Quaternary Solar Cells with 12.5% Efficiency Enabled with Non-Fullerene and Fullerene Acceptor Guests to Improve Open Circuit Voltage and Film Morphology.

Designing the bulk-heterojunction structure (binary, ternary and quaternary) is of great fundamental interest for relaxing the trade-off between open circuit voltage (Voc ) and short circuit current density (   Jsc ). Herein, a new quaternary blended material system is reported with a nonfullerene and PC71 BM as the third and fourth component of PBDB-T:ITCT, leading to simultaneously increased Voc and Jsc and maintained fill factor (FF). The guest IT-T-IC has upshifted LUMO energy level helping to obtain a higher Voc . The further addition of 0.2 PC71 BM as the fourth component yields an even higher Voc because the LUMO of PC71 BM is higher than that of ITCT. Interestingly, the blend of PC71 BM leads to the formation of an unprecedented neuron-like morphology, which acts as new centers not only performing light absorption and charge separation but also charge transport through their surrounding donor and acceptor fibers. The increased hole and electron mobilities and the reduced bimolecular loss results in an even larger Jsc and FF. These results indicate that a combination using a structurally similar higher-LUMO-level non-fullerene acceptor and PCBM is a simple yet effective quaternary material approach to simultaneously increase Voc and Jsc while maintaining FF, improving final device performance.