between 1.04 and 1.68 ev. The current high-efficiency devices are prepared 282

6 with bandgaps in the range ev, this corresponds to a [Ga]/[InþGa] ratio between 25 and 30%. The phase diagram is characterized by a congruently melting intermediate phase,55 -, which forms at 50 at% Te. It has a cubic zincblende (sphalerite) structure. Under pressure or in thin films, two other phases of cubic or hexagonal structure can form.46,56 The deviation from stoichiometric composition is negligible, the Material PV Type Material Requirement (MT/GW) Material Availability (MT/yr width of the stability region of the stoichiometric phase above 400C is 106 at.%. The high liquidus temperature results from a strong ionic binding between Cd and Te atoms. These features make as a robust material suitable for high-depositionrate industrial processes BACK CONTACT Various metal contacts to p-type CIS were examined by Matson et al.144 concluding that only Au and Ni ensure an ohmic contact. Recently, Orgassa et al.145 fabricated solar cells with different back-contact materials, emphasizing the role of the back contact as an optical reflector. Early results by Russell et al.146 suggested that Mo back contacts for CIS form a Schottky-type barrier. But recently, a work of Shafarman et al.147 who analyzed the Mo/CIS interface separately from the cell, shows the contact to be ohmic. Nowadays, Mo growth by sputtering or e-beam evaporation is the most commonly used back contact for solar cells. To form an ohmic contact on p-, metals with a work function greater than 5.7 ev are required. Such metals are not available and the formation of a Schottky barrier at the back contact would be unavoidable. To overcome this problem a heavily p-doped surface is created by chemical etching and a buffer layer of high carrier concentration is often applied.154 Subsequent postdeposition annealing diffuses some buffer material into where it changes the band edges and interface states. The contact barrier is lowered, resulting in a quasi-ohmic contact. VI. EMERGING FUTURE ON CGIS AND Potential PV Capacity (GW/yr) Current (2010) Material Requirements and Current (2010) Material Availability Tellurium Indium 52 1, Reduced (Future) Material Requirements and Current (2010) Material Availability Tellurium Indium 5 1, Reduced (Future) Material Requirements and Increased (2015) Material Availability Tellurium 19 1, Indium 5 1, Table 6 Annual PV Capacity Supply Based on Current and Potential PV Material Requirements and Material Availability 1.The reduced material requirement estimates listed here are not projected to a specific year, e.g., not to Rather, they 283

7 represent estimates of practical minimum limits on tellurium and indium requirements for and PV technologies. Accelerated R&D may reduce the time required to reach these levels. Current production module efficiencies have been demonstrated to be as high as 11.7% (First Solar 2011a), with layers that are 2 3 μm thick (Green 2011). Process materials use is about 90% for current module production techniques (with in -process recycling), which implies tellurium requirements of about 7 10 grams (g)/m2 and, correspondingly, MT/GW of tellurium. According to Woodhouse et al. (2011), if R&D-driven improvements could increase efficiency to 18% and decrease layer thickness to about 1 μm roughly the amount of semiconductor thickness needed to efficiently fully absorb the solar spectrum (without significant drops in photocurrent) tellurium requirements could drop to 19 MT/GW. The indium requirements are from Goodrich et al. (2011b). The 2010 (co evaporation technique) indium requirement of 52 MT/GW is based on current estimates of material yield losses, 10% module efficiency, and a 1.5-μm absorber. The reduced indium requirement of 5 MT/GW includes estimated material yield losses, 20.8% module efficiency, a 1.0-μm absorber, and a high gallium to - indium ratio. 2. Current/2010 and increased/2015 material availability are from DOE (2010); although this report does not project longer - term tellurium and indium availability, availability may be higher beyond The large projected increase in tellurium availability between 2010 and 2015 is based, in part, on assumptions about greatly increasing the rate of tellurium recovery from copper refining; the added cost of this increased recovery rate is unknown, thus it is unknown whether the process will prove economically viable. V. CONCLUSION However, during the last 15e20 years the photovoltaic world has been enriched with other interesting materials such as and CuInSe2. Both these materials are considered very suitable for the fabrication of solar cells because of their direct band gap. As a consequence of the direct energy gap, the absorption edge is very sharp and thus, more than 90% of the incident light is absorbed in a few micro meters of the material. REFRENCES 1. M. Grätzel, Photovoltaic and photoelectrochemical conversion of solar energy Phil. Trans. R. Soc., 365, 993 (2007). 2. R. L. Stolk, H. Li, C. H. M. van der Werf and R. E. I. Schropp, Tandem and triple junction silicon thin film solar cells with intrinsic layers prepared by hot-wire CVD Thin Solid Films, 501, 256 (2006). 3. M. A. Arturo, Can we improve the record efficiency of CdS/ solar cells Sol. Energy Mater. Sol. Cells, 90, 2213 (2006). 4. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf,C. L. Perkins, B. To and R. Noufi, 19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor Prog. Photovolt. Res. Appl., 16, 235 (2008). 5. Ramanathan K, Contreras MA, Perkins CL, Asher S, Hasoon FS, Keane J, Young D, Romero M, Metzger W, Noufi R, Ward J, Duda A. Properties of 19_2% efficiency ZnO/CdS/Cu(In,GA)Se2 thin-film solar cells. Progress in Photovoltaics: Research and Applications 1999; 11: u X, Kane JC, Dhere RG, DeHart C, Albin DS, Duda A, Gessert TA, Asher S, Levi DH, Sheldon P. 16_5% Efficiency CdS/ polycristalline thin-film solar cells. Proceedings of the 17th European 284

8 Photovoltaic Solar Energy Conference and Exhibition, Munich, 2002; Tiwari AN, Krejci M, Haug F-J, Zogg H. 12_8% Efficiency Cu(In, Ga)Se2 solar cell on a flexible polymer sheet. Progress in Photovoltaics: Research and Applications 1999; 7: Tuttle JR, Szalaj A, Keane J. A 15_2% AMO/1433 W/kg thin-film Cu(In,Ga)Se2 solar cell for space applications. Proceedings of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, 2000; Matulionis I, Han S, Drayton JA, Price KJ, Compaan AD. Cadmium telluride solar cells on molybdenum substrates. Proceedings of the 2001 MRS Spring Meeting, San Francisco, 2001; H8.23_ Romeo A, Arnold M, Ba tzner DL, Zogg H. Development of high efficiency flexible solar cells. Proceedings of the PV in Europe From PV Technology to Energy Solutions Conference and Exhibition 2002;