Shockley–Queisser limit
<p>Nov 8, 2009 8:53 PM  pronounced  KWEI zer
 
max theoret limit of a solar cell using p-n junction.
 
limit is around 30% using a p-n band gap of 1.1 eV typical of silicon.
 
of all power falling on a silicon solar cell  abt 1000 Watts/m2  only abt 30% can ever be turned into electrcity.
 
In a traditional <a href="/wiki/Solid_state_(electronics)" title="Solid state (electronics)">solid-state</a> <a href="/wiki/Semiconductor" title="Semiconductor">semiconductor</a>, a solar cell is made from two doped crystals, one with a slight negative bias (<a href="/wiki/N-type_semiconductor" title="N-type semiconductor">n-type semiconductor</a>), which has extra free <a href="/wiki/Electron" title="Electron">electrons</a>, and the other with a slight positive bias (<a href="/wiki/P-type_semiconductor" title="P-type semiconductor">p-type semiconductor</a>), which is lacking free electrons. When placed in contact, some of the electrons in the n-type portion will flow into the p-type to "fill in" the missing electrons, also known as an electron hole. Eventually enough will flow across the boundary to equalize the <a href="/wiki/Fermi_level" title="Fermi level">Fermi levels</a> of the two materials. The result is a region at the interface, the <a class="mw-redirect" href="/wiki/P-n_junction" title="P-n junction">p-n junction</a>, where charge carriers are depleted and/or accumulated on each side of the interface. In silicon, this transfer of electrons produces a <a class="mw-redirect" href="/wiki/Potential_barrier" title="Potential barrier">potential barrier</a> of about 0.6<a href="/wiki/Volt" title="Volt">V</a> to 0.7V.<a href="#cite_note-how-2"><sup class="reference" id="cite_ref-how_2-0">[</sup><sup class="reference" id="cite_ref-how_2-0">3]</sup></a>
When placed in the sun, <a href="/wiki/Photon" title="Photon">photons</a> in the sunlight can strike the bound electrons in the p-type side of the semiconductor, giving them more energy, a process known technically as <a href="/wiki/Photoexcitation" title="Photoexcitation">photoexcitation</a>. In silicon, sunlight can provide enough energy to push an electron out of the lower-energy <a href="/wiki/Valence_band" title="Valence band">valence band</a> into the higher-energy <a href="/wiki/Conduction_band" title="Conduction band">conduction band</a>. As the name implies, electrons in the conduction band are free to move about the silicon. When a load is placed across the cell as a whole, these electrons will flow out of the p-type side into the n-type side, lose energy while moving through the external circuit, and then back into the p-type material where they can once again re-combine with the valence-band hole they left behind, producing a lower-energy photon. In this way, sunlight creates an electrical current.<a href="#cite_note-how-2"><sup class="reference" id="cite_ref-how_2-1">[</sup><sup class="reference" id="cite_ref-how_2-1">3]</sup></a></p>