Crystal face dependent charge carrier extraction in TiO₂/perovskite heterojunctions

Solar cells based on metal-halide perovskite/TiO₂ heterojunction have demonstrated exceptional high power conversion efficiencies. However, the electron extractor TiO₂ is typically a polycrystalline film processed with solution methods. The crystal face dependent electron extraction from perovskite to this TiO₂ film is still unclear, which is essential for the device performance further optimization. Herein, the ultrafast charge carrier dynamics in CH₃NH₃PbI₃/TiO₂ heterojunctions with rutile TiO₂ single crystals terminated at (100), (001) and (110) faces were systematically investigated. Our results clearly show that the charge carrier extraction from perovskite to TiO₂ is strongly dependent on the TiO₂ crystal face. The (100) face shows the best charge carrier extraction performance with an electron extraction efficiency over 98%. In contrast, the (001) face demonstrates an electron extraction efficiency less than 40%. The hot electron extraction from photo-excited perovskite to TiO₂ has also been clearly revealed with transient absorption. The first-principle density function theory calculations indicate that the electron transfer from CH₃NH₃PbI₃ to rutile TiO₂ is more energetically favoured for the (100) and (110) faces than for the (001) face. These findings reveal the hidden role of the TiO₂ crystal facets in electron extraction and its impact on the perovskite solar cell efficiency.