Light-Soak and Dark-Heat Induced Changes in Cu(In, Ga)Se2 Solar Cells: A Macroscopic to Microscopic Study

We report the effects of light-soaking and dark-heat on Cu(In,Ga)Se₂ solar cells from the macroscopic level of module and cell performance down to the microscopic level of carrier concentration and sodium profile changes in the absorber layer. We find that solar cell performance degradation from each stress is driven by a fundamentally different mechanism. In particular, we find that the Lany-Zunger (L-Z) framework, namely an increase in carrier concentration associated with changes in the charge state of a copper-selenium divacancy complex in the absorber, can adequately explain light-soak induced degradation. Dark-heat induced degradation, on the other hand, is driven by an increase in voltage dependent collection (VDC). Atomic redistribution of sodium at various stages of light-soak and dark-heat is reported, and its potential impact on the observed device performance is discussed.