Solar Electricity

Solar Cells - How They Work

    Solar cells are made of silicon, one of the most abundant elements on Earth (after oxygen and iron). Pure silicon forms tetrahedronal crystal lattices (because of its four outer valence electrons). We can stress the electron clouds in crystalline sheets of silicon by adding trace amounts of elements that have 3, or 5, outer shell electrons. The nuclei of these elements fit well in the crystal lattice; but with only three outer shell electrons, there are too few electrons to balance out, and "positive holes" float in the electron cloud. With five outer shell electrons, we have too many electrons to comfortably fit in the tetrahedronal lattice, and so have some very loose electrons.

     When the two layers are joined, the postive holes and electrons at the junction immediately (within milliseconds) cross over, forming an electrostatic field, so there is a thin lip of negative charge on the p-side (got positive holes), and a thin lip of positive charge on the n-side (got loose electrons). The p-side overall has too few electrons to fit the electron cloud pattern comfortably, and therefore has more positive holes; the n-side is the one with too many electrons to fit the lattice, and so has more freely roaming electrons.

    The electrostatic field immediately formed at the junction prevents electrons from the n-side (where electrons are in the majority) from crossing over easily; though it does accelerate loose electrons from the p-side if they happen to be near the junction (and this is the most important point).

    Sunlight comes in. Its electromagnetic energy can break electrons free from their lattice bonds, creating loose electrons and positive holes on both sides of the joined crystal lattice sheets. If an electron is freed near the junction on the p-side (where free electrons are in the minority already), it will be whipped over to the n-side (where electrons are in the majority) by the electrostatic field. Same with a positive hole in the n-side near the junction...it will be whipped over to the p-side. With incoming sunlight energy, the layers build up a net electrical charge.

    It turns out that it is easier for the electrons in the n-side to migrate to the p-side through wires we handily lay out connecting the two layers than it is for them to get back across the electric field. The electric potential difference created at the junction, and by the difference in energy levels of free and bound electrons, is around 0.5 volts; so a direct electrical current will flow. Electrons in the p-side accelerated by the electrostatic field at the junction do cross over, so the solar cell never runs out. The n-side is thinner than the p-side.

    By wiring solar cells in series, we can increase the voltage; or in parallel, the amperage; or both ways at once, increase both...creating any size and voltage of solar panel we need. Solar cells are wired together to make a solar panel. Solar panels can be joined together to make a solar array.

    A solar panel doesn't store energy like a battery...it directly converts the Sun's radiated electromagnetic energy to a flow of electrons (electricity). Without sunlight, the photo-electric effect stops. The process in normal commercial solar cells is about 12 - 15% efficient. Though receiving 850 - 1000 watts of power from the sun, a square metre of solar panels might produce up to 130 watts of electricity, depending on its efficiency. However, sunlight may be used to directly heat air and water, often at 70% and up to 90% efficiencies. Think about that.

    Some factors influencing a solar cell's output are: angle of sunlight through atmosphere, angle of panel to sun, season of year, atmospheric haziness, cleanliness of panel's surface, temperature (less efficient when hot), and amount of light focussed or reflected onto it.

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