Accelerated Ageing Tests Of Solar Cells And Encapsulations

Abstract

Today’s power PV module is a complete, enclosed package of solar cells, inter- connects, power leads and a transparent cover or optical concentrator, depending on the type of module. The estimated lifetime of cells used as solar converters is ∼ 25 years. Encapsulation materials must be highly transparent, resistant against thermal and UV oxidation (degradation) at low and high temperatures, resistant against humidity, have good mechanical and electrical properties (resistance), and be thermally consistent with the cell. At present, the most viable materials for industrial use seem to be ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), poly/n/butyl acrylate (P-n-BA) and aliphatic polyether urethane (PU). EVA and EMA are dry films for vacuum-bag lamination at temperatures up to 150°C. P-a-BA and PU are liquid casting systems. Both the cells themselves and the encapsulation of the photovoltaic panels undergo degradation during the device lifetime. As a particular panel degradation mechanism, electrochemical corrosion of the cell metallization leads to encapsulant deterioration, reducing the insulation resistance and especially its electrical stability [1]. The voltage between two electrified cells or between a cell and the grounded metallic frame may result in dissolution of the cell metallization into the surrounding encapsulant, which then becomes insufficiently resistive to prevent electrical breakdown. Humidity is very effective in promoting this process. Also, discoloration effects and a browning of the encapsulant can reduce the efficiency, because of reduced light transmittance [2].