Solar Power And Multi-junction Solar Cells

By Author: Johanson
Total Articles: 139
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The sun emits a wide spectrum of radiation towards the earth every day. This radiation ranges in wavelength from 100 nm to 1mm. The low energy photons are part of the infrared spectrum, the high energy photons are part of the ultraviolet spectrum and in between these two exists visible light of all colours. There is thus a wide variety of solar power available to absorb and convert into electricity through the photovoltaic effect. The photovoltaic effect refers to the excitation of electrons by radiation; this occurs within certain materials. The electrons then flow in a circuit, producing direct current. Since photovoltaic materials are semiconductors, electrons flow in one fashion, creating direct current electricity.

Semiconductors are devices that include a p-n junction. The junction is the interface between two materials and behaves very differently than either material individually. The p-material contains “holes” which are essentially a lack of electrons within the crystal lattice; the n-type material contains additional electrons. In a semiconductor, the energy response is tuned to the band gap of the material. ...
... The bandgap refers to the energy level gap between the valence band (electrons bound to atoms) and conduction band (energy level available for electron transfer). A large band gap will exclusively respond to high energy excitation, such as UV radiation. A smaller band gap will respond to lower energy excitation, such as red light. With every p-n junction, there is a band gap, and the energy of this band gap varies depending on the semiconductor material used.

Thus the number of band gaps determines the efficiency limit for solar power panels. The theoretical efficiency limit for a single junction solar power cell was calculated to be approximately 33.7%, this is called the Shockley-Queisser Limit. For an infinite junction solar cell, the limit is 86% when using a solar concentrator. Thus a finite junction solar cell must use materials with band gaps spanning the solar spectrum to obtain the most possible solar power. For these solar cells, a solar concentrator is required to squeeze the maximum amount of power possible. One sun is not enough.

No matter how efficient one’s cell is, the solar power available drops drastically if it is not absorbing maximum sunlight at all times. The most efficient style of solar concentrator is a two-axis tracking parabolic dish, such as the SolarBeam, made by Solartron Energy Systems. The geometry of the dish focuses all solar radiation onto an absorber many times smaller than the dish. There will be a multi-junction solar cell add-on available for this solar thermal system in the near future. The use of higher efficiency multi-junction solar cells with a solar concentrator represents a new paradigm in power production. Fields of flat panels can be replaced by great metallic solar concentrators with highly efficient receivers. This technological union is extremely promising and one should expect many great developments in the field in coming years.