What are the technologies and their respective manufacturing process?

Depending on the technologies used to produce the PV cell, different manufacturing process is taken place. As mentioned earlier, semi-conducting materials are the fundamental elements in making up a solar cell. By different choice of semiconductors, crystalline silicon in a wafer form, thin films of other materials, and concentrated PV (CPV) are the technologies used.

1. Crystalline silicon (c-Si). Mainstream technology 85-90% market share.

There are generally two types of this semiconductor: mono-crystalline silicon and polycrystalline silicon. Polycrystalline is composed of a number of smaller silicon crystals. The multiple crystals create boundaries for electrons resulting in less efficiency comparing to mono-crystalline silicon. However, polycrystalline can be produced at a lower cost than the mono-crystalline and it is used most in the solar industry.

Manufacturing process:

Raw material: silicon

Sand is mostly made up of silica. At the first step, silica goes through carbothermic reduction process and become metallurgical grade silicon (MG-Si).The MG-Si then goes through refining and casting scratch process to become poly-silicon.

Ingot growing or Brick Casting

The poly-silicon material goes into two different production process for mono-crystalline silicon and multi-crystalline silicon production

For mono-crystalline silicon, the poly-silicon is used for ingot growing either through the Czochralski(CZ) or the Float Zone (FZ) process.

For the multi-crystalline silicon, the poly-silicon material is melted and then cast into bricks.

Slicing

The mono-crystalline silicon ingot is then slice into wafers. As for the poly-crystalline silicon, the silicon brick is first diced into bars and then sliced into wafers. Since monocrystalline silicon grows in a cylinder shape, their wafers are not in perfect squares.

Doping

For both mono and multi-crystalline Silicon, a semiconductor junction is formed by diffusing n-type or p-type onto the top surface of the silicon wafer to form the p-n junction in a solar cell. Depending on the process, it might start out with an n-type waver, followed by a p-type layer.

Wiring

Solar cells are wired together to form a circuit.

Coating

Contacts are applied to the front and rear of the cell to protect the cells and increase efficiency in absorbing light.

Assembling

With the necessary functions to generate electricity, the solar cells are assembled to form solar modules. The modules are the final solar products that can be arrange in arrays for larger output.

In addition, there is a new technology producing quasi mono semiconductors. They have similar appearance and electrical properties to the mono-crystalline silicon. The quasi-mono is produced based on poly-crystalline ingots but mono-crystal seed is used partially in the crystal growth process.

2. Thin film: thin film is an alternative technology that uses less or no silicon in the manufacturing process. The thin film PV cells are constructed by depositing extremely thin layers of the semi-conducting materials onto a low-cost backing such as glass, stainless steel or plastic. A conducting layer is then formed on the front electrical contact of the cell, and a metal layer is formed on the rear contact.

Different types materials used in thin films are amorphous silicon (a-Si), CIGS/CIS and CdTe.

l Amorphous silicon (a-Si): most common and developed. It is the non-crystalline form of silicon. The cell structure has a single sequence of p-i-n layers. When exposed to sun, their power output is significantly decreased. A-Si type thin film solar cells are commonly found in calculators. A-Si type thin film is manufactured in 6 steps. First the glass substrate is coated with a TCO (transparent conductive oxide) layer as front contact, followed by P1 laser scribing. Then a layer of a-Si is deposited followed by P2 laser scribing. Then a metal conductive layer is placed as back contact with the relative P3 laser scribing.

l CIGS/ CIS: it is the semiconductor material composed of copper, indium, selenium, and/or gallium. In thin film technology, CIGS has the highest PV conversion efficiency. CIGS/CIS has similar manufacturing process as a-Si thin films. However, as opposed to a-Si thin film, the glass substrate on CIGS/CIS is at the rear instead of the front. In addition, Cds is applied as a buffer layer.

l CdTe: it is formed from cadmium and tellurium. It is usually combined together with cadmium sulfide to form a p-n junction PV cell. the composition is similar to a-Si solar cell with an additional Cds layer for buffer. First Solar is the largest manufacturer.

3. CPV

The technology is to build the solar cells into concentrating collectors that use a lens to focus the sunlight onto the cells. As a result, less semi-conducting materials are used for solar cells decreasing material costs while collecting as much sunlight as possible. Efficiencies are in the range of 20 to 30%.