Amorphous BaTiO 3 Electron Transport Layer for Thermal Equilibrium‐Governed γ‐ CsPbI 3 Perovskite Solar Cell with High Power Conversion Efficiency of 19.96%

Compared to organic–inorganic hybrid perovskites, the cesium‐based all‐inorganic lead halide perovskite (CsPbI 3 ) is a promising light absorber for perovskite solar cells owing to its higher resistance to thermal stress. Nonetheless, additional research is required to reduce the nonradiative recombination to realize the full potential of CsPbI 3 . Here, the diffusion of Cs ions participating in ion exchange is proposed to be an important factor responsible for the bulk defects in γ‐CsPbI 3 perovskite. Calculations based on first‐principles density functional theory reveal that the [PbI 6 ] 4− octahedral tilt modifies the perovskite crystallographic properties in γ‐CsPbI 3 , leading to alterations in its bandgap and crystal strain. In addition, by substituting amorphous barium titanium oxide (a‐BaTiO 3 ) for TiO 2 as the electron transport layer, interfacial defects caused by imperfect energy levels between the electron transport layer and perovskite are reduced. High‐resolution transmission electron microscopy and electron energy loss spectroscopy demonstrate that a‐BaTiO 3 forms entirely as a single phase, as opposed to Ba‐doped TiO 2 hybrid nanoclusters or separate domains of TiO 2 and BaTiO 3 phases. Accordingly, inorganic perovskite solar cells based on the a‐BaTiO 3 electron transport layer achieved a power conversion efficiency of 19.96%.