Role of the Cu-Deficient Interface in Cu(In,Ga)Se2 Thin-Film Photovoltaics with Alkali-Metal Doping

$\mathrm{Cu}$-deficient phases of chalcopyrite $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS), such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_{3}{\mathrm{Se}}_{5}$ and $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_{5}{\mathrm{Se}}_{8}$, which are occasionally referred to as ordered vacancy compounds and often unintentionally form on CIGS film surfaces and at grain boundaries, are among the most important subjects in developing chalcopyrite thin-film energy-conversion materials and devices. Here, we revisit the role of the $\mathrm{Cu}$-deficient surface layer (CDL) present at the p-CIGS/n-$\mathrm{Cd}\mathrm{S}$ interface in photovoltaic devices with the effects of alkali-metal doping. The device structure used is alkali-containing soda-lime glass or alkali-free zirconia substrate/$\mathrm{Mo}$/CIGS/${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_{2}$-based CDL/$\mathrm{Cd}\mathrm{S}$/intrinsic and $\mathrm{Al}$-doped $\mathrm{Zn}\mathrm{O}$. The photovoltaic device performance deteriorates with increasing CDL thickness when no alkali metal is added. However, the CDL thickness significantly affects the results of alkali-halide $\mathrm{Rb}\mathrm{F}$ postdeposition treatment (PDT) and the use of a thick CDL enhances the beneficial effects of $\mathrm{Rb}\mathrm{F}$ PDT, resulting in photovoltaic performance enhancements manifested by improvements in the open-circuit voltage and fill-factor values. On the other hand, $\mathrm{Rb}\mathrm{F}$ PDT leads to degradation of the device performance when the CDL is thin. High $\mathrm{Rb}$ concentration and significant $\mathrm{Ga}$ diffusion from the CIGS layer to the surface CDL with $\mathrm{Rb}\mathrm{F}$ PDT are observed when the CDL is thick. The formation of metastable acceptors, namely, an increase in the nominal carrier density in CIGS, is observed with light-soaking (LS) treatments, regardless of the thickness of CDL. Nevertheless, improvements in photovoltaic efficiency with LS treatments are observed only from devices grown with a thick CDL.