Effect of Extended Conjugation on the Photosensitizers for DSSC: DFT and TD-DFT Study

Abstract

Dye sensitized solar cells have achieved more than 12% efficiency[1][2] since 1991 when first metal organic
photosensitizer was made by Gratzel based on ruthenium complex[3].Here we have discussed the effect of
extended conjugation on absorption spectra of the photosensitizers[4]. The designed dye molecules are based on
donor- π -acceptor model. The designed dyes are named as S1 [naphthalene-pyrrole-butanoic acid] and S2
[naphthalene-pyrrole-thiophene-butanoic acid]. In both the systems naphthalene is the donor and butanoic
acid is the acceptor moiety. The pyrrole is the bridging molecule in S1 and pyrrole along with thiophene are the
bridging moiety in S2. Both the systems are optimized under the framework of density functional theory using
B3LYP hybrid functional with 6-31+G(d,p) basis set and absorption spectra is calculated utilizing time dependent
density functional theory at CAM-B3LYP functional on Gaussian 16W software. The optimized geometries
are also checked for their global minima on the potential energy surface through frequency calculation. The
calculated absorption maxima of the S1 are at 286.66 nm and for S2 the absorption maxima are observed at
296.89 nm. The frontier molecular orbital energy gap HOMO-LUMO for S1 is found at 4.34 eV and for S2 it
is 3.75 eV. This shows that the HLG energy reduces on increasing the π bridge molecule in the system which
enhances the absorption maxima towards the red shift of the visible spectrum. The red shift of 10.23 nm in
the absorption spectra is observed in the S2 system as compared to the S1. Thus, the effect of bridging on the
absorption maxima shows that conjugation length plays a key role in enhancing the absorption spectra for the
designed photosensitizer. Therefore, this study would help in designing the novel conjugated organic molecules
as better photosensitizers for high-performance DSSCs.