Significantly Increasing the Power Conversion Efficiency by Controlling the Orientation of Nonfullerene Small Molecular Acceptors via Side Chain Engineering

Three S,N‐heteroacene nonfullenere small molecular acceptors (NF‐SMAs) with different branching positions on side chains named SNBDT1‐F, SNBDT2‐F, and SNBDT3‐F are synthesized to understand the relationship among branching positions, molecular orientations of NF‐SMAs, and photovoltaic performance of nonfullerene organic solar cells (OSCs). When the branching position is moved close to the conjugated backbone, the proportion of face‐on orientation increases gradually from 4% (SNBDT3‐F), 16% (SNBDT2‐F), to 44% (SNBDT1‐F), suggesting that the molecular orientation changes from edge‐on to face‐on, which has a positive influence on exciton dissociation and charge transport. As a result, the device based on PBDB‐T:SNBDT1‐F shows the highest exciton dissociation probability and carrier mobilities due to the highest proportion of face‐on orientation of SNBDT1‐F. Therefore, the highest power conversion efficiency (PCE) of 12.70% with a high short‐circuit current ( J sc ) of 20.97 mA cm −2 and a fill factor of 70.4% is obtained for SNBDT1‐F‐based device, which is much higher than PCEs of SNBDT2‐F‐based (6.57%) and SNBDT3‐F‐based devices (5.87%). Overall, these results provide deep insight into the relationship among the branching positions, molecular orientation, and photovoltaic performance in nonfullerene OSCs, which is important for designing new NF‐SMAs with face‐on orientation and promoting the development of nonfullerene OSCs.