The modules are composed of an array of cells, which are made of semi- conducting metals that have been treated and assembled in a manner similar to the technology underpinning transistors and computer ‘chips’. The most common type of solar cell is silicon crystalline, but there are also several other types. Silicon cells
A standard silicon cell is composed of two layers of silicon, one of the world’s most abundant elements. Atoms of silicon have the property that when a photon – a quantum particle of light – hits them, its energy is transmitted to an electron in the outer ring of the atom, knocking it into a higher energy band. The amount of energy required to achieve this is determined by the ‘band gap’ of the material, which itself affects what portion of the solar spectrum a PV cell absorbs. Ordinarily the electron would soon fall back, as its negative charge is attracted to the positive charge of the atom’s nucleus. So, the two-layered structure of the cell is designed instead to capture that electron with minimum energy loss and make it flow in a circuit.
To achieve this, the upper layer of silicon is ‘dosed’ or injected with atoms of phosphorous, and the lower layer is ‘dosed’ with boron. These give the layers respectively a negative and a positive potential. The electron that was knocked off the upper layer is now attracted to the lower layer, leaving behind a hole. If the cells are connected in a circuit, this electron, and all the millions of others in a similar state, will flow around it, producing a current. Voltage is created by a reverse electric field around the junction between the layers – which is known as a p–n junction. Returning electrons will fill the original holes – and the process is ready to start again.
Other metals and compounds besides silicon can be manufactured to have this same property, and modules composed of them are both available and in development. More on this later. Silicon cells are the most widely available at present.
Figure 6.5A monocrystalline solar cell.
The effectiveness of the process rests upon matching the exact frequency of the light hitting the cells, to their composition. As we saw in the introduction, sunlight is made up of a spectrum of frequencies. The efficiency of a cell is therefore partly dependent on the range of frequencies it can respond to. Photons with insufficient energy will not excite the electrons to jump the band gap. (The higher the frequency, the more energy.) Those photons with more energy than that required will lose this as heat, causing the cell to heat up, reducing its efficiency. In addition, the greater the range of frequencies whose energy can be captured, the greater the cell’s efficiency and the more power that will be generated. Extending this range is another significant area of research and development that is reducing the cost of PV. Similarly, since average light quality varies with climate and location, the more the module’s characteristics can be matched with the average qualities of the light falling upon it, the more efficient it will be.
Types of silicon cell
Silicon cells can be of three types: monocrystalline, polycrystalline and amorphous. The names refer to the arrangement of the silicon crystals, which is determined by the manufacturing process.
Monocrystalline cells are grown from high quality pure silicon. They are efficient (up to 24 per cent, under laboratory conditions) but more expensive. Polycrystalline cells are made from silicon that is melted and cast, and contains Figure 6.6 When photons strike
a silicon PV cell on the negative side of the junction, they dislodge electrons of the same energy level, which move into the positive side of the junction. This creates a voltage difference between the front and rear sides of the cells. If they are connected in a circuit, then a current will flow.
many crystals but are slightly less costly to make and slightly less efficient but are the most common type, representing about 85 per cent of the market. Thin-film modules
So-called ‘thin-film’ modules are a more recent development that are gaining in popularity. They work in a similar way to silicon cells but are constructed differently, by depositing extremely thin layers of photosensitive materials onto a low-cost backing, such as glass, stainless steel or plastic, making them potentially flexible and useful in consumer applications including mobile phones. They are more tolerant of shade or higher temperatures than other sorts of cells.
Thin film modules are less costly to manufacture than the more material intensive crystalline technology, but they are less efficient – 5–13 per cent
(requiring 11–13m2 (118–140ft2) for 1kWp). Therefore, to obtain the same
output of power, double the surface area of thin film modules would be needed compared to crystalline modules, which is not always available. Thin film modules are produced in sheets sized for specified electrical outputs. Three types of thin film modules (depending on the active material used) are commercially available at the moment:
amorphous silicon (a-Si);
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cadmium telluride (CdTe);
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copper indium selenide (CIS) / Gallium diselenide/disulphide (CIGS).
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Amorphous silicon cells have no crystals and are less efficient, but they are cheaper as they use fewer raw materials. They are used in pocket calculators and watches. A film of the amorphous silicon is deposited as a gas on a surface such as glass, aluminium or plastic. Thin-film, commercially-produced CdTe and CIGS modules achieve around 10 per cent efficiency. Tellurium (used in CdTe) is a rare substance, and cadmium and gallium do not occur naturally in sufficient quantities to support a mass roll-out of cells utilizing them.
Multi-junction cells (a-Si/m-Si) include not one but several p–n junctions connected in series. The purpose of this is to capture more frequencies of sunlight. Different materials are used at each junction so the junctions have different band gaps and can therefore receive photons with different wavelengths. Efficiencies are currently in the region of 9 per cent outside of the laboratories, but could go much higher in the future.
Table 6.1 Surface area needed for different module types to generate 1kWp under
standard test conditions (STC).
Material Surface area needed for 1kWp
Monocrystalline silicon 5–7m2 (54–75ft2)
Polycrystalline silicon 6–8m2 (65–86ft2)
Amorphous silicon 11–17m2 (119–183ft2)
CdTe thin film 9–11m2 (97–119ft2)
CIS/CIGS thin film 8–10m2 (86–108ft2) Source: IEA