How Solar Cells Generate Electricity

For traditional PV solar panels a semiconductor PN junction is manufactured in which two halves of one pure silicon crystal are doped with two different dopants (e.g. arsenic, gallium, aluminium, phosphorus). One half of the crystal is left electron deficient (the p-type layer), and one half is left with an excess of electrons (the n-type layer). The dopants in the semiconductor lead to an electric field across the junction between the two halves of the crystal with electrons able to travel in one direction only - from the electron rich half to the electron poor half.

Where the two halves of the crystal meet, there is a depletion region, so called because it is depleted of charge carriers (electrons and holes). Electrons move from the n-type (negative) side to the p-type (positive) side of the crysal recombining with holes. Likewise holes move from the p-type side to the n-type side. As the silicon atoms themselves do not move, any holes which remain uncovered by electrons in the n-type side are left positively charged, and any electrons without holes to cover in the p-type side remain negatively charged. This leaves positive material close to the junction in the n-type side, and negative material close to the junction in the p-type side with a potential between the two sides of around 0.6-0.7 volts in a silicon pn junction. This potential barrier between the p and n-type sides of the crystal prevents further electrons and holes from travelling across the junction until sunlight hits the solar cell and releases electrons with enough energy to overcome the barrier.

The light from the sun is made up of packets of energy called Photons. Each photon carries an amount of energy corresponding to its wavelength of light. When a photon strikes a solar cell it can do one of three things: pass straight through, be reflected, or be absorbed. If the photon is absorbed, its energy is absorbed by an electron in an atom of the solar cell enabling it to escape from its normal position, cross the junction and fill a hole. The electrons then flow through a load (e.g. charging a battery, lighting a light, or powering a motor), and complete the circuit by recombining with the holes they left behind. In so doing energy from the sunlight has been extracted and used at an efficiency of around 5-15%. This process can be repeated over and over again over the decades of lifetime of solar cells.