α-Particle Detection and Charge Transport Characteristics in the A₃M₂I₉ Defect Perovskites (A = Cs, Rb; M = Bi, Sb)

We have investigated the defect perovskites A₃M₂I₉ (A = Cs, Rb; M = Bi, Sb) as materials for radiation detection. The phase purity of Bridgman-grown A₃M₂I₉ single crystals was confirmed via high-resolution synchrotron X-ray diffraction, while density functional theory calculations (DFT) show surprisingly dispersive bands in the out-of-plane direction for these layered materials, with low effective masses for both holes and electrons. Accordingly, each of the four A₃M₂I₉ defect perovskites showed response to ²⁴¹Am α-particle irradiation for hole and electron electrode configurations, a remarkable ambipolar response that resembles the 3D halide perovskites. The electron response spectra were used to estimate the mobility-lifetime product (μτ)_e for electrons in these materials, with Rb₃Bi₂I₉ showing the lowest (μτ)_e value of 1.7 × 10^-6 cm² V^-1 and Cs₃Bi₂I₉ the highest (μτ)_e of 5.4 × 10^-5 cm² V^-1. The rise time of the α-particle-generated pulse was used to estimate the electron mobility μ_e of the A₃M₂I₉ defect perovskites, which ranged from 0.32 cm² V^-1s^-1 for Rb₃Sb₂I₉ to 4.3 cm² V^-1s^-1 in Cs₃Bi₂I₉. Similar analysis of the hole response spectra yielded (μτ)_h values for each A₃M₂I₉ compound, with Cs₃Bi₂I₉ again showing the highest (μτ)_h value of 1.8 × 10^-5 cm² V^-1, while Rb₃Bi₂I₉ showed the lowest (μτ)_h with 2.0 × 10^-6 cm² V^-1. Rise time analysis gave hole mobilities ranging from 1.7 cm² V^-1 s^-1 for Cs₃Bi₂I₉ to 0.14 cm² V^-1 s^-1 for Cs₃Sb₂I₉. Comparing the experimental electron and hole mobilities to the effective masses obtained from DFT calculations revealed sizable discrepancies, possibly indicating self-trapping of charge carriers due to electron-phonon interactions. The α-particle response of the A₃M₂I₉ defect perovskites demonstrates their potential as semiconductor radiation detectors, with Cs₃Bi₂I₉ and Cs₃Sb₂I₉ showing the most promise.