Shockley–Queisser limit

The Shockley - Queisser limit, also called Shockley - Queisser limit, is a term used in solid state physics. The formulated by William B. Shockley and Hans-Joachim Queisser limit describes, among other things, limiting the efficiency of solar cells on the basis of the absorption and remission processes.

Description

In a solar cell wherein light is converted into electrical energy, the light excites electrons from the valence band into the conduction band. The decisive factor for the energy that you can win per excited electron, here is the size of the band gap of the semiconductor. Regardless of how far the electron is excited by the lower edge of the conduction band, it receives a maximum per electron, the energy of the band gap as electrical energy. Wherein electrical power is obtained from all of the excited electrons is necessary to consider that, for a small band gap, more electrons are produced. With a large band gap for each electron has more energy. It is therefore necessary to find a compromise for the following limiting cases:

• Large band gap: Only high-energy light (blue and ultraviolet light) can produce electrons, because longer wavelengths are not absorbed. Because of the large band gap, each electron has a large energy.
• Small band gap: Even long-wavelength light can excite electrons, so that a total number of electrons are excited into the conduction band. However, these lose by collisions with the crystal lattice in a few hundred femtoseconds part of their energy until they have only the energy of the band gap.

The energy of the electromagnetic (sun) radiation from the energy of a single photon, and the total number of photons of the frequency, i.e., where the spectrum.

Since only photons whose frequency is higher than can be absorbed and each generates an electron has an energy of about his relaxation processes, the electric energy of the electrons results in overall

The efficiency is the ratio of to. Its size depends crucially on the band gap and the spectrum. For an illumination under normal, unconcentrated sunlight ( AM1.5, opening angle 0.5 ° ) to a maximum efficiency of about 31 % results in a band gap of 1.3 eV. The light is focused with a lens to a maximum of the solar cell (equivalent to 46,200 suns), the maximum efficiency increases to 41 % with a band gap of 1.1 eV.

These considerations apply only to the case of a cell with only one pn junction. With so-called tandem solar cells (English: multi -junction solar cell), where multiple pn junctions are combined with different band gaps, higher efficiencies can be achieved in principle, see Section multijunction solar cells.