Design, Synthesis, and Optical and Electrochemical Properties of D-π-A Type Organic Dyes with Carbazole-Based Donor Units for Efficient Dye-Sensitized Solar Cells: Experimental and Theoretical Studies

Significant progress has been made in developing organic compounds by modifying carbazole (Cz) based donor-spacer-acceptor (D-pi-A) type dye molecules. The Cz-1-Cz-3 dyes were theoretically designed and experimentally synthesized successfully. Herein, we report the synthesis, photophysical properties and electrochemical characterization (quasi-reversible oxidation processes) of three Cz-based compounds. The calculated results agree well with the available experimental data of absorption spectra, HOMO-LUMO energy levels and band gaps using the time-dependent density functional theory. Furthermore, the results from natural bond orbital calculations were analyzed with the computational electronic structure and charge transfer (conjugative interaction) in molecular systems. The result shows fluorescence time-correlated single-photon counting between 4.32 ns, 4.25 ns, and 13.2 ns, and green to blue (lambda(PL) = 431-881 nm) emission for complexes Cz-1-Cz-3 in DMF solution. The Cz-3 compound showed excellent photovoltaic properties, with power conversion efficiency of 5.68%. These results clearly reveal that modification of the electron-withdrawing capability in D-pi-A conjugated metal-free organic dyes is an effective way to improve the optical and electrical properties of organic photovoltaic (PV) devices.