Unraveling the Absorption Spectra and Relaxation Energy of NF-SMAs Using Multiparameter Franck Condon Models—A Structure-Function Study

Understanding correlations among the molecular structure, conformational diversity, and relaxation energy of a nonfullerene small molecule acceptor (NF-SMA) used in organic solar cells (OSCs) and their functional, optoelectronic properties is important to rationally design new materials with controlled properties. Although, correlations among molecular structure, optical and electronic gaps, and electron affinity of NF-SMAs have been studied extensively, an analysis of absorption spectra that classifies the NF-SMAs based on their electronic and vibrational finestructure is still missing. Here, we study the absorption spectra of 10 high-performing NF-SMAs in dilute solutions at room temperature, the most common tool utilized to uncover structure-spectra relations. Utilizing multiparameter Franck Condon (FC) analysis and quantum-chemical calculations, we show that the absorption spectra of NF-SMAs can be categorized into three types based on the molecular structures-spectra correlation: 1) The absorption spectrum of a linear ITIC-like molecular structure can be described by a single vibrational progression with one electronic transition and one effective vibrational mode. 2) The absorption spectra of curved, Y6-like structures can be described using a multiparameter FC model with two electronic transitions and two effective vibrational modes. Our results show that Y6 exhibits a highest conformation uniformity and the smallest intra-molecular relaxation energy, characteristics that may contribute to the Y6 success in state-of-the-art OSCs. 3) In contrast, structures like EH-IDTBR with π-bridges between the core and end groups have broader absorption spectra. The result of FC analysis and quantum-chemical calculations confirm the coexistence of different conformations and larger intra-molecular relaxation energies for these systems when compared to the other NF-SMAs in this study. These results provide insights for the design of new NF-SMAs, in particular a need for a simultaneously small number of molecular conformations and energetic disorder, and small relaxation energies, critical parameters that if controlled should improve the efficiency of organic photovoltaics or organic devices in general. The results clearly demonstrate that DFT calculations are able to provide excellent relative relaxation energies that can be utilized to assess molecules prior to synthesis.