Opto-electronic compatibility of cadmium telluride (CdTe) thin films on ultra-thin glass substrate for flexible solar cell application

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Doroody Camellia, Dr.
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In recent years, bendable and role-to-role production of low-cost and light-weight thin film solar cells have been emerging rapidly. Among all commercialized thin film solar cells, cadmium-telluride (CdTe) showed the highest efficiency in superstrate configuration on the rigid glass as deposited by high-temperature techniques. However, this kind of CdTe thin film solar cell on rigid substrate, i.e. glass, is heavier in weight with limited applications. Several flexible materials have been tested as substrate material for flexible CdTe thin film solar cells, where the conversion efficiency as high as 16.4% was found for bendable glass substrate produced by high-temperature deposition method called close-spaced sublimation (CSS). However, majority of the available flexible or semi-flexible substrates are incompatible with the process temperature above 500 °C. Moreover, most of the available substrates’ coefficient-ofthermal-expansion (CTE) have huge mismatch with the CTE of the CdTe absorber material, except for ultra-thin-glass (UTG) of particular kind that has narrow difference to be the best choice for CSS-grown CdTe thin film solar cells. Another prominent limitation lies in the difficulty of the ohmic contact formation with CdTe layer irrespective of substrate types. No detailed investigation on the formation of ohmic contacts to CdTe layers on UTG substrates has been reported to date. Consequently, no detailed research has been conducted on the optimization of transparent conductive oxides (TCOs), window layer or other limitations of CdTe thin film deposition on UTG. Therefore, there are research scopes for a more compatible UTG upon systematic investigation on layer-by-layer growth of feasible materials with optimum thickness, optimum deposition process as well as optimized properties of the deposited layers and eventually the verification of p-n junction formation. In this study, deposition of CdTe as absorber layer is proposed on D263T eco (0.1 mm, manufacturer: The Schott Company) ultra-thin glass substrate that has the optimal thermal expansion coefficient matching with CdTe and the ability to endure the high-temperature process while keeping the high transparency. Three typical transparent conductive oxides, namely fluorine-doped tin oxide SnO2:F (FTO), aluminium-doped zinc oxide ZnO:Al (AZO), and indium tin oxide (ITO) on top of UTG substrate are simulated and characterised for optical properties using OPAL2 software. Around 200 nm of ITO layer is found to be the most stable TCO material for UTG substrate. Furthermore, CdS layer with thickness range of 50 nm to 150 nm with bandgap above 2.3 eV is found as the most successive n-type window layer for CdTe thin films. Hence, CdS thin film is deposited on UTG using Chemical Bath Deposition (CBD) and Radio-Frequency (RF) Sputtering methods, whereby sputtering shows better suitability of CdS layer on UTG substrate. Thereafter, CdTe is investigated in regard to film deposition by Close‐Spaced Sublimation (CSS), whereby stable cubic phased polycrystalline CdTe thin film has been found on UTG substrate. Upon optimizing the CSS process parameters, deposition pressure of 1 Torr of and substrate-source temperature of 500-600 ˚C were found to be the optimized parameters for CdTe film growth on UTG substrate. Moreover, the optimum annealing temperature of 390 ˚C during CdCl2 activation treatment was sorted out for CdTe film on UTG substrates. Finally, a rudimentary solar cell structure of UTG/ITO/CdS/CdTe/C:Cu/Ag stack is prepared following the optimum materials and methods as the outcome of this study, whereby favourable p-n junction with diode ideality factor of 1.23 is confirmed under dark current-voltage measurement condition. These findings are believed to substantially contribute on the conformity of CdTe thin film growth on ultra-thin glass substrates for flexible thin film solar cell application.
Opto-electronic compatibility of cadmium telluride (CdTe) thin films