Potential Induced Degradation of CIGS Solar Cells

Abstract: This thesis studies the effects of Na diffusion in Cu(In,Ga)Se2 (CIGS) solar cells,caused by electrical Potential Induced Degradation (PID) and how to prevent it. Thiswas done by subjecting CIGS solar cells a temperature of 850C and an electrical biasfrom the backside of the glass substrate to the Mo back contact of the CIGS cell.When the bias was negative at the back contact the Na diffused in to the CIGS(degradation) and when it was positive the ions diffused out again (recovery). TheCIGS samples were electrically characterized with IV- and EQE-measurements duringthese conditions and compositional depth profiling was used to track the Nadistribution.This study showed that during degradation Na seemed to accumulate in the interfacesbetween the different layers in the CIGS cell. The buffer and window layers arestrongly affected by Na diffusion. Zn(O,S) buffer layer showed a clear difference inrecovery behavior compared to CdS buffer layer. The introduction of an Al2O3barrier layer between the CIGS and Mo back contact increased the degradation timefrom 50 h to 160 h. During this study it was also found that in some cases the CIGSsolar cells efficiency could be improved by degrading the cells and then recoveringthem, in the best case from 13% average energy efficiency to 15% efficiency.