FORMATION AND DEGRADATION OF COPPER(2-X)SULFIDE / CADMIUM SULFIDE SINGLE CRYSTAL HETEROJUNCTIONS: A HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE STUDY (INTERFACE)

The Cu(,2-x)S/CdS photovoltaic device is the most thoroughly studied and developed polycrystalline thin-film solar cell. However, many of the fundamental relationships between processing, microstructure, and device performance are essentially unknown. Further improvement of this device will require an understanding of the effects of surface texture, heat treatments, and long term exposure on device efficiency and stability. Critical to understanding these effects is the determination of the Cu(,2-x)S/CdS interface morphology and the phase distribution in the Cu(,2-x)S absorber.

In this study electron-optical methods are applied to the structural characterization of single-crystal Cu(,2-x)S/CdS heterojunctions prepared by the aqueous ion-exchange process. Results obtained by scanning electron microscopy, transmission electron microscopy and diffraction, and cross-sectional high-resolution transmission electron microscopy (XHRTEM) are combined to develop a detailed description of the structural evolution of the Cu(,2-x)S/CdS heterojunction.

The XHRTEM images reveal the two-phase nature of the Cu(,2)S absorber. The tetragonal phase, a high-pressure polymorph of low chalcocite (Cu(,2)S), is found to form the metallurgical junction with CdS if the local surface orientation is nearly basal. However, only low chalcocite is detected in the absorber layer if the surface is steeply inclined to the basal plane. These results are rationalized on the basis of lattice misfit, structural compatibility, and the nucleation of h.c.p.-to-f.c.c. transformation dislocations. Lattice misfit considerations are also found to be useful in explaining the morphology of degraded heterojunctions. In particular, the observed shape and orientation of djurleite (Cu(,1.97)S) domains in low chalcocite are consistent with the minimization of lattice mismatch. The implications of these results for the reproducible fabrication of high efficiency Cu(,2-x)S/CdS solar cells are discussed.