Mechanisms for light-soaking induced carrier concentration changes in the absorber layer of Cu(In, Ga)Se<inf>2</inf> solar cells

Abstract

We report evidence supporting two separate mechanisms that can change the device performance of Cu(In, Ga)Se 2 (CIGS) solar cells upon light-soak through changes in the carrier concentration of the absorber layer. The first mechanism involves the Lany-Zunger (L-Z) copper-selenium divacancy complex (V Se -V Cu ), where this metastable defect is theoretically predicted to change from a shallow donor state to an acceptor state upon light-soak. We show that growing CIGS under selenium rich conditions can mitigate open-circuit voltage (Voc) change, consistent with the L-Z framework. Furthermore, we show that while carrier concentration increases with light-soak, Voc can either increase or decrease depending on the carrier concentration of the relaxed state. The second mechanism involves sodium redistribution in the CIGS layer, where sodium is known to affect absorber carrier concentration in CIGS solar cells. However, we argue that sodium redistribution by itself cannot account for the total light-soak induced carrier concentration changes observed in our devices.