Α-Si solar cell technology is very different from crystalline silicon technology, since most of these cells consist of a very thin layer of non-crystalline material. Large cell production and low requirements in cell material, has as a result low energy cost manufacturing. This fact gives a great advantage to this technology, although these panels seem to be les popular due to their low efficiency. Their manufacturing process is simpler since smaller temperatures are needed and control demand level is sufficiently lower (Alsema, 1996).
Most companies have utilized p-i-n device structures on glass substrates coated with tin oxide, like the one presented on figure 2.13 (Carlson, 2003). The greatest percentage of the first generation commercial products (1970-1980) was fabricated in a single junction p-i-n configuration with natural i-layer (thickness 0.25-0.3 μιη) (Carlson, 1989). The p-layer consisted of an amorphous silicon-carbon (a-SiC) alloy doped with boron and the n-layer of phosphorous doped a-Si. Aluminium was the material used for the back contact which was deposited by sputtering. In earlier technology, deposition of the a-Si layers took place by using plasma-enhanced
chemical vapour deposition (PECVD) in pure silane atmospheres (SiH4) (Carlson,
Most of current a-Si PV modules use multijunction p-i-n structures with highly reflective back contacts. Since a-Si cells can be made at lower temperatures, this allows the fabrication of the cells on a wide variety of substrates and either using tandem junction cell or even triple-junction cell (Vijh et al., 2006).
2003).
Figure 2.13: BP solar tandem junction cell [16]
A progress of tandem structure manufacturing is described on figure 2.14. For the fabrication two sheets of glass are needed (one coated with texture tin oxide and one uncoated). The uncoated sheet is used as a back cover and the other one as the substrate. Then a driller is used for forming holes which are used as electrical contacts on the back plate and the tin oxide glass is then cut to the size that is desired (Alsema, 1996; Arya and Carlson, 2002).
Figure 2.14: A schematic showing of the a-Si manufacturing layout [19]
In next steps a series of laser scribing and washing is taking place. The tin oxide coated glass is washed and scribed by a doubled frequency Nd-YAG laser into a series of strips. This step defines the operating voltage and current of the panel. Then, washing takes place and the deposition of the a-Si/a-SiGe on the scribed substrates starts with various layers of the tandem structure. The rate of this step determines the production rate of an amorphous panel factory and limits the total capacity (Carlson, 2003; Arya and Carlson,2002).
After deposition of the tin oxide layer, another laser scribe takes place close to the previous scribe and then magnetron sputtering is used for the aluminium deposition. Finally two more lasers are applied. A third laser scribe for removing locally the back contact and complete the series interconnection of the cells and a fourth laser scribe for cleaning any films around the perimeter of the module to provide electrical isolation. Last stage includes an electrical curing which applies a reverse bias to each solar cell. This includes locally heating so that defects that can cause shorts and shunts are removed. Then the front plate is encapsulated, power leads, junction box and frames are applied and the module is ready for testing, packaging and shipment.
References
E.A. Alsema, “Environmental Aspects of Solar Cell Modules, Summary Report” (August 1996), A study in commission by the Netherlands Agency for Energy and the Environment (NOVEM)
D.E. Carlson, “Monolithic amorphous silicon alloy solar modules” (2003), Solar Energy Materials & Solar Cells 78: 627-645
D.E Carlson, A. Catalano, “Optoelectronics 4” (1989) 185, as sited in D.E. Carlson, Monolithic amorphous silicon alloy solar modules (2003), Solar Energy Materials & Solar Cells 78: 627-645
R.R. Arya, D.E. Carlson, “Amorphous Silicon PV Module Manufacturing at BP Solar” (2002), Progress in Photovoltaics: Research and Application, 10: 69-76