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Module Materials for
Selective Emitter PV Cells

Dr Lucie Garreau-Iles, DuPont Photovoltaic Solutions, Geneva, Switzerland, presented research findings on selective emitter technology at the 2011 PVSEC Conference. Her presentation, titled "Module Materials for Selective Emitter PV Cells, addressed how encapsulants made with modified UV absorbers and stabilizer packages can significantly enhance power output.

Abstract

Selective emitter technology offers improvements of up to 1% absolute efficiency over homogeneous emitters at the current sheet resistivity, measured on cells in air. A significant part of the gain is achieved by increasing the efficiency at the short wavelength part of the solar spectrum. Encapsulants used by the PV industry today are formulated with UV absorbers to protect the polymer from degradation.

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These absorbers filter out a significant part of the spectrum which would otherwise be available to the cell, and consequently a significant part of the gain observed at cell level is not realized in modules made from SE cells. Module makers are therefore calling for encapsulants with the same high overall transmission and durability but lower UV cutoff than the current offerings.

We have prepared encapsulants based on polymers similar to those already used by module makers but with modified UV absorber and stabilizer packages. The UV cutoffs are lower and the transmission is better in the 300-500nm range. Power gains compared to industry standard encapsulants were measured with a variety of cells under both under artificial and outdoor light, and found to be significant.

Several types of module constructions were built, aged and tested. In particular we used experimental encapsulants between the solar glass and the cell, in combination with themselves behind the cell or standard EVA. Several different types of backsheet including TPT were used. Modification of the encapsulant requires confirmation of the adhesion and durability of a number of materials and interfaces in the module construction. UV light may fall to a greater extent on the backsheet if more transmissive encapsulants are used both behind and in front of the cells. Where interfaces that have not been used before are created, the durability and compatibility of those interfaces need to be validated. The contructions with the experimental encapsulants were aged under ultraviolet, damp heat and thermal cycle aging. Parameters including the efficiency, light transmission and mechanical properties especially adhesion were measured after ageing.

The results we obtained with encapsulants specifically designed for an enhanced response in the low wavelength region increase our confidence that their adoption will allow module makers to take full advantage of the enhanced power output of selective emitter cells.

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