TD-DFT investigations on structural modification in Carbazole based organic Photosensitizers to improve electron injection in DSSC

Time-dependent density functional theory approach implemented at hybrid-B3LYP, GGA-PBE and DFTB levels of theory was used to model photoinjection in organic-dye/TiO2 quantum-dot to explore the prospects of improvement of DSSC. The photosensitizer used in this study consisted of six carbazole based organic dyes having acceptor as cyanoacrilic acid group and oligothiophene π-bridge spacer. The modifications were made in the dyes by increasing length of the spacer by adding thiophene and oxadiazole rings at different positions of the donor-acceptor bridge. The structural variations appeared to alter the electronic and optical properties of dyes studied via energy levels and excitation spectra. The UV-Vis spectra calculated for all the dyes in solvents exhibited a red shift in spectral peaks with increase in polarity of the solvents. The findings of the study pointed towards photoinjection of indirect nature studied in dye-(TiO2)96 complex for six different dyes. The substitution of oxadiazole ring in center and addition of a thiophene ring at the edge of the spacer produced two dyes which exhibited lowest injection energies of 0.11eV and 0.17 eV along with the regeneration energies of 1.18 eV and 1.12 eV respectively. The dyes reported herein may have promising applications in photoanode for enhancing the performance of DSSC.