Single crystal Cu 2S/CdS photovoltaic devices with optimum performance before a post barrier air bake

Abstract

The separate stages in the preparation of Cu 2S/CdS photovoltaic devices on single crystal substrates have been investigated carefully using the techniques of reflecttion electron diffraction and scanning electron microscopy. Mechanical polishing of hexagonal single crystal CdS produces a polycrystalline surface layer with cubic structure which, for efficient cells, has to be removed by chemical etching before the topotaxial Cu 2S layer is formed. Different types of etched surfaces have been produced and found to be closely related to the polishing procedure. The nature of the etched surface has a strong influence on the cell characteristics. Devices made on surfaces with many ledges and kink sites on the sides of the etch hillocks have poor diode characteristics and photovoltaic behaviour in the as-made condition, whereas cells formed on surfaces with faceted hillocks have good rectifying and photovoltaic properties at this stage. While a post barrier air-bake is essential to improve cells which have poor characteristics in the as-prepared condition, it has a deleterious effect on devices with good characteristics in the as-made state. The highest efficiencies are obtained with as-plated cells fabricated on surfaces etched to produce hillocks with well developed facets on substrates with optimum resistivity.