Structural dynamics in CdS–CdTe thin films

Abstract

The performance of CdS–CdTe heterojunction solar cells depends critically upon the structures formed during thin film deposition and any subsequent processing. We have undertaken a detailed examination of solar cell materials (in particular CdTe and CdS) which has enabled some correlation between their fundamental properties and structural behaviour as thin films. In particular we have determined the Vegard coefficients and phase diagram for the CdS–CdTe system. We have also examined the diffusion characteristics of both single-crystal and polycrystalline CdTe and CdS with respect to Te and S in order to define the rate at which any intermixed region may grow. Thus we have determined several fundamental properties of CdTe and CdS which were either not available or apparently anomalous. These data have been used to underpin and interpret findings from studies of the structural and electronic changes that occur during the type conversion anneal of CdTe. In particular, we have shown how an intermixed region forms during the heat treatment and that this could be mediated by the initial, as-deposited structures. We have also been able to contrast the behaviour of CdTe films produced by PVD and electrodeposition. In order to characterise the structure of these thin films it has been essential to develop novel depth profiling methods based upon our primary analytical methods, i.e. X-ray diffraction and ion-beam analysis. These techniques, when used with the fundamental material properties, are shown to provide complementary information that has allowed us to build models of the CdTe and CdS layers that may allow the formation of the intermixed region to be controlled during the fabrication process.