This program has involved investigations of the stability of CdTe and copper-indium-gallium-diselenide (CIGS) solar cells under damp heat conditions and effects of barrier coatings. CdTe and CIGS cells were subjected to environmental conditions characterized by 60 degrees C and 90% relative humidity (60/90), and to some extent 85 degrees C and 85% relative humidity (85/85). Barrier coating technology developed at PNNL for OLEDs was used to investigate approaches to encapsulation of the two types of thin film solar cells. Moisture barriers consisting of multi-layer coatings involving alternating layers of polymer and aluminum oxide were deposited directly onto the front surface of CIGS cells and the rear surface of CdTe devices, with the front surface defined by the light receiving side of the cell. Most of the studies were conducted with directly deposited barrier coatings.