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This article describes a method to have a better knowledge of barrier performances needed for encapsulating materials, particularly in the case of organic solar cells devices. We have developed a high sensitivity permeameter which enables simultaneous measurements of water and oxygen permeation. Various polymers and inorganic coatings on polymer substrates have been measured. Experimental barrier parameters have been plotted considering the steady and transient states of permeation curves and compared to theoretical values. In addition, we have performed ageing experiments on encapsulated organic solar cells to establish a barrier requirement directly related to the device. Finally, we have performed such experiments using different cathode materials and encapsulating materials.

Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C 61 butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm 2 illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm 2 illuminations and at high temperature (60°C).

Though being much less efficient than silicon cells, organic solar cells exhibit a unique combination of interesting properties: low cost, flexibility, and the possibility of large surface coverage. Large progresses have been made over the last years using MDMO-PPV (Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) reaching efficiencies of 2.9% and recently efficiencies over 3%, using poly(3-hexyl thiophene). A great deal of research however has still to be invested to improve the current state of the art. Among the main key-points to be addressed are namely the stability and lifetime of such devices. We are currently working on bulk heterojunction solar cells made from MDMO-PPV and PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester). Different batches of MDMO-PPV, originating from different synthesis modes (classical "Gilch" synthesis and "Sulphinyl" synthesis led by IMEC-IMOMEC) have been tested. Evolution of the power efficiency following continuous illumination (AM1.5, 80 mW.cm-2) was characterized under controlled atmosphere of nitrogen. In parallel, photodegradation studies are also investigated and electrical modeling is under way in order to get a better understanding of the relations between photochemical and electrical parameters of the diode that can be deduced from I/V curves.