Synthesis of new conjugated small-molecule-dyes based on 2-(2-methyl-4H-chromen-4-ylidene)malononitrile as the electron-withdrawing group and their application in photovoltaic devices

Abstract

A series of new acceptor-donor-acceptor (A-D-A) type conjugated small-molecule-dyes based on indacenodithieno[3,2-b]thiophene (IDTT) and 2-(2-methyl-4H-chromen-4-ylidene)malononitrile (CMCN), named IDTT-CMCN and IDTT-HT-CMCN, have been synthesized for application in organic solar cells. A-D-A type small-molecules with CMCN can effectively form a powerful push-pull structure that induces effective intramolecular charge transport (ICT) and enhanced optical absorption. In comparison with IDTT-CMCN, IDTT-HT-CMCN has 3-hexylthiophene (HT) as a π-bridge unit between the IDTT and CMCN moieties to increase the solubility and extend the π-conjugation length. The resulting dye (IDTT-HT-CMCN) with a hexylthiophene ring as a π-bridge showed a smaller band gap and better solubility. Thus, a power conversion efficiency (PCE) as high as 7.60% was achieved with a short circuit current of 12.7 mA/cm2, open circuit voltage of 0.94 V, and fill factor of 63.8%. To the best of our knowledge, small-molecule-based donors with CMCN as an electron-withdrawing unit have never been reported before. In addition, the PCE of 7.60% reported in this work is highly efficient among the PCE reported for an organic solar cell using an IDTT-based small-molecule-donor material.