Effect of electrolyte conductivity, co-additives and mixed cation iodide salts on efficiency enhancement in dye sensitized solar cells with acetonitrile-free electrolyte

Abstract

Electrolyte modifications in dye-sensitized solar cells (DSSCs) provide a convenient way to enhance their photovoltaic performance. In this work, the synergistic effect of the two mixed cation iodide salts KI and Pr4NI and the two co-additives 4-tertiary butyl pyridine (TBP) and guanidinium thiocyanate (GuSCN) has been used successfully to enhance the photovoltaic performance of DSSCs fabricated with non-volatile propylene carbonate (PC) solvent based electrolyte. The reference DSSC made with electrolyte using only TPAI as the iodide salt exhibited an efficiency of 5.26 % while the DSSC made with optimized electrolyte with TPAI and KI exhibited an efficiency of 6.43 %. This efficiency increase of 22 % has been attributed to the mixed cation effect. It was observed that, the addition of TBP alone to the electrolyte increases the photovoltage of the cells but decreases their photocurrent density. However, the addition of GuSCN increases both the photocurrent density and the photovoltage. While the addition of TBP to the mixed cation electrolyte increased the efficiency from 6.43 % to 6.84 %, the addition of GuSCN increased the cell efficiency from 6.43 % to 6.76 %. However, the addition of the combination of TBP and GuSCN as co-additives in the optimized ratio of 65:35 enhanced the cell efficiency to 7.70 %. After compensating for the loss of iodine due to complexation with TBP the efficiency of DSSCs with co-additives reached an impressive 8.01 %. The overall efficiency enhancement has been explained by the mixed cation effect on the basis of the shifting of the conduction energy band edge of TiO2 due to the adsorption of cations from the two iodide salts and the adsorption of ionic species from the two co-additives by TiO2 leading to the enhancement of photocurrent density as well as photovoltage. The ionic conductivity increase in the electrolyte due the presence of the co-additives does not appear to be directly contributing to the increase in the photocurrent in the DSSC. However, the efficiency of all TBP added solar cells could also be improved further by compensating for the decrease of iodide ion content in the electrolyte due to complex formation.