Perfect silicon crystals have a diamond structure where the atoms are surrounded by four nearest neighbors in a periodic arrangement of all the atoms. In real crystals, obtained by directional solidification, large deviation from the crystal symmetry occurs; these are called defects. There are many kinds of defects, including: Point defects and precipitates, grain boundaries and dislocations. Dislocations are important features for solar cell performance and will be treated here.
Dislocation Structures
Dislocations are responsible for the plastic behaviour of crystalline materials, but they may also significantly influence the electrical properties, especially of semiconductors. Two main classes of dislocations exist; edge and screw dislocations. The Burgers vector of an edge dislocation is normal to the line of the dislocation, and for a screw dislocation it is parallel to the line of the dislocation. Most dislocations are, however, a mixture of the two.
A dislocation line cannot end within an otherwise perfect region of a crystal, but must terminate at a free surface, another dislocation line, a grain boundary, or some other defect. A dislocation generated from a source within a crystal with diamond structure usually assumes a hexagonal shape if the dislocation is isolated from other dislocations. The loop consists of segments of 60° and screw dislocations. The hexagonal shape is also presumed in crystals with relatively high impurity content.
Geometrically, dangling bonds, or unpaired electrons, are aligned along the dislocation core. It is, however, commonly accepted that such geometrically dangling bonds are reconstructed to make bonds between two atoms neighbouring each other along the dislocation line.
Dislocation Mobility
The velocity of dislocation motion under a given stress depends very sensitively on the temperature, but it is not so sensitive to stress under a given temperature. The dislocation velocity in any semiconductor under a given stress increases as the temperature increases. There are two basic types of dislocation movement: Glide and climb. A dislocation able to move by glide is called glissile, while a dislocation which is not is called sessile.
Impurities and dislocations in Silicon
Silicon used for solar cells is very pure (For information on purification of Silicon: Silicon Production and Purification, Alternative Refining Methods to Obtain Solar Grade Silicon), but impurities are present and they are attracted to dislocations. The impurity mobility is higher on dislocations, and pipe diffusion will lead to clusters of impurity atoms and often precipitation. Foreign atoms may influence the dislocation mobility, and precipitates may pin the dislocations.
Solar Cell Performance
Localized regions with high dislocation densities are known to be rather detrimental for solar cell performance. Sopori et al. (2005) claim an efficiency loss due to defect clusters in solar cells of more than 3-4 absolute percent points.
It is therefore crucial for a solar cell producer to understand the behaviour of dislocations during the production of silicon for solar cells.
References:
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Birgit Ryningen - Birgit (born 01.11.77) is a judo player with one Nordic and several Norwegian Champion titles. She holds a PhD in material science and ...
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