Compared to organic–inorganic hybrid perovskites, the cesium-based all-inorganic lead halide perovskite (CsPbI₃) 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 CsPbI₃. Here, the diffusion of Cs ions participating in ion exchange is proposed to be an important factor responsible for the bulk defects in γ-CsPbI₃ perovskite. Calculations based on first-principles density functional theory reveal that the [PbI₆]⁴⁻ octahedral tilt modifies the perovskite crystallographic properties in γ-CsPbI₃, leading to alterations in its bandgap and crystal strain. In addition, by substituting amorphous barium titanium oxide (a-BaTiO₃) for TiO₂ 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-BaTiO₃ forms entirely as a single phase, as opposed to Ba-doped TiO₂ hybrid nanoclusters or separate domains of TiO₂ and BaTiO₃ phases. Accordingly, inorganic perovskite solar cells based on the a-BaTiO₃ electron transport layer achieved a power conversion efficiency of 19.96%.

Electron transport layer (ETL) is pivotal to charge carrier transport for PSCs to reach the Shockley–Queisser limit. This study provides a fundamental understanding of heterojunction electron transport layers (ETLs) at the atomic level for stable and efficient perovskite solar cells (PSCs). The bilayer structure of an ETL composed of SnO₂ on TiO₂ was examined, revealing a critical factor limiting its potential to obtain efficient performance. Alteration of oxygen vacancies in the TiO₂ underlayer via an annealing process is found to induce manipulated band offsets at the interface between the TiO₂ and SnO₂ layers. In-depth electronic investigations of the bilayer structure elucidate the importance of the electronic properties at the interface between the TiO₂ and SnO₂ layers. The apparent correlation in hysteresis phenomena, including current density–voltage (J–V) curves, appears as a function of the type of band alignment. Density functional theory calculations reveal the intimate relationship between oxygen vacancies, deep trap states, and charge transport efficiency at the interface between the TiO₂ and SnO₂ layers. The formation of cascade band alignment via control over the TiO₂ underlayer enhances device performance and suppresses hysteresis. Optimal performance exhibits a power conversion efficiency (PCE) of 23.45% with an open-circuit voltage (V_oc) of 1.184 V, showing better device stability under maximum power point tracking compared with a staggered bilayer under one-sun continuous illumination.