The effects of various preparation conditions and CdS microstructure on the photovoltaic properties of sintered CdS/CdTe solar cells have been investigated to obtain efficient thin film solar cells for low cost terrestrial applications. Sintered CdS films were prepared by coating a CdS slurry containing $CdCl_2$ as a sintering aid and propylene glycol on amorphous glass substrates and sintering at 600℃ for 1 hour in nitrogen. All-polycrystal CdS/CdTe films were fabricated by coating a CdTe slurry on sintered CdS films and sintering in nitrogen. A solar cell was fabricated by evaporation of indium onto the CdS and coating of carbon paint on the CdTe. Although the solar cell shows considerable amount of photocurrent and photovoltage, the results of efficiency was rather poor due to the high series resistance of solar cell. Heat treatments of the solar cell improved the cell performance by lowering the contact resistance between the carbon and CdTe films. The best photovoltaic property of the solar cell was obtained after a heat treatment at 350℃ for 5 min in nitrogen.
Sintered CdS/CdTe solar cells were fabricated by changing the sintering temperature of CdTe films from 580℃ to 700℃. Compositional interfaces and n-p junctions are formed during the sintering of the CdTe films. A maximum efficiency was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625℃ for 1 hour. Fabrication temperatures below 610℃ resulted n poor efficiencies due to the porous structure of the CdTe films and above 650℃ also resulted in poor efficiencies due to the formation of a thick $CdS_{1-x}Te_x$ layer at the interface and a large n-p junction depth.
Sintered CdS/CdTe solar cells with various thicknesses of CdS films were prepared in an attempt to optimize the thickness of the sintered CdS films whose role is to be a window as well as front contact for the CdS/CdTe solar cell. The thickness of the CdS film was varied 14 um to 55 um by changing the screen mesh-size of a screen printer and/or solid-liquid ratio of slurry which consisted of CdS powder, 9 w/o of $CdCl_2$ and propylene glycol. Average grain size of the sintered CdS films increases and porosity decreases with an increase in film thickness. Electrical resistivity of the sintered CdS films shows a minimum value in 35 um-thick CdS film. The highest optical transmittance is observed in 20 um -thick CdS film. The $CdCl_2$ remaining in thick CdS films after the sintering causes an increase in the thickness of $CdS_{1-x}Te_x$ solid solution layer at the interface between the CdS and CdTe films. The combined effects of the optical transmittance and solid solution layer resulted in a maximum efficiency in a CdS/CdTe solar cell with 20 um-thick CdS layer.
Sintered CdS films with various microstructures and electrical properties were prepared by changing sintering conditions and additional treatments. In order to investigate the effects of CdS microstructure on the cell performance, CdS/CdTe solar cells were fabricated with these CdS films. Solar efficiency increases with increasing average grain size of the sintered CdS layer via the increase in short circuit current due to the increase in optical transmittance of the CdS film and via the decrease in reverse saturation current due to the better microstructure of the heterojunction. An all-polycrystal CdS/CdTe solar cell with an efficiency of 8.12% under 72.4mW/㎠ sunlight was obtained as the results of improving the microstructural characteristics of the CdS film and optimizing the various cell fabrication conditions.
