A theoretical analysis shows that a purely optical effect accounts for most of a puzzling infrared signature and suggests how to capture the true molecular response.
SUMMARY
• A purely optical effect in thin films can mimic light-induced changes in molecular vibrations.
• The analysis reproduces most of a puzzling infrared signal in a perovskite film without any change in molecular vibrations.
• The effect can appear as increased absorption, a peak shift, or bleaching, depending on the charge response.
• Measuring dilute nanocrystal solutions is predicted to eliminate the misleading signal.
• The work provides a framework for interpreting past and future infrared studies and lays the groundwork for quantitative nanoscale infrared spectroscopy.
An infrared signal thought to reveal how molecules in perovskite solar materials respond to light-generated charges can arise largely from the way light travels through the thin film itself, a new theoretical study shows. Jun Nishida, Assistant Professor at the Institute for Molecular Science (IMS), Japan, found that light-generated charges alter how infrared light is reflected and transmitted within the film layers, producing sharp features that resemble changes in molecular vibrations even when the vibrations themselves are unaltered. Published in The Journal of Chemical Physics and selected as an Editor's Pick, the study names this mechanism carrier-induced photonic resonance (CIPR) and shows that it must be distinguished from genuine molecular responses when interpreting such measurements.
Time-resolved infrared spectroscopy, in which a visible light pulse excites the material and an infrared pulse probes its molecular vibrations, is widely used to study how light-generated charges interact with molecules in solar materials. In lead halide perovskites, however, several studies have reported vibrational absorption that appears to grow dramatically upon excitation, which is hard to explain because the organic cations are not directly bonded to the inorganic framework that carries the charges. Nishida analyzed earlier measurements on a perovskite film using multilayer transmission theory, which accounts for the multiple reflections of infrared light at each interface of the layered sample. The model included only the broad optical response of the charges and assumed no change in the vibrations of the organic cations. Remarkably, it reproduced most of the narrow, blue-shifted signal observed in the experiment, including both its position and amplitude, showing that an apparently molecular feature can be dominated by a purely optical effect.
The analysis further shows that CIPR can masquerade as a variety of molecular events. Depending on the spectral character of the charge response, the same optical mechanism can mimic increased absorption by excited molecules, a shift of a vibrational peak, or the bleaching of an existing absorption band. Calculations also predict that an adjacent silica layer can generate additional resonances even though it is not itself excited, features that could be mistaken for charge trapping or charge transfer into that layer. These findings make clear that the spectral shape of an infrared signal alone cannot establish a molecular change; the propagation of light through the entire stack of film and substrate must also be taken into account.
Importantly, the study also proposes a practical way to separate the optical effect from genuine molecular dynamics: measuring perovskite nanocrystals dispersed in a dilute solution instead of a densely packed film. In the calculations, CIPR weakened as the concentration was lowered. While dilution suppresses the unwanted resonance, a correspondingly longer optical path through the sample keeps the molecular vibrational signal strong. Although this approach has yet to be tested experimentally, it offers a clear control experiment for future studies of how charges interact with molecules.
Nishida also revisited earlier measurements using ultrafast infrared nanospectroscopy, which probes the same perovskite with a sharp metallic tip at a spatial resolution below 100 nanometers. Here the picture is more complex. The predicted CIPR contribution depended strongly on how the interaction between the tip and the sample was modeled, and in the models considered most realistic, the optical effect accounted for only a minor part of the observed resonance. This suggests that a genuine shift in molecular vibration, possibly linked to light-induced expansion of the perovskite lattice, may also contribute. Resolving the discrepancy between conventional and nanoscale measurements will require further work. By identifying the optical origin of a previously puzzling signal and proposing a way to test for it, the study lays a firmer foundation for interpreting infrared measurements of perovskites and other photovoltaic thin films.
Information of the paper
Authors: Jun Nishida
Journal Name: The Journal of Chemical Physics
Article Title: Photonic effects in visible-pump infrared-probe vibrational spectroscopies on photovoltaic thin films
DOI: 10.1063/5.0350300
Funders
JSPS KAKENHI
JP24K01443, JP25K24852
JST FOREST program
JPMJFR236L
5018
