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.