The Intermediate band concept
RSLE is developing a product, the IBand, based on a revolutionary new Nitride Thin Film semiconductor material technology. It was recognized over thirty years ago that an introduction of a narrow band of states in a semiconductor band gap presents an alternative to the multijunction designs for improving the power conversion efficiency of solar cells.  Such a band plays a role of a stepping-stone for sub-bandgap photons to excite carriers across the gap via a two-step absorption process.  A simplified band diagram of a material with an Intermediate Band (IBand™) is shown in Fig. 1 with the three possible absorption steps indicated.  Detailed theoretical calculations (performed for 46,000x concentration) indicate that with the proper location of a narrow band of gap states a single junction cell can achieve an ideal power conversion efficiency of 63.2%, much larger than the 55% ultimate limit for two junction tandem cells. Since the IBand cell only requires a single junction structure, it is inherently much simpler and therefore can be fabricated at a much lower cost than the multijunction cells. 

Fig. 1 illustrated on the right is a schematic diagram of a intermediate band solar cell with a partially filled intermediate band acting a stepping stone for the absorption of low energy photons.

Press Release:

RoseStreet Lab Scientists Announce Breakthrough Multiband Solar Cell Technology