Monocrystalline Solar Cells

by Clint Ouma

If you see a solar panel, chances are it’s made of monocrystalline solar cells. Monocrystalline, also commonly known as crystalline silicon or single crystalline silicon, are by far the most widely used solar photovoltaic technology.

This article takes a detailed look at how monocrystalline solar panels work. If you’re looking for a more simple explanation of solar photovoltaics, you may wish to read the article on how solar panels work.

Monocrystalline cells were first developed in 1955 [1]. They conduct and convert the sun’s energy to produce electricity.

When sunlight hits the silicon semiconductor, enough energy is absorbed from the light to knock electrons loose, allowing them to flow freely. Monocrystalline silicon solar cells are designed in such a way that the free electrons can be directed, within the cell’s electric field, in a path or a circuit as electricity which is used to power various appliances [2].

The power (measured in watts) of the cell is determined by the current and the voltage of the cell. The voltage depends on the cell’s internal electric field [2].

Monocrystalline vs Polycrystalline Solar Panels

Crystalline silicon solar cells derive their name from the way they are made. The difference between monocrystalline and polycrystalline solar panels is that monocrystalline cells are cut into thin wafers from a singular continuous crystal that has been grown for this purpose. Polycrystalline cells are made by melting the silicon material and pouring it into a mould [1].

The uniformity of a single crystal cell gives it an even deep blue colour throughout. It also makes it more efficient than the polycrystalline solar modules whose surface is jumbled with various shades of blue [1].

Apart from the crystal growth phase, their is little difference between the construction of mono- and polycrystalline solar cells.

The cells are usually laminated using tempered glass on the front and plastic on the back. These are joined using a clear adhesive and then the module is framed with aluminium. Single crystal modules are usually smaller in size per watt than their polycrystalline counterparts [1].

Why is silicon used in solar cells?

The atomic structure of silicon makes it one of the ideal elements for this kind of solar cell. The silicon atom has 14 electrons and its structure is such that its outermost electron shell contains only four electrons. In order to be stable, this shell needs to have eight electrons.

In its normal state or pure form, each silicon atom attaches itself to four other silicon atoms to form a stable silicon crystal [2].

In the silicon crystal’s pure state, there are very few free electrons available for carrying the electric current. In order to alter their electrical conductivity, other elements are introduced to the silicon as useful impurities in a process known as Doping [2].

Doping of silicon semiconductors for use in solar cells

Doping is the formation of P-Type and N-Type semiconductors by the introduction of foreign atoms into the regular crystal lattice of silicon or germanium in order to change their electrical properties [3].

As mentioned above, electricity is generated when free electrons are directed to carry a current within the cell’s electric field [2].

When the silicon atoms share their electrons, they can attain equilibrium easily because each atom needs four electrons and each can give four electrons.

The elements used in doping however, either have 5 or 3 electrons that they can share which are also known as valence electrons [3].

When elements with five valence electrons are introduced to the silicon crystals, the normal sharing of electrons begins, but the fifth electron remains unattached or unbound [2]. This unbound electron can easily be dislodged from the atomic shell when energy is introduced to the crystal causing it to be negatively charged.

This makes the crystal an N-Type semiconductor where the “N” stands for negative [2].

Some of the elements with 5 valence electrons include phosphorus, antimony and arsenic; phosphorus is the most commonly used element in crystalline solar cells.

On the other hand, when elements with three valence electrons such as boron, aluminium and gallium are introduced, there is a deficiency of electrons and instead, holes are formed [3].

This means that the crystal will carry a positive charge because it needs extra electrons to fill the hole left thus making it a P-Type Semiconductor where the “P” stands for positive. The holes move around seeking to be filled just as the free electrons move around ‘looking’ for holes to fill [2].

The P-Type and N-Type silicon semiconductors are combined to make the solar cell. When in contact, these two semiconductors generate the electric field which is necessary for electricity to flow in the solar cell [2].

The extra or free electrons in the N side are attracted to fill the holes in the P side. Unfortunately, at the point of contact, also called the junction of the two semiconductors, the electrons and holes mix to form a barrier preventing the electrons on the N side from crossing to the P side.

This barrier becomes an electric field separating the two sides when equilibrium is reached and acts as a diode allowing the flow of electrons in one direction, from the P-type semiconductor to the N-type [2].

Electricity generation at cell level

All that is needed for the electricity to be generated is the flow of electrons through a path provided within the electric field. However, we have seen that the flow of electrons has been localized and limited by the electric field which acts as a barrier between the cells. Nevertheless, this flow can be achieved when sunlight hits the solar cell.

