Modulation of End Groups for Low-Bandgap Nonfullerene Acceptors Enabling High-Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high-performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near-infrared region, it is important to develop ultralow-bandgap SMAs that have extended absorption in the spectral range of 800-1000 nm to maximize light absorption and efficiencies. In this work, three low-bandgap SMAs, namely, IXIC, IXIC-2Cl, and IXIC-4Cl, are designed and synthesized with same fused terthieno[3,2-b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E-g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V-oc of the devices is reduced but the J(sc) is significantly increased. In addition, the addition of chlorine atoms can enhance pi-pi stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine-substituted IC and OSCs based on PBDB-T:IXIC-2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low-bandgap SMAs.