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.
The research group added a variety of 3d transition metals other than Mn and obtained similar results in many cases. A 3d transition metal is an element whose number of electrons increases in the 3d orbit, which is inside the outermost orbit, as its atomic number (the number of protons in the atomic nucleus) increases. Specifically, scandium (Sc), titanium (Ti), vanadium (V), chrome (Cr), Mn, iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn) are 3d transition metals.
By appropriately choosing those additive elements, even aluminum nitride (AlN), which has a very large bandgap, can possibly have an absorbing region in the visible light range, Sonoda said.
This time, the PV cell was prototyped by adding cobalt to p-type GaN. Its Voc is 2V or more at 1 sun. In general, when a unijunction cell has a Voc of 2V or more, its bandgap is large, and only the short-wavelength part of visible light (blue, green, etc) can be converted into electricity. However, it does not apply to the new PV cell.
On the other hand, the short-circuit current density of the PV cell is about 10μA/cm2, which is about 1/1,000 that of a normal crystalline silicon PV cell. Because the cell and electrodes are separated, the electric resistance of the p-type GaN connecting them is very large, Sonoda said.
This time, it was not possible to accurately measure the output current because photolithography machines could not be used for designing the cell. As a result, the current cell conversion efficiency is only slightly higher than 0.01%.
Recently, many researchers are adding indium (In) to GaN-based PV cells in the aim of narrowing the bandgap and enabling to absorb visible lights. However, in such cases, multi-junction cells using materials with, for example, different ratios of indium are necessary for converting a wide wavelength band of light into electricity. The findings of the research group are expected to pave the way to a GaN-based PV cell with a totally different mechanism.
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