The nitrides are notoriously difficult materials to work with. InGaN is one of the worst. Why?

1) InN has a very high "vapor pressure" - it decomposes (with nitrogen being given off) at rather low temperature (e.g. around 400 C). This makes it a very difficult material to "grow" into the type of layers you'd need for solar cells.

2) In order to control the bandgap, you have to control the In/Ga ratio in the material. The amount of In you can get in the material is a strong function of the temperature at which you grow the material - the higher the temperature the less In. So these two-junction solar cells would have to be grown using two different substrate temperatures, risking contamination of the interface between them (i.e. it takes time to change the temperature and during that time the critical interface between them is exposed to potential contamination).

3) You'd like to grow the material hotter to improve its structural and electrical quality (fewer defects, fewer unintentional impurities).

4) There's no good substrate to grow the nitrides on. Although lots of progress has been made, there are no large area (e.g. > 2" diameter) crystalline nitride substrates that are suitable for epitaxial growth. Most of the nitride LEDs (e.g. the green LEDs used in stop lights) are grown on sapphire (crystalline Al2O3) because it's cheap (about $100 for a 2" wafer) and relatively inert, but it has a huge mismatch in the crystal lattice spacing with the nitrides (and leads to the huge number of defects mentioned in the article). Other groups use SiC, which has a closer lattice match and better thermal properties, but it's very expensive (up to $4000+ for a 2" diameter high-resistivity wafer).

5) Solar cells are much more dependent on crystalline perfection than LEDs. Especially if you want high efficiency, you can't tolerate defects that suck up the electrons and holes that the light generates. In nitride LEDs, you have more losses than you'd like, but you can just drive the LED with more current to make up for it and get the light that you need.

It's neat and important work, but it's going to be a while before you see nitride solar arrays based on InGaN. Just like blue LEDs and lasers were made years ago but you still don't see DVDs, etc., that use them.

More info on the nitride semiconductors can be found at the [link|http://nsr.mij.mrs.org/|MRS Internet Journal of Nitride Semiconductor Research].

Cheers,
Scott.