Ga_2O_3 Alloys As Potential Transparent Conducting Oxides (TCO) Materials for CdTe Photovoltaics - a DFT Study

摘要

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 V_(oc). Improving the front TCO/absorber interface is key to realize higher V_(oc) along with achieving required bulk carrier density and lifetime.