Organic photovoltaic solar cells and lithium microbatteries are easily degraded in atmosphere. Indeed, these devices require encapsulation with a barrier material, which exhibits extremely low permeation rates for water vapor and oxygen. These transmission rates are in the range of 10-3 cm3/m2/day for oxygen and 10-6 g/m2/day for water vapor.
To obtain such a high barrier, we chose to direct our studies on layers deposited by low frequency plasma-enhanced chemical vapor deposition (PECVD) at room temperature, and more particularly on amorphous materials like silica (SiOx).
Barrier properties were compared using the "Lithium Test". A study was made to correlate oxidation of Li protected by an encapsulation to the permeation rates of it.
This test enabled to demonstrate that a single layer is not enough a good barrier. So we studied a multilayer composed of silica layers separated by a plasma treatment, which creates an interface. Indeed, tests showed that a multilayer, deposited in the same conditions but without plasma treatment, had the same barrier properties that a single layer. Different gases were compared to find the best treatment.
Then different number and thickness of layer were studied and the importance of interfaces was revealed. Indeed, the multiplication of layers, and thus of interfaces, increases barrier properties.
A comparison between a single layer and a multilayer of silica, having the same total thickness, shows an increase of 33 for lifetime. The multilayer has a lifetime higher than 800hrs in climatic chamber (85°C and 85%RH).
Morphological analyses (AFM, MEB-FEG), density measurements and chemical composition in depth have been made to explain the effect of plasma treatment and the improvement of results.
|