Chapter 5 Terrestrial Silicon Solar Cells Today

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The crystalline silicon solar cell and module dominate the terrestrial solar market ... until 1975 when Bill Yerkes left Spectrolab and formed Solar Technology.
Chapter 5 Terrestrial Silicon Solar Cells Today

The crystalline silicon solar cell and module dominate the terrestrial solar market today. While the silicon cell had been used in space since 1958, it was not brought down to earth for terrestrial applications until 1975 when Bill Yerkes left Spectrolab and formed Solar Technology International (STI). The ownership of that pioneering company has changed hands several times and it has now become SolarWorld [1]. STI developed the screen printed grid solar cell and the laminated glass module used by everybody today. Figure 1 shows a time-line for the terrestrial silicon module technology and market development. Figure 2 shows a photograph of a section of an early STI crystalline silicon (c-Si) module circa 1980.

Figure 1: Timeline of Solar Industry Firsts at SolarWorld [1]

Figure 2: Photo of 1980 STI solar panel (courtesy Jim Avery)

Figure 3 shows photographs of today’s evolved c-Si modules. A SolarWorld 14.9% efficient module [2] is at the left with the best available Chinese 15.9% efficient Yingli [3] module in the middle. These two modules are almost identical. The module at the right is a SunPower 19% efficient module [4] with innovative higher efficiency c-Si cells.

Figure 3: C-Si modules with SolarWorld (Left), Yingli (Middle), and SunPower (Right). While SolarWorld has definitely been a pioneer in the c-Si module technology and market development, in recent years, Chinese module manufacturers have taken market share and they now dominate the c-Si solar market as shown in figures 4 and 5.

Figure 4: PV Production by region in 2010 [5]

Figure 5: China is capturing share in PV module manufacturing [6]. As shown in table 1, the reason that the Chinese module manufacturers have taken market share is because the Chinese government has given major financial support to the Chinese solar industry. Loans and Credit Agreements by Chinese Banks to Chinese solar companies in 2010 totaled $40.7 billion [7]. Table 1

It is easy to understand why SolarWorld, the pioneering solar company, filed AntiDumping and Anti-Subsidy cases against China. What is involved here is a clash between the Western free enterprise model and the Chinese Government Industrial planning model. One can see both points of view here. The Chinese Government support model has advanced the world case for renewable energy but it has also been unfair to Western innovators like SolarWorld. Problems like this will continue but hopefully can be worked out amicably. Meanwhile, there are now multiple suppliers of c-Si modules at low cost and there are now multiple terrestrial applications for these modules as shown in figures 6 and 7.

Figure 6 SunPower modules on a home [8]

Figure 7: Partial view of 1.5 MW SunPower Oasis Power Block in 250 MW California Valley Solar Ranch project at San Luis Obispo, CA [9]

The modules shown in figures 6 and 7 are now a mass produced commodity. The module production processes subdivide into silicon wafer production steps followed by solar cell production steps followed by module assembly steps as shown in figure 8. Professor N. Cheung from UC Berkeley has posted an excellent presentation on Solar Cell Fabrication Technologies on the web [10]. This presentation is the source for figures 8 to 11.

Figure 8: c-Si solar cell and module fabrication [10] More detail on the wafer fabrication steps is shown in figure 9. Wafer fabrication subdivides into either crystal growth or ingot casting with the result being either single crystal wafers or large grain size polycrystalline wafers.

Figure 9: There are two paths for Si wafer fabrication resulting in single crystal or multicrystalline wafers [10]. For either wafer case, the standard cell fabrication then begins involving a diffused step to create the P/N junction, an electric current collection grid fabrication step, an antireflection coating step, and a back contact metallization step as shown in figure 11. Once the cells are prepared, they are then wired together in series with soldered leads and laminated with glass into the final module. An advantage for China is that this can be done by hand with low cost labor. Alternatively, it can be done with automated equipment as shown in figure 12.

Figure 10. Generic Crystalline Silicon Cell processing using diffusion for junction formation and screen printing for front grid formation [10].

Figure 11: Module Packaging with Automated Equipment, Source: Spire Corporation

So, c-Si module prices have dropped dramatically but what is now possible for the future? Today’s modules made via the processes outlined in figures 9 to 11 have efficiencies in the 15% range. However, higher module efficiencies are possible using higher purity silicon feedstock and cell fabrication modifications. Cell efficiencies in the 22% range are now available. One example is the HIT module described in the next chapter. Another example is the interdigitated-back-contact cell [11] made by SunPower shown in figure 12. Higher efficiency cells will produce more energy and reduce system costs provided that they are not too expensive themselves. However, there is a solution if the higher efficiency c-Si cells are more expensive in that one can break these cells into smaller cells and use lenses or mirrors to concentrate the sunlight as shown in figures 13 and 14. Figure 13 shows this concept for a 2x concentration and figure 14 shows SunPower’s 7x sunlight concentration C7 system [12]. This is a very promising approach as discussed later in Chapter 7.

Figure 12

Figure 13: Low conceentration PV modules substitute lower cost mirrors for some of the expensive single crystal Si cell material.

Figure 14: SunPower C7 field installation [12] As shown in figue 15, SunPower believes that the C7 using their 24% efficient c-Si cells will produce lower cost electricity (LCOE) than either the standard planar c-Si modules or any thin film PV module.

Figure 15: SunPower believes that the LCOE for C7 solar fields will be lower than for planar c-Si or thin film solar systems [13].

References 1.) 2.) 3.) 4.) 5.) 6.) 7.) 8.) 9.) 10.) 11.) 12.) 13.)

http://www.solarworld-usa.com/about-solarworld/history-of-solar http://www.solarworld-usa.com/~/media/www/files/datasheets/sunmoduleplus/sunmodule-solar-panel-250-mono-ds.pdf http://www.yinglisolar.com/assets/uploads/products/downloads/2012_PANDA_60.pdf http://us.sunpower.com/cs/Satellite?blobcol=urldata&blobheadername1=ContentType&blobheadername2=Content-Disposition&blobheader http://www.solarnovus.com/europes-role-in-the-worldwide-pv-market_N3570.html http://en.wikipedia.org/wiki/List_of_photovoltaics_companies http://www.prosun.org/en/fair-competition/trade-distortions/subsidies.html 2/19/2014 http://gigaom.com/2012/08/08/sunpower-looks-to-solar-leases-as-a-bright-spot/ http://us.sunpower.com/power-plant/products-services/oasis-power-plant/; http://www-inst.eecs.berkeley.edu/~ee143/fa10/lectures/Lec_26.pdf Fraas, L and Partain, L. Solar Cells and Their Applications, 2nd Edition, Ch 4, Wiley (2010) http://www.solardaily.com/reports/SRP_and_SunPower_Dedicate_Completed_C7_Track er_Solar_Power_System_at_ASU_Polytechnic_Campus_999.html http://www.slideshare.net/HitReach/sun-powerpresentation?utm_source=slideshow02&utm_medium=ssemail&utm_campaign=share_sl ideshow