Linking Device Performance to Nanoscale Photoactivation of Grain Boundaries in 2D Hybrid Halide Perovskites

2D halide perovskites are emerging as stable and low-noise materials for photodetection, yet the role of grain boundaries in governing their optoelectronic response remains debated. Here, we demonstrate that grain boundaries act as photoactive centres that dominate the photoconductive gain in polycrystalline (PEA)2PbBr4 films. By engineering the crystallization process, we fabricate films with grain areas spanning several orders of magnitude and systematically correlate grain boundary density with device responsivity. While dark transport is unaffected by morphology, films with higher grain boundary density exhibit enhanced responsivity. Using correlative X-ray fluorescence and X-ray beam–induced current nanomapping, we directly grain boundaries act as preferential charge collection paths with extended carrier collection lengths. Kelvin probe force microscopy further reveals that grain boundaries selectively accumulate trapped minority carriers under illumination, activating a photoconductive gain mechanism. Our results establish grain boundaries as functional photoactive elements rather than detrimental defective sites, providing a clear design strategy for optimizing 2D perovskite photodetectors through controlled microstructural engineering.