Increasing Molecular Packing Density and Dielectric Constant in Non-Fullerene Acceptors Enables to Boost the Fill Factor Approaching 0.82 in Organic Solar Cells

Abstract

SUMMARYThe fill factor (FF) of organic solar cells (OSC), a critically important photovoltaic parameter, is yet sub-optimal, often less than 0.8. To further reduce the FF gaps with regard to the Shockley-Queisser upper limit, we present a study unveiling the impacts of dielectric properties on obtaining high FFs and photovoltaic efficiencies in OSCs. With increasing the dielectric constant (e) of non-fullerene acceptors (NFAs) and bulk heterojunction (BHJ) films, afforded by the increase of molecular packing density (MPD) of NFAs in solid states, we show that the FF penalties related to non-geminate charge recombination in OSCs can be appreciably mitigated. This conceptual modulation on FFs is embodied by a series of NFA-based BHJs where the differentiated MPDs and resultant changes of e in BHJs are reflected by the difference in FFs. In particular, as exemplified by an NFA model system L8-BO tailored with branched side chains, the enlargements of MPD and e allow to achieve an impressive FF of 0.815 with efficiency of 18.23% in OSCs featuring an impressively low bimolecular recombination and reduced sensitivity of FF on the BHJ film thickness.