Influence Of Ga Back Grading On Voltage Loss In Low-Temperature Co-Evaporated Cu(In,Ga)Se2 Thin Film Solar Cells

The performance of Cu(In,Ga)Se-2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (V-OC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (Delta GGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the Delta GGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both V-OC deficit and time-resolved photoluminescence lifetime are observed with increasing Delta GGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a Delta GGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high-performance CIGS solar cells and devices.