| The main goals of this thesis were to compare Cu(,2)S/CdS and Cu(,2)S/ZnCdS cells using Cu(,2)S/CdS cells as a reference, and to understand the operation and properties of Cu(,2)S/ZnCdS cells in order to improve cell performance. Four different measurements were used: electrical, spectral, capacitance and deep trap.;Spectral response with and without bias light were measured for both Cu(,2)S/CdS and Cu(,2)S/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/C('2) 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 10('16) cm('-3) 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 Cu(,2)S/ZnCdS is attributed to the filling of the interface traps near the junction.;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 the above results, several differences between the Cu(,2)S/CdS and the Cu(,2)S/ZnCdS cells can be seen. The Cu(,2)S/ZnCdS cells show stronger red quenching, smaller electron lifetime at the interface near the junction, and deeper traps than the Cu(,2)S/CdS cells. These differences can account for the decline of I(,sc) and the V(,oc) decay. The smaller I(,sc) for the Cu(,2)S/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(,oc) decay for the Cu(,2)S/ZnCdS cells is mostly due to long decay of charge. Longer decay could be attributed to deeper traps. (Abstract shortened with permission of author.). |
