Ternary organic solar cells with enhanced charge transfer and stability combining the advantages of polymer acceptors and fullerene acceptors

Organic solar cells (OSCs) based on highly efficient non-fullerene acceptors (NFAs) have recently developed rapidly, and achieved effective charge separation and high open-circuit voltage (VOC) under low driving force. Nonetheless, the low driving force inescapably reduces the short-circuit current (JSC) and potentially restricts the improvement of external quantum efficiency (EQE). Herein, we introduce the fullerene acceptor PC71BM into the efficient NFA system. And the PM6:PY-IT system is selected as a typical NFA system for its high VOC above 0.90 V. Benefiting from the deeper energy level, excellent electron transport characteristic, and unique spherical structure of PC71BM, the driving force for charge transfer is effectively improved. Accordingly, the ternary OSC based on PM6:PY-IT:PC71BM demonstrates a high efficiency of 16.36%, a much-enhanced JSC of 25.06 mA cm−2, and the maximum EQE value is successfully increased to 87%, which is superior to binary OSC (efficiency of 14.84%, JSC of 23.10 mA cm−2 and maximum EQE value of 80%). Meanwhile, the introduction of PC71BM improves the molecular stacking structure, resulting in excellent stability in non-encapsulated ternary OSCs, which retain 80% initial efficiency after 6 h of continuous illumination treatment and 85% initial efficiency after 33 days of storage (the efficiencies decrease to 68% and 76% respectively in binary OSCs). In addition, similar conclusions can be drawn in another classical NFA system PM6:Y6, with an optimal efficiency of 17.49%. This work proves that PC71BM has unique advantages and great research potential in improving the driving force for charge transfer.