A study of thick films of indium-tin-oxide (ITO) and the feasibility of using ITO for fabricating photovoltaic cells
Thin films of In203-Sn02 transparent semiconducting oxides (ITO) have been prepared by conventional thick-film method normally employed in microelectronics technology. When fired at approximately 650 C in a continuous air flow, a continuous thick-film of the semiconducting oxides (about 3000~ thick) is obtained. The sheet resistance of the as-deposited film is about 6kQ/sq. which can be reduced to about 300 Q/sq. upon heat o -3 treatment at 300 C in a vacuum of 10 -3 torr. A slight increase of the sheet resistance to about 1 kQ/sq. occurs on ageing at ambient atmospheres for a few days. It is thought that the origin of the conductivity in the as-deposited films is due to the presence of defects resulting from the non-stoichimetric composition of the material - i.e. oxygen vacancies. and/or intestitial tin ions. The remarkable increase in conductivity when the sample is heated in a vacuum is a result of an increase in the amount of native defects from shallow donor levels in the tin-oxide. At ambient atmosphere, the film loses tin due to gradual oxidation process which again leads to an increase in resistivity. The Hall effects measurements shows that changes due to annealing are caused primarily by change in mobility, and a slight change in carrier concentration. Vacuum annealing also eliminates the effects of annealing in air. This rules out any structural changes in the film due to annealing Nature of the annealing characteristics shows that the presence of oxygen is the cause of the changes in electrical properties. Structural studies show that the films are polycrystalline with crystal sizes of 100-200 R. There is no obvious change in crystal sizes due to 11 annealing process. Electron diffraction studies also show no obvious change due to the annealing process. This, together with the vacuum annealing and mobility studies might suggest that the conductivity in the films is due to non-stoichiometric effects. Auger electron spectroscopy (which allows accurate compositional analysis) and in-depth profiling of the elements in the film was carried out. The studies show that there is a slight increase in the In/O and Sn/O ratio for the annealed films compared to the as-deposited films. All the samples show light transmissivity in the visible region of the spectrum. The fundamental absorption edge appears near 3000R which corresponds to an optical band-gap of ~4.0eV. The fundamental optical absorption edge shifts slightly towards the lower wavelength for the more conductive samples. This shift is thought to be due to Burstein shift. There is no remarkable absorption observed up to about 2pm for the unfired and unannealed films. For the annealed films however, there is an increase in absorption as the wavelength increases, possibly due to free carrier absorption. Although these results do not indicate conductivity and mobility as high as that obtained by using thin-film techniques, a feasibility study has been undertaken to fabricate heterojunction solar cells (HJSC's) of ITO-SiO -Si (single crystals). x In this structure, the ITO thick film acts not only as a conducting surface layer that induces the SIS junction but also acts as an antireflection coating. The experimental results on the working cells have shown a V = 400 mV, and J = 0.5 mA/cm2 , cc Sc and efficiency of 2 ~ 0.2% under a total insulation of = 800 W/m. The dark and illuminated I-V characteristics have been compared with published SIS solar cell data and attempts have been made to explain the mechanism of the cells.