Back Surface Recombination Effect on the Ultra-Thin CIGS Solar Cells by SCAPS

Abstract

the impact of the back surface recombination velocity (S_R) and the presence of the Electron Back Reflector (EBR) on the performance of CIGS solar cell when varying the absorber thickness from 0.3 to 2 µm is illustrated by the diverse results obtained by simulation using SCAPS. Thinner devices are more affected than thick ones by the variations of the EBR and S_R. The gain in efficiency due to the reducing S_R is increased as the absorber thickness is reduced. The results revealed that for thin absorbent layer less than 1µm the efficiency of the CIGS cell can be increased by 1-3% depending on the thickness if the S_R is reduced to lower than 10³ cm/s. This leads to enhance the V_oc and efficiency which become comparable to those obtained for standard devices (2µm). For high S_R the electron back reflector plays much more significant role and becomes beneficial. However the high band gap of EBR does not necessary result in high performance where the results show that 0.2 eV of EBR height is sufficient to enhance the performance. Independently to the absorber thickness the efficiency increased sharply, especially for thinner device, when an EBR with thickness around 5% corresponding of the total CIGS thickness was added at the back surface. The gain in efficiency increases as the thickness of the layers is reduced and reaches the same levels as the standard devices. As the thickness of EBR is increased, the reduction of J_sc is fairly recovered by the augmentation of V_oc which leads to a slight reduction in efficiency especially for thinner device.