phide (InP) as a new high efficiency solar cell material. The bandgap, 1.35 eV, is reasonably close to the optimum value. Cells with efficiency as high as 18.8% AMO [13] have been produced, with 20% efficiency confidently predicted as a future goal [14]. A major reason for the interest in the material is that InP is considerably more resistant to radiation damage than silicon or GaAs. A difficulty with InP is the extremely high cost of the material. It is likely, though, that the cost will be reduced if the cells are ever manufactured at production levels. Single Crystal Film Technologies. Another approach to making low mass cells is to grow a thin epitaxial layer of a single crystal semiconductor on a thick substrate, make a solar cell on the layer, then remove it from the substrate (which, optimally, can then be reused). By the process of CLEFT (Cleavage of Lateral Epitaxy for Transfer), the epitaxial layer is mechanically removed from the substrate along a pre-defined cleavage plane [15]. Solar cells on 10 micron thick layers of GaAs have been produced with efficiency as high as 19.5% AMO. An alternative peeled film technology uses a selective etchant to separate the film from the substrate. This technology has been demonstrated to produce laser-quality peeled films on GaAs [16], but has not been used for solar cells to date. InP films have also been produced by this technology. High Efficiency Cascades. Another approach to high efficiency is to use a cascade structure (described in more detail in section 5 below). The cascade combination of CLEFT GaAs and thin-film CuInSe has produced cells with total efficiency of 21.3% AMO. This will result in a specific power of 620 W/kg, including 50 micron substrate plus coverglass [17]. The best monolithic cascade produced to date has an efficiency of 22.3% at AMO 1 sun. This cell, produced by Varian, utilized an Al035Ga0 65As 1.93 eV top element on a GaAs bottom element. The cell was current-matched at AMO [18], Concentrators. Finally, it should be noted that efficiency can be increased by concentrating the incident sunlight, either by means of a mirror or a lens. Concentration systems can be designed for high (100 X plus), medium (10-20 X), and low (2-5 X) concentration ratios, each with different advantages and problems. Both reflective and refractive [19] concentrators can be made. This approach will not be discussed in detail here. 3. Thin-film Solar Cells An alternative to conventional single crystal solar cell is the thin-film solar cell. Thin- film solar cells are made from thin (1 to 5 micron) polycrystalline or amorphous semiconductor layers deposited on an inert substrate or superstrate material. The materials used are high absorption direct bandgap semiconductors; the high absorption constant allows essentially complete absorption of the light within the first micron or so of the material. Recently thin-film solar cells have been the topic of intense research for low-cost terrestrial electricity production; the low materials usage and potential for high-throughput, automated deposition allows the production cost to be extremely low. Initial research efforts focused on amorphous silicon; recently copper indium selenide and cadmium telluride have shown extremely good experimental results. For space technology, little work has been done. Research into the potential use of thin-film solar cells for space will be a topic of research under the surface power task of the NASA ‘Pathfinder' program to develop enabling technology for future NASA missions.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==