Defect mediated pulse terahertz emission from thiocyanate-treated hybrid perovskite nanoparticles: role of the orientation of built-in surface electric field

Hybrid perovskites have demonstrated exceptional versatility as optoelectronic materials since their introduction as light absorbers and charge transport media in photovoltaic devices. These materials are now utilized in photodetectors, photocatalysis, and light-emitting devices, offering emission capabilities spanning from the ultraviolet to the mid-infrared spectrum. Recent studies in the far-infrared regime have shown that pulsed terahertz (THz) radiation can be effectively generated, enhanced, and detected. Nevertheless, the fundamental emission mechanisms is still an ongoing debate, which have been ascribed to nonlinear optical effects, surface electric fields, photo-Dember effect, bulk photovoltaic phenomena, or a combination of these factors. Here, we demonstrate that increasing the thiocyanate concentration in formamidinium lead iodide (FAPbI₃) thin films—a widely used strategy for defect passivation—leads to a commensurate increase in the peak-to-peak amplitude of the generated THz pulse. Spectroscopic and microscopic analyses of both surface and bulk properties reveal that THz emission is predominantly influenced by the orientation of the built-in surface electric field, the degree of crystallinity, and the dynamic interplay between these factors, rather than solely by defect concentration. Our results offer valuable insight into optimizing hybrid perovskite-based THz emitters and detectors through surface defect engineering while also clarifying the limitations of such approaches.