New metal free organic dyes incorporating heterocyclic Benzofuran core as conjugated spacer: Synthesis, Opto-electrochemical, DFT and DSSC studies

Synthesis of a series of new organic photosensitizers, designated as BZ1-BZ4, was achieved by incorporating benzofuran core as pi-spacer, triphenylamine (TPA) and 4-methoxy triphenylamine (4-MeO-TPA) as donors and cyanoacrylic acid (CAA) and rhodanine-3-acetic acid (RAA) as acceptor/anchoring groups. Structurally, dyes BZ1 and BZ2 carry TPA and 4-MeO-TPA as donors, respectively, whereas CAA constitutes as a common acceptor. On the other hand, BZ3 and BZ4 incorporate TPA and 4-MeO-TPA as donors, respectively, and RAA serves as a common acceptor. Key steps to access BZ1-BZ4 involved Wittig olefination, reduction of cyano groups to aldehyde, and finally Knoevenagel condensation. In UV-visible spectra, dyes BZ1-BZ4 exhibited intramolecular charge transfer (ICT) maxima in the range of 503 nm to 534 nm and their extinction coefficients varied from 22,600 to 40,400 M-1 cm(-1). Using cyclic voltammetry and UV-visible data, we calculated the optical band gaps, (E0-0) of BZ1-BZ4 to be 2.09, 2.00, 2.01, and 1.93 eV, respectively. DFT studies revealed that HOMOs of dyes were localized essentially on the donors triphenylamine groups, whereas the LUMOs are largely confined over electron acceptors CAA and RAA as well as partly diffused into the benzofuran spacer. DSSCs, configured using BZ1-BZ4 as photosensitizers, showed fill factors (ff) in the range of 0.57 to 0.76, comparable to standard N3 dye (0.67). In addition, BZ1 and BZ2 dyes with CAA as an acceptor also exhibited decent open circuit voltage (V-oc) in the range of 0.61 to 0.63 relative to the 0.66 observed for N3 dye. Dyes BZ1-BZ2 carrying CAA as an acceptor exhibited an efficiency (eta) of 2.03% and 1.49%, respectively, against the benchmark N3 dye, showing eta of 5.02%. Due to poor V-oc and short circuit current density (I-sc), dyes BZ3-BZ4 carrying RAA as acceptor/anchoring group performed significantly poor, showing eta of 0.08%, and 0.32%, respectively.

Automated summary

A series of new organic photosensitizers, BZ1-BZ4, were synthesized and exhibited good optical properties in UV-visible spectra. They also showed certain efficiency in dye-sensitized solar cells.