NEAR-TERM HIGHER EFFICIENCIES WITH MECHANICALLY STACKED 2-COLOR SOLAR BATTERIES

Two-band-gap (or two-color) solar batteries have the potential for much higher efficiencies than can be achieved with single-junction devices. While two-terminal monolithic stacks with two active junctions and a series interconnect junction grown sequentially onto a single substrate are conceptually elegant, they require the development of three new components in a complex multi-element III-V alloy system. Although such devices have been demonstrated, each of the three novel components (the two cells and the interconnect) must be optimized so that it will perform near its theoretical limit in order for the two-color device to outperform the more developed single-junction devices. Tandem mechanically stacked two-color solar batteries offer a shorter path to commercialization. We previously observed that, if the Si cell is chosen as the well-developed cell, a GaAs//0//. //7P//0//. //3 cell on a GaP substrate would be a logical choice for the novel component in a solar battery. In studying this particular option, we recently noted that the four terminals available in a mechanical-stack battery can be used in a series-parallel interconnect scheme, allowing voltage matching at the module level. This interconnect scheme makes the module energy-conversion efficiency quite insensitive to variations in the solar spectrum and opens up a broader range of materials that are usable in mechanical stacks.