What can be done to make solar cells more efficient? We have already described the main classes of solar cells that are now available. These can be characterized in terms of the number of distinct energy gaps, according to the physical form, and according to the use for single sun illumination versus concentrated illumination.
New conceptual approaches, which may become available in practice, may lead to higher efficiency and lower cost. These were suggested in Figure 5.6 in the category “advanced thin films.” The agreed criterion for “third generation” designation is that the efficiency exceeds that for the single-junction cell, about 30%. At present, the only cells to achieve “third-generation” efficiency are, in fact, single-crystal tandem cells and spatially dispersed multijunction cells, at most ~41%. The tandem cell is a proven device, as we saw in Chapter 7, but at high demonstrated efficiency it has been available neither in thin-film form nor at low cost.
The first new concept that may alter this situation is “the intermediate band cell.” The second is some form of carrier multiplication, as a means of harvesting in the external circuit the photon energy in excess of the bandgap.