A Japanese research group prototyped a photovoltaic (PV) cell that can generate electricity from a wide wavelength band of light including ultraviolet light, visible light and infrared light.
The group, which is led by Saki Sonoda, associate professor at the Kyoto Institute of Technology, made the announcement March 19, 2010, at the 57th Spring Meeting of the Japan Society of Applied Physics.
The PV cell was realized by adding "3d transition metals" including manganese (Mn) to transparent composite semiconductors with a wide bandgap such as gallium nitride (GaN). It could enable to develop a highly-efficient PV cell by using a simply-joined cell without making a multi-junction cell.
Currently, the conversion efficiency of the new PV cell is low, but its open voltage (Voc) is as high as 2V.
The research group delivered a 90-minute lecture on the cell under the title "Nitride Semiconductor Added With Transition Metals as a Photoelectric Conversion Material for Ultraviolet, Visible and Infrared Lights ~ In the Aim of Realizing the Next-generation Super-efficient PV Cell With a Simple Element Structure."
Sonoda found that when Mn is added to GaN, which is transparent because its bandgap is as large as 3.4eV, until its component ratio reaches several to 20%, the absorbing coefficient of the GaN becomes continuously high for a wide wavelength band of light including ultraviolet, visible and infrared lights. In fact, a PV cell made by adding Mn to p-type GaN is black and transparent unlike an element that does not contain Mn.
Sonoda explained the "impurity band" model, which is mainly composed of Mn's energy levels in the 3d orbit. There has been a technology to set a ladder to a forbidden band, to which electrons with small energy levels cannot climb, by adding impurities to a semiconductor material with a large bandgap so that light with a longer wavelength can be absorbed. And such a band-gap structure is commonly called "intermediate band." However, it is not clear whether the new mechanism is the same as that of the intermediate band, Sonoda said.
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