In2O3−Ga2O3 Alloys as Potential Buffer Layers in CdTe Thin-Film Solar Cells

Abstract

The efficiency of state-of-the-art $\mathrm{Cd}\mathrm{Te}$ solar cells remains limited by the relatively low open-circuit voltage (${V}_{OC}$). Improving the front interface is key towards realizing a higher ${V}_{OC}$ after achieving the necessary bulk carrier density and lifetime. Recent efforts in identifying buffer layers beyond $\mathrm{Cd}\mathrm{S}$ have focused on ${\mathrm{Mg}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$, which offers tunability of the band offsets, but often suffers from high interfacial defect densities. ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-based buffer layers demonstrate tremendous improvements in interfacial defect passivation in crystalline silicon and dye-sensitized solar cells, leading to record high ${V}_{OC}$, yet remain largely unexplored in $\mathrm{Cd}\mathrm{Te}$-based devices. Here, we perform hybrid density-functional-theory calculations to investigate pure ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ alloys as a window layer in $\mathrm{Cd}\mathrm{Te}$ photovoltaics. We report calculated band offsets for several pairs of solid-solid interfaces comprising transparent conducting oxide (TCO) and $\mathrm{Cd}\mathrm{Te}$ heterojunctions. The results support a large conduction band offset spike of 0.67 eV for the $\mathrm{Cd}\mathrm{Te}/{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ (100) interface, while the offset is reduced to 0.18 eV for the ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ alloy and matches closely with the preferred optimum value of 0.2 eV. Device-level modeling tests of $\mathrm{Cd}\mathrm{Te}$ solar cells integrating our results indicate that the highest efficiency is achieved with ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_{3}$ acting both as a buffer layer and TCO. Our results suggest that alloys of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ may be attractive alternatives to ${\mathrm{Mg}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$ for tailoring optimal conduction-band offsets of the buffer and TCO layers in high-efficiency $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells.