Unilateral π‐Expanded Self‐Assembled Hole Transport Multilayers for Binary Organic Solar Cells With Efficiency of 20.77%

ABSTRACT The π‐extension strategy represents a highly effective approach toward improving the charge transport capability of self‐assembled monolayers/multilayers. Nevertheless, highly conjugated molecular backbones readily trigger intense intermolecular interactions, which further cause severe molecular aggregation and incomplete surface coverage of the assembled films. Here, we synthesized a unilateral π‐expanded self‐assembled molecule, namely (4‐(3‐(2‐chlorothiophen‐3‐yl)‐9H‐carbazol‐9‐yl)butyl)phosphonic acid) (Cl‐4PAThCz). The unilateral π‐extension design of Cl‐4PAThCz disrupts the molecular symmetry, which in turn optimizes its critical micelle concentration and micelle size in solution. This unique solution property is beneficial to the subsequent formation of a uniformly covered film without large‑scale molecular aggregation. Such a high‐quality film enables effective passivation of interfacial defects, accompanied by enhanced electrical conductance and hole transport capacity. Consequently, a champion efficiency of 20.77% (certified to 20.31%) with an outstanding fill factor of 83.16% was achieved, which is among the state‐of‐the‐art efficiencies for OSCs. This work clarifies the mechanism behind asymmetrical self‐assembled molecules, affording valuable guidance for the materials design of organic semiconductor.