Effect of annealing ambient on the structural, optical and electrical properties of tio2:ge thin films for photovoltaic application.

Abstract

Titanium dioxide is a wide bandgap semiconductor widely used in optoelectronics. Despite its potential application in photovoltaics, the overall efficiency of fabricated solar cells using TiO2 is usually low due to losses through electron-hole recombination. Introduction of defects such as the use of dopants in the TiO2 structure has been used to overcome some of these limitations. However, some of the dopants used still acts as sites of recombination for the generated electron-hole pair. In this study, pure TiO2 and TiO2:Ge composite thin films of varying Ge concentration (5%, 10%,and 15%) were deposited on conducting glass substrates by radio frequency sputtering technique at ambient temperature of 23 ℃ -25 ℃ under optimum sputtering power of 200 W and argon flow rate of 35 sccm. Post annealing heat treatment was done on the films at 450 ℃ in air, argon and nitrogen atmosphere respectively, in an attempt to tune the structural, optical and electrical properties of TiO2:Ge films. The effect of annealing ambient on structural, optical and electrical properties for the films was later analyzed. The films annealed in nitrogen and argon appeared dark brown indicating an increase in oxygen vacancies while those annealed in air were utterly transparent. The X-ray diffraction patterns of both pure TiO2 and TiO2:Ge films showed that the films were composed of anatase and rutile phases irrespective of the annealing atmosphere with crystallite sizes ranging between 19-21 nm. Scanning Electron Microscope images of the films showed crack- free structures that had good adherence to the substrates, with the films annealed in nitrogen showing larger crystals compared to those annealed in air and argon. This is an indication of improved crystallinity. It was observed that, increase in Ge content in TiO2 matrix decreased both the optical properties and electrical properties. TiO2:Ge ratio of 85:15 recorded the lowest transmittance average of 70% in wavelengths 400-700 nm. The bandgap decreased from 3.64 eV to 3.57 eV while the electrical resistivity decreased from 21110−× cmΩ− to 22.2410−× cmΩ−. On the other hand, TiO2:Ge thin films annealed in nitrogen recorded the best optical and electrical properties with the bandgap and resistivity averaging about 3.55 eV and 25.2310−× cmΩ− respectively. Generally, good films were obtained at optimal condition of 10% Ge concentration in TiO2 matrix and annealed in N2. It was therefore recommended that TiO2:Ge thin films with 10% Ge concentration and annealed in nitrogen atmosphere could be considered as a potential photoanode in Dye Sensitized Solar Cells applications.