Investigating the Optical and Electrical Properties of Cu2O and ZnS: Al Thin Films for Solar Cell Applications
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Date
2019
Authors
Otieno, Philip Jeremia
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Journal ISSN
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Publisher
Kenyatta University
Abstract
Semiconductor materials have been used in fabrication of a variety of electronic devices for example solar cells, photo-detectors, integrated circuits (ICs), light emitting diodes (LED) among others. A lot of focus is on thin film solar cell due to the rising need for low cost and renewable sources of energy. A variety of semiconductor materials like Ge, Si, Al, GaAs, CuInSe2, have been employed in fabrication of thin film solar cells. Cu2O and aluminum doped ZnS thin films are promising materials for the development of future generation low cost and higher efficiency thin film solar cells. Cu2O thin film has a low band gap and high absorption co-efficient while ZnS:Al has good optical transmission in the visible spectral range. However, very little data in literature is available on Cu2O-ZnS:Al p-n junction. Therefore in this work, Cu2O and ZnS:Al thin films were coated on glass slide by DC reactive sputtering and evaporation methods, respectively using Edward Auto 306 evaporation system. Cu2O- ZnS:Al p-n junction was fabricated by DC reactive sputtering evaporation methods on a glass substrate. Transmittance and reflectance of both Cu2O and ZnS:Al thin films were measured using Spectrophotometer 3700. Absorption of the thin films was calculated from the transmittance and reflectance data. The transmittance data was analyzed by scout software models to obtain band gap. Transmission of Cu2O films decreased from 43% to about 30% for films with thicknesses 50 nm to 250 nm. All the films showed high absorption of above 50% within the visible region. Absorption of Cu2O films increased with thickness, with highest absorption of about 60% for thin films of thickness 200 nm. For ZnS:Al, transmission increased from 68% for undoped ZnS to 85% for Al dopant concentration of 6 at% dopant concentration. The band gap of Cu2O decreased from 2.452 eV to 2.402 eV for film thicknesses range 50 nm to 250 nm. Electrical properties of Cu2O and ZnS:Al thin films, at room temperature were measured using four point probe method whereby measurements were made using Keithley 2400. The electrical resistivity of ZnS:Al thin films decreased from 1.54 x 105 Ωcm for undoped ZnS thin film to 69 Ωcm for Al concentration of 6 % dopant concentration. Resistivity of Cu2O films increased from 7 Ωcm to 53 Ωcm with increase in thicknesses from 50 nm to 250nm. The I-V measurements were obtained using Solar Simulator. From I-V characteristics of Cu2O- ZnS:Al p-n junction, fill factor (FF) was found to be 0.629, conversion efficiency (ŋ) was 0.62%, Short-circuit current (Isc) was 1.0x10-2A and open circuit voltage (Voc) was 0.592V. ZnS:Al and Cu2O thin films are therefore suitable for making solar cells. In addition to optical and electrical characterization of Cu2O and ZnS:Al thin films, studies on the morphology of the glass used for the slides and its effects on the efficiency of the solar cell also need to be done
Description
A thesis submitted in partial fulfillment of the requirements for the award of the degree of Master of Science (Electronics and Instrumentation) in the School of Pure and Applied Sciences of Kenyatta University, November, 2019