Optoelectrical and stability characterization of SnxSey/ZnO:Sn solar cell prepared by resistive evaporation
Nyaga, Peter Kinyua
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Thin films for solar cell application have been found to be unstable when used in extreme weather conditions. Therefore there is a quest for the study of effects on light soaking and exposure to outdoor conditions on SnxSey/ZnO:Sn solar cell. A p-n junction thin film solar cell was prepared using SnSe as the absorber layer and ZnO:Sn as a window layer. Tin selenide thin films were prepared at different ratios using pure tin pellets and selenium powder. The glass tube containing mixed tin with selenium at a specific ratio was filled with argon gas and heated at temperature of 350oC until all the metals melted. Zinc doped with tin was prepared using tin pellets and zinc granules in a similar procedure. The resulting material was cut into ingots, respectively and used in preparing thin films by resistive evaporation. The evaporation was done using Edwards 306 AUTO coating unit at a chamber pressure of 5.0 × 10-5 mbars. The prepared thin films of tin selenide and tin doped zinc oxide were characterized for optical and electrical properties. The optical measurements were done using a Shimadzu type 3700 DUV UV-VIS-NIR spectrophotometer. The transmittance, reflectance and absorptance spectra data obtained were analyzed using the scout software. The calculated optical band gap for tin doped zinc oxide ranged between 3.72eV and 4.08eV while for tin selenide increased from 0.70eV to 1.63eV with increase in Sn:Se ratio. The calculated refractive index for tin selenide decreased from 3.57 to 2.77 with increase in selenium in the sample, while that of tin doped zinc oxide increased from 0.73 for undoped zinc oxide to 1.07 at 4% then reduced to 0.84 at 8% tin doping. The electrical measurements were carried out using a four point probe configuration at room temperature (25oC) to obtain the sheet resistivity. Measurement of sheet resistivity was done for both prepared and annealed samples. The sheet resistivity of tin selenide decreased as the ratio of tin to selenium increased reaching a minimum of 19.11Ωcm at 1:0.6 for as prepared samples and 15.13Ωcm for annealed samples. Sheet resistance for tin doped zinc oxide reduced with increase in tin doping to a minimum of 11.92Ωcm at 4% tin doping for as prepared samples and 11.89Ωcm for annealed samples. The SnSe/ZnO:Sn p-n junction solar cell was fabricated by double depositing the optimized films on a microscope glass slide. The current voltage (I-V) characteristics of the p-n junction diode was obtained using a solar simulator connected to Keithley 2400 series high voltage source meter and the results were analyzed using Lab view software. The data obtained was used to determine solar cell parameters. The parameters calculated were, Isc = 1.1304mA/cm2, Voc = 0.5847V, FF = 0.6359 with efficiency η = 0.4204%. The stability test on the solar cell was performed by subjecting the solar cell to both low and elevated temperature. The short circuit current increased from Isc = 1.1655mA/cm2 to Isc = 1.1730mA/cm2 at 298K and 423K respectively. At low temperature of -4oC the Isc = 0.9563mA/cm2. The solar cell was also subjected to uncontrolled outdoor conditions for one week. The Isc increased from 1.0764mA/cm2 after one day to 1.1039mA/cm2 in the third day. On damp environment, also Isc increased from 1.0902mA/cm2 to 1.1771mA/cm2 in the third day. The solar cell was also subjected to light soaking for 120 minutes. The Isc increased from 1.0704mA/cm2 after 15 minutes to 1.1894mA/cm2 in 120 minutes. The efficiency of the solar cell ranged between 0.4203% at 75 minutes and 0.4202% in 120 minutes. The solar cell was stable on light soaking but greatly affected by exposure to outdoor conditions. More research on the same is recommended for a treated solar cell, use of appropriate anti-reflective coating (ARC) as well as structural analysis of the thin films.