Extracting disorder parameters from optical spectra of non-fullerene acceptors.

Organic solar cells have seen significant advancements through the use of non-fullerene acceptors, yet understanding the impact of molecular design on energetic disorder remains critical for optimizing material performance. In this work, we investigate three methodologies for quantifying static and dynamic excitonic disorder by analysing the temperature dependence of spectral features in thin film absorption and photoluminescence spectra. Our results demonstrate that fitting the temperature dependence of the first emission peak energy is the most reliable approach for assessing static disorder, while linewidth fitting of absorption spectra is best suited for quantifying dynamic disorder. Comparative case studies reveal that linear n-octyl side chains (e.g., in O-IDTBR and IDIC-4Cl) improve aggregation and induce the lowest static disorder, whereas bulkier side chains (e.g., 2-ethylhexyl and phenylhexyl) result in static disorder parameters which are approximately 50% larger in magnitude. For materials exhibiting strong aggregation, such as Y6, the limitations of current models underscore the need for caution when interpreting disorder metrics. These findings highlight the importance of side chain engineering in controlling the excitonic energetic landscape and provide guidance for the accurate assessment of the related disorder parameters in organic semiconductors.