Solar cells are semiconductor structures made from a p-n junction with wide area that absorbs light and deliver electricity. They are made usually of Silicon, and they are engineered in such a way to absorb a large part of light into their bulk. The working principle of solar cells is the internal photoelectric effect, an effect very close to the external photoelectric effect: when a photon is absorbed at the surface of a material it releases an electron from the material provided there is enough energy for this. Into the bulk of material a photon releases instead a pair electron-hole. This pair is separated by the electric field of the pn junction and travel further towards the contacts. Initially solar cells were made of Silicon, which is a very convenient material (it absorbs very good light, and it has also a good efficiency), but also other materials are used now. All work that has been done is about increasing efficiency of the solar cells. Initially the solar cells were having efficiencies of about 2-5% but now they can have efficiencies up to 30%.
Advances in solar cells mean new materials to improve their efficiency and new ways to manufacture them. Initially the solar cells were made up of thin films of differently doped Silicon that were making the p-n junction. These films need to be transparent to allow light to enter the structure but also thick enough to absorb the light that entered.
Now there are increasingly new materials used. Quantum dots and quantum wires are used that are very easily made and that are very cheap to be produced. A solar cell made up of quantum dots involves one or two thin layers of dots that make the junction. Also there are made tandem solar cells from multiple layers to improve the absorption of light and also the efficiency. Efficiencies of up to 40% have been attained. This is very promising if we consider that the solar power available is about 1000 W/sq.m. and for an usual efficiency of 20% about 200 W can be obtained from one square meter.