Japan's National Institute for Materials Science (NIMS) announced Dec 6, 2013, that it has succeeded in utilizing the 450-750nm waveband of sunlight by forming multiple quantum dot layers (intermediate bands) in gallium indium nitride (InGaN), which is a material for solar cells.

Before the success, only sunlights with shorter wavelengths could be used for InGaN. Because it became possible to convert all the wavelengths of sunlight to electricity, conversion efficiency is expected to drastically improve, NIMS said.

There are two methods to improve the conversion efficiency of a solar cell. One is to increase conversion efficiency by making improvements to material quality and cell structure. The other is to use a wider wavelength range of sunlight.

The usable wavelength range for a compound semiconductor solar cell depends on the bandgaps of the compound semiconductor's elemental species and crystal structure. And it can use only a certain wavelength range of light. Therefore, there are ongoing researches on new cells such as quantum dot solar cells that incorporate a quantum dot structure and tandem structure formed by stacking multiple semiconductor materials with different bandgaps to use sunlights with longer wavelengths.

However, it has been difficult to drastically improve conversion efficiency with the existing structures because they have different lattice shapes and can use only limited types of semiconductor materials.

This time, NIMS paid attention to the fact that GaN and InN have a similar structure and their wavelength range includes all the wavelengths of sunlight. And it considered that if intermediate bands can be formed centering on gallium indium nitride mixed crystal whose indium composition is controlled (InxGa1-xN), conversion efficiency will be improved by using not only light equivalent to the bandgap energy but also lights on the longer wavelength side such as green, yellow and other visible lights that are the main wavelengths of sunlight.

NIMS used a metal-organic chemical deposition method to make an intermediate band solar cell having a structure in which an InGaN quantum dot is embedded in each quantum well. When it measured the external quantum of the solar cell, it confirmed that the cell absorbs 450-750nm light, which cannot be utilized with the existing InGaN, and converts it to electricity.