Exceptional Hole‐Selective Properties of Ta2O5 Films via Sn4+ 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 (Ta2O5) 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 Ta2O5 films exhibit a high hole selectivity, which blocks electrons and promotes hole transport simultaneously. Through the ozone pre‐treatment of Ta2O5/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 cm2. Moreover, the Sn4+ 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 Ta2O5/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. The study first demonstrates that solution‐processed Ta2O5 films have a high hole‐selectivity capability, which is strengthened by the interfacial UV‐O3 pre‐treatment and the Sn4+ doping, leading to the conversion efficiency of Ta2O5/p‐Si heterojunction solar cells significantly improved from 14.84% to 18.47%.