Characterization of SnxSey-ZnO:A1 P-N junction for solar cell applications
Mugah, Charles Onyango
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Thin film semiconductors of Sn.Se, and aluminum doped ZnO are potential candidates for the development of future generation improved efficiency and low cost solar cell devices. This is due to their high absorption co-efficient and good optical properties in the visible region of the solar spectrum. Optical characterization of the two thin films under varied composition and deposition conditions was done by optimizing them for an improved solar cell. Deposition of both Sn.Se, and ZnO: Al thin films on glass substrate was done by evaporation technique using An Edward Auto 306 RF/DC Magnetron deposition chamber. Synthesized specific ratios of p-material i.e. tin (Sn) and selenium (Se) metal samples were both mixed together and evaporated under these conditions: Temperature of 500K, current of 3.5A, and a pressure of 3.5x10-5 millibars. For aluminium (AI) and zinc (Zn) samples, they were similarly mixed together in specific ratios under the same conditions. However this n-material had an additional oxygen gas flow rate of 20sccmlmin let into the chamber so as to form ZnO:Al thin films. Transmittance and reflectance data in the range 300nm-2500nm was obtained using UV -VIS NIR Spectrophotometer Solid State 3700 DUV for all the samples that were prepared. A solar cell was then fabricated and characterized from the optimized thin films. The resulting optical measurements were analyzed using scout software to determine optical constants for thin film solar cell. The band gap energy of optimized SnSe thin film was found to be 1.18±0.05 eV. For the optimized 4% doped ZnO:Al thin film, the band gap energy was found to be 3.28±0.05 eV. Electrical measurements of the thin films was done by four point probe method in which measurements were obtained using Keithley 2400 Source Meter interfaced with a computer using Lab View program. The sheet resistivity at room temperature of unannealed and 150°C annealed SnSe thin films was 26.98±0.05 ncm and 18.25±0.05 nem respectively. For ZnO:Al thin films, the sheet resistivity was 24.42±0.05 nem for unannealed film and 20.38±0.05 ncm for the 150°C annealed film. XRF and XRD analysis of the thin films were done by use of MiniPal 2 spectrometer and PW 3040160 X'Pert diffractometer respectively. XRF analysis of the thin films gave their respective elemental composition and XRD analysis of the thin films showed that as-deposited SnSe thin films were amorphous in nature whereas ZnO:Al thin films were crystalline. The solar cell had an open circuit voltage (Voc) of 0.59V, short circuit current (lsc) of 1.06mA1cm2, fill factor (FF) of 0.65 and conversion efficiency of 0.41 % as calculated from current density-voltage (J- V) values obtained by the solar simulator.