Does Fluorination Necessarily Lead to Efficiency Gains in Non-fullerene-Based Organic Solar Cells? A Theoretical Exploration

Fluorination of donors and acceptors has been recognized as an effective strategy to enhance the power conversion efficiency (PCE) of organic solar cells. However, improper fluorination may weaken the effect. Specifically, we identify that the stable active-layer cluster structures obtained after the annealing of the PM6/Y6 combination with fluorination and the PBDB/Y5 combination without fluorination not only enhance the intermolecular interactions within the cross-linked structures but also improve donor-acceptor energy level alignment and achieve near-neutral average electrostatic potential distributions. Importantly, this work underscores the importance of promoting the singlet energy transfer process while inhibiting the triplet energy transfer process since they exhibit positive and negative correlations, respectively, with the PCE. It inspires us to propose a descriptor combining the singlet energy transfer rate with the electron-hole recombination distance to predict PCE. Our findings demonstrate that strategic fluorination, rather than indiscriminate fluorine substitution, is crucial for optimizing organic photovoltaic materials.