Design Principles for Narrow-Gap Hybrid Semiconductors: Insights from Viologen-Tin and Viologen-Lead Iodides

Hybrid metal halide semiconductors containing "electronically active" organics are an arising subclass of materials that derive remarkable emergent optical properties from blending organic orbitals with the photophysics of metal halide semiconductors. Although this subclass has been known for some time, the majority of reported compounds are lead halides, and there is a paucity of systematic studies investigating the influence of structure and composition on organic energy levels in these materials. Herein we report the first viologen tin hybrids to be published in the form of: HVSnI₄ (HV = hydroviologen), MeVSn₂I₆ (MeV = methylviologen), and EtVSn₂I₆ (EtV = ethylviologen). These materials exhibit emergent electronic structures where charge-transfer from the inorganic lattice to viologen LUMO states results in optical gaps as narrow as 1.10 eV. Through comparison of these compounds with their lead analogs and viologen iodide salts, we develop a systematic understanding of energy levels and lead/tin systems, which allows us to identify key chemical principles for future narrow-gap hybrid materials. We find that organic LUMO states in these materials are relatively immobile under the exchange of lead with tin but are strongly influenced by substituent choice, conformation, π-π stacking, and the polarizability of the inorganic lattice. We further study the energetic landscape of these materials in a set of lead/tin alloys (EtVPb_2-xSn_xI₆) that do not exhibit the anomalous band-bowing typically associated with lead/tin alloys, providing a new point of evidence that this phenomenon generally relies on concomitant motions of the inorganic conduction and valence band. Photoresponse of HVSnI₄ pressed pellet devices to 1064 nm light establishes the potential of these materials for NIR optoelectronic applications.