Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO₂ as an Electron Selective Layer for Efficient Perovskite Solar Cells

The high-efficiency photocarrier collection at the interfaces plays an important role in improving the performance of perovskite solar cells (PSCs) because the photocarrier effective diffusion lengths in the lead halide perovskite absorbers usually surpass the incident depths of light. Developing the electron selective layer (ESL) that has good interfaces with photoactive perovskite and current collector layer-like fluorine-doped tin oxide (FTO) is actively pursued. Here, an unusual dense film of faceted rutile TiO₂ single crystals with a gradient of the Sn⁴⁺ dopant grown heteroepitaxially on the FTO layer is obtained by a hydrothermal route and subsequent thermal treatment. Owing to the global features including low concentration of defects, atomically smooth coherent interface with FTO, and gradient doping-induced built-in electric field to promote the collection of photoelectrons in it, an optimal PSC with such a film as the ESL exhibits an efficiency of 17.2% with an open-circuit voltage of 1.1 V and fill factor of 76.1%, which are among the highest values of the PSCs with rutile TiO₂ films as ESLs.