Structural dynamics in CdS-CdTe thin films

The performance of CdS-CdTe heterojunction solar cells depends critically u pon the structures formed during thin film deposition and any subsequent pr ocessing. We have undertaken a detailed examination of solar cell materials (in particular CdTe and CdS) which has enabled some correlation between th eir fundamental properties and structural behaviour as thin films. In parti cular we have determined the Vegard coefficients and phase diagram for the CdS-CdTe system. We have also examined the diffusion characteristics of bot h 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 w e 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 electro nic changes that occur during the type conversion anneal of CdTe. In partic ular, we have shown how an intermixed region forms during the heat treatmen t 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 b ased upon our primary analytical methods, i.e, X-ray diffraction and ion-be am analysis. These techniques, when used with the fundamental material prop erties, 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. (C) 2000 Published by Elsevier Science S.A. All rights reserved.