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Verifying my understanding of solar cells and semiconductors

by themadquark
Tags: cells, semiconductors, solar, verifying
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Aug1-14, 01:45 AM
P: 19
My base understanding of the solar cell is as follows: Light hits metal in solar cell and emits electron via photoelectric effect. There is n-type silicon which has an overall negative charge due to excess electrons. There is also p-type silicon which has an overall positive charge due to a deficiency of electrons. A voltage is produced by this, but in order to keep current flowing the electrons being emitted via the photoelectric effect supply the n-type silicon with electrons to keep current flowing.

My understanding is that an element such as Phosphorous can be added to silicon to produce excess electrons due to it's five valence electrons and silicon's four. Whereas an n-type semiconductor and that an element such as Boron can be used to produce a deficiency in electrons as Boron only has three valence electrons.

Could somebody please check my understanding and correct me where I am incorrect?
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Aug1-14, 09:22 PM
P: 145
Solar cells are not made out of metals but rather out of semiconductors, with a band gap around 1 eV to match the solar spectrum. Light from the sun hits the semiconductor and is absorbed, producing an electron-hole pair known as an exciton. Ultimately, the electron must be taken out one end of the cell to contribute to the current - it recombines with the hole after completing the circuit.

A common way to separate the electron and the hole is with a p-n junction, similar to what you describe. Exactly as you say, phosphorous can "dope" silicon and add an electron (producing n-type) and boron can dope silicon and remove an electron, or add a "hole" (producing p-type). Importantly, n-type and p-type semiconductors are still neutral! Neutral P + neutral Si must be neutral overall. However, when you bring n-type and p-type semiconductors together, the electrons from the n-type material flow into the p-type material to combine with holes (or to replace the electron deficiency). *This* leaves the n-type material positively charged (it lost electrons) and the p-type material negatively charged (it gained electrons). The two layers of charges produce an electric field (known as the built-in electric field), and that electric field can separate the electron and hole produced by an absorbed photon, thereby driving the current in the solar cell.

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