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How Photovoltaic Cells Work Chris Greacen

©1991 Chris Greacen

Photovoltaics are indeed magical devices - who would think, really, that you could put a shiny blue flat thing out in the sun and get electricity from it? They do work. Moreover, they need not be mysterious. It does take a little patience (you may need to read over this twice or more to get comfortable with the terms) but you do not need to be a semiconductor physicist to understand qualitatively how PVs convert light into electricity.

photon

silicon atom

valence electron

Photovoltaics

Atomic Model for Semiconductors

Ninety-nine percent of today's solar cells are made of silicon (Si), and other solar cells are governed by basically the same physics as Si solar cells. Since it is helpful to be concrete, I'll explain solar cells with reference to silicon. A silicon atom has 14 electrons. Four of them are valence electrons, meaning they are available to associate with other atoms. In a pure silicon crystal, each atom shares these valence electrons with four neighbor atoms in covalent bonds. This fairly strong electrostatic bond between an electron and the two atoms it is helping to hold together can be broken by input of sufficient energy: 1.1 electron volts (eV) or more. This corresponds to a photon of light of wavelength 1.12μm or less - all colors in the visible spectrum, and well into the infrared. This freed electron now roams the crystals much the way an electron in a metal travels freely, not attached to any one atom. It is free to accelerate in the presence of an electric field; that is to say it takes a part in the conduction of electricity. In making this transition it leaves behind a "hole", a place lacking an electron. Neighboring electrons can leave their bonds to fill the hole, essentially switching places with it. Hence both electron and hole can move through the crystal. This is called the photoconductive effect.

If nothing is done, within a certain time t, called the minority carrier lifetime, the electron is expected to recombine with a hole, producing a photon (heat). This is not very exciting, and it certainly is not useful for creating electricity. Loosely, what is needed is a way to separate the electrons and the holes so that they won't recombine in the crystal, and a path to funnel these electrons out to do work on a load. The former is provided by a semiconductor junction between two semiconductors with different electrostatic charges. The latter, simply by metal contacts to the cell on opposite side of the junction.

freed electron

freed hole

Figure 1 Photoconductive effect in silicon

Doping

If we add a small amount (on the order of one part per million) of phosphorous to the silicon crystal as it is forming so that the phosphorous atoms fill sites in the silicon crystal lattice, then we are said to have 'doped' the crystal with phosphorous. Phosphorous is group V on the chemical chart, so it has five valence electrons - one more than silicon. The phosphorous nucleus and inner electrons settle happily into the lattice site, and four of phosphorous's electrons participate in the covalent bonding with electrons from the four neighboring silicon atoms. But in the crystal the fifth electron is very loosely bound to the phosphorous atom, so loosely in fact that at room temperatures it is thermally excited into the wandering free state. Doping with elements like phosphorous with one valence electron more than the original atom is called n type doping (n for 'negative'), and the dopant is called a 'donor' because it easily gives up electrons.

Doping silicon with boron has exactly the opposite effect. Boron is group III, so it has three valence electrons - one less than silicon. It fills a silicon lattice site, but has enough electrons for only three covalent bonds with

Home Power #23 • June / July 1991 37

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