Crystal face dependent charge carrier extraction in TiO2/perovskite heterojunctions

Abstract 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.