Light soaking effect driven in porphyrin dye-sensitized solar cells using 1D TiO₂ nanotube photoanodes

Light soaking (LS) effect on porphyrin (GD2) dye-sensitized solar cells (DSSCs) using well-ordered TiO₂ nanotube (TNT) photoanodes was studied with different lengths of TNTs and LS treatment time. The TNT array possessing longer length and larger tube diameter had improved photoelectrochemical (PEC) property by generating larger photocurrent, and besides provided a larger surface area to yield more dye loading. The LS effect on GD2-applied DSSCs using TNT photoanodes was triggered off in the first 5 min of the LS treatment, and gradually increased during 60 min LS treatment. The improved PEC property and dye loading by longer TNT array (22 μm) led to the noticeable enhancement rate of power conversion efficiency from 0.7% to 1.88% (168% increase) after 60 min LS treatment, which was more prominent compared to the enhancement of GD2-applied DSSCs using mesoporous TiO₂ films. Especially, morphological property of well-ordered 1D TNT photoanodes offers more spaces to facilitate the cation exchange in electrolyte system, leading to enhancing electron injection and reducing recombination under the LS condition. Electrochemical impedance spectroscopy (EIS) results confirmed the contribution of well-aligned 1D TNT structure to significant LS effect in GD2-applied DSSCs. Bode phase and Nyquist plots in EIS results showed the elongated electron lifetime and diffusion length with an increase in TNT length, followed by improving power conversion efficiency with significantly increased J_sc.