Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics

Recent efforts in organic photovoltaics (OPVs) have been devoted to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent generation. However, the low-lying lowest unoccupied molecular orbital (LUMO) level in narrow bandgap NFAs typically results in a small energy difference (Delta E-DA) between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, leading to low open-circuit voltage (V-oc). The trade-off between Delta E-DA and photocurrent generation significantly limits the simultaneous enhancement of both V-oc and short-circuit current density (J(SC)). Here, we report a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing N-ethyl thiabarbituric acid terminal group on each end of the indacenodithienothiophene (IDTT) core. When paired with a benchmark low-bandgap PTB7-th polymer donor, simultaneous enhancement of both Delta E-DA and absorption spectral coverage was realized. The OPV devices yield a V-OC of 1.01 V, corresponding to a low energy loss of 0.57 eV in around 10% efficiency single-junction NFA OPVs. The design demonstrates a working principle to concurrently increase Delta E-DA and photocurrent generation for high V-OC and PCE in bulk fullerene-free heterojunction OPVs.