First-principles calculations of Na and K impurities in CuInSe2 and their effect on Cd incorporation

Recent experimental work has revealed the distinct and beneficial role of K incorporation on the fabrication of increasingly efficient thin-film photovoltaic devices with Cu(In, Ga)Se2 (CIGS) absorber layers. This has been attributed, in part, to improved CdS/CIGS heterojunction quality due to the enhanced diffusion of Cd into the near-interface region of CIGS. In this work, we try to distinguish the role of K compared to Na in enhancing Cd incorporation in CuInSe2 (CIS) based on first-principles calculations. Using a canonical method for calculating defect concentrations as a function of temperature and material stoichiometry, we identify experimentally relevant conditions under which a simple model for Na and K kinetics can lead to such an effect. We argue that a sufficiently low migration barrier for K diffusion mediated by Cu vacancies can lead to Cu-depletion near the CdS interface, allowing Cd to occupy greater numbers of vacant Cu sites.