Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodizationof Ti sputtered on conductive glass

Abstract

Self-organized porous TiO2 nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titaniumin an NH4F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 °C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, withan average external diameter of 92 nm, were grown; this gave a high conversion rate ofoxide from titanium (1.9), with a 220 nm thick layer of titanium, which was notoxidized, located at the base of the tubes. The NTs revealed a mainly amorphousstructure, which transformed mostly to anatase upon thermal treatment in air at450 °C.The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporatedinto liquid dye solar cells (DSCs) and further tested under back-side illumination. High values ofVoc (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude andlarge recombination resistance and electron lifetime. In addition, typical values of fillfactors (of the order of 0.62) were attained, in agreement with the estimated ohmicresistance of the cells in combination with low electron transfer resistance at theplatinum/electrolyte interface. The overall moderate power conversion efficiency(of the order of 0.3%) was mainly due to the low short-circuit photocurrents (Jsc = 0.68 mA cm−2),which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitudeof Jsc was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading(due to the limited thickness of anodic titania) and the high charge transfer resistance at theTiO2 /conductive substrate due to the presence of barrier layer(s) underneath the tubes.These preliminary results encourage the DSC community to explore further thegalvanostatic anodizing of titanium in order to produce highly efficient porousTiO2 NTs directly on conductive glass. Current work is focusing on achieving complete anodizingof the metal substrate and full transparency for the photoelectrode in order to increase andoptimize the resultant cell efficiencies.