Cu₂S/CdS solar cells have been studied extensively for the past two decades due to their potentially high efficiencies per unit cost. The operation and characteristics of Cu₂S/CdS solar cells are fairly well understood. However, the properties of the newer Cu₂S/ZnCdS cell type are not well understood.
The main goals of this thesis were to compare Cu₂S/CdS and Cu₂S/ZnCdS cells using Cu₂S/CdS cells as a reference, and to understand the operation and properties of Cu₂S/ZnCdS cells in order to improve cell performance. Four different measurements were used in this research to achieve these goals. They were; electrical, spectral, capacitance and deep trap measurements.
I-V measurements give important electrical parameters of the cells; cell efficiency, fill factor, short circuit current, open circuit voltage, shunt resistance and series resistance are reported. From a In(I<sub>SC</sub>) versus V<sub>OC</sub> measurement, the diode factor, A, was found to be about 1 for Cu₂S/CdS, Cu₂S/Zn<sub>0.11</sub>Cd<sub>0.89</sub>S, and about 1.2 for Cu₂S/Zn<sub>0.25</sub>Cd<sub>0.75</sub>S cells. The relation between In(J<sub>oo</sub>) (current density) and ϕ (potential barrier height) is linear for both types of cells. The slope of this linear relationship increases as the content of Zn increases in Zn<sub>x</sub>Cd<sub>1-x</sub>S. Under air mass 1 (100 mW/cm²) illumination, it was found that V<sub>OC</sub> decays and capacitance increases for Cu₂S/ZnCdS cells. This is attributed to electron relaxation from deep traps near the junction.
Spectral response with and without bias light were measured for both Cu₂S/CdS and Cu₂S/ZnCdS cells. White and blue bias light enhance the spectral response, while red bias light quenches the response. This is attributed to ionization and filling of deep traps near the junction.
Capacitance measurements on both cell types show that 1/C² versus voltage is quite flat, which indicates the existence of an i-layer (insulation layer) in the CdS or ZnCdS near the junction.
Three methods–photocapacitance, space-charge-limited current, and thermally stimulated. current techniques–were used for deep trap measurements. Photocapacitance measurements indicate one deep donor energy and two deep acceptor energy levels. These trap energies become larger as the content of Zn in ZnCdS increases. Space-charge-limited current measurements give a trap density of the order of 10¹⁶ cm³ for both cell types. The shallow energy trap is found to be 0.26 eV below the conduction band edge of CdS. The occurrence of a current-saturated region for Cu₂S/ZnCdS is attributed to the filling of the interface traps near the junction. Thermally stimulated current measurements give two energy levels below the conduction band of CdS; 0.05 eV and 0.26 eV.
From the above results, several differences between the Cu₂S/CdS and the Cu₂S/ZnCdS cells can be seen. The Cu₂S/ZnCdS cells show stronger red quenching, smaller electron lifetime at the interface near the junction, and deeper traps than the Cu₂S/CdS cells. These differences can account for the decline of I<sub>SC</sub> and the V<sub>OC</sub> decay. The smaller I<sub>SC</sub> for the Cu₂S/ZnCdS cells can also possibly result from smaller electron lifetime at the interface, larger interface recombination velocity, different deep trap levels, and enhanced Zn concentration near the junction. The V<sub>OC</sub> decay for the Cu₂S/ZnCdS cells is mostly due to long decay of charge. Longer decay could be attributed to deeper traps. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/54740 |
| Date | January 1985 |
| Creators | Chang, Shang-wen |
| Contributors | Electrical Engineering, Burton, Larry C., Bostian, Charles, Gilmer, Thomas E., Lee, Fred, Chen, D.D. |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xiii, 138 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 13174994 |
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