Fabrication of long TiO2 nanotube arrays in a short time using a hybrid anodic method for highly efficient dye-sensitized solar cells

We report a simple hybrid anodic method, with initial potentiostatic anodization followed by galvanostatic anodization, to grow much longer titania nanotube ( TNT) arrays in a much shorter anodization period (t). The length of the TNT arrays ( L) depends linearly on t and is controlled by the electric current; the growth rates are 5.3, 10.7 and 20.3 mu m h(-1) for current densities 3.7, 5.6 and 7.5 mA cm(-2), respectively. The produced TNT films of L = 15-57 mu m sensitized with N719 dye were fabricated into devices for photovoltaic characterization. The NT-DSSC devices show systematically improved cell performance depending on L, reflecting the excellent intrinsic light-scattering property of the NT-DSSC devices to harvest increased sunlight with long TNT arrays. The great effective surface area inside TNT arrays has been shown to significantly increase the dye loading, which might help to enhance the cell performance of the device with co-sensitizing of different dyes for improved efficiency of light harvesting in the future. The best performance of the NT-DSSC device was achieved at L similar to 30 mm with a spacer of similar thickness, giving J(SC) = 14.63 mA cm(-2), V-OC = 0.741 V, FF = 0.70, and eta = 7.6%, which is unprecedented for a back-illumination DSSC.