Tuning band structures of dyes for dye-sensitized solar cells: effect of different π-bridges on the performance of cells

Abstract

Three organic dyes (LJ101, LJ102, and LJ103) with different π-bridges such as thiophene, 3-hexylthiophene and 3, 4-ethyldioxythiophene, were prepared and applied for the dye-sensitized solar cells (DSSCs). The effect of the different π-bridges on the photovoltaic performance of DSSCs were studied with the aid of quantum chemical calculation and electrochemical-impedance spectroscopy (EIS). The introduction of π-bridges to construct dyes makes the absorption spectra red shift and enhances the molar extinction coefficient compared to the parent structure of TH305 without π-conjugated bridge. However, the increasing conjugation length of the molecules decreases the performance of DSSCs, due to the two reduced gaps: one is between the LUMO levels of dyes and the conduction band edge of TiO₂, the other one is the energy gap between the HOMO levels of dyes and the redox potential of iodide/triiodide. The conversion efficiencies of DSSCs based on LJ series dyes ranged from 6.1 to 6.4%, which are lower than that of TH305 (7.5%).