Age and heat stabilized photovoltaic cells

#The present invention relates to age and heat stabilized photovoltaic solar cells. More particularly, the invention relates to heterojunction photovoltaic cells having a stabilizing metal electrode.

#The preparation of thin film photovoltaic cells is well known. However, a great deal of research effort is still expended in the development and improvement of photovoltaic cells in order to provide greater cell efficiency and economic feasibility. In particular, there is a continuing need for improvement of the age and heat stability of heterojunction photovoltaic cells such as cadmium sulfide photovoltaic cells.

#Solar cells prepared from CdS as well as other solar cells prepared from compound semiconductors are known to degrade during operation. The degradation in cell efficiency is thought to occur by thermally activated diffusion. The Cu.sub.x S-CdS solar cell is particularly sensitive to degradation through thermal effects. Cu.sub.x S indicates a copper sulfide composition wherein the value of x is in the range of 1.95 to 2.0. The thermal and age degradation of this cell is thought to be caused by diffusion of copper from the Cu.sub.x S layer into the CdS layer, and to changes in the composition of the Cu.sub.x S layer.

#The nature of the improved photovoltaic cells of the present invention will be more readily understood when taken into consideration with the accompanying drawings.

#FIG. 1 shows a schematic cross-sectional view of a photovoltaic cell of the "backwall" type. In this cell the light enters through the substrate and passes through the conducting electrode and the semiconductor layer.

#FIG. 2 shows a schematic cross-sectional view of a photovoltaic cell of the "frontwall" type in which the light enters through the stabilized metal electrode grid and passes through the barrier layer and the semiconductor layer.

#The present invention applies to all solar cells which contain a diffusing metallic element in one of the semiconducting layers of the solar cell, in particular, those cells comprising successively a semiconductor electrode layer, a compound semiconductor layer, a metal-containing barrier layer and a barrier electrode. The diffusing metallic element is referred to herein as M.sub.1. In embodiments of this invention, M.sub.1 is present in both the barrier layer and the barrier electrode. However, if the tendency of M.sub.1 to diffuse from the barrier electrode into the barrier layer is approximately zero, the barrier electrode is called a "stabilizing electrode". It should be recognized that at equilibrium, although the tendency of M.sub.1 to diffuse between the stabilizing electrode and the barrier layer is zero, there are actually as many M.sub.1 atoms diffusing from the stabilizing electrode into the barrier layer as there are M.sub.1 atoms diffusing from the barrier layer into the stabilizing electrode. Therefore, by "tendency to diffuse being approximately zero" is meant approximately zero net transfer of M.sub.1 across the interface between the stabilizing electrode and the barrier layer, i.e., the net diffusion of M.sub.1 between the electrode and the barrier layer is approximately zero.

#A convenient measure of the tendency of M.sub.1 to diffuse between layers is the difference in chemical potential of M.sub.1 between those layers. In embodiments of the present invention, the difference in chemical potential of M.sub.1 between the stabilizing electrode and the barrier layer will be approximately zero at a given temperature and so will the tendency for M.sub.1 to diffuse between same. We assume the temperature, T, is the same in the barrier layer and the electrode. To within a constant, the chemical potential of species M.sub.1 in the ith layer is RT 1n a.sub.i, and in the jth layer is RT 1n a.sub.j, where a.sub.i and aj are the activities of M.sub.1 in the ith and jth layers and R is the gas constant. Consequently, we may express the condition for embodiments of the present invention as a.sub.i =a.sub.j, i.e., the activities of M.sub.1 are the same in both the stabilizing electrode and the barrier layer. It has not heretofore been realized that the critical stoichiometry of M.sub.1 in the barrier layer should be stabilized by the choice of a barrier electrode containing M.sub.1 at the same activity as the activity of M.sub.1 in the barrier layer at its critical stoichiometry, hence a stabilizing electrode.

#The activities of a given constituent of two layers, or phases, are equal at equilibrium (the activities being referred to the same standard state, S. Glasstone, Thermodynamics for Chemists, D. Van Nostrand, New York, 1947). The standard state for the metal M.sub.1 is the pure solid, and the value of its activity in the standard state is unity. There are many ways of measuring the activity of a species such as M.sub.1 in the stabilizing electrode and barrier layer, several of which are mentioned hereinbelow, i.e., thermodynamic measurements (S. Glasstone, cited, Page 363, illustrates the determination of the activity of one metal dissolved in another, e.g., a liquid alloy, by EMF measurements of the appropriate cell).