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)Se₂ (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)Se₂ (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.