TiO₂ electron transport bilayer for all-inorganic perovskite photodetectors with remarkably improved UV stability toward imaging applications

High ultraviolet (UV) stability and low dark current (I_dark) are necessary for high-quality perovskite photodetectors (PDs). TiO₂ thin film is known as effective electron-transport-layer (ETL) for perovskite devices. However, common spin-coated TiO₂ ETLs endow many surface defects and have strong UV photocatalytic effect to decompose perovskite materials, resulting in inferior stability of devices. In this work, TiO₂ bilayer film (Bi-TiO₂) has been fabricated by combining spin-coating and atomic-layer-deposition process and its positive effects on UV stability and I_dark of Cs₂AgBiBr₆-based PDs have been revealed for the first time. It is demonstrated that Bi-TiO₂ possesses fewer surface defects and smoother morphology with type Ⅱ band alignment, which is beneficial to suppress photocatalytic activity of TiO₂ and reduce carrier recombination at the interface. After accelerated strong UV aging treatment, the PD with Bi-TiO₂ maintains excellent performance, whereas the PD with spin-coated TiO₂ film dramatically deteriorate with on-off ratio drops from ∼10² to ∼2. Besides, the I_dark of PD remarkably decreases from ∼10⁻⁸ A to ∼10⁻¹⁰ A after bilayer optimization. Furthermore, we have integrated the corresponding PDs into a self-built imaging system adopting diffuse reflection mode. This work suggests a feasible approach to fabricate TiO₂/Cs₂AgBiBr₆-based PDs with remarkable UV tolerance for imaging applications.