Light- and bias-induced metastabilities in Cu(In,Ga)Se2 based solar cells caused by the (VSe-VCu) vacancy complex

We investigate theoretically light- and bias-induced metastabilities in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) based solar cells, suggesting the Se–Cu divacancy complex $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ as the source of this hitherto puzzling phenomena. Due to its amphoteric nature, the $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ complex is able to convert by persistent carrier capture or emission from a shallow donor into a shallow acceptor configuration, and vice versa, thereby changing in a metastable fashion the local net acceptor density inside the CIGS absorber of the solar cell, e.g., a CdS/CIGS heterojunction. In order to establish a comprehensive picture of metastability caused by the $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ complex, we determine defect formation energies from first-principles calculations, employ numerical simulations of equilibrium defect thermodynamics, and develop a model for the transition dynamics after creation of a metastable nonequilibrium state. We find that the $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ complex can account for the light-induced metastabilities, i.e., the “red” and “blue” illumination effects, as well as for the reverse-bias effect. Thus, our $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ model implies that the different metastabilities observed in CIGS share a common origin. A defect state in the band gap caused by $\left(V_{\mathrm{Se}}\text{-}{V}_{\mathrm{Cu}}\right)$ in the acceptor configuration creates a potentially detrimental recombination center and may contribute to the saturation of the open circuit voltage in larger-gap $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ alloys with higher Ga content. Therefore, the presence of metastable defects should be regarded as a concern for solar cell performance.