Wafer cell manufacture process

For making silicon wafer, pure silicon is needed which is obtained by reduction and purification of silicon dioxide. The reduction process is carried out in a furnace where a high temperature arc is generated between a carbon electrode and the silicon dioxide, producing molten silicon.

(a) Crystal growth

Purification is then carried out though passing the molten silicon in a heated tube. The 100% pure silicon is inserted in the Czochralski apparatus for crystal growth. This is known as the Czochralski process or the floatzone where the seeds are extracted from the melt. For the crystal growth of multi-crystalline Si, another process called the block casting is used.

(b) Dicing and slicing

A pure cylindrical silicon ingot is obtained and is sliced into wafers of mono-crystalline Si. Blocks are diced first into bricks and then sliced into wafers.

(c) Texturing

It is the cleaning of the wafers to remove damaged surface layers. This is important because it reduces reflection losses of the incident light and enhances efficiency of a solar cell. For mono-crystalline Si, etching is performed such that the surface of the wafer is covered with aligned upward-pointing pyramid structures. Incident light bounces back onto the surface and since there are reduced boundary effects, the electron flow is increased, providing more efficiency.

(d) Diffusion of dopants to form p-n junction

These are of two types: solid state diffusion and emitter diffusion. For the first one, the addition of p- and n-type impurities are added to create a p-n junction while for the other type, a thin n-type dopant is coated on the pre-doped with p-type wafer in a diffusion coating furnace to form the p-n junction.

(e) Metalisation

It is the formation of rear and front contacts.

(f) Formation of an anti-reflection (AR) SiN coating

This is performed to further reduce the reflective losses of incident light by adding a thin AR coating (Si3N4 or TiO2) [1].

Fig.1: Silicon wafer production [2]

For wafer based module manufacture, low iron glass is used to protect the cells which are encapsulated with Ethyl Vinyl Acetate (EVA). The rear layer is covered with Tedlar for additional strength to the cells. The whole structure is surrounded by metal frames and a connection box is attached at the back of the module.


References:

[1] “Solar Cell Production: from silicon wafer to cell.” [Online]. Available: https://sinovoltaics.com/solar-basics/solar-cell-production-from-silicon-wafer-to-cell/.

[2] “PPT - IC Fabrication Overview Procedure of Silicon Wafer Production PowerPoint Presentation - ID:2097489.” [Online]. Available: https://www.slideserve.com/keren/ic-fabrication-overview-procedure-of-silicon-wafer-production.