Photovoltaic properties of Cu(In,Ga)(Se,Te)2 thin film solar cells with different tellurium amounts and a copper-poor stoichiometry

Abstract

In this study, the impact of tellurium addition on the microstructure of the copper indium gallium selenide absorber layer with a copper-poor stoichiometry and the photovoltaic properties of SLG/Mo/CIGS/CdS/ZnO/ITO/Ni-Al-Ni solar cells was investigated. Absorber layer, CdS buffer, ZnO and ITO layers, and the Ni-Al-Ni front contact were produced using three-stage co-evaporation, chemical bath deposition, RF magnetron sputtering, and e-beam evaporation techniques, respectively. The thickness and the composition of the absorber layer were controlled in situ. NaF post deposition treatment were applied to the absorber layer. The addition of tellurium improved the crystal quality by increasing the average grain size and decreased the surface roughness. Decreasing surface roughness increased reflection and thus decreased the amount of sunlight absorbed, which in turn reduced current collection. Open-circuit voltage was effected by impurity level and the grain boundry recombination. While moderate tellurium addition reduced grain boundary recombination, excessive tellurium addition created stress, caused crack formation, and increased recombination by reducing crystal quality. The optimum tellurium amount in the copper-poor CIGS structure was found to be 1.1 atomic percent. The control of the microstructure of the absorber and the efficiency improvement of the solar cell were achieved successfully.