Most solar cells used today are made of crystalline silicon. The silicon can be either mono‑crystalline or poly‑crystalline. Monocrystalline material is produced by a Czochralski pulling process, while polycrystalline material is usually produced by molding.
In both cases the material is sliced into wafers by wire saws, which then serves as the substrate material for the solar cells.
The solar cell essentially consists of bulk silicon and is therefore also called a thick‑film cell, as opposed to thin‑film cells where the semiconductor layers are deposited onto a substrate of a different material. Silicon is typically lightly p‑doped, i.e. conductive for positive charge carriers (holes). On the front side, a thin, heavily n‑doped layer must be created by doping so that it conducts negative charge carriers (electrons). This forms a p/n junction that separates the charge carrier pairs generated when sunlight is absorbed. Metallic contacts must then be applied on the front and back to collect the solar current. On the back side a full‑area metallization with aluminium is applied, while on the front only silver contact fingers are applied to let most of the sunlight pass through. Finally, an anti‑reflective coating of silicon nitride is applied to the front to increase light absorption. The last manufacturing step is the assembly of the solar cells into modules.
The doping of the top, strongly n‑conductive silicon layer is performed with phosphorus as the dopant. There are two methods for this that use diffusion furnaces:
• Doping from the gas phase with phosphorus oxychloride POCl3.
• Doping using a screen‑printed doping paste.
