Fraunhofer's New PV Technologies Exceed 23% Efficiency
The Fraunhofer Institute for Solar Energy Systems has created a silicon solar cell prototype with an efficiency of 23.4% through new concepts for the manufacture of n-type cells.
Aaron Hand, Executive Editor -- PV Society, 9/30/2009
Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE, Freiburg, Germany) have created a silicon solar cell prototype with an efficiency of 23.4%. It is based on new methods and cell concepts for the manufacture of n-type silicon solar cells, which could result in higher efficiencies and power production levels for commercial solar cells.
"Most commercial silicon solar cells are currently p-types," said Martin Hermle, Fraunhofer ISE's group manager. Traditionally, when manufacturers use less expensive silicon (with higher impurity levels), they must sacrifice conversion efficiencies for the lower manufacturing costs. But the n-type silicon has a greater tolerance for most impurities, Hermle explained. In addition, p-type Czrochalski (Cz) silicon suffers from light-induced degradation, which does not occur with n-type silicon.
 |
|
Fraunhofer ISE has produced 4 cm2 n-type silicon solar cells with efficiencies of up to 23.4%. |
Silicon solar cells consist of two areas with negative (n) and positive (p) conductions. The thicker substrate layer is considered the base material and determines the cell's type. Conventional cells have a p-type base and a thin n-conductive layer - the emitter, or the charge carrier. In n-type solar cells, the emitter is p-doped, either through boron diffusion or the addition of aluminum.
Experiments have been conducted on n-type silicon as a base material for some time, but production was very complicated. With n-type solar cells, in which the emitter is on the side facing the sun, was the
Passivation of the emitter (which faces the sun in n-type solar cells) has been a problem for example. The emitter was usually doped with boron, and could not be optimally passivated with conventional layers, such as SiO2 or SiNx. In collaboration with the Technical University of Eindhoven (the Netherlands), the problem of front passivation was solved through the use of Al2O3.
Jan Benick, who is working on a doctorate in the group for high-efficiency silicon solar cells, successfully developed a highly efficient cell process for n-type cells that uses boron diffusion to make the emitter; the efficiency is 23.4% on 2Ã2 cm2 â the highest efficiency ever reached for this cell type.
Christian Schmiga, project leader in the group for high-efficiency silicon solar cells, has also reached 18.2 % efficiency on 12.5Ã12.5 cm2 by using much simpler process stages close to industry practice, including a screen printing process to apply the aluminum alloy emitter.
Fraunhofer ISE is further developing the process technology for n-type solar cells to make it more suitable for commercial production. Industrially manufactured silicon solar cells should be able to quickly exceed 20% efficiencies.
