Understanding the Essential Role of PbI₂Films in a High-Performance Lead Halide Perovskite Photodetector

Hybrid lead halide perovskites fabricated via solution processing have been regarded as a promising candidate for next-generation optoelectronic devices owing to their outstanding optoelectronic properties and low fabrication cost. However, the morphology, location, and effects of residual PbI2 in the solution process-based perovskite films are still ambiguous. Herein, by applying the time-of-flight polarized neutron reflectometer (TPNR), we uncover that the highly uniform PbI2 film locates at the bottom of perovskite films. First-principle calculations demonstrate that the PbI2 film in the interface between the perovskite film and contact layer can significantly reduce interface charge recombination via the carrier confinement effect owing to the higher conduction band minimum (CBM) and lower valence band maximum (VBM) of PbI2 than those of the perovskite. To explore the essential role of PbI2 films in the perovskite, we fabricated a photodetector based on perovskite/PbI2 (∼534 nm/40 nm) and found its improved performances with a high optical responsivity of ∼1302 AW-1 and a rapid response speed of ∼2 ms, revealing the critical passivation role of PbI2. This work provides a rational design strategy for PbI2 film thickness control in the high-performance planar optoelectronic devices with a varied structure.