Optimizing Ga-profiles for highly efficient Cu(In, Ga)Se 2 thin film solar cells in simple and complex defect models

Highly efficient Cu(In,Ga)(S,Se) 2 photovoltaic thin film solar cells often have a compositional variation of Ga to In in the absorber layer, here described as a Ga-profile. In this work we have studied the role of Ga-profiles in four different models, based on input data from electrical and optical characterizations of an in-house state-of-the-art Cu(In,Ga)Se 2 (CIGS) solar cell with power conversion efficiency above 19 %. A simple defect model with mid-gap defects in the absorber layer was compared with models with Ga-dependent defect concentrations and amphoteric defects. In these models optimized single-graded Ga-profiles have been compared with optimized double-graded Ga-profiles. It was found that the defect concentration for effective Shockley-Read-Hall recombination is low for high efficiency CIGS devices and that the doping concentration of the absorber layer, chosen according to the defect model, is paramount when optimizing Ga-profiles. For optimized single-graded Ga-profiles the simulated power conversion efficiency, depending on the model, is 20.5-20.8 %, and the equivalent double-graded Ga-profiles yield 20.6-21.4 %, indicating that the bandgap engineering of the CIGS device structure can lead to improvements in efficiency. Apart from the effects of increased doping in the complex defect models, the results are similar when comparing the complex defect models to the simple defect models.