Study on the Change in Photovoltage by Control of Cell Gap in Dye-Sensitized Solar Cells

Photo-electrochemical properties of dye-sensitized solar cells (DSSCs) were investigated by changing the gap between working and counter electrodes. The open-circuit voltage $(VOC)$ of DSSCs was significantly increased from 616 mV to 776 mV by about 26% with 1-methyl-3-butyl imidazolium iodide (BMII) based electrolyte and from 428 mV to 513 mV by 20% with lithium iodide (LiI) based electrolyte as the cell gap increased from $16μm$ to $224μm$⁠. From the electrochemical impedance spectroscopy, it was found that the resistance of the electrolyte was increased as the cell gap widened. This resulted in the reduction in the dark current associated with the $VOC$ enhancement. The transient photovoltage spectroscopic measurement confirmed that the time constant for charge recombination between $TiO2$ and electrolyte became slower as the cell gap of the DSSC with LiI electrolyte increased, which could be an additional reason for the $VOC$ enhancement. The optimal cell gap was determined to be around $31.3μm$ for the BMII electrolyte system, and around $75.5μm$ for the LiI electrolyte system in terms of the energy-conversion efficiency.