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At the 57th Spring Meeting of the Japan Society of Applied Physics last week, a Japanese research group led by Kyoto Institute of Technology associate professor Saki Sonoda presented a prototype nitride-based photovoltaic (PV) cell that can generate electricity from a wide wavelength band including ultraviolet light, visible light and infrared light, according to a report in Nikkei Electronics.
Since its bandgap energy is large (3.4eV), gallium nitride (GaN) only absorbs short wavelengths and is transparent to longer visible wavelengths. Recently, many researchers have added indium (In) to GaN-based PV cells in the aim of narrowing the bandgap and enabling it to absorb visible light. However, in such cases, multi-junction cells using materials with different ratios of indium, for example, are necessary for the conversion of a wide wavelength band of light into electricity.
However, Sonoda found that adding 3d transition-metal elements including manganese (Mn) to transparent wide-bandgap compound semiconductors such as GaN enabled the development of a highly efficient PV cell by using a single-junction cell rather than a multi-junction cell. Specifically, when Mn is added until its component ratio reaches between several percent and 20%, the absorbing coefficient is increased for a wide wavelength band of light, including ultraviolet, visible and infrared. Currently, the conversion efficiency of the new PV cell is low, but its open voltage (Voc) is as high as 2V.
The research group also added a variety of 3d transition metals other than Mn and obtained similar results in many cases. By choosing the additive elements appropriately, even aluminum nitride (AlN) — which has a very large bandgap — could be made to absorb in the visible wavelength range, Sonoda says.
This time, the PV cell was prototyped by adding cobalt to p-type GaN. Its Voc is 2V or more at 1 sun (i.e. no light concentration). In general, when a single-junction 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, this does not apply to the new PV cell.
On the other hand, the short-circuit current density of the PV cell is about 10 microAmps/cm2, which is about one-thousandth that of a typical crystalline silicon PV cell. Because the cell and electrodes are separated, the electrical resistance of the p-type GaN connecting them is very large, Sonoda says.
In this case, it was not possible to accurately measure the output current, because photolithography systems could not be used to fabricate the cell. As a result, the conversion efficiency is only slightly higher than 0.01%. Nevertheless, improvements are expected.
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