Completing the Picture of 2‑(Aminomethylpyridinium) Lead Hybrid Perovskites: Insights into Structure, Conductivity Behavior, and Optical Properties

Hybrid
perovskites have evolved into an exciting materials platform
supporting a wide variety of optoelectronic applications including
solar cells and light-emitting devices. In spite of their rapid deployment
in devices, a detailed understanding of their structure–property
relationships is scarce. In this study, we comprehensively analyze
the crystal and electronic structures as well as thermal, optical,
and electronic properties of a series of 2-(aminomethyl­pyridinium)
lead halides including the isotypic hybrid perovskites (C6H10N2)­PbX4 (X = Cl, Br, and I) and
the hybrid compound (C6H10N2)6IPb5I21·3H2O. The thermal
transformation of (C6H10N2)6IPb5I21·3H2O into (C6H10N2)­PbI4 was studied by thermal
analysis and powder X-ray diffraction and used to reverse engineer
a synthesis route for phase-pure (C6H10N2)­PbI4. The very broad PL emission of (C6H10N2)6IPb5I21·3H2O is traced back to the largest octahedral distortion
found in this compound among all studied 2-(aminomethyl­pyridinium)
lead halides. We further find that (C6H10N2)­PbI4 and (C6H10N2)6IPb5I21·3H2O are
mixed ionic–electronic conductors and identify the diffusing
ionic species as iodine and protons, respectively, by combining solid-state
NMR measurements with a.c. impedance spectroscopy and d.c. galvanostatic
polarization measurements.