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

Thomas J. Aldrich, Micaela Matta, Weigang Zhu, Steven M. Swick, Charlotte L. Stern, George C Schatz, Antonio Facchetti, Ferdinand S. Melkonyan, Tobin J Marks

Research output: Contribution to journalArticle

80 Citations (Scopus)

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-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., 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.

Original languageEnglish
JournalJournal of the American Chemical Society
DOIs
Publication statusPublished - Jan 1 2019

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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