Investigation of effect of doping concentration in Nb-doped TiO2 thin films for TCO applications

Nb-doped TiO2 (referred hereafter as NTO) thin films were deposited by sol-gel spin coating method to investigate their use as transparent conducting electrode (TCE). A range of NbxTi(1- x)O(2) (x = 0, 0.003, 0.005, 0.008, 0.01, 0.015, 0.02) compositions were synthesized and deposited as thin films via spin coating. The films were deposited at room temperature and showed crystallization on annealing at 550 degrees C for 1 h in air. The X-ray diffraction confirms formation of anatase TiO2 by showing dominant peak at 2 theta similar to 25.5 degrees corresponding to (101) reflection plane. Raman spectroscopy shows the characteristics modes of TiO2. Surface topography and morphology measured by atomic force microscopy and field-emission scanning electron microscopy exhibit smooth and uniform deposition of films. Optical transmittance reduces from 85 to 70% as Nb content increases from 0 to 2 at.%. Meanwhile, electrical resistivity attained a minimum value of 3.65 Omega cm for 2 at.% Nb doping. X-ray photoelectron spectroscopy analysis exhibits shifting of Ti 2p peak which confirms substitution by Nb5+ in TiO2 lattice and Nb-O-Ti bond formation. Transmission electron microscope and selected area electron diffraction patterns reveal that films consist of Nb and crystalline phase of TiO2. All films were characterized by Fourier transmission infrared spectroscopy and thermogravimetric analysis. Present study reports low-cost and effective fabrication of TCE for optoelectronic applications.