Modulating electronic properties of pyridinium lead halide perovskites via fluorinated methyl substituents

We study the effect of variation in donor/acceptor strength of aromatic ring's substituent on electronic properties of novel low-dimensional 3-(R)pyridinium lead halide perovskites, R = CH_(3-n)F_n (n = 0–3). First, the series of hypothetical quasi 1D perovskites having Pbca crystal structure of actual 3-(methyl)pyridinium lead halides is considered. Regardless of halide anion (Br⁻ or I⁻), Density Functional Theory calculations show monotonic decrease in electronic bandgap with increase in the degree of their methyl group fluorination. In contrast, actual meta-substituted pyridinium lead halides have spatial symmetries other than Pbca. Moreover, two of them form quasi 2D structures as opposed to the 1D model compounds. Therefore, the synthesized meta-substituted pyridinium lead halides do not demonstrate the found theoretical bandgap trend. We conclude that although bandgap modulation of organometal halide perovskites with aromatic cations via ring substitution is feasible theoretical band structure predictions could be severely biased by a hard to predict structure of actual perovskites.