At Solargex we are constantly asked “How do these panels work?” and “How are these panels made?” Whilst we do not endeavour to provide great detail – for to do so would require many, many pages! – we thought it would be beneficial to put a brief document together to address these queries.
The PV (photovoltaic) industry changes constantly; we are amazed (and of course pleased) that developments have seen efficiencies of output increase greatly. PV supply is now a viable option in applications as diverse as the ubiquitous solar-powered calculator to new-generation
We encourage discussion and debate about PV applications, and welcome any contact from you. At Solargex we aim to grow as the industry also grows, and to keep providing you will accurate information, and unsurpassed service. Join us!
- How are Solar Panels made?
1.1 What is Silicon?
1.2 Crystalline Solar Panel Manufacture.
1.3 Amorphous Solar Panel Manufacture.
1.4 Multi-Junction Solar Panel Manufacture.
- How Do Photovoltaics Work?
- Benefits of Photovoltaic Energy Production.
There are two main types of solar panel: crystalline and amorphous. In both cases the key ingredient is silicon. Amorphous panels are typically lower cost, less susceptible to breakage, and use less silicon. However the power output of amorphous panels is typically lower than crystalline panels meaning that an amorphous solar installation would take up more space than a similarly power-rated crystalline installation. Amorphous solar panels deteriorate faster than crystalline solar panels and so their power output will fall more quickly during the years of use.
1.1 What is silicon?
Silicon (Si) is an abundant non-metallic chemical element which makes up almost 30% of the earth's crust and is the 7th most common element in the Universe. Silicon has two forms - amorphous (brown), and crystalline (dark).
Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom is therefore always highly reactive until this outer shell is filled. In pure silicon, one atom will share electrons with four of its neighbour silicon atoms. That's what forms the crystalline structure, which is important in a photovoltaic cell.
Pure silicon is a poor conductor of electricity because none of its electrons are free to move about. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.
1.2 Crystalline Solar Panel Manufacture.
To make the solar cells which make up a crystalline solar panel, crystalline silicon is sliced into thin disks (wafers) a few millimetres thick or thinner. These are then cut to shape, polished, and any holes in them filled to make the finished wafer uniform.
In order to generate electricity, dopants (contaminants) have to be added to the pure silicon wafer. Typically a layer of phosphorous is coated onto the wafer and the surface is heated. Phosphorous (a common dopant) atoms then diffuse throughout the silicon wafer contaminating it as required. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbour atoms, but the phosphorous has one electron that doesn't form part of a crystalline structure. When doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons.
The other commonly used dopant is boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon ("p" for positive) has absent electrons, so they carry the opposite (positive) charge.
The different types of doped silicon wafers are then aligned together to make up a solar cell, and the solar cells are arranged on a backing panel to make a solar panel (or module). Conductive metal strips are fixed to the sunward face of each solar cell.
An antireflective coating is applied to the top of the cell.
Finally a layer of glass is glued over the top of the collection of solar cells to provide protection from the elements while still permitting sunlight to pass through to the completed crystalline solar module.
1.3 Amorphous Solar Panel Manufacture
Unlike crystalline solar panels, amorphous panels are not made up of a collection of interconnected solar cells manufactured from expensive crystalline silicon. Instead a very thin homogenous layer of silicon atoms and dopants is simply sprayed onto a backing material - typically glass or metal, but also on plastic surfaces to make flexible solar panels, or on roofing tiles to make solar roof tiles. The silicon layer produced can be 100 times thinner than the silicon wafers in a typical crystalline solar cell greatly reducing material costs.
An entire solar module is made in one go, so manufacturing costs are reduced by not having the expense of making silicon wafers, and by the greatly simplified assembly.
1.4 Multi-junction Solar Panel Manufacture
Multi-junction solar cells are a more recent development in amorphous solar technology using multiple thin layers of doped silicon to capture energy across the whole light spectrum. Multi-junction cells are the most efficient solar cells currently generally available.
We provide more information on multi-junction solar cells in the next section.
2. How do Photovoltaics Work?
Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, electric current results that can be used as electricity.
The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel Prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications, particularly for remote locations.
Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely from the P-type silicon to the N-type silicon. These electrons are called free carriers. Photovoltaic cells also all have one or more electric fields that act as a diode to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell to electrons to flow through the contacts back to their original side (the P-type side), we can draw that current off to use externally. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.
A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.
Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.
Today's most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. Light striking the solar cell is comprised of photons of a wide range of energies; some of the photons won't have enough energy to create the necessary movement of electrons from the P-type to the N-type layers (the PN junction). These photons simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts (eV) and defined by the particular cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.
One way to get around this limitation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as "multi-junction" cells (also sometimes called "cascade" or "tandem" cells). Multi-junction devices can achieve higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.
The ideal solar cell in theory would have hundreds of different layers, each one tuned to a small range of light wavelengths all the way from ultraviolet to infrared. Although this would lead to fantastic efficiencies of over 70% it is not possible in practice due to difficulties in manufacturing such complicated crystals. Therefore researchers have focussed their attentions on multi-junction solar cells with just a few different layers
Much of today's research in multi-junction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 40% under concentrated sunlight.
3. Benefits of Photovoltaic Energy Production.
- The 89 petawatts of sunlight reaching the earth's surface is plentiful - almost 6,000 times more than the 15 terawatts of average power consumed by humans. Additionally, solar electric generation has the highest power density among renewable energies.
- Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development.
- Facilities can operate with little maintenance or intervention after initial setup.
- Solar electric generation is economically superior where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
- When grid-connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by net metering. Time-of-use net metering can be highly favourable to small photovoltaic systems.
- Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses.
- Once the initial capital cost of building a solar power plant has been spent, operating costs are extremely low compared to existing power technologies.
- Compared to fossil and nuclear energy sources, very little research-money has been invested in the development of solar cells, so there is much room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40% and efficiencies are rapidly rising while mass production costs are rapidly falling.
With the growth in interest and popularity of solar (and indeed all alternative) energy, there are numerous sources of technical assistance. At Solargex, we strive to stay abreast of the newest technologies; our technicians are trained and experienced in all aspects of PV electrical generation. Below are a few of the links to the text and images provided in this document.
Renewable Energy Website REUK, (2007) “How are Solar Panels Made” http://www.reuk.co.uk/How-are-Solar-Panels-Made.htm retrieved 31 Jan 2009
Knier, G., (n.d.) “How do Photovoltaics Work?” http://science.nasa.gov/headlines/y2002/solarcells.htm retrieved 30 Jan 2009
HowStuff Works, Inc (n.d.), “Solar Cells” http://science.howstuffworks.com/solar-cell1.htm retrieved 1 Feb 2009
HowStuff Works, Inc (n.d.), “How Silicon Makes a Solar Cell” http://science.howstuffworks.com/solar-cell2.htm retrieved 1 Feb 2009
HowStuff Works, Inc (n.d.), “Anatomy a Solar Cell” http://science.howstuffworks.com/solar-cell3.htm retrieved 31 Jan 2009
HowStuff Works, Inc (n.d.), “Energy Loss in a Solar Cell” http://science.howstuffworks.com/solar-cell4.htm retrieved 31 Jan 2009
Renewable Energy Website REUK, (2007) “How do PV Solar Panels Work” http://www.reuk.co.uk/How-Do-PV-Solar-Panels-Work.htm retrieved 31 Jan 2009
Wikipedia contributors. (2009) “Photovoltaics.” Wikipedia, The Free Encyclopedia. http://en.wikipedia.org/w/index.php?title=Photovoltaics&oldid=266879494 retrieved Jan 31 2009
Renewable Energy Website REUK, (2007) “Multi-Junction Solar Cells” http://www.reuk.co.uk/Multi-Junction-Solar-Cells.htm retrieved 31 Jan 2009
Beckman, William A. and Duffie, John A., (n.d.) “Solar Engineering of Thermal Processes”. 2nd Ed. John Wiley and Sons, Inc. 1991, pp.768-793.
Zweibel, Ken. (1990) “Harnessing Solar Power: The Photovoltaics Challenge”. Plenum Press,
