19.35% Efficient Binary Bulk‐Heterojunction Organic Photovoltaic Enabled by Optimizing Bromine‐Substituted Self‐Assembled Carbazole Based Molecules

Abstract A bromine‐substituted [2‐(9H‐Carbazol‐9‐yl) ethyl] phosphonic acid, 1Br‐2PACz, is designed as hole‐selective self‐assembled monolayers (SAMs), contributing to an outstanding power conversion efficiency (PCE) of 19.35% for binary bulk‐heterojunction (BHJ) based organic solar cells (OSCs). As compared to the previous high‐performance 2Br‐2PACz SAMs, 1Br‐2PACz molecules can effectively reduce the interaction of the SAM with the BTP‐eC9 nonfullerene acceptors with a decreased binding energy, resulting in the suppressed vertical self‐aggregation of BTP‐eC9 small molecules as the bottom side of PM6:BTP‐eC9 BHJ during the solidification process. There is decreased energetic disorder within photoactive layer together with more efficient charge transfer and suppressed non‐radiative recombination. Furthermore, five additional binary BHJ systems are applied in 1Br‐2PACz SAM‐based OSCs, exhibiting continuously superior performance as compared to the reference cells with conventional PEDOT:PSS hole transport layer. This work underscores the potential of advancing OSCs by fine‐tuning SAMs through halogenation strategies to improve active layer morphology and overall device performance.