The sun’s rays carry their energy in form of photons. Each photon usually has enough energy to dislodge one electron when it hits the solar cell [2].

In freeing one electron, it automatically causes a hole to be freed simultaneously. Due to the barrier’s disposition to allow the flow of electrons to the N-side only, the freed electron – if within the range of the electric field – will be sent to the N-Type semiconductor while the hole is sent to the P-Type.

Since this motion does not restore balance, the electron that was just displaced will seek to return to the P-Side so that neutrality is restored. Since the electron cannot cross back to the P Side from the N-Side via the barrier between the semiconductors, an external current path is built to allow this electron to return to the P – Side.

While the electrons flow through the external current path, we can make use of its energy to power electrical appliances [2].

Crystalline silicon solar cell efficiency

One of the major subjects of research into crystalline silicon solar cells is their efficiency. It’s widely believed that the absolute limit is that 25% of the solar energy that hits a crystalline cell can be converted to electricity [2].

Researchers are hard at work to reach this efficiency with some companies like Sunpower and Sanyo achieving high efficiencies of up to 24% [4].

The wide spectrum of sun light wavelengths is responsible for the loss of about 70% of the energy that hits the solar cell [2].

The light from the sun has different wave lengths which we see as different colours. The different wavelengths also differ in energy content; some have more energy than the solar cell needs to produce electricity while others have less energy.

The crystalline silicon cell needs about 1.1 eV (Electron Volts) of energy to release an electron in the semiconductor; any energy that is more or less than this simply goes through the cell with no effect [2]. This energy used to release the electron is unique for each material and is known as the material’s band gap.

The band gap also determines the voltage of the cell. If the band gap is low, the voltage is also low. Therefore, although using a material that has a low band gap can increase the current of the cell, it lowers the cell’s voltage. Since Power is the product of current and voltage, the power output of the cell cannot be improved in this way. The optimal band gap for a solar cell made from one material has thus been found to be 1.4 eV so as to balance the effect of the current and voltage [2].

The Future of Monocrystalline Silicon Solar Cells

Having been in the market for more than 50 years, silicon solar cells are approaching if not passing their peak potential.

As such, extensive research has gone into improving the efficiency and lowering production costs of these systems.

Now, new technology is hitting the market. The introduction of thin film solar modules, for example, has attracted a lot of attention.

Market forces in countries like the U.S. have made it unprofitable for many companies to continue manufacturing the traditional silicon solar cells with companies like GE opting to shift its resources to manufacture thin film solar modules while it closes down its silicon crystalline solar cell plants [5].

This closure of firms has greatly been caused by the fact that manufacturing the traditional solar panels is a lot cheaper in other countries such as China, which is why some American companies are shifting production to the oriental super power [5].

The main difference between the traditional silicon cells is in the materials used to make them. The modifications that go into silicon for use in solar cells make it very expensive [1].

New construction methods and materials are much cheaper and although the efficiencies of the new technologies are much lower with thin film efficiencies ranging from 4% – 12% [6], there is still a lot of room for improvement in the new systems.

Multi-junction solar modules, which combine up to four different elements in their construction, have even surpassed the maximum efficiency of crystalline silicon modules with Spectrolab achieving 41.9% efficiency in the NREL Lab Test [4] while a commercial subsidiary of Boeing set a practical record of 39.2% [4].

Article References

[1] Wholesale Solar – Three Photovoltaic Technologies: , Polycrystalline and thin film: http://www.wholesalesolar.com/Information-SolarFolder/celltypes.html

[2] The Solar Plan – Converting Photons to Electrons: http://www.thesolarplan.com/articles/how-do-solar-panels-work.html

[3] Hyper physics – Doping of Semiconductors: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html

[4] The Green World Investor – Efficiency of Solar Cells Made From Silicon (, Multicrystalline), Thin Film (CIGS, CIS, aSi, CdTe, CZTS) and Multijunction: http://www.greenworldinvestor.com/2011/04/17/efficiency-of-solar-cells-made-of-siliconmulticrystalline-thin-film-cigscisasicdtecztsmultijunction/

[5] The Environmental News Service – Modern Solar Technologies Shading Out Silicon Solar Panels: http://www.ens-newswire.com/ens/nov2009/2009-11-10-094.html

[6] Civic Solar – Thin Film vs. Crystalline Silicon PV Modules: http://www.civicsolar.com/resource/thin-film-vs-crystalline-silicon-pv-modules

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