Life prediction for CIGS solar modules part 2: degradation kinetics, accelerated testing, and encapsulant effects

Copper indium gallium di-selenide (CIGS) solar cells from Global Solar Energy, Inc. and development cells from another manufacturer are laminated in non-barrier packaging and exposed to a variety of temperature and humidity conditions, and measured degradation rates fit to a kinetic expression. The kinetic constants are then used in a life model that is based on time-dependent mass and energy balances governing module temperature and the diffusion of water through a module package subjected to weather conditions. Cells with an aluminum-doped zinc oxide window layer degrade approximately 25× faster than cells having an indium tin oxide window layer, and so require much lower permeability barrier films in a flexible package. The electrically conducting adhesive used to connect bottom and top contact to solder-coated ribbons can be a major factor in degradation. This difference may only be apparent at lower temperatures, which drastically affects real-world lifetime. Preliminary Florida and Arizona data confirm this. The key assumption in the model is that the instantaneous rate of degradation is proportional to the relative saturation of the encapsulant with water (0–100%), and not the absolute concentration of water in the encapsulant (10−3–10−2 g/cm3). This leads to the model prediction that it is only the diffusion time constant tc = LESE/WVTR that determines the degradation and life of a packaged cell and that thicker encapsulants with higher water solubility will significantly extend cell life. Experimental data for three different encapsulant materials with and without a moisture barrier film confirm these predictions. Copyright © 2011 John Wiley & Sons, Ltd.