Light‐Soaking Effects in High‐Efficiency Cu(In,Ga)Se₂ and (Ag,Cu)(In,Ga)Se₂ Solar Cells

ABSTRACT Metastable behaviours with respect to light‐soaking of Cu(In,Ga)Se 2 (CIGS) solar cells have long been known and studied, but no explanation has yet been fully agreed on. In this study, silver alloying and its impact on light‐soaking effects is explored in four CIGS and six (Ag,Cu)(In,Ga)Se 2 (ACIGS) samples with high efficiency (17% to 21% before light‐soaking). All were produced by co‐evaporation and similar depositions protocols as the current world record device but with some variation. A variety of opto‐electronic characterisation methods were used to explore the response to 24 h of light‐soaking at 50°C. We found that (i) the open‐circuit voltage increased for all ACIGS devices but decreased for the CIGS devices, (ii) the cells entered a state with higher doping and more tail states, and (iii) the effect was metastable and partially reverted after dark storage. While the improvement of the ACIGS cells saturates after 24 h in one‐third of the irradiance at one sun, months are needed to reverse the open‐circuit voltage change. Despite a higher doping after light‐soaking, none of the samples' short‐circuit current showed significant changes and the efficiency after light‐soaking ranged from 15% to 22%. No long‐range change in sodium or rubidium distributions was observed using glow‐discharge optical emission spectroscopy. In general, external radiative efficiency measurements showed that the nonradiative recombination loss is reduced after light‐soaking in the ACIGS devices. However, the correlation to the measured voltage was not always straight forward, presumably due to the graded bandgap of the absorber.