Achieving High Fill Factor via Increasing Interfacial Disorder to Inhibit Bimolecular Recombination for Efficient Organic Solar Cells

The inferior fill factor (FF) is one of main reasons impeding further improvement of power conversion efficiencies (PCEs) in organic solar cells (OSCs). But no theoretical framework for high FFs has been established yet. Herein, an efficient strategy is developed to enhance FFs via introducing a small molecule, CNDT, into active layer. CNDT increases the electron donor/acceptor interface disorder, raises the energy barrier for charge back transfer, and thus reduces bimolecular recombination rate constant (k_rec). We show that CNDTs distribute across electron donor/acceptor interfaces of D18:Y6+, disturbe the molecular stacking of Y6, lead to a more disordered interfaces, while maintaining higher crystal quality compared to D18:Y6. This disorder, along with a higher energy of charge transfer states magnifies the energy barrier for charge recombination, decreases charge recombination rate/ratio, reduces k_rec, and inhibits bimolecular recombination in devices. Therefore, FFs of OSCs are improved from 75.78% (D18:Y6) to 81.13% (D18:Y6+), yielding a higher PCE of 19.45%. Moreover, D18:L8-BO+ based OSCs feature FFs over 83%, a record for OSCs so far, with PCEs reaching 19.80%. This work demonstrates that increasing interface disorder, without sacrificing crystal quality, enhances energy barriers of charge recombination and inhibits bimolecular recombination to efficiently improve FFs for higher PCEs.