Impact of 4-Tertiary-butylpyridine in Imidazolium Iodide/Triiodide Redox Couple-Based Dye-Sensitized Solar Cells

The effect of 4-tertiary-butylpyridine (TBP) in improving the performance of imidazolium iodide/triiodide redox couple-based dye-sensitized solar cells (DSCs) is studied. The presence of N-I-N vibration is detected in the FT-IR spectrum, and the formation of TBT2I+ ion is predicted. The increased TBT2I+ concentration helps increase the transport number of iodide ions and consequently the ionic conductivity of the electrolyte. It is found that when TBP is introduced to a DSC utilizing 1-methyl-3-propylimidazolium iodide, the open-circuit voltage (V-OC), short-circuit current density (J(SC)), and conversion efficiency (eta) are significantly increased until the formal concentrations of TBP and triiodide ions are equal, beyond which the values are slightly lower. Increase in V-OC is due to the mixed potential of I-/I-3(-) and I-/TBT2I+ redox couples and the maximum value at [TBP] = [I-3(-)] is due to the presence of only the I-/TBT2I+ redox couple. Increase in J(SC) is due to decreased recombination because of surface passivation by the adsorbed TBP molecules and increased ionic conductivity of the electrolyte. Consequently, maximum values are obtained when [TBP] = [I-3(-)], beyond which excess TBP tends to give slightly lower constant values. This is due to increased series resistance by the passivated TiO2 surface. The mechanism proposed based on the chemistry of TBP and I-2 accounts for all the effects observed. The optimized DSC shows a conversion efficiency of 7.94%, with a high V-OC of 0.709 V and J(SC) of 17.22 mA cm(-2), where the conversion efficiency has been increased by 35% from 5.15% when TBP is not present to 7.94% at this optimum TBP formal concentration.