Development of Spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene]-Based A-π-D-π-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells

Three acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules (STFYT, STFRDN, and STFRCN) with spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene] (STF) as the central donor unit, terthiophene as the π-conjugated bridge, indenedione, 3-ethylrhodanine, or 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit are designed, synthesized, and characterized as electron donor materials in solution-processing organic solar cells (OSCs). The effects of the spiro STF-based central core and different acceptors on the molecular configuration, absorption properties, electronic energy levels, carrier transport properties, the morphology of active layers, and photovoltaic properties are investigated in detail. The three molecules exhibit desirable physicochemical features: wide absorption bands (300–850 nm) and high molar absorption coefficients (4.82 × 104 to 7.56 × 104 M–1 cm–1) and relatively low HOMO levels (−5.15 to −5.38 eV). Density functional theory calculations reveal that the spiro STF central core benef...