Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response

Abstract

Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2′-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3- d:2′,3′- d ′]-s-indaceno[1,2- b:5,6- b ′]dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1 H -indene-2,1(3 H )-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC- n F ( n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC- n F molecules cofacially interleave with ITIC-6F having the shortest π–π distance of 3.28 Å. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-0F, and ITIC-4F, and track a positive correlation between the ITIC- n F space-charge limited electron mobility and n . Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-groups (EGs) of IDTT-based NFAs, i.e., EG 1 -IDTT-EG 1 + EG 2 -IDTT-EG 2 ⇌ 2 EG 1 -IDTT-EG 2, with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.