Spirobifluorene-based acceptors for polymer solar cells: Effect of isomers

• Two spirobifluorene-based isomers (SBF1, SBF2) with 1,1-dicyanomethylene-3-indanone as acceptor groups were synthesized.
• SBF1 and SBF2 were used as acceptors to fabricate fullerene-free polymer solar cells.
• Effects of isomer configuration on optical and electronic properties and device performance were investigated.

Two spirobifluorene-based isomeric acceptors (SBF1 and SBF2) with electron-withdrawing 1,1-dicyanomethylene-3-indanone as end groups were theoretically studied, synthesized and applied as electron acceptors for solution-processed polymer solar cells (PSCs). SBF1 possessed three-dimensional molecular structure while SBF2 was more planar. The effect of molecular configuration on absorption, energy level, charge transport, morphology and PSC performance was investigated. These molecules exhibited good thermal stability with decomposition temperatures of 345–348 °C, broad absorption in visible region (400–650 nm), and proper HOMO (−5.96 to −5.94 eV) and LUMO (−3.86 to −3.78 eV) energy levels. The mobility of SBF2 (7.2 × 10−4 cm2 V−1 s−1) was one order of magnitude higher than that of SBF1 (6.1 × 10−5 cm2 V−1 s−1). When blending with low-bandgap polymer donor PBDTTT-C-T, the power conversion efficiencies of SBF2-based PSCs were ca. 50% higher than those of SBF1-based devices.

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