Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) have developed prototype n-type silicon solar cells with conversion efficiency exceeding a record 23.4%.
September 22, 2009 – Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) have developed prototype n-type silicon solar cells with conversion efficiency exceeding a record 23.4%.
P-type silicon solar cells are the most common ones today, but n-type cells offer better PV electricity production thanks to properties like greater impurity tolerance, and it does not suffer from light-induced degradation as does p-type Cz silicon. The end result is better efficiency or lowered manufacturing costs using less-expensive silicon, notes Martin Hermle, Fraunhofer ISE’s group manager, in a statement.
P-type solar cells have a p-type base and thin n-conductive layer; in n-type solar cells the emitter is p-doped through boron diffusion or added Al. Among key problems in using n-type silicon as a base solar cell material is that the sun-facing emitter’s passivation, doped with boron, can’t be optimally passivated with conventional layers such as SiO2 or SiNx. Working with the Technical University of Eindhoven, Fraunhofer says it has solved front passivation by using aluminum oxide (Al2O3).
A 2×2cm2 n-type cell was shown to achieve 23.4% conversion efficiency; a bigger cell (12.5×12.5cm2, “using much simpler process stages close to industry practice” — including a screen-printing application of the aluminum alloy emitter — reached 18.2% efficiency. The researchers say they will further tweak the process technology to push the commercially-viable silicon solar cells to exceed 20% efficiency rates.
|Four-inch wafers with seven highly efficient 4 cm2 n-type silicon solar cells that reach efficiencies of up to 23.4%. (Source: Fraunhofer ISE)|