A Comparison of Indoline Dyes as Photosensitizers in Dye-Sensitized Solar Cells

Abstract

Properties of four indoline dyes were studied by means of density functional theory (DFT) and time- dependent density functional theory (TD-DFT) with the goal of finding an excellent photosensitizer for use in dye-sensitized solar cells. Theoretical results showed that the frontier molecular orbital structures of indoline dyes are suitable for electron injection from the excited states of indoline dyes to a TiO2 electrode. Calculated UV-visible absorption spectra of indoline dyes in vacuum match well with solar radiation spectra. The calculated energy levels of these dye molecules demonstrate that indoline dyes can be used as photosensitizers for TiO2 nanocrystalline solar cells together with the I-/I3- electrolyte. The lowest unoccupied molecular orbital (LUMO) energy levels of indoline dyes are higher than the conduction band edge of the TiO2 crystal, which ensures a high efficiency of electron transfer from indoline dyes to TiO2 electrodes. As the highest occupied molecular orbital (HOMO) energy levels of indoline dyes are lower than those of I-/I3- , molecules that donated electrons can receive electrons from the electrolyte. By comparison with the experimental data, the transfer efficiency of dye-sensitized solar cells may be determined mainly by the LUMO energy levels. The working lifetime of a dye-sensitized solar cell depends mainly on the stability of the dye molecule. From an analysis of the bond length of chemical bonds, we find that the stability of the four indoline dye molecules is basically the same. We further show that indoline dye 1 (ID1) has the highest LUMO energy level and the highest molecular stability. Its absorption spectra match solar radiation spectra well in an ethanol solution, therefore, it is the best photosensitizer among the analyzed dyes for application in dye-sensitized solar cells.