What Is a Solar Cell?

A solar cell is a device that converts electromagnetic radiation from sunlight into electric current. This energy can then be used to generate power for commercial, industrial or residential use.

Most solar cells are made from crystalline silicon. The crystalline structure provides an organized structure that makes the conversion of light into electricity more efficient. Textured front surfaces and anti-reflection coatings also improve efficiency by reducing internal losses.

Photovoltaic effect

When light with the right wavelength strikes the surface of a PV cell, it absorbs energy. This energy causes electrons in the semiconductor material to jump from their valence band to their conduction band. In the process, they leave behind holes in the valence band. This interplay of electrons and holes is what gives rise to a current in the solar cell.

The efficiency of a solar cell depends on the incident irradiance G and the working temperature T of the p-n junction. The latter influences the concentration, lifetime and mobility of the intrinsic carriers (electrons and holes) inside the cell.

The p-n junction is composed of two layers of different semiconductor materials. The first layer, called n-type, has abundant electrons while the second, p-type, has abundant holes. Contacting these two layers creates a built-in electric field that draws electrons toward the n-side and holes toward the p-side of the junction.

Electrons

In solar cells, electrons flow through a semiconductor material that converts sunlight into electricity. This flow of electrons is aided by the fact that one side of the semiconductor contains a positive charge while the other is negative. Special treatment of the semiconductor (known as doping) creates this electric field, which enables the loosened electrons to move from one layer to the other and generate current.

When photons in sunlight strike a solar cell, they cause electrons to be excited from their valence band to the conduction band. This produces electron-hole pairs near the junction between p-type and n-type semiconductor materials. The local electric field sweeps the pairs apart across the p-n junction, increasing the diffusion current and decreasing the drift current. These opposites balance each other, resulting in net current flowing out of the solar cell at equilibrium.

Semiconductor material

The material used in a solar cell can affect its performance. Most commercial solar cells are made of crystalline silicon (Si). Less expensive thin-film solar cells use amorphous or polycrystalline Si, Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS) or Gallium Arsenide (GaAs).

Semiconductor materials have an energy level called the band gap that determines how much light they can absorb and convert. Crystalline silicon is in group IV of the periodic table and has a band gap of about 1.15 eV.

An n-type layer of semiconductor material can be doped with an impurity from a different group to have more or less electrons in the valence band. When sunlight strikes a solar cell, the electrons in the n-type layer are ejected into the adjacent p-type semiconductor, leaving behind positively charged holes. This breaks inversion symmetry and creates an electric field that can carry photogenerated holes and electrons to the metal contacts at either side of the junction, producing electricity.

Conductors

When light strikes the cell, some of it is reflected and some passes straight through. The rest of it has enough energy to separate electrons from their atomic bonds and create electric current. This is known as photovoltaic effect.

The semiconductor material in solar cells is usually crystalline silicon. This is made by wire-sawing block-cast silicon ingots into wafers. It is lightly p-type-doped by diffusion of boron or gallium atoms on the front side.

The resulting layer is several hundred nanometers thick, and forms the p-n junction with the rest of the cell. The layer also contains passivation materials such as silicon nitride, which prevents surface-recombination by blocking holes and electrons at the interface. Temperature plays a key role in solar cells, affecting both their voltage and current output. High temperatures cause the semiconductor properties to shift, leading to a decrease in open-circuit voltage and maximum power point voltage.

Energy conversion

Sunlight contains energy in the form of photons with a range of wavelengths, from ultraviolet through visible light and infrared. A solar cell absorbs a portion of these photons, turning some into electric current. This current is fed through metal contacts to a load, producing electricity.

Efficiency losses limit solar cells’ energy conversion efficiencies to the Shockley–Queisser (S-Q) limits. These losses come from photons with energies less than the threshold (sub-bandgap losses) and from thermalization of holes and electrons at the p-n junction (thermal conversion losses).

A solar cell’s efficiency is measured as its power output at standard test conditions, specified by IEC 61215: open circuit voltage VOC, short-circuit current ISC, and resistive load RPC. The output at this point is called its maximum power point.

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