Post-Heat Treatment on Cu(In,Ga)Se2 Solar Cells with CBD-ZnS Buffer Layers as a Function of ITO Growth Temperature

We fabricated Cu(In,Ga)Se-2 (CIGS) solar cells with a the chemical bath deposition-ZnS buffer layer by varying the substrate temperature for indium tin oxide (ITO) growth (T-sub.ITO) from room temperature (RT) to 200 degrees C. The CIGS solar cell efficiency increased with increasing T-sub.ITO. After light soaking (LS), the CIGS solar cell performance improved noticeably, with a rise in fill factor, except for T-sub.ITO at RT, due to the high resistivity of ITO film. Post heat treatment (PHT) was carried out on the CIGS solar cells, in ambient air at 200 degrees C, with increasing annealing time, from 10 min to 1 h. After PHT for more than 10 min, cell performance was superior to that after LS at T-sub.ITO <= 100 degrees C, with substantially increased cell efficiency. This was due to simultaneously enhancing the quality of the ITO film with supplementing thermal energy, and curing a defect at the p-n junction. At T-sub.IOT >= 150 degrees C, cell performance improved after LS, compared to after PHT, regardless of the annealing time. After LS, photoexcited carriers were generated, which was beneficial for curing defects at the p-n junction, resulting in elevating cell performance However, after PHT, excessive thermal energy was injected into the solar cell, which induced Zn diffusion into the CIGS absorber layer, forming different defect states, such as Zn-Cu, and Zn-i, from the defect located at the p-n junction.