A STUDY OF THIN-FILM POLYCRYSTALLINE CADMIUM-SULPHIDE - COPPER-SULPHIDE SOLAR CELLS AND COPPER-SULPHIDE THIN-FILMS

Our work was done as two distinct parts. In the first part we studied the effects of heat treatments on three Cu(,x)S thin films (1.995 (LESSTHEQ) x (LESSTHEQ) 2). Our results suggest that initial heat treatments cause copper in grain boundaries to diffuse irreversibly into the Cu(,x)S crystallites. Subsequent heating in hydrogen causes a reduction in surface oxides while the reverse process occurs in an oxygen atmosphere. At a given elevated temperature (usually 150(DEGREES)C), the resistivity ((rho)) and charge density ((RHO)) vary with time according to the expressions (RHO) = (RHO)(,o)e('(+OR-)SQRT.(t/(tau)) and (rho) = (rho)(,o)e('(+OR-)SQRT.(t/(tau)). On the other hand, the mobility is found to be approximately constant at a given temperature.;The second part of our work was concerned with studies of the capacitance of completed CdS/Cu(,x)S solar cells. Measurements of the dark capacitance of CdS cells as a function of the frequency of the applied signal voltage have shown that the capacitance varies with frequency according to the relation.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;where (tau) is the time constant associated with interfacial and deep bulk states. Photocapacitance studies show that the CdS cell can be treated according to the frequency dependent model of Schibli and Milnes. Under the influence of light C (TURN) 1/SQRT.((omega). Further work demonstrates that the simple planar junction model most often used to describe the CdS cell is accurate at high frequencies. Also, we studied A.C. capacitance measurements taken as a function of reverse bias voltage and signal frequency on four thin-film CdS/Cu(,x)S solar cells and one single crystal cell. Our results suggest the presence of at least two trapping states in the i-layer formed by the presence of Cu impurities in CdS. The deep traps produce anomalies in the normally linear 1/C('2) versus V plots. The energies of the states were found by measuring the particular bias voltages at which anomalies were observed. Other results concerning the anomalous behavior in CdS cells and their relation to the work of Roberts and Crowell are also discussed. These include variations of capacitance with frequency and temperature, and changes in shunt conductance with frequency.;Changes in Cu(,x)S stoichiometry were correlated with the sheet resistance of the Cu(,x)S layer in completed cells. Results indicate that heat treatment in a hydrogen atmosphere causes an increase in resistivity (corresponding to an increase in stoichiometry) while oxygen causes the reverse. The effects of hydrogen heat treatments are reported on CdS/Cu(,x)S polycrystalline thin-film solar cells. The short-circuit current (I(,sc)) of the cells changes in an exponential manner such that I(,sc) = I(,sc)(t=0)e('SQRT.(t/(tau)). The principle physical mechanism responsible for this change appears to be copper diffusion through the Cu(,x)S layer as copper oxides at the surface are reduced or formed. We were able to maximize the cell efficiency with an error of less than 5 per cent by monitoring changes in cell parameters during heat treatments.;*All degree requirements completed in 1980, but degree will be granted in 1981.