A theoretical investigation was made of light- and bias-induced metastabilities in Cu(In,Ga)Se2 (CIGS)-based solar cells, suggesting that the Se–Cu divacancy complex (VSe-VCu) was the source of this phenomenon. Due to its amphoteric nature, the (VSe-VCu) complex was 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 (VSe-VCu) complex, defect formation energies were determined from first-principles calculations, numerical simulations of equilibrium defect thermodynamics were employed, and a model was developed for the transition dynamics after creation of a metastable non-equilibrium state. It was found that the (VSe-VCu) complex could accounted for the light-induced metastabilities, i.e., the so-called red and blue illumination effects, as well as for the reverse-bias effect. Thus, the (VSe-VCu) model implied that the various metastabilities observed in CIGS shared a common origin. A defect state in the band gap caused by (VSe-VCu) in the acceptor configuration created a potentially detrimental recombination center and might contribute to the saturation of the open circuit voltage in larger-gap Cu(In,Ga)Se2 alloys with higher Ga content. Therefore, the presence of metastable defects should be regarded as a concern for solar cell performance.

Light- and Bias-Induced Metastabilities in Cu(In,Ga)Se2-Based Solar Cells Caused by the (VSe-VCu) Vacancy Complex. S.Lany, A.Zunger: Journal of Applied Physics, 2006, 100[11], 113725 (15pp)