All solar cells have the same basic structure. Light enters the system via an optical coating or antireflection layer that minimises the amount of light lost by reflection. This traps the light and promotes its transmission to the energy conversion layers below. This top antireflection layer is typically an oxide of silicon, tantalum or titanium and is formed by spin-coating or vacuum deposition.
Below the top antireflection layer are three energy conversion layers. These are the top junction layer, the absorber layer and the back junction layer. There are also two additional electrical contact layers to carry the electric current to an external load and then back to the cell to complete the electric circuit.
The top electrical contact layer on the surface of the cell uses a grid pattern composed of a good conductor material such as metal. However, since metal blocks light, the grid lines are thin and widely spaced to allow light through while also allowing the collection of the electrical current. The back electrical contact layer has no such restrictions and is usually made solely of metal.
To maintain high efficiency, a solar cell absorber needs to be able to absorb electromagnetic radiation at the wavelengths of visible light. Materials able to absorb this visible radiation are called semiconductors and can manage this at thicknesses of just one-hundredth of a centimetre or less. The junction forming and contact layers are even thinner, meaning that the thickness of a solar cell is basically that of the absorber. Semiconductor materials used in solar cells include copper indium selenide, gallium arsenide, indium phosphide and silicon.
When light meets a solar cell, electrons in the absorber layer go from a lower-energy ‘ground state’ where they are bound to specific atoms in the solid, to an ‘excited state’ where they are able to move freely through the solid. The junction forming layers create a built-in electric field that produces the photovoltaic effect. This electric field creates a collective electron motion so that they flow past the electrical contact layers and into an external circuit. The two junction forming layers need to be dissimilar to the absorber to produce the electric field to carry the electric current. As a result, these can be different semiconductors (or the same ones with different conduction types) or a metal and a semiconductor. Solar cells need to cover as large an area as possible since the amount of power produced is proportional to the illuminated area.
Since solar cells cannot produce power in darkness, they store some of the energy so it can be used when light is not available. This can be by charging electrochemical storage batteries and is similar to the process of photosynthesis in plants.
- Sunlight shines on the surface of the cell
- Energy is carried through the layers of the cell as photons
- The photons give their energy over to electrons in the lower layer
- The electrons use this energy to jump back into the upper layer and escape into the circuit
- The electrons flowing around the circuit provide the power to a device
Solar cells can be divided into three broad types, crystalline silicon-based, thin-film solar cells, and a newer development that is a mixture of the other two.
1. Crystalline Silicon Cells
Around 90% of solar cells are made from crystalline silicon (c-Si) wafers which are sliced from large ingots grown in laboratories. These ingots take up to a month to grow and can take the form of single or multiple crystals. Single crystals are used to create monocrystalline solar panels and cells (mono-Si), while multiple crystals are used for polycrystalline panels and cells (multi-Si or poly c-Si).
These solar cells use an n-type ingot, which are made by heating silicon chunks with small amounts of phosphorus, antimony or arsenic as the dopant. The n-type ingot is coupled with a p-type silicon layer, which uses boron as the dopant. The n-type and p-type ingots are fused to create a junction in a process that was first devised in 1954.
Monocrystalline cells have a distinctive appearance and are often coloured as well as tending to have a cylindrical shape. These cells are cut into shape, which can be wasteful, but do provide the highest levels of efficiency. Polycrystalline cells do not need to be cut to shape as the silicon is melted and poured into square moulds. Polycrystalline solar panels are seen as being a mid-range option both in terms of price and efficiency.
2. Thin Film Solar Cells
Crystalline silicon cells are made from wafers that are just a fraction of a millimetre deep (around 200 micrometers, 200μm), however thin-film solar cells, also called thin-film photovoltaics are around 100 times thinner. These thin film solar panels and cells are made from amorphous silicon (a-Si), in which the atoms are randomly arranged rather than in an ordered crystalline structure. These films can also be made from cadmium-telluride (Cd-Te), copper indium gallium diselenide (CIGS) or organic PV materials.
These cells are produced by layering photovoltaics to create a module and are the cheapest option for producing solar panels. The cells can be laminated onto windows, skylights, roofing tiles and other substrates, including glass, metals and polymers. However, despite this flexibility, they are not as efficient as regular crystalline silicon cells. Where crystalline silicon cells can produce a 20% efficiency, these thin film cells only reach around 7% efficiency. Even the very best CIGS cells barely reach 12% efficiency.
4. Third Generation Solar Cells
The latest solar cell technologies combine the best features of crystalline silicon and thin-film solar cells to provide high efficiency and improved practicality for use. They tend to made from amorphous silicon, organic polymers or perovskite crystals, and feature multiple junctions made up from layers of different semiconducting materials.
These cells have the potential to be cheaper, more efficient and more practical than other types of cell, and have been shown to be able to achieve around 30% efficiency (with a perovskite-silicon tandem solar cell).
Solar cells can only produce electricity based on the light they receive and are able to process. Most cells convert just 10-20% of the energy they receive into electricity, with the most efficient cells laboratory cells reaching around 45% efficiency under the perfect conditions. The reason for this is that solar cells are optimised to only capture photons from within a particular frequency band, with those outside this band being wasted. In addition, of those within the frequency band, some photons lack the required energy to create electrons while others have too much and so the excess is wasted.
Most real-world solar panels only reach 10-20% efficiency as real-world factors such as panel construction, positioning, alignment, shadows, heat and lack of cleanliness can all reduce the optimum efficiency.
This overall solar cell efficiency is determined by a combination of charge carrier separation efficiency, conductive efficiency, reflectance efficiency and thermodynamic efficiency.
