${\mathrm{In}}_2{\mathrm{O}}_3\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ Alloys as Potential Buffer Layers in $\mathrm{Cd}\mathrm{Te}$ Thin-Film Solar Cells

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 ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}$, 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 ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}$ for tailoring optimal conduction-band offsets of the buffer and TCO layers in high-efficiency $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells.

Figure 1

Representative $\mathrm{Cd}\mathrm{Te}$ thin-film structure is shown. Reproduced from Ref. [3] with the permission of AIP Publishing.

Figure 2

Representative conduction-band offsets for (a) $\mathrm{Cd}\mathrm{S}/\mathrm{Cd}\mathrm{Te}$ interface showing a “cliff” feature. Optimal recommended emitter and $\mathrm{Cd}\mathrm{Te}$ conduction-band offset values for (b) “spike” and (c) “large spike” features. Reproduced from Ref. [3] with the permission of AIP Publishing. IF, Interface.

Figure 3

(a) ${\mathrm{Ga}}_2{\mathrm{O}}_3/\mathrm{Cd}\mathrm{Te}$ and (b) ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_3/\mathrm{Cd}\mathrm{Te}$ interface structures are shown. Green, red, blue, magenta, and brown spheres represent $\mathrm{Ga}$, $\mathrm{O}$, $\mathrm{In}$, $\mathrm{Cd},$ and $\mathrm{Te}$ atoms, respectively. $\mathrm{In}$ atoms in ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_3$ in (b) only occupy the octahedral sites in the β-gallia structure. Effective strain on $\mathrm{Cd}\mathrm{Te}$ unit cell is reported.

Figure 4

Average electrostatic potential variations along the direction normal to the interface plane for (a) $\mathrm{Cd}\mathrm{Te}/{\mathrm{Ga}}_2{\mathrm{O}}_3$ and (b) $\mathrm{Cd}\mathrm{Te}/{\mathrm{In}\mathrm{Ga}\mathrm{O}}_3$. Green line shows a macroscopic average, which reaches a constant value within the bulk $\mathrm{Cd}\mathrm{Te}$, ${\mathrm{Ga}}_2{\mathrm{O}}_3,$ and ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_3$ regions along the interface.

Figure 5

Representative band structures of (a) $\mathrm{Cd}\mathrm{Te}/{\mathrm{Ga}}_2{\mathrm{O}}_3$ and (b) $\mathrm{Cd}\mathrm{Te}/{\mathrm{In}\mathrm{Ga}\mathrm{O}}_3$ interfaces, showing conduction- and valence-band offsets. Dashed lines represent the average electrostatic potential for bulk ${\mathrm{Ga}}_2{\mathrm{O}}_3$, ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_3,$ and $\mathrm{Cd}\mathrm{Te}$. E_VBM and E_CBM represent valence-band maxima and conduction-band minima, respectively, for ${\mathrm{Ga}}_2{\mathrm{O}}_3$, ${\mathrm{In}\mathrm{Ga}\mathrm{O}}_3,$ and $\mathrm{Cd}\mathrm{Te}$. ΔE_C and ΔE_V represent conduction- and valence-band offsets, respectively. ΔV is the band alignment, as calculated earlier.