Barrier Layer Effect on the Electron Transport of the Dye-Sensitized Solar Cells Based on TiO2 Nanotube Arrays

We have studied the effect of the barrier layer on the electron transport of dye-sensitized solar cells (DSSCs) based on TiO 2 nanotube arrays. The barrier layer of TiO 2 nanotubes, corresponding to the underlayer of the detached TiO 2 nanotube films, was ion-milled to reduce its thickness. The energy conversion efficiencies (measured by back-side-illumination) of DSSCs based on the TiO 2 nanotube films—whose barrier layers were ion-milled for 0, 10, 20, and 30 min—were 2.5, 2.7, 2.9, and 3.1%, respectively. The conversion efficiency increased significantly with reducing the thickness of the barrier layer without opening the bottom tips. The improvement of the conversion efficiency was mainly due to the improvement of the electron transfer efficiency by reducing the thickness of the barrier layer. Also, electrochemical impedance spectroscopy (EIS) analysis showed that the thickness of the barrier layer has a pronounced impact on the electron transport resistance of the cells. It was concluded that the electron transport is hindered considerably by the barrier layer.