How do photovoltaic cells convert sunlight into electricity?

A photovoltaic (PV) cell, commonly called a solar cell, is a non-mechanical device that converts sunlight directly into electricity. Some PV cells can convert artificial light into electricity.

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 may reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the absorbed photons 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 material 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.

Photovoltaic cells, panels, and arrays

The photovoltaic cell is the basic building block of a PV system. Individual cells can vary from 10 mm to about 100 mm across. However, one cell only produces 1 or 2 watts, which is only enough electricity for small uses, such as powering calculators or wristwatches. Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and they are the fundamental building blocks of PV systems. The cells are connected electrically in series and/or parallel circuits to produce higher voltages, currents, and power levels. Photovoltaic panels include one or more PV modules assembled as a pre-wired, field-installable unit. These come is standard sizes that are easily carried and installed on a rooftop or ground-mounted system. PV panels vary in size and in the amount of electricity they can produce. PV panel electricity-generating capacity 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 photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels. A PV array can be composed of as little as two to hundreds of PV panels. The number of PV panels connected in a PV array determines the amount of electricity the array can generate. A solar farm is a collection of large scale PV arrays over a large area of land.

Generating electricity

Photovoltaic cells generate direct current (DC) electricity. DC electricity can be used to charge batteries that power devices that use direct current 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 arrays to convert the DC electricity to AC electricity.

PV cells and panels will produce the most electricity when they are directly facing the sun. PV panels and arrays can use tracking systems that 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 — directly north in the southern hemisphere — and at an angle that optimises the physical and economic performance of the system.

How efficient are photovoltaic systems?

The efficiency at which 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-1980s, increased to around 15% by 2015, 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.

Applications of photovoltaic systems

The smallest photovoltaic systems power calculators and wristwatches. Larger systems can provide electricity to pump water, power communications equipment, supply electricity for a single home or business, or supply electricity to thousands of electricity consumers.

Some advantages of PV systems are:

• PV systems supply electricity directly to the electric power grid at distribution or transmission level
• PV systems can supply electricity in locations where electricity distribution systems (power lines) do not exist
• PV systems can also supply power directly to a campus or building
• PV arrays can be installed quickly and can be any size.