Electron transport patterns in TiO₂ nanotube arrays based dye-sensitized solar cells under frontside and backside illuminations

TiO₂ nanotube arrays (NTA), of 17-37 μm in thickness, detached from anodic oxidized Ti foils were used as photoanodes for dye-sensitized solar cells (DSSCs). Photovoltaic measurements under frontside and backside illumination showed that frontside illumination geometry provided better cell performance than backside illumination did. A cell assembled with 30 μm thick NTA film produced the greatest photocurrent and light conversion efficiency. Despite an advantageous architecture for electron transport, electron trapping remained a limiting factor for both illumination geometries, due to the presence of crystal grains in the NTA walls. Intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that electron transport in the front-illuminated cells comprises both trap-free and trap-limited diffusion modes, whereas electrons in the back-illuminated cells travel only by trap-limited diffusion. The trap-free diffusion mechanism determines front-illuminated cell performance. Electrochemical impedance spectroscopy analysis showed the front-illuminated NTA-based DSSCs have a charge collection efficiency of better than 90%, even at 30 μm NTA film thickness. Large crystal size results in low trap state density in the NTA film, and this effect may result in a more extensive trap-free diffusion zone in the films, which facilitates charge collection.