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.