Decoding the Role of Molecular Orientation in Conjugated Self-Assembled Monolayers for High-Performance Binary Organic Photovoltaics Approaching 20% Efficiency

Abstract

Molecular orientation stands as the quintessential hallmark of conjugated self-assembled monolayers (SAMs), which have recently catalyzed noteworthy advancements in organic photovoltaics (OPVs). Nevertheless, an unambiguous understanding of these directional arrangements and their impact on optoelectronic properties remains elusive. To address this issue, herein three SAMs with representative orientations, i.e., edge-on (BCZ-1), tilt-on (4PACz) and face-on (BCZ-2) are meticulously designed. These orientations have been rigorously validated by sum frequency generation vibrational spectroscopy and first-principles calculations. Remarkably, an unequivocal correlation between the molecular orientation and the device performance is discerned. Particularly, the edge-on oriented BCZ-1 exhibits the largest dipole moment normal to the electrode, accompanied by a dense and uniform coverage. These features collectively contribute to its strongest work function increment for ultra-fast hole extraction and minimum interfacial carrier recombination. As a result, a champion power conversion efficiency of 19.93% is achieved in devices based on BCZ-1 with D18:L8-BO as the active layer, representing one of the highest values reported for binary bulk heterojunction OPVs. Besides, BCZ-1 shows great potential for practical applications due to its superior up-scalability and enhanced device shelf-stability. Overall, this work offers in-depth insights into the orientation behaviors of SAMs, opening new avenues to unlock the efficiency potential of OPVs.