Facilities, equipment and manufacturing operations for circuit deposition and testing : R. E. Cote. Solid-St. Tech. January 1975. p. 49

Abstract

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 very 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, primarily because one of the two active cells can be an already-developed Si or GaAs cell and because the interconnect problem can be easily solved with conventional wire-bond technology. Thus, the problem of developing three new components for the monolithic-device option is replaced by the problem of developing one new component for the mechanical-stack option. We previously observed that, if the Si cell is chosen as the well-developed cell, a GaAs0.7P0.3 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. In particular, it may be desirable to choose a GaAs cell with a band gap of 1.42 eV as the well-developed cell and a GaSb cell with a band gap of 0.72 eV as the novel cell in a two-color solar battery. The advantage of this option is that the high-band-gap GaAs cell currently holds the world record for energy-conversion efficiency, and the GaSb cell would simply boost this record. Moreover, GaSb is advantageous in comparison to GaAsP because it is a binary compound rather than an alloy and would not require compositionally graded transition layers between the active junction and the substrate. Both of these considerations should greatly improve the chances for higher performance. GaSb photodiodes with high quantum efficiencies have already been described in the literature, and available data allow the projection of stack efficiencies over 30% (AM 1.5) with this option.