Novel Design of Carbazole-Based Donor-Acceptor Molecules for Fullerene-Free Organic Photovoltaic Devices

Small molecular weight donor-acceptor molecules composed of the carbazole donor and cyanobased acceptor units were newly synthesized by the Knoevenagel condensation or formal [2 + 2] cycloaddition-retroelectrocyclization reactions. The resulting donor-acceptor molecules were fully characterized by H-1-, C-13-NMR, and IR spectroscopies, MALDI-TOF MS spectrometry, and elemental analysis. The energy levels, determined by electrochemical redox potentials, suggested the deep lowest unoccupied molecular orbital (LUMO) levels, indicating the potential use as an n-type semiconductor. The narrow band gaps were also suggested by the well-defined visible and near-IR absorptions. Therefore, the mixed films of poly(3-hexylthiophene) (P3HT) and the carbazole-based donor-acceptor molecules were investigated. The mixed films were hardly fluorescent due to the energy transfer from P3HT to the donor-acceptor molecules. A preliminary test of the bulk-heterojunction solar cells based on the donor-acceptor molecule prepared by the [2 + 2] cycloaddition-retroelectrocyclization showed a dark current flow, while the device based on the donor-acceptor molecule prepared by the Knoevenagel condensation displayed a weak photoresponsive current.