Three-Dimensional Lead Iodide Perovskitoid Hybrids with High X-ray Photoresponse

Large organic A cations cannot stabilize the 3D perovskite AMX₃ structure because they cannot be accommodated in the cubo-octhedral cage (do not follow the Goldschmidt tolerance factor rule), and they generally template low-dimensional structures. Here we report that the large dication aminomethylpyridinium (AMPY) can template novel 3D structures which resemble conventional perovskites. They have the formula (xAMPY)M₂I₆ (x = 3 or 4, M = Sn²⁺ or Pb²⁺) which is double of the AMX₃ formula. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (xAMPY)M₂I₆ to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. DFT calculations reveal that the compounds are indirect band gap semiconductors with direct band gaps presenting at slightly higher energies and dispersive electronic bands. The indirect band gaps of the Sn and Pb compounds are ∼1.7 and 2.0 eV, respectively, which is slightly higher than the corresponding AMI₃ 3D perovskites. The Raman spectra for the compounds are diffuse, with a broad rising central peak at very low frequencies around 0 cm^-1, a feature that is characteristic of dynamical lattices, high anharmonicity, and dissipative vibrations very similar to the 3D AMX₃ perovskites. Devices of (3AMPY)Pb₂I₆ crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (μ) and long carrier lifetime (τ). The devices also exhibit sizable X-ray generated photocurrent with a high μτ product of ∼1.2 × 10^-4 cm² /V and an X-ray sensitivity of 207 μC·Gy^-1·cm^-2.