Effect of lithium iodide addition on poly(ethylene oxide)-poly(vinylidene fluoride) polymer-blend electrolyte for dye-sensitized nanocrystalline solar cell

The effect of lithium iodide concentration on the conduction behavior of poly(ethylene oxide)-poly (vinylidene fluoride) (PEO-PVDF) polymer-blend electrolyte and the corresponding performance of the dye-sensitized solar cell (DSSC) were studied. The conduction behavior of these electrolytes was investigated with varying LiI concentration (10-60 wt % in polymer blend) by impedance spectroscopy. A polymer-in-salt like conduction behavior has been observed in the high salt concentration region. The transition from salt-in-polymer to polymer-in-salt conduction behavior happened at the salt content of 23.4 wt %, which is much lower than 50 wt % as generally reported. The electrolyte shows the highest ionic conductivity (similar to 10(-3) S cm(-1)) at the salt concentration above 23.4 wt %. From the evaluation of salt effect on the performances of corresponding DSSC, we find that increasing LiI concentration leads to increased short-circuit photocurrent density (J(sc)) caused by enhanced 13 diffusion up to an LiI content of 28.9 wt %. Above this limitation, the J(sc) decreases as a result of increased charge recombination caused by the further increased 13 concentration. The open-circuit voltage (V-oc) increases gradually with LiI concentration owing to the enhanced I- content in DSSC. The optimized conversion efficiency is obtained at a salt content of 28.9 wt % in the polymer-in-salt region, with high ionic conductivity (1.06 x 10(-3) S cm(-1)). Based on these facts, we suggest that the changes of conduction behavior and the changes of I-3(-) and I- concentrations in the electrolytes contribute to the final performance variation of the corresponding DSSC with varying LiI concentration.