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 (ΔEDA) between the
LUMO of the acceptor and the highest occupied molecular orbital
(HOMO) of the donor, leading to low open-circuit voltage (VOC). The
trade-off between ΔEDA and photocurrent generation significantly
limits the simultaneous enhancement of both VOC and short-circuit
current density (JSC). 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 ΔEDA and
absorption spectral coverage was realized. The OPV devices yield a VOC 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 ΔEDA and photocurrent generation for high VOC and PCE in bulk fullerene-free heterojunction OPVs.