Microcrystalline silicon solar cells have been used as bottom cells, in a tandem struc-ture, together with amorphous silicon top cells, to form what was called the “micro-morph” tandem solar cell. The corresponding cell structure is shown in Fig.6.23.
These tandem cells were, in the period 2005–2009, considered to be one of the most promising options for future Photovoltaics. The reasons being the following:
(a) These cells use a very low quantity of silicon base material (b) They have potentially a very low manufacturing cost (c) No toxic materials are involved in the cell structure (d) Very low-cost mass production seemed imminent
(e) The combination of microcrystalline silicon and amorphous silicon constitute theoretically the ideal combination of two different bandgaps for a tandem cell [17].
Indeed in 2016, relatively high module efficiencies (of over 10%) were reported by Industrial R and D laboratories [18].
In the years 2007–2009, many Industries invested heavily in the purchase of equipment for the large-scale production of modules based on micromorph tandems.
ZnO
μc-Si:H a-Si:H ZnO
glass
Micro-morph
Fig. 6.23 a-Si:H/µc-Si:H or “micromorph” tandem solar cell: a basic structure; b electron micrograph. Reproduced from [1], with the kind permission of the EPFL Press
160 A. Shah However, none of these Industries were able to make a profit from their investment.
Indeed, the period after 2009, led to the complete abandon of the micromorph tandem.
In retrospect we can analyse the reasons for this fiasco:
A. The equipment for the large-scale production of modules based on micromorph tandems was sold at an exaggeratedly high price, leading to commercial prices for micromorph tandem modules, which were far too high and could not compete with the prices of other modules, especially c-Si modules.
B. We all, who were then working in this field, failed to notice what was happening at the same moment in the sector of c-Si modules:
(a) Unprecedented rapid technological progress in the c-Si sector, with a remarkable improvement, both in the increase of efficiencies and in the reduction of the material used.
(b) The entry of Chinese manufacturers into the field. These manufacturers, brought, with massive government support, c-Si modules onto the market12, with prices well below those of all European and U.S. module producers.
C. We also failed to notice the shift of PV market segments during the same period—
from relatively small PV units to very large installations, where area was the dominant factor.
In conclusion, amorphous silicon solar cell development taught us a great deal about thin film solar cells in general and what is necessary to produce a useful, large-scale commercial solar module technology. At present, the only use of these types of solar cells and modules by themselves is in niche markets. The R&D work on a-Si:H also taught us a great deal about the use of disordered materials in electronic devices, and it led to their use as passivation layers in crystalline silicon solar cells, such as HJT.
References
1. A. Shah (ed.),Thin-Film Silicon Solar Cells(EPFL Press, Lausanne, 2010)
2. R.C. Chittick, J.H. Alexander, H.F. Sterling, Preparation and properties of amorphous silicon.
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17, 1193–1196 (1975)
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5. D.L. Staebler, C.R. Wronski, Reversible conductivity change in discharge produced amorphous silicon. Appl. Phys. Lett.31, 292–294 (1977)
12This led, in the years to come, to the breakdown of almost all European PV module manufacturers, both those for thin-film silicon modules and those producing c-Si modules. Thus, the Chinese Government attained their goal, which was to dominate the PV market—a market considered by them, to be of strategic importance.
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Jpn. J. Appl. Phys.43, 3257–3268 (2004)
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9. S. Guha, J. Yang, A. Banerjee, B. Yan, K. Lord, High-quality amorphous silicon materials and cells grown with hydrogen dilution. Sol. Energy Mater. Sol. Cells78, 329–347 (2003) 10. C. Ballif, S. De Wolf, A. Descoeudres, Z. C.Holman, Amorphous Silicon/Crystalline Silicon
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Thesis at the “Faculté des Sciences” of the University of Neuchâtel (2008), Section 4.6 12. A. Shah,Thin-Film Silicon Solar Cells”, Chapter IC-1 inPractical Handbook of Photovoltaics,
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17. A. Shah et al., Basic efficiency limits, recent experimental results and novel light trapping schemes in a-Si:H,µc-Si:H and “micromorph tandem” solar cells. J. Non-Crystall. Solids, 338–340, 639–645 (2004)
18. J.S. Cashmore et al., Improved conversion efficiencies of thin-film silicon tandem (MICRO-MORPH™) photovoltaic modules. Sol. Energy Mater. Sol. Cells144, 84–95 (2016)
Arvind Shah is the Founder of the Photovoltaics Research Lab-oratory (PV-Lab), at the Institute of Microtechnology (IMT), in Neuchâtel, Switzerland. PV Lab Neuchâtel has done pioneer-ing work in the establishment of low-cost production methods for solar cells based on silicon: It introduced a novel plasma-assisted deposition method called “VHF deposition” permitting a significant increase in the deposition rate for thin-film sili-con layers. It also introduced microcrystalline silisili-con, deposited by VHF plasma, and with very low oxygen content, as novel absorber layer, within thin-film solar cells. From 1979 to 2005, Arvind was Professor at the University of Neuchâtel. From 1987 to 2005, he was additionally part-time professor at the EPFL Lausanne. In 1975 he founded and co-directed the Centre for Electronics Design and Technology (CEDT) at the Indian Insti-tute of Science in Bangalore. CEDT is now one of India’s lead-ing University Centres in the field of Electronics. Since 2006, Arvind has been active as scientific consultant to the PV Lab and to various Industries, in Europe, India and the USA. Arvind received the Swiss Solar Prize, together with Johannes Meier in 2005. He received the Becquerel Award in 2007.