Characterization of cdxse1-xs/pbs thin films deposited by chemical bath deposition for p-n junction solar cell application
Njeru, Ephantus Nyaga
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The CdS and PbS are of great research interest due to their outstanding electronics and optical properties such as winder energy band gap phase and good antireflective properties respectively. CdS has been earlier doped by Zn and used as a window layer combined with PbS as the absorber material using the CBD method. It realised an efficiency of 0.9. It is for this reason in my research, I doped CdS with Se in the fabrication of CdxSe1-xS/PbS PN junction for solar cell application to see whether the efficiency could be enhanced still employing the CBD method. The chemicals used in preparation of CdxSe1-xS were; Solutions of 0.004M Cadmium nitrate, 0.008M Ammonium nitrate, and 0.008M Thiourea. Doping of the CdS films with Se was done using varying volumes of uniform concentration of Sodium Selenosulphate (Na2SeSO3) solutions by volume at a constant deposition temperature of 80 ± 20C. The PbS films were prepared from an alkaline bath using aqueous solutions of lead nitrate solution (Pb(NO3)2.3H2O) and Thiourea (CS(NH2)2) which acted as a source of Pb2+ and S2- ions, respectively. The Tri-ethanolamine solution was used as a complexing agent during the deposition process. The CdxSe1-xS thin film deposition was done at a temperature of 80oC ± 2oC for 25 minutes while PbS deposition was done at room temperature of 27oC ± 2oC, both at normal atmospheric pressure utilizing aqueous conditions for approximately 120 minutes. In the deposition of both window and absorber layers, chemical bath deposition method (CBD) was employed. The precursor solution pH level was maintained at 9 using pH meter. Optical optimization of the thin films was done using DUV UV-VIS-NIR spectrophotometer 3700. Electrical characterization was done using four point probe connected to a Keithley2400 source meter interfaced with computer respectively for both thin films. The Cd0.5Se0.5S film was selected as the best candidate for cell fabrication with Eg of 2.83 eV and transmittance of 89.7%, low absorbance of 1.77% and Resistivity of 1.97×104 Ω-cm in the VIS region was noted. The PbS from a 0.5M concentration was selected as the best candidate with a transmittance of 26.8%, Absorbance of 38.79% and band gap of 1.43eV. The Cd0.5Se0.5S/PbS fabricated cell had the following cell parameters: Open voltage (VOC) of 0.36V Short circuit current (Is) of 0.031A, Fill factor (FF) of 0.65 and cell efficiency of 1.15%. In conclusion, the Cd0.5Se0.5S/PbS P-N junction is appropriate for photovoltaic applications and especially in the VIS and IR region of the electromagnetic spectrum.