Origin of the Improved Performance of Cu ( In , Ga ) ( S , Se ) 2 Solar Cells by Postdeposition Treatments: Effect of Band Offsets

Postdeposition treatment (PDT) with alkali metals has profoundly improved the performance of $\mathrm{Cu}(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})(\mathrm{S},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Se}{)}_{2}$ solar cells. Several mechanisms have been proposed to explain the improved performance, but the exact origin is still under debate. Here, using first-principles calculations, we demonstrate that alkali metals tend to accumulate at the interface, which significantly improves the band offset between the absorber and the alkali-metal-doped ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga}){\mathrm{Se}}_{2}$ buffer layer. Our results show that, as the fraction of $\mathrm{Na}$ increases, the valence-band maximum (VBM) of the alloy layer decreases, while the conduction-band maximum (CBM) increases. The drop of the VBM introduces a hole barrier, repelling holes away from the interface, and a moderately higher CBM forms a benign spikelike conduction-band offset (CBO) against the absorber layer independent of the $\mathrm{Ga}$ concentration. Both effects are beneficial for the solar-cell efficiency and exhibit extra advantages over the ordered vacancy compound (OVC), which commonly exists before the PDT, because, although the OVC can introduce the hole barrier, it also introduces a detrimental clifflike CBO, especially at a high $\mathrm{Ga}$ concentration. Such understandings of band offsets introduced by PDT can further be extended to other alkali metals, such as $\mathrm{K}$, $\mathrm{Rb}$, and $\mathrm{Cs}$, and the design principles can also be extended to other types of thin-film solar cells.