Holly Ann Blaydes

Degradation kinetics of CIGS solar cells

The moisture-induced degradation rate of CIGS solar cells at 22–85oC and 0#x2013;100% RH has been measured and fit to a kinetic rate expression. This expression is coupled to a model of moisture diffusion into a package and typical meteorological input data to create a cumulative damage model to predict lifetime of packaged cells. Analytic solutions are also derived. In the diffusion-controlled regime, the life depends on the square root of package permeability and cell degradation rate. Early results indicate that a barrier of ≪10−4 g/m2/day or better will be required to assure 20 year lifetime in Miami for the Global Solar cells used here. Other less stable cells will require 10–100X better moisture barrier. Estimated acceleration factors for damp heat (85C/85%RH) vs. Miami range from 15X to 50X, depending on the package, since diffusion through the package is accelerated differently than the cell degradation kinetics. These low accelerations are a direct result of the square root dependence in this diffusion-controlled process.

On the development of CdS properties upon processing CdTe devices

CdS plays an important role in CdTe devices through both the thickness impact on Jsc and the less well understood effect on FF and Voc by impacting the minority carrier lifetime in the CdTe near the interface. In this study, two techniques are explored, chemical bath deposition and radio frequency sputtering, effectively exploring the role of film density. A multitude of characterization techniques have been exploited on CdS films after various stages in the process of device fabrication as the CdS properties change significantly upon processing of the films. For this purpose we compared the properties for as-deposited films, films that are exposed to a CdTe deposition heating profile without the actual CdTe deposition, as well as CdS films as they appear within completed devices. Especially the CdCl2 process turned out to result in huge changes within the CdS film. From this study it is concluded that significant changes occur to the CdS film depending on its starting density. Change in electrical properties and intermixing occur during the CdTe deposition and are thus temperature related, while a change in optical properties and carrier lifetime near the interface is established after CdCl2.