Ga2O3 Alloys As Potential Transparent Conducting Oxides (TCO) Materials for CdTe Photovoltaics – a DFT Study

Abstract

Thin film photovoltaic cells based on Cu (In, Ga) (S, Se)2 and CdTe have achieved significantly improved power conversion efficiencies (~ 20 %). These solar cells typically exhibit a layered structure with absorber material (CdTe) at the middle. Commercial CdTe solar cells incorporate a CdS buffer layer between the CdTe and transparent conducting oxide (TCO) layer to achieve high conversion efficiency. With advent of MgZnO (MZO), CdS buffer layer is not essential. Wide band gap of MZO ensures appropriate band offset at the TCO/absorber interface. A conduction band offset of ~ + 0.2 eV which is representative of a small spike across the TCO/CdTe interface is preferred to deter interface recombination. Mg content in MZO controls the band gap of MZO and allows easy tunability of the band offsets across MZO/CdTe interface. Despite these advantages, the MZO layers suffer from interface recombination due to unfavorable band offsets, and high interfacial defect density. These factors adversely affect the conversion efficiencies. The efficiency of CdTe solar cells is also limited by low Voc. Improving the front TCO/absorber interface is key to realize higher Voc along with achieving required bulk carrier density and lifetime.As a result, recent studies, are focused on identifying TCO layers beyond MZO. Ga2O3 based TCO layers have demonstrated tremendous improvements in interfacial defect passivation in crystalline silicon and dye sensitized solar cells, leading to record high Voc. As a result, Ga2O3 can be a potential alternative to MZO buffer layers in CdTe photovoltaics. Ga2O3 shows a transparency to wavelengths ~ 250 nm. Additionally, Ga2O3 also shows a relative ease of fabrication in both bulk crystals as well as thin films structures along with wider control over electrical properties from semi-insulating to degenerate doping. All these features make Ga2O3 as a promising candidate for TCO layers in CdTe photovoltaics.In line with the objective, we performed first principles calculations to investigate Ga2O3 and Ga2O3 based alloys as a TCO layer in CdTe solar cells. Density functional theory (DFT) method was applied to model solid-solid interfaces across TCO/CdTe. A very large band gap for Ga2O3 (~ 4.6 – 5.0 eV) would lead to a significantly large conduction band offset across the TCO/CdTe interface. Hence, we have performed DFT calculations for Ga2O3 and InxGa2-xO3 alloys to investigate the impact of In-doping on the conduction band offsets. The DFT calculations determine the conduction and valence band offsets across several pairs of solid-solid interfaces for CdTe thin film solar cells. The results show a preferred conduction band offset of +0.22 eV representative of a small spike across TCO/CdTe interface for InGaO3 (50:50 in alloy). For pure Ga2O3, the calculated conduction band offset is ~ 0.94 eV. The above results indicate that the conduction band offset for Ga2O3 can be lowered to a preferred value by suitable In-alloying. Overall the DFT calculations are beneficial in designing optimum In-alloying content in Ga2O3 to achieve high conversion efficiencies (> 20%) in future CdTe photovoltaics.