Nb-doped TiO 2 Thin Films Prepared through TiCl 4 Treatment for Improvement of Their Carrier Transport Property
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.425
Nb-doped TiO2 Thin Films Prepared through TiCl4 Treatment for Improvement of Their Carrier Transport Property Takaki Kimura, Kan Hachiya and Takashi Sagawa* Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, JAPAN
ABSTRACT
Nb-doped TiO2 thin-films were prepared on fluorine-doped tin oxide (FTO) coated glass directly with niobium ethoxide and TiCl4 in water under the acidic conditions with several concentrations of HCl at 70-90 °C for 45 minutes or 1 hour followed by rinsing with water and annealing at 100 °C for 1 hour. Thin films of 0-1% Nb-doped TiO2 with rutile phase on FTO were obtained, which were confirmed through X-ray diffraction analyses and measurements of energy dispersive X-ray spectroscopy (EDS). Scanning electron microscopy observations equipped with EDS revealed that higher growth temperature over 90 °C is required for doping of Nb. While higher concentration of HCl resulted in much amount of Nbdoping. Band gap of rutile TiO2 gradually reduced from 3.3 eV to 3.23 eV through Nb-doping from 0% to 1%, which were estimated from uv-vis absorption spectroscopic analyses. Hall effect measurements by taking van der Pauw method confirmed that 2.26 times increase of the carrier density and 1.78 times enhancement of the conductivity have been achieved in the case of 1% Nb-doping.
INTRODUCTION TiO2 has been intensely studied as an electron transport layer (ETL) for photovoltaic devices, such as for dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs), because of its low cost, nontoxic, high chemical stability and high refractive index [1,2]. Sol-gel method is one of the most common ways to obtain high quality of TiO2 ETL. However, sol-gel method requires relatively high annealing temperature (t 500 °C) and it is disadvantageous for low-cost manufacturing and flexible devices fabrication [3,4]. As a low temperature approach ( 100 °C), TiCl4 treatment has been reported, and the perovskite solar cells based on rutile TiO2 layer exhibited 12.62% 2665
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power conversion efficiency [5]. Furthermore, TiCl4 treatment has been applied for preparation of various kinds of ETL itself in addition to the surface modification of ETL, such as compact TiO2 layer, TiO2 nanoparticles, TiO2 nanorod array and ZnO [6-9]. It has been reported that TiCl4 treatment fills surface cracks and uncovered area of FTO and restricts a thermal instability of perovskite on ZnO [6,7,9]. To enhance the electron transport property, modification of TiO 2 has been widely studied [10]. Ion doping is the one method to improve the electronic properties. Niobium ion is expected as dopant due to similar ionic radius (64 pm) to that of Ti (60.5 pm) and improved electron conductivi
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