When light strikes a photovoltaic (PV) cell, also known as a solar cell, the light may be reflected, absorbed, or flow through the cell. The PV cell is comprised of semiconductor material; “semi” indicates that it conducts electricity better than an insulator but less efficiently than a metal. PV cells utilise a variety of different semiconductor materials.
When exposed to light, the semiconductor absorbs the light’s energy and converts it to electrons, which are negatively charged particles in the material. This additional energy permits electrons to flow as an electrical current through the material. This current is retrieved via conductive metal contacts — grid-like lines on solar cells — and may be utilised to power your home and the rest of the electric grid.
The efficiency of a photovoltaic (PV) cell is simply the ratio between the quantity of electrical power flowing out of the cell and the amount of energy from the light shining on it, which reflects how well the cell converts one type of energy into another. The quantity of energy generated by PV cells is dependent on the properties (such as intensity and wavelengths) of the available light and a variety of cell performance factors.
The bandgap, which determines which wavelengths of light the material can absorb and convert into electrical energy, is an essential feature of PV semiconductors. If the bandgap of the semiconductor corresponds to the wavelengths of light striking the PV cell, then the cell can efficiently utilise all the available energy.
Learn more about the most prevalent semiconductor materials used in PV cells below.
SILICON
Silicon is by far the most prevalent semiconductor used in solar cells, accounting for nearly 95% of modules marketed today. It is also the second most plentiful substance on the planet (after oxygen) and the most prevalent semiconductor used in computer chips. Silicon atoms interconnected to create a crystal lattice compose crystalline silicon cells. This lattice’s structured structure improves the efficiency of light-to-electricity conversion.
Currently, silicon solar cells provide a mix of high efficiency, low cost, and long lifetime. Modules are anticipated to survive at least 25 years and provide over 80% of their original power after this period.
THE USE OF THIN-FILM PHOTOVOLTAICS
One or more thin layers of photovoltaic (PV) material are deposited on a supporting material such as glass, plastic, or metal to create a thin-film solar cell. Cadmium telluride (CdTe) and copper indium gallium diselenide are the two most prevalent thin-film PV semiconductors on the market today (CIGS). Both materials can be put directly on the front or rear surface of the module.
CdTe is the second-most popular PV material after silicon, and CdTe cells may be produced by low-cost techniques. While this offers them a cheaper alternative to silicon, their efficiency is not quite as good. CIGS cells offer perfect qualities for a PV material and good efficiency in the laboratory, but the intricacy of integrating four components makes the move from the laboratory to production more difficult. CdTe and CIGS require greater protection than silicon for outdoor operation to last longer.
PEROVSKITE PHOTOVOLTAICS
Perovskite solar cells are a form of thin-film solar cell that derives their name from their distinctive crystal structure. Perovskite cells are constructed with layers of materials that are printed, coated, or vacuum-deposited onto the substrate, an underlying support layer. Typically, they are simple to construct and may achieve efficiency comparable to crystalline silicon. In the laboratory, the efficiency of perovskite solar cells has increased faster than any other PV material, from 3% in 2009 to over 25% in 2020. In order for perovskite photovoltaic (PV) cells to be economically viable, they must be able to withstand the elements for 20 years, thus researchers are attempting to make them more robust and to create large-scale, low-cost production procedures.
ORGANIC PHOTOVOLTAICS
Organic photovoltaic, or OPV, cells are constructed of carbon-rich (organic) molecules and can be modified to improve a particular PV cell property, such as bandgap, transparency, or colour. OPV cells are now around half as efficient as crystalline silicon cells and have shorter operational lifetimes, although high-volume production might be less expensive. Additionally, they may be applied to a variety of supporting materials, such as flexible plastic, allowing OPV to serve a wide range of purposes. PV
QUANTUM DOTS
Quantum dot solar cells carry energy through quantum dots, which are nanometer-sized particles of various semiconductor materials. Quantum dots offer a novel method for processing semiconductor materials, but it is difficult to build an electrical connection between them, thus they are not particularly efficient at the moment. However, they are simple to manufacture as solar cells. They can be placed into a substrate using spin-coating, spraying, or roll-to-roll printers similar to those used for printing newspapers.
Quantum dots may gather difficult-to-capture light and can be coupled with other semiconductors, such as perovskites, to maximise the performance of a multijunction solar cell due to their variable size and bandgap (more on those below).
MULTIJUNCTION PHOTOVOLTAICS
Layering numerous semiconductors to create multijunction solar cells is another method for enhancing PV cell efficiency. These cells are basically stacks of several semiconductor materials, as opposed to single-junction cells, which only contain a single semiconductor. Each layer has a unique bandgap, so they each absorb a separate portion of the solar spectrum, allowing them to utilise sunlight more efficiently than single-junction cells. Multijunction solar cells can achieve record levels of efficiency due to the fact that the light that is not absorbed by the top semiconductor layer is collected by a layer underneath it.
While all solar cells with more than one bandgap are multijunction solar cells, a tandem solar cell has precisely two bandgaps. Multijunction solar cells consisting of semiconductors from columns III and V of the periodic table are referred to as multijunction III-V solar cells.
Multijunction solar cells with efficiency greater than 45 percent have been proven, but they are expensive and difficult to build, therefore they are saved for space exploration. The military use III-V solar cells in drones, and scientists are investigating additional applications where great efficiency is essential.
CONCENTRATION PHOTOVOLTAICS
Concentration PV, also known as CPV, employs a mirror or lens to concentrate sunlight onto a solar cell. By concentrating sunlight on a tiny region, less PV material is used. CPV cells and modules have the highest overall efficiencies due to the fact that photovoltaic materials grow more efficient as light becomes more concentrated. However, more expensive materials, manufacturing procedures, and the capacity to watch the sun’s position are required, making it difficult to demonstrate the requisite cost advantage over today’s high-volume silicon modules.
Learn more about photovoltaics research at the Solar Energy Technologies Office, peruse these informational materials on solar energy, and discover how solar energy works.