Spectroscopic Evidence of Localized Small Polarons in Low-Dimensional Ionic Liquid Lead-Free Hybrid Perovskites

Rational
design is an important approach to consider in the development
of low-dimensional hybrid organic–inorganic perovskites (HOIPs).
In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole
(API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical
ionic liquid components in bismuth-based HOIPs. Element-sensitive
X-ray absorption spectroscopy measurements of BMPBiBr4 and
APIBiBr5 reveal distinct resonant excitation profiles across
the N K-edges, where contrasting peak shifts are observed. These 1D-HOIPs
exhibit a large Stokes shift due to the small polaron contribution,
as probed by photoluminescence spectroscopy at room temperature. Interestingly,
the incorporation of a small fraction of tin (Sn) into the APIBiBr5 (Sn/Bi mole ratio of 1:3) structure demonstrates a strong
spectral weight transfer accompanied by a fast decay lifetime (2.6
ns). These phenomena are the direct result of Sn-substitution in APIBiBr5, decreasing the small polaron effect. By changing the active
ionic liquid, the electronic interactions and optical responses can
be moderately tuned by alteration of their intermolecular interaction
between the semiconducting inorganic layers and organic moieties.