Synthesis and characterization of metal based Chalcogenide thin films for solar cell application

Abstract

To reduce the cost of expensive solar cells, new, cheap, nontoxic and more abundant new materials have been proposed as suitable to use as absorber/window layers in solar cells. One such new material as absorber layer is tin antimony sulphide. We have attempted two methods for the growth of these films. A two stage process involving combinatorial sputtering of metallic targets tin and antimony followed by sulphurization by heating libraries in the presence of elemental sulphur in vacuum thermal evaporator. The sulphurized films were annealed for 1 hour in sealed quartz ampoule containing argon gas at low pressure (̴ 1 atm) at 425°C, 450°C, 475°C, 500°C and 525°C in tube furnace The 2nd method of combinatorial tin antimony sulphide thin films deposition is vacuum thermal evaporator. Thin films of Sn-Sb-S were synthesized on soda lime glass substrate from SnS and Sb2S3 binary compounds. SnS and Sb2S3 were evaporated in vacuum chamber at 10-4 torr without substrate heating simultaneously. The films were annealed in argon atmosphere at 85 °C, 105°C, 150°C, 275°C and 325°C inside glass ampoules. The elemental composition of the films was characterized by EDX and the XRD analysis was done for crystallographic phase’s confirmation. The XRD pattern of combinatorial tin antimony sulphide thin films shows that the as deposited films are amorphous while the low annealed temperature thin films are poly crystalline. The optical properties and thickness of the films were measured by ellipsometry techniques. Electrical properties were calculated from photoconductivity and hot point probe measurement. The photoconductivity of the library was calculated by photoconductivity spectrometer while hot point probe was used for the type of conductivity. It was found that Sb2Sn5S9 is a good candidate for photovoltaic application with a band gap of 1.15-2.5eV, absorption coefficient above 105cm-1, transmittance above 700nm and whose conductivity changing from n-type to p-type at high annealing temperature (325°C). The effect of air annealing of tin antimony sulphide was also studied in the current study. Tin antimony sulphide (SnSb2S4) thin films were deposited on glass substrate and annealed thermally at 150°C, 200°C and 300°C. The 300°C annealed films have good photoconductivity response and low transmittance. The band gap calculated by ellipsometry technique was found in the range of 2.65eV-1.45eV. The absorption coefficient of the films is ~105cm-1 while the refractive index and other optical properties of the library presented have good results. The influence of temperature dependence on Cadmium Sulfide deposited on corning 7059 glass substrate (1500C to 3000C) was also studied in this study. Transmittance, absorbance, band gap and reflectance were obtained by UV spectroscopy. The transmittance for 300nm to 1100nm thickness was grater then 80%. The resistivity and mobility was calculated by Vander Pauw method which were 10-80 Ωcm and 2-60 cm2V-1S-1 respectively. The thermoelectric properties of the film were measured by hot and cold probe method which shows the n-type nature of the film