Predicted THz-wave absorption properties observed in all-inorganic perovskite CsPbI₃ thin films: Integrity at the grain boundary

The realization of terahertz (THz) detectors using organic-inorganic hybrid perovskite (OHP) materials presents a challenge because of the high real conductivities and broad THz absorption spectra required. The control of phonon vibration modes in OHPs depends on the control of defect states such as the defect-incorporated perovskite structure and the δ/α-mixed interfacial structure observed in MAPbI₃ and δ/α-mixed FAPbI₃, respectively. Difficulties with defect control in particular materials may amplify relative risk factors and invite strong resistance to commercialization. As an alternative to OHPs, the all-inorganic perovskite material CsPbI₃ (lacking the hydrophobic organic cations that generate structural defects) constitutes a good candidate for THz detectors. We investigated the phonon vibration modes of thin films of the all-inorganic perovskite γ-CsPbI₃ containing no atomic or chemical defect states. We observed the three expected major phonon vibration modes at 0.9, 1.5, and 1.8 THz that originate from the transverse I–Pb–I frame, the Cs–I–Cs optical vibration, and the longitudinal I–Pb–I frame, respectively, finding good agreement with theoretical simulations. Significantly, the real conductivity of these all-inorganic perovskite thin films ranged from approximately 10–40 S/cm across a broad frequency range of 0.5–3.0 THz, demonstrating the material's considerable potential for application as a THz-band detector.