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
(EtVPb2–xSnxI6) 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.