Solar Cell

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Silicon solar cells have been the workhorse of the PV industry for many years and currently account for well over 80% of world production. Modules based on these cells have a long history of rugged reliability, with guarantees lasting 20 or 25 years that are exceptional among manufactured products.

Although cells made from other materials are constantly being developed and some are in commercial production, it will be hard to dis-lodge silicon from its pedestal. The underlying technology is that of semiconductor electronics: a silicon solar cell is a special form of semiconductor diode.

Fortunately, silicon in the form of silicon dioxide (quartz sand) is an extremely common component of the Earth ’ s crust and is essentially non - toxic. There is a further good reason for focussing strongly on silicon cells in this chapter: in its   crystalline  form silicon has a simple lattice structure, making it comparatively easy to describe and appreciate the underlying science.

There are two major types of crystalline silicon solar cell in current high volume production:

• Monocrystalline.The most efficient type, made from a very thin slice, or wafer, of a large single crystal obtained from pure molten silicon. The circular wafers, often 5 or 6 inches (15cm) in diameter, have a smooth silvery appearance and are normally trimmed to a pseudo - square or hexagonal shape so that more can be fitted into a module. Fine contact fingers and busbars are used to conduct the electric current away from the cells which have a highly ordered crystal structure with uniform, predictable, properties. However, they require careful and expensive manufacturing processes, including  ‘ doping ’  with small amounts of other elements to produce the required electrical characteristics. Typical commer-cial module effi  ciencies fall in the range 12 – 16%. The module surface area required is about 7  m2 /kWp .
• Multicrystalline,      also called   polycrystalline.  This type of cell is also produced from pure molten silicon, but using a casting process. As the silicon cools it sets as a large irregular multicrystal which is then cut into thin square or rectangular slices to make individual cells. Their crystal structure, being random, is less ideal than with monocrystalline material and gives slightly lower cell eficiencies, but this disadvantage is offset by lower wafer costs. Cells and modules of this type often look distinctly blue, with a scaly, shimmering appearance. Multicrystalline modules exhibit typical eficiencies in the range 11 – 15% and have overtaken their monocrystalline cousins in volume production over recent years. The module surface area is about 8 m2 /kWp .     

You have probably already gathered that the eficiency  of any solar cell or module, the percentage of solar radiation it converts into electricity, is considered one of its most important properties. The higher the eficiency, the smaller the surface area for a given power rating. This is important when space is limited, and also because some of the additional costs of PV systems  –  especially mounting and fixing modules  –  are area related. Crystalline silicon cells, when operated in strong sunlight, have the highest efficiencies of all cells commonly used in terrestrial PV systems, plus the promise of modest increases as the years go by due to improvements in design and manufacture. But it is important to realize that other types of cell often perform better in weak or diffuse light, a matter we shall return to in later sections.

Research laboratory cells achieve considerably higher efficiencies than mass - produced cells. This reflects the ongoing R & D effort that is continually improving cell design and leading to better commercial products. In some applications where space is limited and effi ciency is paramount  –  for example, the famous solar car races held in Australia  –  high - quality cells made in small batches are often individually tested for effi  ciency before assembly.

Module efficiencies are slightly lower than cell efficiencies because a module’s surface area cannot be completely filled with cells and the frame also takes up space. It is always important to distinguish carefully between cell and module efficiency.

There is one further type of silicon solar cell in common use:

• Amorphous. Most people have met small amorphous silicon (a - Si) cells in solar - powered consumer products such as watches and calculators that were first introduced in the 1980s. Amorphous cells are cheaper than crystalline silicon cells, but have much lower efficiencies, typically 6 – 8%. Nowadays, large modules are available and suitable for applications where space is not at a premium, for example on building facades. The surface area required is about
  16 m2/kWp .

We focus initially on crystalline silicon solar cells for two main reasons: their comparatively simple crystal structure and theoretical background; and their present dominant position in the terrestrial PV market. Their wafer technology has been around for a long time and is often referred to as  ‘first generation’ ; they are the cells you are most likely to see on houses, factories, and commercial buildings. 

However, it is important to realize that many other semiconductor materials can be used to make solar cells. Most come under the heading of   thin film  –  somewhat confusing because a - Si is also commonly given this title  –  and involve depositing very thin layers of semiconductor on a variety of substrates. Thin  film products are generally regarded as the ultimate goal for terrestrial PV since they use very small amounts of semiconductor material and large - scale continuous production processes without any need to cut and mount individual crystalline wafers. Thin  film modules based on the compound semiconductors   copper indium diselenide (CIS) and cadmium telluride (CdTe)  are in commercial production. Often referred to as ‘second generation’, they currently have efficiencies lower than those of crystalline silicon, but they represent a highly significant advance into thin film products. We will discuss them, and several types of specialized cells and modules later.