Compared with other semiconductor materials, gallium arsenide (GaAs) has a band gap energy (Eg — 1.42eV) that is virtually ideal for both the AM1.5 and AM0 spectra. The theoretical efficiency ZTin the AM1.5 spectrum at ambient temperature is around 30%, and is nearly 27% for the AM0 spectrum (see Figure 3.25). As GaAs is a directly absorbent semiconductor, full light absorption is attainable with ultrathin material, which means that GaAs is suitable for both crystalline and thin-film solar cells.
Hence GaAs solar cells for the AM1.5 spectrum and 1 kW/m2 provide the highest absolute efficiency among all types of cells with only a single p-n junction (values achieved using small cells where AZ~ 1cm2: zpv — 26.1% for both crystalline and thin-film GaAs solar cells, according to [3.2]). Moreover, owing to their higher band gap energy, GaAs solar cells are able to withstand far higher
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Figure 3.42 The workflow from wafer to finished solar cell (Based on AEG documents)
temperatures than silicon solar cells and have lower temperature coefficients for Voc and Pmax than silicon solar cells (around —0.2% per K for GaAs according to [Hu83]). Hence such cells make very good concentrator cells, i. e. solar cells that use highly concentrated sunlight (up to well over 100kW/m2).
According to [3.11], in early 2009 such a GaAs concentrator cell with Az ^ 0.05 cm2 and 232-fold sunlight concentration attained 28.8% efficiency. Even higher efficiency is attainable with GaAs containing tandem and triple concentrator cells (see Section 18.104.22.168).
crystallized silicon ribbon solidifying Si
liquid Si rising in a narrow capillary
– capillary (SiC plates) molten silicon
___ high frequency
That said, it should also be noted that GaAs solar cells have two major drawbacks. First, unlike abundantly available silicon, the base material for GaAs solar cells is relatively scarce, and thus obtaining the requisite ultrapure quality is very cost intensive (according to [3.5], 27% of the mass of the Earth’s crust is composed of silicon, whereas only 0.000 15% is made of Ga and only 0.000 05% of As). The GaAs manufacturing process is also very cost and energy intensive on account of the extremely high purity standards that must be met. Another drawback is that As is highly toxic, which means that disposing of GaAs solar cells at the end of their service life poses a major ecological problem. Moreover, if such cells catch fire they release highly toxic and carcinogenic substances such as As2O3 [Hu83]. Thus in view of the environmental friendliness of solar cells, the use of such problematic substances should be avoided despite their excellent efficiency.
Inasmuch as GaAs solar cells are very cost intensive for the reasons described above, they are used almost exclusively as concentrator cells with very high concentration factor, or for space exploration applications on account of their excellent radiation resistance properties. Owing to the preponderance of diffuse radiation in Central Europe, the use of GaAs solar cells fortunately is not particularly worthwhile. Hence they will not be discussed further in this book. For more on these cells see [Hu83], [Joh80], [Gre86].