Future Students - What are Photovoltaic Devices
Buried Contact Solar Cells
In a conventional silicon solar cell, the front contact is formed by screen printing a silver-loaded paste onto the top surface. These "screen printed" solar cells are reliable and effective, but are able to achieve energy conversion efficiencies of only 12-15%. Buried contact solar cells, however, are made using a laser or mechanical scriber to form grooves in the top surface, into which copper is plated. This structure allows efficiencies of over 20% to be achieved at potentially lower cost than with screen printed cells.
Processing Sequences
Conventional Solar Cells
Conventional Solar Cell
In a conventional solar cell, the top contact is formed by screen printing a silver loaded paste onto the top surface.
- Texture
- Top Surface Diffusion
- Top Surface Grid Screen
- Bake and Fire
- Rear Surface Screen
- Bake and Fire
- Antireflection Coating
- Cleaving
Buried Contact Solar Cells
Buried Contact Solar Cell
For a buried contact solar cell, the top contact is formed by grooving the surface with a laser or scriber and plating metal into the grooves.
- Texture
- Top Surface Diffusion
- Oxide Growth
- Laser Scribing and Etch
- Groove Diffusion
- Aluminium Deposition and Sinter
- Metal Plating
- Cleaving
The structures of screen printed and buried contact solar cells are compared in the diagrams above. The BCSC structure has several advantages over the screen printed structure, including:
- The top metal lines can be made about 10 times narrower-less than the thickness of a human hair-with most of the metal deposited below the surface of the cell. This reduces the amount of light lost because of shading by these lines.
- Because the metal lines are much finer, they can be more closely spaced. This reduces the electrical resistance of the solar cell and improves its efficiency.
- To make a good electrical contact between the metal and silicon of a screen printed solar cell, the top surface must be heavily "doped"; that is, it must be loaded with a high concentration of impurities (labelled n++ on the diagram). In a normal cell, this produces a "dead layer" across the entire cell surface that absorbs blue light and reduces its efficiency. In a BCSC, this problem is avoided since only the groove walls are heavily doped.
- Copper is used instead of silver for the front contact metal, reducing material costs.
The UNSW team has taken the Buried Contact Solar Cell (BCSC) concept from a laboratory device and developed it into a highly successful commercial product. The technology has been licensed to most of the world's largest commercial solar cell manufacturers, including BP Solar in Spain, Solarex in the USA, Samsung in Korea, Central Electronics Laboratories in India and ASE in Germany. BP Solar is currently producing four megawatts per year of solar modules primarily for the European market. With present demand greatly exceeding supply, there are plans to rapidly expand global production of this technology in the near future.
The BCSC has also proven itself in the field. Large grid-connected solar facilities using BCSC technology have been installed at locations such as Toledo (Spain), Gelsenkirchen (Germany), Bern Parliament House (Switzerland) and Kalbarri (Western Australia). Field tests have also shown that BCSC modules, apart from being more efficient, also produce more energy per installed watt than other technologies, because of their comparatively better performance in low light and overcast conditions.