Photovoltaic cells convert sunlight into electricity
A photovoltaic (PV) cell, commonly called a solar cell, is a nonmechanical device that converts sunlight directly into electricity. Some PV cells can convert artificial light into electricity.
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Sunlight is composed of photons, or particles of solar energy. These photons contain varying amounts of energy that correspond to the different wavelengths of the solar spectrum.
A PV cell is made of semiconductor material. When photons strike a PV cell, they will reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the photons that are absorbed provide energy to generate electricity. When the semiconductor material absorbs enough sunlight (solar energy), electrons are dislodged from the material's atoms. Special treatment of the PV cell's surface during manufacturing makes the front surface of the cell more receptive to the dislodged, or free, electrons so that the electrons naturally migrate to the surface of the cell.
The flow of electricity in a solar cell
The movement of electrons, which all carry a negative charge, toward the front surface of the PV cell creates an imbalance of electrical charge between the cell's front and back surfaces. This imbalance, in turn, creates a voltage potential similar to the negative and positive terminals of a battery. Electrical conductors on the PV cell absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as a battery, electricity flows through the circuit.
PV cells, panels, and arrays
The PV cell is the basic building block of a PV system. Individual cells can vary from 0.5 inches to about 4.0 inches across. However, one PV cell can only produce 1 or 2 Watts, which is only enough electricity for small uses, such as powering calculators or wristwatches.
PV cells are electrically connected in a packaged, weather-tight PV panel (sometimes called a module). PV panels vary in size and in the amount of electricity they can produce. Electricity-generating capacity for PV panels increases with the number of cells in the panel or in the surface area of the panel. PV panels can be connected in groups to form a PV array. A PV array can be composed of as few as two PV panels to hundreds of PV panels. The number of PV panels connected in a PV array determines the amount of electricity the array can generate.
PV cells generate direct current (DC) electricity. DC electricity can be used to charge batteries that power devices that use DC electricity. Nearly all electricity is supplied as alternating current (AC) in electricity transmission and distribution systems. Devices called inverters are used on PV panels or in PV arrays to convert the DC electricity to AC electricity.
PV cells and panels produce the most electricity when they are directly facing the sun. PV panels and arrays can use tracking systems to keep the panels facing the sun, but these systems are expensive. Most PV systems have panels in a fixed position that are usually facing directly south in the northern hemisphere&#;or directly north in the southern hemisphere&#;at an angle that optimizes the physical and economic performance of the system.
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Solar photovoltaic cells are grouped in panels, and panels can be grouped into arrays of different sizes to power water pumps, power individual homes, or provide utility-scale electricity generation.
Source: National Renewable Energy Laboratory (copyrighted)
PV system efficiency
The efficiency that PV cells convert sunlight to electricity varies by the type of semiconductor material and PV cell technology. The efficiency of commercially available PV panels averaged less than 10% in the mid-s, increased to around 15% by , and is now approaching 25% for state-of-the art modules. Experimental PV cells and PV cells for niche markets, such as space satellites, have achieved nearly 50% efficiency.
PV system applications
When the sun is shining, PV systems can generate electricity to directly power devices such as water pumps or supply electric power grids. PV systems can also charge a battery to provide electricity when the sun is not shining for individual devices, single homes, or electric power grids.
- PV systems can supply electricity in locations where electricity distribution systems (power lines) do not exist, and they can also supply electricity to electric power grids.
- PV arrays can be installed quickly.
- The environmental effects of PV systems located on buildings are minimal.
Source: National Renewable Energy Laboratory (copyrighted)
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Source: National Renewable Energy Laboratory (copyrighted)
History of PV systems
The first practical PV cell was developed in by Bell researchers. Beginning in the late s, PV cells were used to power U.S. space satellites. By the late s, PV panels were providing electricity in remote, or off-grid, locations that did not have electric power lines. Since , most PV systems in the United States are grid-connected&#;they are connected to an electric power grid. These PV systems are installed on or near homes and buildings and at utility-scale power plants that have at least 1 megawatt of electric-generation capacity. Technological advances, lower costs for PV systems, and various financial incentives and government policies, especially tax credits and net metering, have helped to greatly expand PV use since the mid-s. Millions of grid-connected PV systems are now installed in the United States.
Electricity generation at utility-scale PV power plants increased from 6 million kilowatthours (kWh) (or 6,000 megawatthours [MWh]) in to about 162 billion kWh (or 161,651,000 MWh) in . About 74 billion kWh (or 73,619,000 MWh) were generated by small-scale, grid-connected PV systems in , up from 11 billion kWh (or 11,233,000 MWh) in . Small-scale PV systems have less than 1,000 kilowatts of electricity-generation capacity. Most small-scale PV systems are located on buildings and are sometimes called rooftop PV systems.
Last updated: May 24, , with preliminary data for from the Electric Power Monthly, February .
By Mary-Russell Roberson
One reason is that today's photovoltaic cells are relatively inefficient. They put out only about a quarter of the energy they take in from the sun. If that efficiency could be improved, solar panels could take up less real estate while pumping out more electricity.
&#;At the end of the day, efficiency is the challenge with solar energy,&#; says Adrienne Stiff-Roberts, PhD, who is the Jeffrey N. Vinik Professor of Electrical and Computer Engineering. &#;If solar cells are more efficient, then the technology is less expensive [per unit of electricity produced] and you're talking about renewable energy being a replacement for carbon-based energy sources.&#;
At Duke, Stiff-Roberts and her colleagues are working on new photovoltaic technology that could one day capture more energy from sunlight.
One way to increase the efficiency of solar cells is to change their chemical makeup. Current solar cells use silicon, an inorganic element that's long lasting, great at transporting electric charges, and satisfactory at absorbing light energy. Certain organic molecules, on the other hand, are great at absorbing light energy, but can degrade quickly in the presence of moisture and oxygen.
The benefits of organic molecules go beyond their light-absorbing properties. &#;An organic chemist can design organic molecules that have all sorts of functions,&#; Stiff-Roberts says. &#;They can be flexible.&#;
Combining inorganic and organic compounds into one solar cell, Stiff-Roberts says, could offer &#;the best of both worlds.&#; But working with these kinds of hybrid materials isn't easy.
For one thing, the hybrid material must be deposited as a film, nanometers thick, that functions as a semiconductor. Thin-film semiconductors are already ubiquitous in devices we use every day, like cell phones, computers, and televisions. But often those semiconductors are made of inorganic minerals. The technique by which they are deposited in thin films doesn't work for organic compounds.
There are ways to deposit organic molecules in a thin film, but either they only work for small organic molecules or they are challenged to deposit multiple layers required for solar cells.
Now, Stiff-Roberts and her team have developed and demonstrated a technique for depositing hybrid materials made of both inorganic and large organic compounds. &#;My group contributed a novel approach that is fundamentally different from what everyone else was doing,&#; she says.
&#;Our deposition is very gentle. The [organic molecule] gets transferred from source to substrate with no change.&#;
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