Exploration of sub-bandgap states in 2D halide perovskite single-crystal photodetector

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA(2)PbBr(4)) single crystals with optical bandgap of similar to 413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA(2)PbBr(4). As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA(2)PbBr(4) single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA(2)PbBr(4) single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

Automated summary

The greater stability and excellent emission properties of low-dimensional halide perovskites make them popular candidates for optoelectronic applications. In this study, the electronic response of the edges of two-dimensional perovskites was elucidated using Kelvin Probe Force Microscopy. Sub-bandgap states were found to exist, and the behavior of charge carriers was found to depend on the edge height. A photodetector was fabricated using a millimeter-sized single crystal, and broadband photodetection response was observed.