Understanding the photo-electrochemistry of metal-free di and tri substituted thiophene-based organic dyes in dye-sensitized solar cells using DFT/TD-DFT studies

In this work, two 2, 5-disubstituted and three 2, 3, 5-trisubstituted thiophene-based organic dyes have been investigated using the density functional theory. Although substitution at the 3-position of thiophene ring may retard the back electron transfer, the loss of coplanarity affected the intramolecular charge transfer. The natural bond orbital (NBO) analysis of dye-(TiO2)(8) cluster has been performed to study the feasibility of electron injection. The highest driving force of dye regeneration, higher negative NBO value of cyanoacrylic acid (CA) attached to the (TiO2)(8) cluster (CA-(TiO2)(8) moiety), and reasonably higher open-circuit voltage make (E)-2-cyano-3-(5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophen]-5-yl)acrylic acid (D1) to perform as an effective light harvester in dye-sensitized solar cells. The outcomes of this theoretical study are in good agreement with the experimental data reported.