Enhancement of intra- and inter-molecular π-conjugated effects for a non-fullerene acceptor to achieve high-efficiency organic solar cells with an extended photoresponse range and optimized morphology

In this work, a new A-D-A type non-fullerene electron acceptor, DF-PCNC, which possesses an electron-donating (D) core constructed by linking a 2,5-difluorobenzene ring with two cyclopentadithiophene moieties and two electron-accepting (A) end-groups of 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NC), is designed and synthesized. Because of the extension of the π-conjugation system, DF-PCNC shows stronger and more red-shifted absorption peaks while compared to those of its counterpart, DF-PCIC, which has the same D core but smaller A terminals of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC). Furthermore, NC groups can enhance the intermolecular π-π stacking of DF-PCNC in the condensed state. Thus, when it is blended with a polymer donor, PBDB-T, to fabricate organic solar cells (OSCs), good morphologies of the blended films are achieved through appropriate optimizations: both donor and acceptor form highly crystalline phase-separation domains with appropriate nanoscaled sizes, which is beneficial to charge generation and transport in OSCs. As a result, the short-circuit current density (J_SC) of the PBDB-T:DF-PCNC device is increased by 16% compared with that of the PBDB-T:DF-PCIC one, and a high fill factor (FF) of 72.62% is maintained, leading to a better power conversion efficiency (PCE) of 11.63%, which is the highest value for OSCs based on non-fullerene acceptors adopting decreased fused-ring D cores to date.