Ultrafast transformation of PbI2 in two-step fabrication of halide perovskite films for long-term performance and stability via nanosecond laser shock annealing

Abstract

A two-step sequential deposition has been a reliable method to synthesize large-area and high-quality perovskite films due to its better reproducibility. However, the long-term performance and stability of thin films are adversely affected by the slow and incomplete transformation of PbI₂ to perovskites. Here, we propose a nanosecond laser shock annealing process to induce ultrafast organic salt diffusion into the PbI₂ layer to modulate the crystalline structure, residual tensile strain, and electron transport kinetics in two-step fabricated halide perovskite films. We found that pulse-laser induced ultrafast diffusion reduces the thickness of the residual PbI₂ layer at the bottom of the perovskite film fabricated by a two-step method, resulting in the reduction of residual tensile strain by over seven times. The shocking moment of the nanosecond laser promotes the diffusion of the organic salt to the PbI₂ layer. Compared to traditional thermal annealing, ultrafast laser shock annealing enhances the molecular interaction, which significantly affects the orbital overlap resulting in band structure changes. Laser shock annealing induced modulation of band structures in perovskites leads to remarkable improvement in their carrier lifetime, producing a responsivity (R) and detectivity (D*) of 2.45 A W⁻¹ and 1.48 × 10¹² Jones, respectively. Besides, the stability testing under various harsh thermal and humid thermal conditions shows that laser shock annealing improves the stability of perovskite thin films as a result of the reduced PbI₂ layer and residual tensile strain. The presented technology that utilizes laser shock annealing to modulate the ultrafast diffusion in the PbI₂ layer provides a guideline for future improvement in the device performance and stability of hybrid organic–inorganic halide perovskites.