Molecular packing tuning via chlorinated end group enables efficient binary organic solar cells over 18.5%

Abstract Designing novel nonfullerene acceptors (NFAs) is of vital importance for the development of organic solar cells (OSC). Modification on the side chain and end group are two powerful tools to construct efficient NFAs. Here, based on the high‐performance L8BO, we selected 3‐ethylheptyl to substitute the inner chain of 2‐ethylhexyl, obtaining the backbone of BON3. Then we introduced different halogen atoms of fluorine and chlorine on 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) malononitrile end group (EG) to construct efficient NFAs named BON3‐F and BON3‐Cl, respectively. Polymer donor D18 was chosen to combine with two novel NFAs to construct OSC devices. Impressively, D18:BON3‐Cl‐based device shows a remarkable power conversion efficiency (PCE) of 18.57%, with a high open‐circuit voltage (V_OC) of 0.907 V and an excellent fill factor (FF) of 80.44%, which is one of the highest binary PCE of devices based on D18 as the donor. However, BON3‐F‐based device shows a relatively lower PCE of 17.79% with a decreased FF of 79.05%. The better photovoltaic performance is mainly attributed to the red‐shifted absorption, higher electron and hole mobilities, reduced charge recombination, and enhanced molecular packing in the D18:BON3‐Cl films. Also, we performed stability tests on two binary systems; the D18:BON3‐Cl and D18:BON3‐F devices maintain 88.1% and 85.5% of their initial efficiencies after 169 h of storage at 85°C in an N₂‐filled glove box, respectively. Our work demonstrates the importance of selecting halogen atoms on EG and provides an efficient binary system of D18:BON3‐Cl for further improvement of PCE.