Phenethylammonium Functionalization Enhances Near-Surface Carrier Diffusion in Hybrid Perovskites

Understanding semiconductor surface properties and manipulating them chemically are critical for improving their performance in optoelectronic devices. Hybrid halide perovskites have emerged as an exciting class of highly efficient solar materials; however, their device performance could be limited by undesirable surface properties that impede carrier transport and induce recombination. Here we show that surface functionalization of methylammonium lead iodide (MAPbI₃) perovskite with phenethylammonium iodide (PEAI), a commonly employed spacer cation in two-dimensional halide perovskites, can enhance carrier diffusion in the near-surface regions and reduce defect density by more than 1 order of magnitude. Using transient transmission and reflection microscopy, we selectively imaged the transport of the carriers near the (001) surface and in the bulk for single-crystal MAPbI₃ microplates. The surface functionalization increases the diffusion coefficient of the carriers in the 40 nm subsurface region from ∼0.6 cm² s^-1 to ∼1.0 cm² s^-1, similar to the value for bulk carriers. These results suggest the PEA ligands are effective in reducing surface defect and phonon scattering and shed light on the mechanisms for enhancing photophysical properties and improving solar cell efficiency.