Realizing over 18% Efficiency for M-Series Acceptor-Based Polymer Solar Cells by Improving Light Utilization

M-series molecules are one kind of promising acceptor-donor-acceptor (A-D-A)-type acceptors for constructing high-performance organic solar cells (OSCs). However, their power conversion efficiencies (PCEs) are lagging behind that of current state-of-the-art OSCs, limited by the relatively low fill factor (FF) and photocurrent. Herein, combined strategies of layer-by-layer (LBL) deposition and interface engineering are conducted to systematically improve light utilization and thus PCEs for M36-based OSCs. Through choosing a proper processing solvent, a PCE of 17.3% with an FF of 77.9% is achieved for the resulting LBL devices, much higher than those (15.9%/74.0%) from the blend-casting devices. The improvement is assigned to the favorable morphological evolution that facilitates carrier generation and transport as well as reduces charge recombination. More importantly, light-harvesting of the active layers can be enhanced upon employing a self-assembled monolayer of (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz) instead of the widely used PEDOT:PSS as the hole-selecting layer, due to the decreased parasitic absorption of the former. Consequently, 2PACz-based LBL devices exhibit significantly increased photocurrent, affording a PCE up to 18.2%, which is the highest among the reported A-D-A-type acceptor-based OSCs. These results deliver important strategies to enhance the performance of OSCs and thus highlight the great potential of M-series acceptors for practical applications.