How Solar Cells are Made

Solar cells are composed of various semiconductor materials. Semiconductor materials become electronically conductive when supplied with light or heat, but which operate as insulators at low temperatures. The process of making solar cells greatly varies depending on the base material and its technology. The most important material has been silicon, which is the second most abundant element in the earth's crust in the form of silica (quartz). It dominates the present world market, particularly in its crystalline form. There are a host of other solar cell technologies like Amorphous Silicon, Cadmium Telluride, Copper Indium Diselenide and Organic Cells, besides Gallium Arsenide and Indium Phosphide.

Crystalline Silicon Solar Cell

The most commercially successful technologies are based on Single (Mono) or Poly (Multi) Crystal, and they imply screen print processes for metallization and are used at all Gaia Solar plants.

The life cycle of Solar Cells start with silica (quartz). Silica is Silicon Dioxide, which is reduced in a blast furnace where it is called metallurgical grade, and than further purified to obtain high purity silicon equivalent to electronic grade, which is used as "Feedstock" for making the silicon wafer. These feedstocks are melted in a crucible and either pulled/grown as a cylinder (single crystal) or directionally solidified (poly crystal).

During the melt process a small quantity of Boron is mixed to make the silicon p-type. They are called ingots or bricks, which is shaped /cut to more appropriate geometries. After shaping, ingots or bricks are sawn into thin slices called wafers by inner diameter blade sawing or continuous wire cutting.

Etching and Texturing

Wafers are cleaned with industrial soaps and then etched using hot sodium hydroxide to remove saw damage. Monocrystalline wafers are further etched in a hot solution of sodium hydroxide and isopropanol to form square-based pyramids also called texture. The texturization helps reduce the reflection of sunlight. The same process doesn't work as well in the case of polycrystalline wafer.

Diffusion and Edge Isolation

Since the wafers are pre-doped with boron (p-type), an n-type material is diffused into the wafer, to achieve n-p junction, Phosphorous is the usual diffusant and is achieved by subjecting the wafers at high temperature to a phosphorous source like phosphoric acid, phosphorus oxychloride, etc... Phosphorous diffuses not only into the desired wafer surface but also into the side and the opposite surface, to some extent. This gives a shunting path between the cell front and rear. Removal of the path around the wafer edge, "edge junction isolation" is performed by "coin stacking" the cells and exposing them to a plasma etching chamber to remove exposed edges.

Anti-Reflection Coating

To further reduce the surface reflection, an anti-reflection coating (ARC) is done on the surface like silicon nitride, titanium oxide, etc... One of the best ARC is silicon nitride deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The process not only deposits a layer of ARC but also improves the electronic properties of the silicon by injecting hydrogen and provides the passivation, useful to improve silicon quality.

Metallization

Silver is the most widely used metal for contact formation due to its solderability. Silver in the form of a paste is screen printed onto the front and the rear. In Addition, aluminum paste is also used onto the rear to achieve Back Surface Field (BSF), which improves the performance of the solar cell. These metal pastes are subsequently heated above the alloying temperature to form a good ohmic contact.

A Simplified Explanation Of How The Solar Cell Works

Essentially, the photons carrying the sun's light energy are absorbed in the cell; some of the photons have enough energy to separate electrons from a silicon atom in the cell. That liberated electrons exits the cell as energy, do some useful work (like light a room) and returns to re-join the solar cell and complete the circuit. The challenge is to maximize that process and generate a lot of electricity.

Under a typical sunny day, one square meter surface of solar cells exposed to the sun around noontime will receive approximately 1,000 W. Gaia Solar's multi/mono silicon cells convert roughly 14%/15% of this into electricity; therefore one square meter of multi cells will generate about 150 electric Watts in full sunshine.