Multi Crystalline Silicon

Techniques for the production of multicrystalline silicon are more simple, and therefore cheaper, than those required for single crystal material. However, the material quality of multicrystalline material is lower than that of single crystalline material due to the presence of grain boundaries. Grain boundaries introduce high localized regions of recombination due to the introduction of extra defect energy levels into the band gap, thus reducing the overall minority carrier lifetime from the material. In addition, grain boundaries reduce solar cell performance by blocking carrier flows and providing shunting paths for current flow across the p-n junction.

Growth of multicrystalline silicon
Growth of a multicrystalline slab of silicon. The animation plays automatically. It is captured from an animation format that is not compatible with modern browsers.

It used to be thought that large grain crystals were the most suitable for multicrystalline silicon solar cells since larger crystals meant fewer grain boundaries. However, in recent years it was found that smaller grains gave lower stress at the ground boundaries so they were less electrically active (lower recombination). Presently, most multicystalline silicon for solar cells is grown using a process where the growth is seeded to produce smaller grains and referred to as "high performance multi"1

Slab of multicrystalline silicon after growth. The slab is further cut up into bricks and then the bricks are sliced into wafers.

To avoid significant recombination losses at grain boundaries, grain sizes on the order of at least a few millimeters are required 2. This also allows single grains to extend from front to back of the cell, providing less resistance to carrier flow and generally decreasing the length of grain boundaries per unit of cell. Such multicrystalline material is widely used for commercial solar cell production.

At the boundary between two crystal grains, the bonds are strained, degrading the electronic properties.

multicrystalline wafer

A 10 x 10 cm2 multicrystalline wafer. The wafer has been textured so that grains of different orientation show up as light and dark.