Effect of Inorganic Iodides on Performance of Dye-Sensitized Solar Cells

In this work, we investigate the effect of iodides based on a variety of cations (Mn+) in an electrolyte system composed of 1-propyl-3-methylimidazolium iodide (0.6 M) and methoxypropionitrile on the photocurrent density−voltage (J−V) characteristics and the kinetics process of electron transfer/transport of corresponding dye-sensitized solar cells (DSCs). It is observed that the ionic conductivity of the electrolyte is reduced slightly after introduction of Mn+-based iodides. The investigation of the performance of DSCs as a function of Mn+ shows that the short-circuit photocurrent density, Jsc, linearly depends on the charge/radius ratio of Mn+ in the electrolyte up to 1.5 Å-1. Beyond this value, Jsc asymptotically reaches saturation. The highest Jsc is obtained with an AlI3-based electrolyte. Open circuit voltage, Voc, is reduced after addition of monocations or trications into the electrolyte, whereas for dication-based iodides such as CaI2, Voc is increased. Electrochemical impedance spectra for the DSCs show that the charge-transfer resistance, Rct, related to the reaction between I3- and electrons at the Pt electrode/electrolyte interface, decreases with the cations displaying higher charge/radius ratio and with increasing cation concentrations in the electrolyte. An inverse dependence of Rct on logarithmic NaI concentrations is observed. Meanwhile, the resistance related to the electron back-transfer reactions from the dyed TiO2 to I3- in electrolyte, Rbr, is significantly influenced by the nature as well as the concentration of different cations. A maximum Rbr is observed with CaI2-based electrolyte, whereas AlI3-based electrolyte displays the lowest Rbr values, consistent with the variation of Voc as a function of Mn+. We explain that electrostatic interactions between cations in the electrolyte system and the TiO2 surface, and also with conduction band electrons, govern the electron injection efficiency and electron mobility in the nanostructured TiO2 film as well as the performance of the DSCs