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-1H-indene-2,1(3H)-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-nF (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-nF 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-nF 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., EG1-IDTT-EG1 + EG2-IDTT-EG2 ⇌ 2 EG1-IDTT-EG2, with implications for the entire ITIC PSC field,
is identified and mechanistically characterized, and the effects on
PSC performance are assessed.