There are seven stages to making solar cells, as follows:
Stage One: Purify the Silicon
Silicon dioxide is placed in an electric arc furnace and a carbon arc is applied to release the oxygen. This leaves carbon dioxide and molten silicon, which will yield silicon with just 1% impurity, but even this is not pure enough to use in solar cells. Rods of the 99% pure silicon is passed several times, in the same direction, through a heated zone in a process called the floating zone technique. Repeating this process pulls all of the impurities to one end of the rod, eventually allowing this impure end to simply be removed.
Stage Two: Creating Single Crystal Silicon
The most common method to create single crystal silicon is called the Czochralski Method, whereby a seed crystal of silicon is dipped into melted polycrystalline silicon. By rotating this seed crystal as it is removed from the melted polycrystalline, a cylindrical ingot or boule is created.
Stage Three: Cut The Silicon Wafers
The boule from stage two is cut into silicon wafers using a circular saw. Diamond is the best saw material for this job, producing slices of silicon that can then be cut further to form squares or hexagons that are easier to fit together onto the surface of a solar cell. The sliced wafers are then usually polished to remove saw marks, although some manufacturers leave these imperfections as it is believed that rougher cells may absorb more light effectively.
Stage Four: Doping
Having purified the silicon in an earlier stage, the material can now have impurities added back in. This process is called doping and usually involves using a particle accelerator to fire phosphorous ions into the ingot. Controlling the speed of the ions allows you to control the depth of penetration. This part of the process can be skipped by using the more traditional method of introducing boron while cutting the wafers.
Stage Five: Add Electrical Contacts
Electrical contacts connect solar cells to one another and act as the receiver for the produced current. These contacts are thin so as not to block sunlight from entering the cell and are made from metals such as palladium or copper. The metal is either vacuum evaporated through a photoresist or deposited on the exposed portion of the cells, which have been partially covered with wax. Once the contacts are in place, thin strips, usually of tin-coated copper, are placed between the cells.
Stage Six: Add the Anti-Reflective Coating
The shiny nature of silicon means that it can reflect up to 35% of the sunlight that hits it. An anti-reflective coating is added to the silicon in order to reduce the amount of sunlight lost by reflection. Titanium dioxide and silicon oxide are commonly used for this, with the material being heated until the molecules boil off and travel onto the silicon where they condense. Alternatively, a high voltage can be used to remove molecules from the material and deposit them on the silicon at the opposite electrode in a process called ‘sputtering.’
Stage Seven: Encapsulate the Cell
Finally the solar cells are encapsulated in silicon rubber or ethylene vinyl acetate and placed into an aluminium frame with a back sheet and glass or plastic cover for protection.
The amount of potential energy that reaches the Earth from the Sun each day is easily enough to meet all of our power generation needs. However, as mentioned above, most solar cells are only able to capture around 15% of the light that reaches them. Of course, the larger a solar panel or array is, the more energy it can capture.
Since monocrystalline, polycrystalline and thin film solar cells have differing efficiencies, we will look at the most common type of crystalline silicon solar cells.
A single solar cell (which is about the size of a compact disc), can generate 3-4.5 watts. By placing 40 of these cells together into a typically sized module, you can generate 100-300 watts. Placing several of these modules together to form several solar panels it is possible to generate several kilowatts of energy, which should be enough to meet the peak energy needs of most homes. Solar farms are able to produce more power still, with estimates saying it would take 22,000 panels across 30 acres to generate 4.2 megawatts of power; enough to power 1,200 homes.
By way of comparison, it would take 500-1000 solar roof installations to match the power generated by a large wind turbine (2-3 megawatts), while it would take some one million solar roof installations to reach the output of a large coal or nuclear plant (which are rated in gigawatts).
This leads to questions about the ability of solar power to meet our energy needs in the future…
Solar power is already providing many benefits for users, while also helping to mitigate the negative environmental impact of fossil fuel power generation. As well as the reduced air pollution and carbon dioxide emissions that come with switching over to solar, there is also benefits on a more local level as it places power generation at the point of use.
On the smallest scale, this has allowed us to power watches and calculators without batteries, while road and railway maintenance signs can also be solar powered so they can be used in even the remotest of locations. Solar power is also being used in some countries to power water pumps, telephone boxes and even refrigeration units in hospitals and health clinics.
Current developments are underway to create self-cleaning coatings for solar panels to improve their efficiency, as well as projects to reduce material waste during manufacture (see the OLEDSOLAR Case Study, below).
In the future, as fossil fuel resources shrink, there will be an increasing need to look towards renewable energy sources including solar. Those in favour of a ‘solar economy’ believe that most of our global energy requirements could be met by solar panels operating at 20% efficiency and covering just 191,817 square miles of the Earth’s surface. This is possible, at least in theory, given that silicon is the second-most abundant element in the Earth’s crust.
While it is tempting to see the cheap, clean and renewable resource of sunlight being used to power humanity, the advances in other renewable resources such as wind power makes this unlikely. Instead, it is more likely that solar power will just be one part of an overall renewable energy mix made up of different sources.
However, solar power will still find local applications on a smaller scale, especially in developing countries with suitable climates. Solar power is also ideal for domestic and small scale commercial use, leading some to envision a time when everybody can go ‘off grid’ and create their own personal energy supplies. While this is a tempting notion, others feel that a grid needs to be maintained so that we can ensure everyone gets the power they need regardless of circumstances.
Solar cells are widely used as a renewable source of energy in scales ranging from the smallest handheld devices right up to powering entire communities. As the move towards achieving net zero carbon emissions continues, solar energy looks set to become part of an overall renewable energy mix. With this will come investment and technological advances alongside reduced costs for solar energy systems.
You can see more about TWI’s work in this area of industry here.
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