Unusual Enhancement of Photocurrent by Incorporation of Bronsted Base Thiourea into Electrolyte of Dye-Sensitized Solar Cell

Thiourea was used as an additive in the iodide/iodine redox electrolyte for dye-sensitized solar cell and its effect was investigated. Thiourea was found to have the simultaneous effect of a positive band edge shift and a decrease in charge recombination rate. Addition of 0.05 M thiourea in the electrolyte comprising 0.7 M 1-methyl-3-propylimidazolium iodide (MPII) and 0.05 M I-2 in acetonitrile enhanced significantly photocurrent density from 7.7 to 10.8 mA/cm(2), while voltage decreased from 0.78 to 0.71 V. As a result, overall conversion efficiency increased from 4.7% to 5.8%, corresponding to increment of 23%. The solution acidity was changed from pK(a) = 18.9 (thiourea in acetonitrile) to pK(a) = 4.9 (thiourea in iodide- and iodine-containing acetonitrile), corresponding to change in pH from 10.1 to 3.1, which was attributed to chemical reaction between thiourea and iodine. As a consequence of the reaction, protons were produced and triiodide concentration was slightly reduced. The generation of protons in the electrolyte, associated with a positive shift of conduction band edge, led to a significant increase in photocurrent density. The unexpectedly small voltage drop, however, was ascribed to a slow recombination rate due to the reduced triiodide concentration. A large increase in photocurrent density along with a small decrease in voltage was also demonstrated from the variation of thiourea concentration.