Effect of Sub-Bandgap States in 2D Halide Perovskite Photodetector

Two-dimensional (2D) perovskite is a promising material for optoelectronic devices due to enhanced stability and high light extinction coefficient and excellent emission properties. However, considering the possibility for formation of edge in 2D perovskite and distinct properties at the edge were reported, studies of edge effect in 2D is crucial for understanding instinct properties and developing device performance based on 2D perovskite. Here, we investigate the edge height effect in butylammonium lead bromide (BA₂PbBr₄) with Kelvin Probe Force Microscopy (KPFM) and elucidate that contact potential difference increases with edge height, implying more charges were accumulated. Surface photovoltage (SPV) spectra by combining external incident laser in KPFM_.As increasing edge height, the SPV value was reduced, which is attributed to the reduction of photo-induced charge accumulation at the edge due to the possibility of more traps. Two direct bandgaps were detected from Ultraviolet-visible spectra and several PL peaks below bandgap were observed from spectral photoluminescence, indicating evidence existence of sub-gap states. To confirm sub-gap states, we operated BA₂PbBr₄ photoreactor devices under red (634 nm), green (515 nm), and blue (447 nm) light. Despite the low on-off ratio under green and red light, remarkable responsivity and specific detectivity were calculated, which is attributed to the existence of sub-gap states. Our observation of edge height-dependent optical properties for BA₂PbBr₄ single crystals could provide a contribution to enhancing the performance of the optoelectronic devices.