Top conversion for Mitubishi solar cells

Mitsubishi Electric says it has achieved what it believes is a world record photoelectric conversion efficiency rate of 18.6% in a 150mm square practical use multi-crystalline silicon solar cell, showing a 0.6% improvement over the company's previous record of 18% for a prototype, which the company announced on May 31, 2007.

The efficiency rate measures the percentage of solar light energy that is converted to direct current electrical energy, and the results were evaluated by the Japanese public standards agency National Institute of Advanced Industrial Science and Technology.


In terms of production volumes of photovoltaic (PV) technology, the company highlighted that systems have been increasing as they have garnered attention as a good source of renewable energy, against the background of increased global environmental awareness.

And while silicon is an essential component in the wafers used to make solar cells, the supply of silicon has not been able to keep up with demand which is driving research into the development of thinner wafers that not only use less silicon, but also have improved efficiency and increased electrical output.

For its 150mm square practical-use multi-crystalline silicon solar cell, Mitsubishi said it achieved the 18.6% rate by adding a low reflectivity surface texture on the multi-crystalline silicon wafer, by optimising the p-n junction to increase electric current generation and by developing a process to print electrodes on the surface of the silicon (metallization) to reduce shade loss of front grid electrodes, all of which contributes to higher efficiency in small installations such as narrow roofs.

Specifically, the company explained that to create highly efficient PV cells, it is necessary to reduce light reflectivity on the surface, and while a honeycomb-textured structure is recognized as suitable for reducing surface reflectivity, it was a challenge to develop a way to apply this technology in production lines. Therefore, Mitsubishi developed a method for fabricating a honeycomb structure on the surface of a 150mm square multi-crystalline silicon by combining laser patterning and wet etching.

In terms of power generation, which relies on the amount of light that approaches the p-n junction, a shallow, lightly doped emitter (n-layer) increased light gain and improved efficiency of power generation.

Also, 25% reduced shade loss was achieved from front grid electrodes and larger effective electrical output surface area using modified screens and front metal electrodes.

Mitsubishi said it would begin introducing this multi-crystal silicon cell technology into its mass-produced photovoltaic modules by fiscal 2011 which runs from April 1, 2010 to March 31, 2011.

Further, the company said it also aims to increase output of solar power generation systems by combining this technology with its PV inverters, which it believes have the industry's highest energy conversion efficiency rate.

There are 18 domestic and 7 international patents pending for this technology, the company noted.

In a separate announcement, the company also said that thanks to a sharp increase in demand for solar power generation systems, it will invest a total of approximately $70.7 million (7 billion yen) to expand annual production capacity of PV cells at its Nakatsugawa Works Iida Factory in Nagano Prefecture, and modules, at its Nakatsugawa Works Kyoto Factory in Kyoto Prefecture, which will expand the company's annual PV production capacity from 150MW to 220MW in October 2008.

Also, the company plans to establish a production system with an annual capacity of 500MW by fiscal 2013, which runs from April 1, 2012 to March 31, 2013.

In related news, last month Leuven, Belgium-based nanoelectronics and nanotechnology research centre IMEC said it had achieved a conversion efficiency of 24.7% for a single-junction GaAs solar cell on a Ge substrate, which has been measured and confirmed by the National Renewable Energy Laboratory.