Air-stable CsPbIBr2 photodetector via dual-ligand-assisted solution strategy *

The CsPbIBr2 perovskite films deposited from the precursor solutions in air, usually suffered from poor surface coverage and air-stability due to the uncontrolled nucleation and the existence of I- during the film formation, resulting in terrible photoelectric characteristics and reproducibility. At present, the preparation process of high-quality CsPbIBr2 films is under nitrogen atmosphere, which increases the cost and impedes its application in air. Herein, we propose a strategy for the growth of perovskite films with low defect density and better stability in air via dual-ligand-assisted (Ligand 1 (LP) and ligand 2 (NH4SCN))solution strategy. These ligands contains some organic molecules which have strong interaction with ions on the surface of perovskite thin films in order to regulate the addition of precursor ions onto the films. The high-quality CsPbIBr2 thin films were carried out in air with relative humidity of ≦60% by spraying method. The results indicate that ligand 1 with hydrophilic group and hydrophobic group, one of kind of surfactants, could effectively reduce the surface tension of perovskite precursor solution, improve the coverage of CsPbIBr2 perovskite film, and form a block layer of water and oxygen. However, the addition of ligand 1 in precursor solution inevitably introduced a large number of grain boundaries and detriment to carrier transport and collect. Thus, ligand 2 was employed to control the nucleation of perovskite films as another ligand, resulting in reducing the point defect formation. Their combination is beneficial to form 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/CsPbIBr2/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 × 10-4 mA cm-2, a fast response time of 20 μs and 21 μs, and high stability (81%, ≥70 days) in air with relative humidity of 40%-60%. Hence, this study provides a simple method to prepare high-quality CsPbIBr2 perovskite thin films with low-density defect and realize air-stable and charge-transport-layer-free CsPbIBr2 photodetectors for practical applications in photoelectric detection field.