Solar cells

A solar cell, a form of photovoltaic cell, is a device that uses the photoelectric effect to generate electricity from light, thus generating solar power (energy). Solar cells are used to power many kinds of equipment, including satellites, calculators, remote radiotelephones, and advertising signs. Most often, many cells are linked together to form a solar panel with increased voltage and/or current. Solar cells produce direct current (DC) which can be used directly, stored in a battery or converted from DC to AC to directly power common household devices or to feed into the utility grid. This DC to AC conversion is done by means of an inverter. Since the solar cell, grid feeding and anti-islanding requires special handling, so called Photovoltaic Inverters are used.

Silicon

The main component of a solar cell is silicon ‘doped’ with trace amounts of impurities. In pure silicon, each atom is fixed in a crystal lattice and bonded to other silicon atoms covalently, sharing the 4 valence electrons in their outer shells with them. There are thus few free electrons or positive charge carriers to carry charge, and pure silicon is thus a bad conductor.

In doped silicon, atoms with 3 or 5 valence electrons are introduced to the lattice. Take arsenic or phosphorus for example, with 5 valence electrons. Since silicon atoms require only 4 of those electrons to form stable bonds, there will be one free electron which can move and thus carry charge. Since there are so many free electrons in silicon doped with arsenic or phosphorus (compared to pure silicon), this sort of silicon is called “n-type silicon”.

If the silicon is doped with boron, which has 3 valence electrons, when it bonds with silicon it will be short of one electron. This ‘hole’ is also free to move. Since there are so many positively-charged holes in silicon doped with boron, this sort of silicon is called “p-type silicon”.

The p-n junction

In a solar cell, a plate of p-type silicon is placed next to a plate of n-type silicon. At the junction between the two, electrons in the n-type plate will migrate to the p-type plate, and vice-versa for the holes in the p-type plate. After a while, enough holes and electrons would have combined to form a barrier at the junction, preventing further flow of holes and electrons.

There is now an electric field across the p-n junction — positive on the n-type silicon plate, since the electrons have crossed over and there are excess protons which do not have corresponding electrons and negative on the p-type silicon plate, because the reverse occurs.

The electric field and the barrier act as a diode — electrons can move from the n-type plate to the p-type plate easily due to the electric field, but the reverse is difficult.

Photons

Light energy is transmitted by photons, and its quantity is given by the formula: E = hν — the energy of a photon equals its frequency multiplied by Planck’s constant (6.626 × 10−34 m²kg/s).

When photons hit the silicon plates, electron-hole pairs are created (with a probability depending on the quantum efficiency) and separated. The electric field across the p-n junction draws the electrons and holes in opposite directions, and they then diffuse to the front and back contacts. If the 2 silicon plates are connected across a load, the electrons and holes can be extracted, driving the load in the process. If nothing is connected, electrons and holes, each being in minority in their respective zone, recombine with the majority carriers. It is the open-circuit voltage (Voc) condition.

Practical Solar cells

Because the current and voltage supplied by any one solar cell is small, many cells are typically coupled in series and parallel to produce the desired level of output.

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