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 (Voc) of 1.184 V, showing better device stability under maximum power point tracking compared with a staggered bilayer under one‐sun continuous illumination.
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%.
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