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.