How do solar cells work?

Introduction
Solar cell convert solar energy directly into electric energy using a phenomenon known as photoelectric effect and that is why there are also known as Photovoltaics. Photoelectric effect in simple terms is release of electron due to absorption of a Photon. As I have already described in one of the podcast here that electrons reside in specifically defined states by rules of quantum mechanics. So, when a photon strikes an atom, one of the electrons can be knocked loose and can be captured into a circuit using electrode thereby converting solar energy into electric energy.


(Source of the gif: http://www.bluffton.edu/courses/TLC/MontelA/Montel/Alternative_Energy_Website/jenaefinal.gif)

Science behind
Typically, number of solar cells are connected to each other and combination is known as Solar panels. Common solar cells are made from a junction which are engineered in a way that photons energy is greater or equal to the band gap. Band gap is an energy range in which no electrons can reside as per rules of quantum mechanics.

(Source of the image: http://en.wikipedia.org/wiki/Band_gap)

Photons with energy lower than the band gap at the junction will not be able to create electrons. Silicon and Gallium Arsenide are the common materials used in solar cells. In the commercial solar cells, multiple junctions are engineered with varying energy gaps to use as much of the sun's spectrum as possible.

Efficiency and future
Efficiency of single junction are limited to 34% but this has been eliminated by using multijunctions and current record is around 44% at the lab scale. For commercial solar panels, efficiency range from 10-17%. Apart from intrinsic characterisitics of the junction, efficiency of solar cells is also dependent on how much sun's spectrum can be used and how much sun light can be concentrated on the solar cells for a given area. So, active area of research in photovolatics is on three area:
- Finding new multijunctions and putting these materials together
- Solar concentrators- to collect more sun light on a given area
- Engineering band gap of material

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