Photovoltaics from Milliwatts to Gigawatts. Tim Bruton

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СКАЧАТЬ the 1973 oil crisis, as discussed in Chapter 2. By 1970, it had already been noted that growth prospects in the space cell industry were limited, and scientists were beginning to look to earth [39]. Sharp Corporation had consistently used reject space cells for terrestrial applications, supplying photovoltaic power to 256 lighthouses between 1961 and 1972 [59]. It demonstrated the first viable silicon module for terrestrial use in 1963 and became a major supplier of photovoltaic modules from 1980 onward. AEG‐Telefunken started terrestrial solar cell manufacture at its Wedel site in the mid‐1970s [57]. Other ‘pure’ terrestrial solar cell companies were founded by individuals leaving space cell companies. Bill Yerkes had been president at Spectrolab, but following the Hughes Aircraft acquisition he left the company to form Solar Technology International, which subsequently became ARCO Solar and was the largest photovoltaics manufacturer for several years [60]. Joseph Lindmayer and Peter Varadi left COMSAT in 1973 to form the Solarex company [61], and this too – after a merger with BP Solar – became the world’s largest photovoltaics manufacturer, in 2000 [61].

      In this way, the development of solar cells for space formed the foundation of the major manufacturing industry and global energy supply that photovoltaics has become.

1.4 Gallium Arsenide and III–V Alloys for Space

Despite its many advantages, silicon also has negative characteristics. Namely, it is an indirect bandgap semiconductor and the bandgap at 1.1 eV is not the optimum. This means that a relatively thick absorber layer is required to absorb the solar spectrum, and the bandgap limits the ultimate efficiency that can be achieved. The semiconductor gallium arsenide (GaAs) does not have these disadvantages. As a direct bandgap semiconductor, it requires only a few microns of absorber, which lowers the mass of the solar cell – an important characteristic in space applications. The bandgap at 1.4 eV is very near the optimum for a single‐junction solar cell [44]. GaAs also has the advantage that a wide range of alloys of various group III and V elements exist with different bandgaps and are lattice‐matched to it [62], enabling the high‐efficiency tandem structures envisaged by Jackson [43]. On the other hand, GaAs is a high‐cost technology, with the cost of substrates and active film deposition being much higher than that for silicon. Nonetheless, the cost of the solar cell is a much less important parameter than kWp/kg for space applications [63]. GaAs also has the advantage that it is more radiation‐hard than silicon [64]. GaAs cells have been used in space since the 1960s, with the Russian Venera 2 and 3 missions to Venus. In 1986, 70 m² of GaAs solar cells were installed on the MIR space station; these functioned for 15 years [65]

The development of III–V cells can be divided into two programmes: single‐junction GaAs cells on either a GaAs substrate or germanium and tandem cells usually on a germanium substrate for use in space but also for concentrating solar cell applications. The bandgaps and lattice constants of the most important III–V compounds are given in Figure 1.12. Silicon solar cells were the preferred space cell technology until the 1990s, when III–V cells began to be used. They remain the dominant technology today.

      1.4.1 Single‐Junction GaAs Solar Cells

GaAs has a history almost as long as that of silicon. One of the first GaAs solar cells was made at RCA Laboratories in 1956, with an efficiency of 6% on very small‐area solar cells [66]. From that point on, significant progress was made, driven by the expectation of higher efficiency and enhanced radiation hardness. By 1981, the 20%‐efficiency barrier had been breached, with an n+/p/p+ GaAs structure on both germanium and GaAs substrates [67]. Early GaAs cells were made by liquid‐phase epitaxy, which had limits in terms of the alloys that could be produced, while the 1981 work was performed using vapour‐phase epitaxy. By the late 1990s, however, metal organic chemical vapour deposition (MOCVD) had become a well‐established technique for producing a wide range of III–V compounds [64]. In 1990, cells of over 25% efficiency were demonstrated using MOCVD for the epitaxial layers – in some cases, as thin as 0.1 μm [68]. The best cell had an efficiency of 25.7% under the AM1.5 Global spectrum; its structure is illustrated in Figure 1.13. The improved efficiency was derived in part from the use of a GaInP₂ window at front and rear, such that the active carrier collection region was separated from the high recombination surfaces at the front and rear of the active GaAs cell.

Figure 1.12 Bandgap and lattice constant for the important III–V alloys [62]

      (Courtesy Royal Society of Chemistry) Source: H. Cotal et al: Energy and Environmental Science 2 (2009) 174‐192

Figure 1.13 Structure of a 25.7% GaAs solar cell under the AM1.5 Global spectrum [68]

      (Courtesy IEEE) Source: S.R. Kurtz, J.M. Olsen and A. Kibbler: Proc 21st IEEE PVSC (1990) 138‐140

      GaInP₂ was used in preference to the GaAlAs₂ previously employed, as this was prone to degradation by the inclusion of oxygen. The work highlighted that the electronic quality of the individual layers was as important as the overall device structure in achieving very high efficiencies. Development of single‐junction GaAs has been relatively slow, as more research has gone into the higher‐efficiency potential triple‐junction cells. In 2008, the record efficiency was 26.1% [69]; in 2018, Alta Devices reported a new record for single‐junction GaAs cells of 28.9% [70], in the form of an ultrathin cell. This was intended not for space applications but for terrestrial ones, where very high efficiency is important (e.g. the Internet of Things, unmanned aircraft).

      1.4.2 Multijunction Solar Cells for Space

The first successful GaAs heterojunction solar cell was demonstrated in 1970 for a GaAlAs/GaAs structure [71]. Good progress was made, and by 1981 an AM0 efficiency of 15% was posted for a GaAs/GaAlAs tandem [72]. At the same time, it was observed that germanium could also be used as a substrate, as it is also lattice matched to GaAs, as shown in Figure 1.12 [67]. At this point in time, liquid‐phase epitaxy was the preferred method for making the tandem cells. Lattice matching is important as if a significant mismatch occurs, the stress at the interface between two semiconductors creates threading dislocations which spread through the epitaxial layers and act as recombination centres. These have a great impact in reducing the solar cell efficiency. Removing the lattice‐matching constraint means a wider range of III–V compounds can be used, with greater potential for high efficiency. In this case, alternating layers with different lattice parameters of stress and compression can prevent the generation of threading dislocations. These are known as metamorphic cells [64]. In general, space cells have been lattice matched, but metamorphic cells have been developed for concentrator applications. A particular challenge with the use of tandem cells is that there is a p/n junction between the top cell and the bottom one, which is rectifying for the direction of current flow. To overcome this, a heavily doped ‘tunnel junction’ is used, in which carriers can travel through the rectifying junction, as illustrated in Figure 1.14 [73]. The requirements of the tunnel junction are that it be a wide‐bandgap semiconductor for good transparency and that it be heavily doped in order for tunnelling to occur. This makes for an added complexity and cost for tandem cells.

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