Highly Improved Ion Diffusion through Mesoscopically Ordered Porous Photoelectrodes

Photoelectrochemical devices rely on porous photoelectrodes because of the formation of heterojunctions with the electrolyte solution. We evaluate the electrolyte ion-diffusion transport in mesoscopic inverse opal (meso-IO) structures with a uniform pore network by comparison with the diffusion in the conventional random pore electrode. The ion diffusivity through porous photoelectrodes was obtained using the modified Fick’s law with the diffusion-limiting current in the current–voltage characteristic. We observe that the ion diffusivity of iodine-based electrolytes through the meso-IO electrode film was 1.5 times greater than that through the random pore structure. More importantly, the diffusion of larger ions, cobalt-based ions, was 58% more enhanced in the meso-IO structure than in the random pore structure. In practice, we confirmed the effect of electrolyte ion diffusion on the photovoltaic performance of the dye-sensitized solar cell, and about 11% higher efficiency at the meso-IO electrode compared to the random pore electrode when the electrolyte contains large cobalt ions. This study suggests that, with bulky electrolyte ions and a highly viscous polymer gel matrix, a uniform pore electrode structure, such as the IO structure, can be advantageous to achieve the best photoelectrochemical performance.