From 20.9 to 22.3% Cu(In, Ga)(S, Se)2 solar cell: Reduced recombination rate at the heterojunction and the depletion region due to K-treatment

Certified efficiency of 22.3% has been achieved for Cu(In, Ga)(Se, S)(2) solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased V-oc, benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance-voltage profiling, temperature-dependent V-oc, Suns-V-oc) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current-voltage curve for the 22.3% cell. The best model was also used to estimate the achievable V-oc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher J(sc) was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination analysis on the 22.8% cell indicates that the interface recombination is further reduced, but the recombination coefficients at the depletion region was higher, pointing out that further improvement on the depletion region recombination could help to achieve a higher V-oc and therefore an efficiency beyond 23%. (C) 2017 The Japan Society of Applied Physics