Absorptive Behaviors and Photovoltaic Performance Enhancements of Alkoxy-Phenyl Modified Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptors

Nonfullerene (NF) small molecular acceptors are very attractive for further improving the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) to overcome the limited absorptive region and fixed-energy-level drawbacks of fullerene-based electronic acceptors (PC₆₁BM and PC₇₁BM). The acceptor-donor-acceptor (A-D-A)-type oligomers (ITIC) containing an electron-rich core (four hexyl-phenyl-substituted indacenodithieno[3,2-b]thiophene) as a donor motif sealed with 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as an acceptor motif has been intensively investigated, because of its excellent absorptive and photovoltaic properties. Side-chain modifications have been proven to be an effective approach to modulate the energy levels and absorptive behaviors of conjugated polymers, as well as conjugated small molecules. Through the introduction of various side-chain and end groups, a series of promisingly modified ITIC-based small molecules have been synthesized and well-studied. Herein, we reported three novel alkoxy-phenyl modified ITIC-type NF acceptors (namely, pO-ITIC, mO-ITIC, and FpO-ITIC), in which 4-hexyloxy-phenyl, 3-hexyloxy-phenyl, and 3-fluorine-4-hexyloxy-phenyl side-chains were connected on the indacenodithieno[3,2-b]thiophene core as the electron-donating segments of the A-D-A molecules. Both three small molecules exhibit good solubility in common solvents, finely tunable energy levels, and adjustable optical bandgaps. The 4-hexyloxy-phenyl and 3-hexyloxy-phenyl-substituted materials possess relatively low bandgaps (1.61 eV for pO-ITIC and 1.63 eV for mO-ITIC) and a 4.7% enhancement in the maximum extinction coefficient, compared to that of ITIC. As the result of the better absorption behaviors, inverted polymer solar cells based on pO-ITIC blended with PTB7-Th achieve an open-circuit voltage (V_oc) of 0.80 V, a short-circuit current (J_sc) of 14.79 mA/cm², and a fill factor (FF) of 59.1%, leading to a high-power conversion efficiency (PCE) of 7.51%, relative to the 7.31% PCE of ITIC-based device. By using a new thiazolothiazole-based wide-bandgap polymer (PTZ-DO, 1.98 eV) with deep HOMO energy level (-5.43 eV) to match the optical absorption and molecular energy levels with the three NF acceptors, excellent PCE values - 9.28% for mO-ITIC and 9.03% for pO-ITIC - are obtained, which show increments of 15.3% and 12.2%, respectively, relative to that of ITIC (8.05%). This finding should offer useful guidelines for the design of novel NF acceptors for highly efficient PSCs through alkoxy-phenyl side-chains modified on the electron-donating moiety of A-D-A organic small molecules.