THIN FILM COPPER-INDIUM-DISELENIDE/CADMIUM(ZINC)SULFUR HETEROJUNCTION SOLAR CELL: CHARACTERIZATION AND MODELING.

摘要

The results of experimental measurements and characterization of high efficiency, thin film CuInSe(,2)/Cd(Zn)S solar cells provided by Boeing are reported. These devices are produced by elemental evaporation; they have very high I(,SC) but lower than expected V(,OC) and FF. An automated, multi-purpose experimental station was developed which can carry out capacitance, current, spectral response measurements as a function of voltage, frequency, wavelength, light bias intensity and temperature. The device is modeled as a p-i-n heterojunction where the interface largely determines the device properties and transport. Long time constants of capacitance transients were observed as light or voltage bias was changed. From such transient data it is deduced that a bulk deep level hole trap exists in the selenide which has a density nearly the same as the free carrier density. It was also observed that the photocurrent is strongly bias dependent in violation of the superposition principle. It is shown, by using chopped light data of collection efficiency and the dark I-V, that the "light" diode can be fully explained on the basis of a voltage dependence of the photocurrent. The crossover of light and dark I-V curves is similarly explained. The effects of such a voltage dependent collection factor on all cell parameters are analyzed and explained. Heat treatments were carried out, and device parameters and cell performance were monitored as a function of baking. It is concluded that baking mainly improves the selenide properties rather than improving the interface. A detailed interface charging model is proposed which successfully accounts for A, I(,O) and the voltage dependence of the photocurrent. It is concluded that higher efficiencies, on the order of 13-14%, are possible if selenide conductivities can be increased, R(,S) lowered, conduction band discontinuity reduced, back contact improved and higher energy photons are utilized in I(,SC) by an electroluminescent front cover.