Efficient inverted solar cells using TiO(2) nanotube arrays.

Using a vertical titania (TiO(2)) nanotube array, an inverted polymer solar cell was constructed with power conversion efficiency up to 2.71%. In this study, self-organized TiO(2) nanotubes arrays were grown by anodizing Ti metal in glycerol electrolyte containing 0.5 wt% NH(4)F and 1.0 wt% H(2)O with 20 V potential. The tube length (∼100 nm) was controlled by the thickness of the sputtered titanium layer on the indium-tin oxide (ITO) substrate. The diameter of the tube was approximately 15-25 nm. After annealing in air at 500 °C for 1 h, nanotubes arrays were crystallized to the anatase phase from the initial amorphous state. Following the infiltration of polymeric semiconductor (poly(3-hexylthiophene) and (6,6)-phenyl C(60) butyric acid methyl ester, P3HT:PCBM), the filled TiO(2) layer had an optical absorption over a range from UV to visible light. The high surface-to-volume ratio of the nanotube arrays structure increased the effective area of the active region. The high efficiency of our solar cell is attributed to the vertical TiO(2) nanotube array's enhanced conduction of photo-induced current due to its charge transport capability.