Synthesis and Characterization of Doped and Undoped Metal Oxide Multilayer Thin Films

Abstract

A variety of multilayer thin films, such as ZnO/TiO2 (ZT), ZnO, TiO2, 1% Cu doped TiO2 (Cu-TiO2), 2% (Cu-TiO2), ZnO/TiO2/ZnO (ZTZ) and TiO2/ZnO/TiO2 (TZT) are deposited by sol-gel technique. In this technique, colloidal particles are dispersed uniformly in a liquid form sol. A gel is a phase in which liquid and solids are dispersed together and form a solid network containing liquid component. The structure, morphology, electrical and optical properties of multilayer films are studied by X-ray Diffractrometer (XRD), Scanning Electron Microscope (SEM), four point probe technique and UV-Vis spectrophotometer, respectively. The properties of the N719 dye sensitized solar cells (DSSCs) of ZTZ and TZT based photoanodes are investigated by current- voltage measurements. XRD patterns of ZT multilayer thin films are confirmed by the presence of all the three phases of TiO2. The average grain size of 1, 3 and 5 stack layers of ZT films is 1.19, 1.22 and 2.56 nm, respectively. Multilayer ZnO films having hexagonal wurtzite structure with average grain size of 1, 3 and 5 layers is 7.96, 11.15 and 17.7 nm, respectively, according to XRD patterns. XRD patterns of TiO2 multilayer films having 1, 2, 3 and 4 stacked layers have shown the anatase phase only with average grain size of 1.06, 1.19, 7.29 and 7.69 nm respectively. Both 1% and 2% Cu doped TiO2 (3, 5 and 7) multilayer films have only the brookite phase of TiO2. The average grain size of 1% Cu doped TiO2 films having 3, 5 and 7 layers is 9.4, 12.9 and 14.4 nm respectively. Similarly, 3, 5 and 7 layers of 2% Cu doped TiO2 thin films have average grain size of 8.85, 14.1 and 14.8 nm respectively. XRD patterns of ZTZ and TZT films have shown anatase phase of TiO2 and cubic phase of ZnO. Average grain size of ZTZ and TZT films is 12.9 and 7.56 nm respectively. Morphology of the multilayer ZT films has shown cracks on the surfaces of 3 and 5 Layers films which may be due to heating and cooling rate of the film. ZnO multilayer films have uniformly distributed grains structures. Both 1% and 2% Cu doped TiO2 multilayer thin films have well defined nano particles with increasing size as the layers increases in SEM micrographs. Both ZTZ and TZT films have densed packed particles due to high viscosity of the sol. Four point probe technique has been used to measure electrical resistivity of films. In all multilayer films, average sheet resistivity is decreased by increasing the layers. The average sheet resistivity of 1, 3 and 5 layers of ZT films is 20.8 106, 9.12106 and 7.71106 ohm-m respectively. The average sheet resistivity of 1, 3 and 5 layers of ZnO thin films is found to be 7.91105, 5.65104 and 1.43104 ohm-m, respectively. For 1, 2, 3 and 4 layers of TiO2 films, the average sheet resistivity is 2.30107, 1.96107, 1.41107, 1.14107 ohm-m, respectively. The average sheet resistivity for 3, 5 and 7 layers of 1% Cu doped TiO2 films is 8×106, 6.5×106 and 1.0×106 ohm-m, respectively. The average sheet resistivity of 3, 5 and 7 layers of 2% Cu doped TiO2 films is found as “2.19 x 107, 1.20 x 107 and 1.11 x 107 ohm-m” respectively. The average sheet resistivity of ZTZ and TZT films is 9.35 x 104 and 5.44 x 105 ohm-m, respectively. Optical properties of films are measured by UV-Vis spectrometer. The band gap energy (Eg) of ZT having 1, 2 and 3 layers is 3.65 eV, 3.59 eV and 3.53 eV, respectively. The Eg of 1, 3 and 5 layers of ZnO thin films is 3.85, 3.81 and 3.72 eV, respectively. The calculated Eg for 1, 2, 3 and 4 layers of TiO2 thin films is 3.65, 3.60, 3.59 and 3.40 eV, respectively. The Eg of 3, 5 and 7 layers of 1% Cu doped TiO2 films is 3.813, 3.822 and 3.843 eV, respectively. The Eg of 3, 5 and 7 layers of 2% Cu doped TiO2 thin films is 3.778, 3.768 and 3.736 eV, respectively. The calculated Eg of ZTZ and TZT is 3.7 eV and 3.71 eV, respectively. “Due to very low sheet resistivity of ZTZ and TZT thin films as compared to other deposited multilayer films, dye sensitized solar cells of these films are prepared. After deposition, the films are annealed at 450 oC for 30 min and graphene coated FTO glasses are used as counter electrodes. “N719 ruthenium dye is used in these cells and all photovoltaic parameters like open circuit voltage (VOC), short circuit current density (JSC), conversion efficiency (η) and fill factor (FF) are calculated”. X-ray diffraction patterns have shown the presence of both ZnO and TiO2 particles in the films. Morphology of the photoanodes revealed the formation of nanoparticles in both the films; also it has shown that TZT film is more porous and consists of large particles as compared to ZTZ film. Due to this porosity, TZT film absorbed more dye as compared to ZTZ film shown in UV-Vis absorbance graph. The TZT cell has high JSC and VOC as compared to ZTZ cell, resulting high efficiency”.