TiO2 nanotube formation by Ti film anodization and their transport properties for dye-sensitized solar cells

J Mater Sci: Mater Electron (2016) 27:6496–6501 DOI 10.1007/s10854-016-4591-5 TiO2 nanotube formation by Ti film anodization and their transport properties for dye-sensitized solar cells Masoud Iraj1 • Mohammadreza Kolahdouz1 • Ebrahim Asl-Soleimani1 • Emad Esmaeili1 • Zahra Kolahdouz2 Received: 22 December 2015 / Accepted: 9 February 2016 / Published online: 11 March 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract In this paper, we present the synthesis of TiO2 nanotube (NT) arrays formed by anodization of Ti film deposited on a fluorine-doped tin oxide-coated glass substrate by direct current magnetron sputtering. NH4F/ethylene glycol electrolyte was used to demonstrate the growth of stable nanotubes at room temperature. TiO2 NTs as long as 4.8 lm with the high expansion factor to the initial sputtered Ti film (2 lm) were obtained, showing little undesired dissolution of the metal in the electrolyte during anodization. The average pore size and wall thickness of NTs were about 70 and 30 nm, respectively. Structural investigations on the transparent NT arrays reveal the presence of anatase phase after annealing. The NTs were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into dye sensitized solar cells (DSSCs). The conversion efficiency of 1.97 % was obtained under AM 1.5 illumination and the open circuit voltage, short circuit current density and the fill factor were 0.59 V, 6.71 mA/cm2 and 0.50, respectively. Investigation of the electron transport of the DSSCs by electrochemical impedance spectroscopy showed that the electron diffusion length (8.6 lm) was higher than NTs’ height. It was also observed that the electron transport resistance measured in & Mohammadreza Kolahdouz & Zahra Kolahdouz 1 School of Electrical and Computer Engineering, University of Tehran, Tehran, Islamic Republic of Iran 2 Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, Delft, The Netherlands 123 NT DSSC was lower compared to the reported TiO2 nanoparticle one. 1 Introduction Titanium dioxide nanotubes have proven to be a highly promising material for application in various fields, such as sensing, photocatalysis and photovoltaics [1–3]. For instance, when used as one-dimensional nanostructure photoanodes in dye-sensitized solar cells (DSSCs), they were found to show higher light harvesting and charge-collection efficiencies compared to their nanoparticle-based counterparts owing to their faster electron transport and slower charge recombination properties [4, 5]. A suitable method for fabrication of TiO2 nanotubes is anodization of metallic Ti. Commonly, high purity Ti foils are used and anodized in aqueous HF-containing solutions or non-aqueous electrolytes containing fluoride ions [6, 7]. However, especially for photovoltaic devices, TiO2 nanotubes on conducting glass are of particular interest. Structures on conducting glass substrates allow front side illumination, i.e. illumination from the substrate side, resulting in reduced loss of incident light intensity compared to cells fabricated from foils [8]. In this study, we fabricated an anodic TiO2 nanotube film on a transparent conductive oxide (TCO) layer by anodization of a sputtered Ti film in an ethylene glycol solution of ammonium fluoride (NH4F). The structure and morphology of prepared samples were characterized by X-ray diffraction and field emission scanning electron microscopy (FESEM). Finally, the photovoltaic performance of the prepared sample as the DSSC photoanode was investigated and the electron transport and J Mater Sci: Mater Electron (2016) 27:6496–6501 recombination properties of TiO2 NTs were characterized using electrochemical impedance spectroscopy (EIS). 2 Experimental details 2.1 Deposition of Ti films on FTO substrate The 2 lm titanium films were deposited on FTO coated glass (TEC-15, 15 ohm per square) using DC magnetron sputtering from a 99.99 % pure titanium target. The sputtering chamber was pumped down to the pressure of 2 9 10-3 mTorr before argon gas was introduced. Sputtering was operated in pure argon (23 sccm) and the working pressure was kept at 1.3 mTorr with the cathode power of 100 W. The substrate temperature was 300 °C, and the deposition time was about 150 min. 2.2 Preparation of TiO2 nanotube arrays on FTO substrate by anodization TiO2 nanotube arrays were fabricated via electrochemical anodization in a two-electrode cell setup using 1.5 9 1 cm2 sample of Ti sputtered FTO/glass as a working electrode and stainless steel foil as the counter electrode. Anodization was performed in the ethylene glycol based electrolyte solution containing NH4F (0.5 wt%) at 50 V. The resulting TiO2 nanotubes on the FTO substrate were rinsed with DI water and ethanol, and then dried in air. Anatase TiO2 was obtained by annealing the anodized samples at 450 °C for 30 min, in air ambient. 6497 Photovoltaic measurements were performed using simulated AM 1.5 sunlight with an output power of 100 mW/ cm2. Electrochemical impedance spectroscopy (EIS) was performed with a potentiostat/galvanostat (PGSTAT 30, Autolab, EcoChemie, the Netherlands) under illumination. The frequency range was explored from 0.01 Hz to 0.1 MHz. The applied bias voltage and ac amplitude were set at open-circuit voltage of DSSCs and 10 mV. 3 Results and discussion Titanium (Ti) films were deposited on FTO substrates using DC magnetron sputtering. The typical SEM images of Titanium films are shown in Fig. 1. The substrate temperature for the deposition of Ti film plays an important role in controlling the adhesion and morphology of TiO2 nanotube growth [9]. The sputter deposited films at 300 °C exhibit columnar structures as seen in the cross-sectional view of Ti films and surface of the Ti films is smooth and dense. The thickness of the Ti film is about 2 lm. Mor et al. [10] have suggested that Ti anodization occurs as a result of a competition between electrochemical 2.3 Dye sensitized solar cell fabrication The DSSCs were fabricated by soaking the nanotube array films in 0.4 mM N719 dye (Solaronix) in acetonitrile for 20 h. Counter electrode was prepared by deposition of Pt on FTO/glass by coating with a drop of H2PtCl6 solution (5 mM in isopropyl alcohol) with repetition of the heat treatment at 400 °C for 15 min. The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with 30 lm surlyn (Dyesol) spacer which was annealed at 120 °C for a few seconds. The cells were filled with a redox electrolyte (I-/I3-). 2.4 Sample characterization and device measurements The crystal structure and morphology of as-grown TiO2 nanorods were investigated by X-ray diffraction (XRD) on a Philips X’pert Pro (the Netherlands) using Cu-ka radiation (k = 1.54 nm), and field emission scanning electron microscopy (FESEM, Hitachi S-4160). Fig. 1 FESEM images of Ti film s (...truncated)