Characterization of SnxSey /SnO2: Co p-n junction deposited by spray pyrolysis for photovoltaic application
Riungu, Geoffrey Gitonga
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Research on utilization of solar energy has been stimulated by Energy crisis in everyday usage of energy. Photovoltaic hold great hopes towards solving energy crisis since they are environment friendly, sunlight is free, abundant and not likely to be exhausted in the near future. In this study SnxSey and SnO2: Co thin films were prepared by spray pyrolysis deposition technique at deposition temperatures of 400 K and 500 K respectively. SnxSey precursor was prepared using 0.05M SnCl4:2H2O and 0.05M 1-1 dimethyl selenourea (C3H8N2Se) in a mixture of deionized water and isopropyl alcohol in the ratio of 1:3. Different SnxSey (y/x) ratios were prepared ranging from 0.4 to 1.4. Cobalt doped SnO2 precursor solution was prepared by dissolving 0.025 M of Tin IV chloride 2 hydrate (SnCl4:2H2O) and cobalt nitrate 6-hydrate (Co (NO3)2 6H2O) in a mixture of deionized water and ethanol in the ratio of 1:1.Doping was done using Co (NO3)2:6H2O from (0-10%) wt. Optical properties of the films were characterized. Transmittance and reflectance data was measured using Shimadzu Solid Spec-3700 UV-VIS-NIR spectrophotometer in the range 280-1200 nm. Scout software was used in data analysis after which optical constants were calculated. The calculated band gap (Eg) of SnSe ranged from 1.98eV to 1.38eV. The SnSe sample of ratio 1:1 had higher absorbance in visible region of up to 47.51% and relatively lower transmittance and reflectance of 44.45% and 8.04% respectively. Transmittance for SnO2: Co thin films was >80% for all films while reflectance and absorbance were relatively low. Upon cobalt doping optical band gap of SnO2: Co thin films decreased from 3.56 eV at 0% doping to 3.22 eV at 6% doping. At 4% Co doping highest transmittance in visible region of up to an average of 86.20% was obtained. Measurement of sheet resistivity was done using four point probe connected to a Keithley source meter interfaced with a computer LabVIEW program. The sheet resistivity at room temperature (25oc) for SnSe was 28.67±0.05 Ωcm and that of SnO2: Co decreased from 48.50 cm±0.05 at 0% doping to 37.53 ±0.05 Ωcm (10%) doping. A ratio of SnxSey [y/x] 1.0 and SnO2: Co (4% doping) was used to fabricate a p-n junction solar cell. The p-n junction solar cell had an open circuit voltage (Voc) of 0.62V, short circuit current (Isc) of 1.16mA, fill factor (FF) of 0.732 and conversion efficiency (η) of 0.525%. Tin Selenide and cobalt doped SnO2 are therefore candidates for solar cell devices.