Regioisomer-Free Chlorinated Thiophene-Based Ending Group for Thieno[3,2-b]thiophene Central Unit-Based Acceptor Enabling Highly Efficient Nonfullerene Polymer Solar Cells with High V_oc Simultaneously

A pair of pure regioisomeric acceptor–donor–acceptor (A–D–A) typed nonfullerene small molecule acceptors (NF-SMAs) (4TTIC and 4TTIC-Cl), containing a central thieno[3,2-b]thiophene-sp³ hybridized “carbon-bridge”-based fused ring core unit and thiophene-based IC or chlorinated thiophene-based IC are synthesized for polymer solar cells (PSCs). Compared with 4TTIC, 4TTIC-Cl not only achieves a red-shifted absorption spectra and lower energy levels but also enhancement of molecular packing and crystallinity. The 4TTIC-Cl-based blend films display higher and more balanced charge carrier mobilities, more favorable morphology, and more efficient exciton dissociation in comparison with the 4TTIC-based blend film. The optimized devices based on PBDB-ST:4TTIC-Cl deliver an impressively high power conversion efficiency (PCE) of 13.11% and fill factor of 74%, much higher than that of the PBDB-ST:4TTIC-based devices. Moreover, a small energy loss of ≈0.54 eV and a decent V_oc of 0.88 V are simultaneously achieved for PBDB-ST:4TTIC-Cl-based devices. Noticeably, the PCE of 13.11% is the highest reported value for NF-SMAs containing the central thieno[3,2-b]thiophene unit with sp³ hybridized carbon-bridged cyclopentadiene fragments in binary PSCs. This study proves that introduction of less electron-deficient thiophene-based IC terminal group into thieno[3,2-b]thiophene central core-based SMAs is a very effective method for making high V_oc and excellent PCE simultaneously.