Enhancing the Photovoltaic Performance of Ladder-Type Dithienocyclopentacarbazole-Based Nonfullerene Acceptors through Fluorination and Side-Chain Engineering

Besides an aromatic polycyclic core with extended π-conjugation, side chains and terminal groups are the two other molecular structure factors which can greatly affect the photovoltaic performance of nonfullerene acceptors. In this work, three dithienocyclopentacarbazole-based nonfullerene acceptors (HCN-C8, HCN-C16, and H2FCN-C16) have been designed and synthesized to investigate the effects of side chain and fluorination on the photovoltaic properties of nonfullerene acceptors. Among the three acceptors, HCN-C8 and HCN-C16 are designed with the same terminal group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) but with different side chains of octyl and 2-hexyldecyl appended on the central nitrogen of the dithienocyclopentacarbazole unit, respectively. H2FCN-C16 and HCN-C16 share the same side chain of 2-hexyldecyl, but the former is designed with the fluorinated INCN in the terminal groups. Although the side chain has a negligible effect on the band gap and energy levels of the resulting nonfullerene acceptors, the replacement of linear octyl chain with the branched 2-hexyldecyl chain leads to a crystallinity change of the resulting acceptors from crystalline to amorphous, thereby affecting their phase separation with donor polymers. On the other hand, the nonfullerene acceptor with fluorination on the ending groups shows a decreased optical band gap with deepened energy levels in comparison with the counterpart without the fluorination. By using a p-type polymer (J71) as the donor material, the best-efficiency polymer solar cell based on H2FCN-C16 exhibits an impressive power conversion efficiency (PCE) of 11.18% with a high short-circuit current density (J_SC) of 18.62 mA cm^-2, a fill factor (FF) of 66.7%, and an open-circuit voltage (V_OC) of 0.90 V. The PCE of 11.18% is the highest among all dithienocyclopentacarbazole-based nonfullerene acceptors reported so far. However, the best-efficiency solar cells based on HCN-C8 and HCN-C16 show low PCEs of 2.38 and 5.51%, respectively. We further elucidate the important structure-property relationships for these dithienocyclopentacarbazole-based nonfullerene acceptors. These results provide useful guidelines for enhancing the performance of nonfullerene acceptors through fluorination and side-chain engineering.