Determination of HOMO levels of organic dyes in solid-state electrochemistry

Abstract

The solid-state electrochemistry of organic dyes, TT(2-cyano-3-(4-hexyl-5-dimethyltriphenylamine-thien-2-y l ) a c r y l i c a c i d ) , T T N a ( 2 -c y a n o -3 -( 4 -h e x y l -5 -dimethyltriphenylamine-thien-2-yl)acrylic acid sodium salt), and TS(2-[5-(4-hexyl-5-dimethyltriphenylamine-thien-2-yl)methylene-4-oxo-2-thioxothiazolidin-3-yl]acetic acid), was investigated. The redox processes of the solid dyes are electrochemically quasi-reversible. The electrochemical oxidation behaviors of these dye are similar because of the identical donor moieties and the highest occupied molecular orbital (HOMO) distribution in the dyes as revealed by density functional theory (DFT) calculation. The AC impedance spectra show that the dye film shows negative differential resistance characteristics in the high-frequency range and finite diffusion responses in the low-frequency range at higher potentials, which indicates the nonlinear charge transport and limited diffusion of counterions in the solid-state film. The onset oxidation potential of the solid dyes in the cyclic voltammetry does not affected by the ion diffusion. The cyclic voltammetry can be employed to derive reliable HOMO and lowest unoccupied molecular orbital (LUMO) levels of the solid dyes.

A perspective on using experiment and theory to identify design principles in dye-sensitized solar cells

“…Furthermore, for heavy elements spin orbit coupling needs to be included [47]. Modelling isolated dye molecules or dye-plus-anchor groups in the gas-phase enables calculation of their HOMO and LUMO levels [48], which can be combined with Langmuir adsorption isotherms to postulate the orientation of the dyes on the TiO 2 surface [49]. …”