Light consists of particles of energy called photons. The energy of a photon is measured in electron volts, or eV. One eV is equivalent to the energy gained by an electron when it accelerates through a one volt potential difference. The electrons in the atoms of a solar cell need a specific amount of energy from photons in order to be propelled into the circuit of a solar power system.
Relationship Between Energy and Wavelength
The relationship between the energy of a photon and the wavelength of light is inverse; as the wavelength of light increases, energy decreases. In the visible light spectrum, the energy of photons ranges from roughly 1.65 eV to 3.1 eV. The corresponding wavelengths of these energies are 750 and 400 nanometers, respectively. These wavelengths correspond to the red and violet ends of the spectrum of visible light.
Energy Required for Solar Cells
A typical silicon solar cell requires 1.1 eV in order for electrons to flow out of the cell and through the circuitry of the solar panel system. This 1.1 eV of energy corresponds to photons in the infrared range of the spectrum with a wavelength of roughly 1,127 nanometers. At energies lower than 1.1 eV, photons do not have enough energy to dislodge electrons. At shorter wavelengths and higher energies, silicon electrons will get energized and current will flow. However, shorter wavelengths and higher photon energies do not correspond with an increase in electrical current. When a photon of higher energy impacts a solar cell, energy above 1.1 eV is given off as heat.
A lot of energy from the sun is not used to create electricity in solar panels. In fact, a typical silicon solar panel will only convert about 15 to 18 percent of the incoming solar energy into electricity. This is partially because of the very specific energy required to dislodge electrons in the silicon atoms of the cell. Any energy above or below that 1.1 eV photon energy is not converted into electricity by the solar panels.
Different Solar Cell Materials
While many consumer-grade solar cells are made from silicon, cells can be made from different semiconductor materials as well. These materials have different atomic properties, which correspond to different energy requirements from photons. For example, a gallium arsenide solar cell requires roughly 1.4 eV of energy from a photon. This corresponds to a wavelength of roughly 886 nanometers. Silicon is still the most commonly used material due to both the cost of production as well as the fact that even a 15 percent efficiency is sufficient for most everyday solar power applications.