Probing Optical Response and Surface Conductance in Mixed-Halide Perovskites Using Terahertz Spectroscopy

Mixed halide perovskite single crystals (MHPSCs) have attracted considerable attention as promising building blocks for next-generation photovoltaic technologies due to their exceptional optoelectronic properties, low fabrication cost, and facile processability. However, their practical implementation remains limited by phase segregation, which severely undermines the material stability and device scalability. Among the various crystal growth strategies, additive engineering has emerged as an effective approach to address these challenges by facilitating the formation of passivating interfacial layers, enhancing nanoscale crystallinity, and reducing the defect density. In this study, the optical response and surface conductance of MHPSCs were investigated by using terahertz spectroscopy in both reflection and transmission geometries. The influence of choline bromide (CB) and the heterocyclic additive 3-aminopiperidine (API) on the phonon modes and terahertz response were also examined. X-ray photoelectron spectroscopy (XPS) revealed that the suppression of phase segregation, associated with stronger chemical bonding, contributes to an enhanced THz transmission in the modified MHPSCs. In addition, the THz response of the crystals was examined after approximately six months of ambient aging, revealing changes in the reflection response compared with measurements performed shortly after synthesis. Since the fresh and aged samples were characterized by using different THz spectroscopy configurations, the aging-related changes are interpreted qualitatively. Overall, combined time- and frequency-domain imaging provides a comprehensive assessment of spatial and spectral heterogeneity, complementing bulk THz spectroscopy and highlighting the impact of additive-induced microstructural modifications on the THz dielectric response.