Deposition of silicon nitride antireflexion layers on crystalline silicon solar cells by PECVD technology.
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Crystalline silicon solar cells c-Si.

Most solar cells which are used today are based on crystalline silicon. The silicon can be mono-crystalline or poly-crystalline. Monocrystalline material is produced by the Czrochalski-process, while polycrystalline material is usually prepared by molding. In both cases the generated material is cut to wafers by wire saws. The wafers serve then as substrate material for the solar cell.

Solarzelle

The solar cells consists mainly of silicon and is called therefore thick film solar cell, in contrary to thin film solar cells where the semiconductor layers are deposited on substrate of a different material. The bulk silicon is usually lightly p-doped, and conductive for positive charge carriers or holes. On the front side a thin heavily n-doped layer has to be formed by doping, which is conductive for negative charge carriers or electrodes. This way a p/n-junction is formed, which can separate the charge carrier pairs, generated by the absorption of sunlight. On the front side and on the back side metallic contacts have to be formed in order to drain the solar current. At the backside a holohedral aluminium layer is deposited, while at the front side silver contact fingers are generated, which allow most of the sunlight to pass into the cell. Finally a silicon nitride antireflection coating ARC is attached to the front side in order to increase absorption of the sunlight. The last production step is the assembly of the solar cells to solar modules.

Solar cell manufacturing overview

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PECVD of silicon nitride Si3N4 layers as antirefective coating.

In order to absorb as much light as possible, it is necessary to minimize light reflection. This can be achieved by coating the solar cell with an antireflective layer ACR. When light waves reflected by the upper side and the lower side of the antireflection layer are interferring, then they can be cancelled. This happens, when the thickness of the anti-reflective layer is 1/4 of the wave length. Sunlight contains a broad range of different wave lengths and the angle of incidence also varies over the day. Therefore a compromis regarding the thickness of the ACR has to be found. Adaption of the refractive index of the antireflection coating can also help to optimize the layer.
In solar technology, silicon nitride Si
3N4 is used as antireflection layer. This layer causes the dark blue color of crystalline silicon solar cells. Deposition is carried out plasma-enhanced in a PECVD system (plasma enhanced chemical vapor deposition).
PECVD technology allows a fast deposition of the silicon nitrid layer. Edge coverage is good. Usually, silane and ammonia are used as feedstock. Deposition can take place at temperatures below 400C.

3 SiH4    +     4 NH3    →    Si3N4     +    24 H2

PECVD-system for coating of solar cells with anti reflection coating ARC PECVD Nitride Reactor
Nitride Plasma Deposition System for Antireflection Layers from SNTEK

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