Fullerene-Based Triads with Controlled Alkyl Spacer Length as Photoactive Materials for Single-Component Organic Solar Cells

Two kinds of dumbbell-shaped acceptor-donor-acceptor (A-D-A)-type triad single-component (SC) photovoltaic molecules based on a benzodithiophene-rhodanine (BDTRh) core and [6,6]-phenyl-C₆₁butyric acid (PC₆₁BA) termini, BDTRh-C₂-PC₆₁BA and BDTRh-C₁₀-PC₆₁BA, were synthesized by modulating the alkyl (C2 and C10) spacer lengths. Both SC photovoltaic structures had similar UV-vis spectra in solution, but BDTRh-C₁₀-PC₆₁BA showed a significantly higher absorption coefficient as a thin film. In films, a more facile intermolecular photo-induced charge transfer was observed for BDTRh-C₁₀-PC₆₁BA in the broad-band transient absorption measurements. BDTRh-C₁₀-PC₆₁BA also exhibited a higher hole mobility (by 25 times) and less bimolecular recombination than BDTRh-C₂-PC₆₁BA. By plotting the normalized external quantum efficiency data, a higher charge-transfer state was measured for BDTRh-C₁₀-PC₆₁BA, reducing its voltage loss. A higher power conversion efficiency of ∼2% was obtained for BDTRh-C₁₀-PC₆₁BA, showing higher open-circuit voltage, short-circuit current density, and fill factor than those of BDTRh-C₂-PC₆₁BA devices. The different carrier dynamics, voltage loss, and optical and photoelectrical characteristics depending on the spacer length were interpreted in terms of the film morphology. The longer decyl spacer in BDTRh-C₁₀-PC₆₁BA afforded a significantly enhanced intermolecular ordering of the p-type core compared to BDTRh-C₂-PC₆₁BA, suggesting that the alkyl spacer length plays a critical role in controlling the intermolecular packing interaction.