Heat-light soaking treatments for high-performance CIGS solar cells on flexible substrates

Abstract Cu(In,Ga)Se2 (CIGS) solar cells are sensitive to heat-light soaking (HLS) treatments performed at elevated temperatures. Such treatments often result in improved open-circuit voltage at the expense of short-circuit current and/or fill factor, and may be reversible. In this contribution, we analyze the changes brought in the devices by heat and light treatments to CIGS, to deliver hints regarding the fundamental reasons behind the observed changes and to ensure the long-term stability of the performance improvement. We apply HLS treatments to improve the performance of low-temperature grown CIGS solar cells on flexible polyimide and on glass substrates. We first investigate the treatment conditions favorable for device performances. A gain in open-circuit voltage is obtained, with no clear statistical impact on short-circuit current and fill factor. The changes are stable over periods of several months. Then we combine material analysis techniques to optoelectronic and electrical characterization on samples subject to a unique HLS treatment at the optimal conditions. The voltage gain arises from a large increase in the doping density in the absorber layer, correlated with increased concentrations of alkali elements Na and Rb. This improvement is partly offset by a reduction in minority charge carrier lifetime. The microscopic origin for the observed changes is discussed. At low temperatures, we also measure device enhancement due to the front interface behavior, however with unclear impact on standard operating conditions. The HLS treatment yielded a CIGS solar cell with certified record 21.4% power conversion efficiency on a flexible substrate.