Self‐Assembled Molecules with Asymmetric Backbone for Highly Stable Binary Organic Solar Cells with 19.7% Efficiency

The hole‐transporting material (HTM), PEDOT:PSS, is the most widely used material in the realization of high‐efficiency organic solar cells (OSCs). However, the stability of PEDOT:PSS‐based OSCs is quite poor, arising from its strong acidity and hygroscopicity. In addition, PEDOT:PSS has an absorption in the infrared region region and high HOMO energy level, thus limiting the enhancement of short‐circuit current density (JSC) and open‐circuit voltage (VOC), respectively. Herein, two asymmetric self‐assembled molecules (SAMs), namely BrCz and BrBACz, were designed and synthesized as HTMs in binary OSCs based on the well‐known system of PM6:Y6, PM6:eC9, PM6:L8‐BO, and D18:eC9. Compared with BrCz, BrBACz shows larger dipole moment, deeper work function and lower surface energy. Moreover, BrBACz not only enhances photon harvesting in the active layer, but also minimizes voltage losses, thus improving interface charge extraction/ transport. Consequently, the PM6:eC9‐based binary OSC using BrBACz as HTM exhibits a champion efficiency of 19.70 % with a remarkable JSC of 29.20 mA cm‐2 and a VOC of 0.856 V, which is a record efficiency for binary OSCs so far. In addition, the unencapsulated device maintains 95.0 % of its original efficiency after 1,000 hours of storage at ambient condition, indicating excellent long‐term stability.