Optical and electrical characterization of CdxSe1-Xs and Cu2Sthin films for solar cell application
Mogunde, Charles Moraro
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There has been a steady increase in demand for clean and renewable energy globally. As a result, more effort has been put into research to harness solar energy from the sun to generate electricity using solar cells. Solar cells convert solar energy directly into electricity. Recent research findings indicate that properties of thin film solar cells strongly depend on the various deposition techniques chemical bath deposition (CBD) being one of them. This technique yields stable, adherent, uniform and hard films with good reproducibility by a relatively simple process. In this research, CBD method was used to deposit thin films of cadmium seleno-sulphide (CdxSe1-xS) and Copper (I) sulphide (Cu2S)on glass substrates. Solutions of copper (II) sulphate, thiourea and tartaric acid were used to deposit Cu2S thin films at constant temperature 40 ± 20C whereas solutions of cadmium nitrate, thiourea and Sodium Seleno-sulphate were used to deposit CdxSe1-xS thin films at constant temperature of 60 ± 2 0C. The optoelectronic properties of both films were investigated. Electrical properties like sheet resistivity (ρ) were investigated using a four point probe connected to a Keithley 2400 source meter interfaced with computer. The electrical resistivity of Cu2S decreased from 479 Ω/⧠ to 87 Ω/⧠ with increasing concentration of copper (II) ions. Resistivity of cadmium seleno-sulphate thin films decreased from 175.54Ω/⧠ to 130.78 Ω/⧠ with increase of selenium concentration. Optical properties like reflectance and transmittance were determined using UV-VIS NIR 3700 spectrophotometer in the range of wavelengthbetween 300-1200 nm. Transmittance of Cu2S thin films varied between 10 and 40% with wavelength of the light energy whereas that of CdSeS varied between 75 and 87%.Transmittance and reflectance were simulated by scout software from which other optical parameters like band gap (Eg), refractive index (n), extinction coefficient (k) and absorption coefficients (α) were calculated. The plots of (𝛼𝑣)2 against 𝑣 were used to determine energy gaps(Eg) of Cu2S and CdSeS thin films.Energy gaps of Cu2S varied between1.68eV to 2.64eV and that of CdxSe1-xS thin films varied from 2.64eV to 3.314eV. The copper (I) sulphide thin film with optimum properties of an active part of a solar cell was selected. The CdSeS with optimum window layer optoelectric characteristicswas selected. Optimized parameters comprised electrolyte pH, bath temperature and concentrations of cadmium and copper ions in the n and p type materials respectively.The fabricated solar cell was characterized by solar cell simulator. The solar cell’s diode characteristics like short circuit current (Isc= 0.0039A), open voltage (Voc= 0.40V), fill factor (FF=0.60) and conversion efficiency (η=0.81%) were obtained. The characteristics obtained shows that the solar cell materials used have properties that can be enhanced to fabricate commercialsolarcells.