CDS-CU(_x)S single crystal and thin film solar cells

Abstract

The work presented in this thesis is concerned with photovoltaic cells formed by plating CdS single crystals and thin films, and Cd(_y) Zn(1 _ y)S single crystals, with copper sulphide. An electroplating technique has been used to control the phase of copper sulphide by changing the electric field during its formation. Different phases of Cu(_x)S have been identified directly using Reflection High Energy Diffraction (RHEED), and indirectly from spectral response measurements. A dramatic change in the spectral response accompanying the reduction in the covellite response associated with an increase in that from chalcocite following argon heat treatment has been achieved. The change from the djurleite phase to that of chalcocite has also been obtained by using argon heat treatment for 5 minutes at 200 C. This effect was found to be reversible in that layers of chalcocite were converted to djurleite when air was used as the ambient for the heat treatment. C-V measurements have demonstrated that with increasing plating bias the donor concentration decreases at first before it assumes a constant value. This led to the effect of decreasing the junction capacitance as the width of the depletion region changed. The problem of the stability of the CdS-Cu(_2)S photovoltaic devices formed by wet plating" is addressed by studying the combined effects of the substrate onto which the CdS is deposited and the ambient used during annealing. Thin film cells have been prepared on both Ag/Cr and SnO substrates, and the device characteristics for each have been investigated as a function of annealing ambient. The results have shown that devices formed on Ag/Cr substrates were more stable following annealing in air than in argon, while the converse was true for cells fabricated on SnO(_x) substrates. The degradation effects of CdS-Cu(_2) S photovoltaic cells have been investigated. While devices stored in the dark showed little or no degradation, those maintained under illumination exhibited a significant deterioration in all operational parameters over a four week period. As far as the combined effect of temperature and ambient on the stability of cells are concerned, it was found that the ageing of devices in argon at room temperature in the dark was negligible, and moreover the fill factor was observed to improve marginally. When the devices were stored in the same ambient conditions at 50 C, they showed a significant improvement in the fill factor, but simultaneously exhibited a considerable reduction in the short circuit current. This process was reversible, since the sensitivity of degraded devices could be restored by annealing them in a hydrogen/nitrogen mixture. By comparing Electron Spectroscopy for Chemical Analysis (ESCA) studies with solar cell device characteristics, it has been shown that the formation of copper oxide on the Cu(_2)S surface plays a significant role in the degradation of CdS-Cu(_2) S devices. The extent of the cross-over between the dark and light J-V characteristics is a function of the period of etching used prior to junction formation. The variation of current and diode factor has been established as a function of the bias value. The dependence of forward current on the temperature at fixed forward voltage has also been investigated. Finally this work has shown that an increase in V(_oc) can be achieved when Cd(_0◦8)Zn(_0◦2)S is used as a base material for solar cells instead of CdS. Different traps were identified through a photocapacitance investigation. An important trap was found at 0.78eV below the conduction band. It has been demonstrated that the effect of this level was found to be diminished much more slowly when the annealing was carried out in argon rather than in air. This level may play an important role in the Cd(0◦8) Zn(0◦2)S-Cu(_2)S solar cell properties.