Surface passivation of solar cells in carbon dioxide filled plasma chamber helps to increase the efficiency

Using surface passivation of solar cells reduces the recombination of the current carriers.

The ability of semiconductors to show electrical conductivity under certain physical and chemical conditions has proved its importance in the past years. Using silicon as the main component of the solar cell is no new thing.

This silicon used in the cells has impurities in it and creates electricity when exposed to sunlight. These impurities at the surface give rise to a problem known as the dangling bond. Dangling bonds at the surface are the sites of high recombination rate of the charge carriers. The chemical process called surface passivation reduced the effect of dangling bonds.

What are dangling bonds?

To attain stability and fill their valence electrons many atoms form covalent bonds with other atoms. When these atoms or molecules somehow lose electrons, they form free radicals. These free radicals in their immobilized state are called dangling bonds.

Doping or introduction of impurities in the intrinsic (pure) semiconductor is done to change the electrical, chemical, and optical properties of the semiconductor. Any defect induced in the semiconductor within the surface promotes recombination. These defects in the semiconductor surface caused by doping gives rise to the dangling bond.

How the problem was resolved?

The researchers from the King Abdullah University of Science and Technology said they used a chemical method called surface passivation. They exposed the silicon surface to a carbon dioxide filled gas chamber in plasma.

Surface passivation of solar cells
schematic representation of Surface Passivation

A layer of silicon oxide was deposited on the surface of the silicon followed by coating another layer of silicon over it. This fabrication process gives improves electrical resistance and allows the panel to work at high voltages.

What is surface passivation?

Surface passivation is a method in which a layer of silicon oxide is deposited over the surface of silicon wafers.

It was developed by Mohamed M. Atalla at Bell Telephone Laboratories (BTL) in 1950. In this process, the surface of the semiconductor is rendered inert by thermal oxidation. Passivating the surface allows the electricity to efficiently penetrate to the conduction silicon below the surface, to overcome surface states. This method of thermal deposition reduced the leakage current by a factor of 10 to 100.

What do the Researchers say?

The researchers stated that there are other methods of achieving stabilization at the surface but surface passivation is the most economical method. They emphasised the deposition of silicon oxide and them overlaying it with another layer of silicon in a single chamber filled with CO2 and plasma.

They are currently working on the integration of this technology into a working solar module and improving its light capturing ability.

Abhinav

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