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As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon.

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. Main articles: semiconductor and p-n junction These higher energy photons will be absorbed by a silicon solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy. However, the solar frequency spectrum approximates a black body spectrum at about 5,800 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon (1.12eV), which is near to the ideal value for a terrestrial solar cell (1.4eV). It can be said that photons absorbed in the semiconductor create electron-hole pairs.Ī photon only needs to have energy greater than that of the band gap in order to excite an electron from the valence band into the conduction band. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the 'hole', leaving another hole behind, thus propagating holes throughout the lattice in the opposite direction to the movement of the negatively electrons. This is known as a hole, and it has positive charge. The network of covalent bonds that the electron was previously a part of now has one fewer electron.

Side view solarcell free#

The energy given to the electron by the photon 'excites' it into the conduction band where it is free to move around within the semiconductor. Usually this electron is in the valence band. When a photon is absorbed, its energy is given to an electron in the crystal lattice.

This generates an electron-hole pair and sometimes heat depending on the band structure.īand diagram of a silicon solar cell, corresponding to very low current (horizontal Fermi level), very low voltage (metal valence bands at same height), and therefore very low illumination

The photon can be absorbed by the semiconductor if the photon energy is higher than the band gap value.

The photon can reflect off the surface.

The photon can pass straight through the semiconductor - this (generally) happens for lower energy photons.

When a photon hits a piece of semiconductor, one of three things can happen:

An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.

The electronic structure of the materials is very important for the process to work, and often silicon incorporating small amounts of boron or phosphorus is used in different layers. Due to their special structure and the materials in solar cells, the electrons are only allowed to move in a single direction.

Electrons (negatively charged) are knocked loose from their atoms as they are excited.

Photons in sunlight hit the solar panel and are absorbed by semi-conducting materials.

Band diagram of a solar cell, corresponding to very low current (horizontal Fermi level), very low voltage (metal valence bands at same height), and therefore very low illumination Working explanation