Spray Pyrolysis Synthesized Cu(In,Ga/Al)(S,Se)2 Thin Films for Photovoltaic Applications

Abstract

Semiconducting CIGS thin-films were deposited using spray-pyrolysis of aqueous precursor solutions of gallium (gallium chloride; GaCl3), copper (indium chloride; CuCl2), indium (indium chloride; InCl3), and Sulphur (thiourea; (SC(NH2)2). In order to maintain good reproducibility in the deposition process, a robotic spray system was used. The processing conditions, such as substrate temperature, spray rate and film thickness on device performance, were investigated thoroughly. The as-sprayed thin films were then selenized at different temperatures and durations to obtain the optimized selenization conditions (500 oC for 10 minutes). The Chemical bath deposition (CBD) process was employed to deposit Cadmium Sulphide using two different cadmium sources (cadmium acetate and cadmium sulphate) at various deposition times followed by post CdS annealing. CIGSSe devices fabricated using the optimized conditions shows 10.54% power conversion efficiency (PCE) on an active area of 0.15 cm2. Large area devices fabricated with the optimized processing conditions show PCE of 5.89% and 5.07% measured on active area of 2.5 cm2 and 4.6 cm2 respectively. The efficiency of copper indium gallium disulfoselenide (CIGSSe) based solar cells is primarily dependent on the band bap of absorbing thin film of CIGSSe. The device performance can be enhanced by fabricating multi band gap layer of CIGSSe. However, the fabrication of multi band gap CIGSSe using non-vacuum techniques is challenging. In this work, we also fabricated solar cell devices which consisted of multi band gap Cu(In,Ga)(S,Se)2 thin films. The CIGS thin films were prepared by the spray-pyrolysis of aqueous precursor solutions of gallium (gallium chloride; GaCl3), copper (indium chloride; CuCl2), indium (indium chloride; InCl3), and Sulphur (thiourea; (SC(NH2)2) sources on Mo-coated glass substrate. The as-sprayed thin films were then selenized at 500 oC for 10 minutes. After selenization, CIGS films were transformed to Cu(In,Ga)(S,Se)2 (CIGSSe). The CIGS films with different composition were deposited again on top of selenized CIGSSe films and selenization process was repeated, hence multi band gap CIGSSe films were fabricated. The Chemical bath deposition (CBD) process was used to deposit cadmium sulphide (CdS) buffer layer. The solar cell fabricated having multi band gap layer of CIGSSe with the device configuration of glass/Mo/CIGSSe/CdS/i-ZnO/AZO showed a power conversion efficiency of 6.51%. We also fabricated Cu(In,Al)(S,Se)2 thin films by the spray pyrolysis of aqueous precursor solutions of copper, indium, aluminium and sulphur sources. The bandgap of the films was engineered by aluminum (Al) doping in CISSe films deposited on molybdenum (Mo) coated glass substrate. The as-sprayed thin films were selenized at 500 oC for 10 minutes. Cadmium sulphide (CdS) buffer layer was deposited by chemical bath deposition process. Solar cell devices were fabricated with configuration of glass/Mo/CIASSe/CdS/i-ZnO/AZO. The solar cell device containing thin film of Cu(In,Al)(S,Se)2 with our optimized composition shows j-V characteristics of Voc = 0.47 V, jsc = 21.19 mA/cm2, FF = 52.88% and power conversion efficiency of 5.27%, under AM 1.5, 100 mW/cm-2 illumination.