Exceptional Hole-Selective Properties of Ta 2 O 5 Films via Sn 4+ Doping for High Performance Silicon Heterojunction Solar Cells.

Carrier-selective passivating contacts using transition metal oxides (TMOs) have attracted great attention for crystalline silicon (c-Si) heterojunction solar cells recently. Among them, tantalum oxide (Ta 2 O 5 ) exhibits outstanding advantages, such as a wide bandgap, good surface passivation, and a small conduction band offset with c-Si, which is typically used as an electron-selective contact layer. Interestingly, it is first demonstrated that solution-processed Ta 2 O 5 films exhibit a high hole selectivity, which blocks electrons and promotes hole transport simultaneously. Through the ozone pre-treatment of Ta 2 O 5 /p-Si interface and optimization of the film thickness (≈9 nm), the interfacial recombination is suppressed and the contact resistivity is reduced from 178.0 to 29.3 mΩ cm 2 . Moreover, the Sn 4+ doping increases both the work function and oxygen vacancies of the film, contributing to the improved hole-selective contact performance. As a result, the photoelectric conversion efficiencies of Ta 2 O 5 /p-Si heterojunction solar cells are significantly improved from 14.84% to 18.47%, with a high thermal stability up to 300 °C. The work has provided a feasible strategy to explore new features of TMOs for carrier-selective contact applications, that is, bipolar carrier transport properties.