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Solar cell unit prototyped by Kyoto University of Technology, Japan. Co is added to the p-type GaN thin film, and an n-type material is laminated. The size of the battery unit with the absorbing layer is 10 mm square. The elongated rectangular pattern around is an electrode. On the left is a p-type GaN film to which Co is not added.

Prof. Yoshida Shoji, an associate professor at the Kyoto University of Technology Fibers, Japan, announced on March 19, 2010 at the 57th Joint Symposium on Applied Physics that the production of ultraviolet light, visible light, and infrared light was experimentally produced. Photoelectric conversion of solar cells. It is said to be realized by adding a "3d transition metal" such as manganese (Mn) to a transparent compound semiconductor having a large band gap such as gallium nitride (GaN). As a result, it is possible to develop a solar cell having a very high conversion efficiency without directly forming a multi-junction battery cell. Although the transfer efficiency is still relatively low, the open circuit voltage is very high and has reached about 2V.

Yuan Tian et al. delivered a speech titled “Adding a Nitride Semiconductor to a Transition Metal to Form an Ultraviolet-Visual-Infrared Photoelectric Conversion Material to Achieve a New Generation of Ultra-Efficient Solar Cell Targets with a Simple Component Structure”. In 8 consecutive days, Sonoda used a 15-minute speech opportunity and gave a 90-minute speech.

Yuantian Research found that adding 5% to 20% of Mn to transparent GaN with a band gap width of up to about 3.4 eV has a sustained high absorption coefficient for a wide range of wavelengths of ultraviolet light, visible light, and infrared light. In fact, a solar cell fabricated by adding Mn to p-type GaN is black opaque unlike a component not containing Mn.

According to Yuan Tian, this can be illustrated by the "impurity band" model with the 3d orbital energy level of Mn as the main component. In the past, there was a similar technique of adding impurities to a large bandgap semiconductor material and building a "ladder" in a forbidden band that cannot be occupied by electrons of a small energy level, so that it can absorb light of a longer wavelength. This band gap structure is generally referred to as an "intermediate band." This time, "whether the mechanism is the same as the original intermediate belt is not clear."

In addition to Mn, attempts have been made to add a variety of other 3d transition metals, and the results obtained are mostly the same. The 3d transition metal refers to an element in which electrons increase in the 3d orbital in the outermost orbital when the atomic number (the number of protons in the nucleus) increases. Specifically, there are strontium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn). ). If the added elements are properly selected, "even aluminum nitride (AlN) with a very large band gap may have a visible light absorbing region."

The prototype solar cell was added with Co in p-type GaN. The open circuit voltage (Voc) is as high as 2V or more at 1 sun. In general, the open voltage of a single-junction cell is as high as 2V or more, which means that the band gap is also large, and only the short-wavelength light (blue and green) in visible light can be photoelectrically converted. In such cases.

On the other hand, the short-circuit current density is about 10 μA/cm 2 , which is three orders of magnitude smaller than that of a conventional crystalline Si solar cell. One of the reasons is that "the battery cell is separated from the electrode, and the resistance of the p-type GaN connecting the two is very large." This is because lithographic equipment cannot be used at present, and a design that can accurately measure the output current cannot be realized. As a result, the current cell conversion efficiency is very low, only about 0.01%.

In terms of GaN-based solar cells, the recent development of visible light absorption by adding In to reduce the band gap has become increasingly prosperous. However, in this case, in order to convert light of a wide range of wavelengths into electricity, it is necessary to develop a multi-junction type battery cell by using a material such as a change in the addition rate. This research will help solar cells that are based on GaN but have completely different mechanisms.

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