Asymmetrical Squaraines Bearing Fluorine-Substituted Indoline Moieties for High-Performance Solution-Processed Small-Molecule Organic Solar Cells

Two novel asymmetrical squaraines
based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two
fluorine substituents, have been developed to investigate the effect
of fluorine substituted on small-molecule bulk-heterojunction (BHJ)
organic solar cells (OSCs). In comparison with non-fluorine-substituted
ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous
absorption band gaps but 0.05 and 0.10 eV lowered highest occupied
molecular orbital (HOMO) energy levels, respectively. Single-crystal
analysis exhibits that ASQ-5-DF shows more desirable intermolecular
packing patterns for the hole-carrier collection than ASQ-5 does;
hence, higher hole mobility could be acquired. Therefore, solution-processed
small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM
and ASQ-5-DF/PC71BM blends exhibit extremely higher power
conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that
of ASQ-5/PC71BM (4.5%). The much improved PCE could be
attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the
ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%)
among squaraine-based solution-processed BHJ OSCs and the highest
PCE in OSCs based on the fluorinated donor segment of small molecules.