A recombination analysis of Cu(In,Ga)Se2 solar cells with low and high Ga compositions

Separation and quantification of recombination losses in thin-film Cu(In1−xGax)Se2 solar cells is paramount to understanding the current state-of-the-art and future improvements, but an effective characterization technique has been lacking. We use the recently developed temperature-illumination-dependent open-circuit voltage method to extract individual recombination rates at the buffer/absorber interface, in the space-charge region, and in the quasi-neutral region, as well as the carrier lifetime and surface recombination velocity for devices with low (x=30%) and high (x=84%) Ga absorbers. In the low-Ga absorber, recombination in the quasi-neutral region dominates. In the high-Ga absorber, interface recombination dominates. The open-circuit voltage deficit of the high-Ga device originates from an inadequacy of band bending in the absorber and a lack of strong inversion at the buffer/absorber interface. As two promising mitigating strategies for the open-circuit voltage deficit problem at high-Ga levels, we highlight a homojunction in the absorber or alternative transparent conducting oxides with low work function.