Air-stable CsPbIBr<sub>2</sub> photodetector via dual-ligand-assisted solution strategy

Abstract

The CsPbIBr<sub>2</sub> perovskite films deposited from the precursor solutions in air, usually suffer poor surface coverage and air-stability due to the uncontrolled nucleation and the existence of I<sup>–</sup> during the film formation, resulting in terrible photoelectric characteristics and reproducibility. At present, the high-quality CsPbIBr<sub>2</sub> films are prepared under nitrogen atmosphere, which results in the increase of the cost and thus impedes their applications in air. Here in this work, we propose a strategy for growing the perovskite films with low defect density and better stability in air via dual-ligand-assisted (ligand 1 (LP) and ligand 2 (NH<sub>4</sub>SCN)) solution strategy. These ligands contain some organic molecules which have strong interaction with ions on the surface of perovskite thin film in order to regulate the addition of precursor ions onto the films. The high-quality CsPbIBr<sub>2</sub> thin films are prepared in air with relative humidity of ≤60% by the spraying method. The results indicate that ligand 1 with hydrophilic group and hydrophobic group, a kind of surfactant, can effectively reduce the surface tension of perovskite precursor solution, improve the coverage of CsPbIBr<sub>2</sub> perovskite film, and form a block layer of water and oxygen. However, the addition of ligand 1 in precursor solution inevitably introduces many grain boundaries, which is unfavorable for carrier transport and collection. Thus, ligand 2 is employed to control the nucleation of perovskite film as another ligand, resulting in reducing the point defect formation. Their combination is beneficial to forming the uniform perovskite film with large-size crystal and low-density defect. The high-quality crystallization of the perovskite film is found to simultaneously enhance the response and the durability of photodetectors. Thus, the unpackaged photodetectors (ITO/CsPbIBr<sub>2</sub>/Au) based on this strategy yield the outstanding photoelectric response under the excitation of 405 nm laser. This device exhibits a low dark current density of 2 × 10<sup>–4</sup> mA/cm<sup>2</sup>, a fast response time of 20–21 µs, and high stability (81%, ≥70 d) in air with a relative humidity of 40%–60%. Hence, this study provides a simple method to prepare high-quality CsPbIBr<sub>2</sub> perovskite thin films with low-density defect and realize air-stable and charge-transport-layer-free CsPbIBr<sub>2</sub> photodetectors for practical applications in photoelectric detection field.