Degradation kinetics of CIGS solar cells

The moisture-induced degradation rate of CIGS solar cells at 22-85°C and 0-100% 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.