21.26% Efficiency Organic Solar Cells Enabled by Mixed Self‐Assembled Multilayers

Insufficient surface coverage persists as a critical limitation for self-assembled monolayers/multilayers (SAMs/SAMULs) in photovoltaic field. The co-assembled SAMULs (Co-SAMULs) strategy, which incorporates a guest self-assembled molecule, offers an effective solution. However, the working mechanism of the guest molecule is still not fully understood. Herein, benzo[b]thien-2-ylboronic acid (BBA) was introduced as a guest material into the host molecule system based on (4-(3,6-di(thiophen-3-yl)-9H-carbazol-9-yl)butyl) phosphonate (4PAThCz) to construct efficient Co-SAMULs. We found that BBA can fused into 4PAThCz micelles rather than forming individual micelles, respectively, thereby inhibiting micelle growth. During the film-forming process, BBA engages in synergistic adsorption with the host molecules, selectively fills the voids between host molecule and substantially improving surface coverage. Such a homogeneous and high-quality film effectively suppresses surface defect states, enhances interfacial charge transport capacity and minimizes non-radiative exciton recombination. As a result, a champion power conversion efficiency (PCE) of 21.26% (certificated as 20.79%) with an outstanding fill factor (FF) of 83.54% is achieved, representing the highest performances reported for organic solar cells (OSCs) to date.