Electrochemical and spectroscopic characterization of dye-sensitized solar cell

Novel porphyrin and triphenylamine based organic dyes with different electron-donating moieties, π-bridging units and cyanoacetic acid as an acceptor (D-π-A) were characterised for use in dye-sensitized solar cells. The electrochemical and spectroscopic properties of the dyes were characterized by cyclic voltammetry, Raman, ATR-FTIR, UV-Vis, fluorescence and XPS. The UV-Vis spectra of all the dyes displayed maximum absorbances in the range 400-500 nm. ATR-FTIR and Raman spectra of all the adsorbed dyes showed asymmetric and symmetric carboxylate stretches which indicated that the dyes were adsorbed to the TiO2 surface via the carboxylate group. The HOMO energy levels of the dyes were in the range of ca. (1.0 – 1.4)V, more positive than the redox potential of the iodide/triiodide couple (0.4 V vs NHE). The LUMO levels of the dyes were more negative, ca. (- 0.6 to -1.5 V), than the conduction band edge (-0.5 V vs NHE) of TiO2. The results showed that all the dyes fulfil the criteria for use in DSSC. The current-voltage (I-V) characteristics of the dye-sensitized solar cells were used to determine the power conversion efficiency of the cells. Electrochemical impedance spectroscopy was used to study the kinetics of electrochemical and photo electrochemical processes in DSSCs under dark and illumination conditions. The charge transfer resistances and the electron lifetime in the mesoscopic film were obtained by using an appropriate equivalent circuit. Among all the dyes, the organic dye (CL1) based DSSC showed the highest efficiency, η ~ 4.3%, compared to C212D (η~2.4%) and L0 DSSC (η~1.9%). The best performance of the CL-DSSC was a result of reduced charge recombination confirmed by the electrochemical impedance data. The overall conversion efficiency of porphyrin dyes (PM1-5) were quite low, ranging from 0.1 to 1.1 %, compared to their counterpart organic dyes. PPy-co-PEDOT copolymers were successfully used as counter electrodes in dye-sensitized solar cells. Cells using PPy-co-PEDOT (at a ratio of 2:1) as a counter electrode exhibited an overall conversion efficiency of 4.6% compared to a value of 5.5% for a DSSC utilising platinum as the counter electrode. Impedance analysis of the response demonstrated that the PPy-co-PEDOT (2:1) counter electrode had enhanced catalytic activity to the reduction of triiodide in comparison to a Pt counter electrode. This study indicates that PPy-co-PEDOT coated FTO represent a good alternative for use as a counter electrode in DSSCs.