Synthesis and photochemical characterization of fumaronitrile-based organic semiconductor and its use in solution-processed small molecule organic solar cells

A novel and efficient symmetrical donor–acceptor–donor organic semiconductor, TFN-bis[ter-HT3-TPA], comprising two triphenylamine (TPA) donors and one fumaronitrile (FN) acceptor linked by π-conjugated hexylated quaterthiophene, was synthesized and its photovoltaic behavior in solution-processed small molecule organic solar cells (SMOSCs) was investigated. Under light illumination, the synthetic intermediate di-bithophenyl-fumaronitrile, containing the FN acceptor moiety, exhibited cis–trans isomerization in solution, while cis–trans photoisomerisation of TFN-bis[ter-HT3-TPA] was not observed, presumably because of the steric bulk of the TPA-hexylated terthiophene in the cis-isomer. The TFN-bis[ter-HT3-TPA] showed strong intramolecular charge transfer between the TPA donor and the FN acceptors. A TFN-bis[ter-HT3-TPA]:PC71BM bulk heterojunction film with an optimized weight ratio of 1:3 resulted in an improved photovoltaic performance and an enhanced hole mobility. After insertion of a TiOx layer between the photoactive layer and the Al electrode, a power conversion efficiency of 2.21% was obtained in devices fabricated from the TFN-bis[ter-HT3-TPA]:PC71BM film resulting in a ∼17% higher efficiency than devices without a TiOx layer.