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.