Bithiazole-based A-π-A'-π-A-type completely non-fused small-molecule acceptors for efficient organic solar cells

Abstract

For the development of completely non-fused small-molecule acceptors (CNF-SMAs), the main focus is on molecular engineering of the terminal groups and side chains, while the central unit lacks attention.Here, we have designed and developed a new series of A-π-A′-π-A-type CNF-SMAs, CBTz-2F, TBTz-2F, and TBTz-2Cl, by using electron-deficient 2,2′-bithiazole (BTz) as the central unit (A′) instead of common electron-rich units (D), where the side chains on BTz have been systematically altered from alkyl to 3,5-dialkylthiophene, and the halogen atoms on terminal A groups have been changed from F to Cl. The effects of the electron-deficient A′unit and the alterations of the side chains and the halogen atoms on solubility, crystallization, molecular structure, physicochemical and photovoltaic properties of these CNF-SMAs have been investigated in detail. Results demonstrate that these A-π-A′-π-A molecules have better planar backbone, larger optical bandgap and higher-lying HOMO/LUMO levels than those of A-π-D-π-A-type tetrathiophene-based analogues with bithiophene as the central unit (D). Moreover, organic solar cells (OSCs) incorporating the chlorinated TBTz-2Cl obtain a higher power conversion efficiency (PCE = 5.76%) than that of its fluorinated counterpart TBTz-2F (5.25%). Encouragingly, ternary OSCs with TBTz-2F as the third component achieve an increased PCE of 19.15% relative to the control device PM6:L8-BO (18.52%). Device physical processes and film morphological analyses reveal that the enhanced performance can be attributed to the improved exciton dissociation, charge extraction and carrier transport, reduced charge recombination and superior nanofibril morphology of the ternary device. This finding indicates that the electron-deficient central unit A′ plays a critical role in tailoring molecular properties, film morphology and device performance, which provides valuable insights into the molecular design of future efficient and low-cost A-π-A′-π-A-type CNF-SMAs.