Basic principles of solar cells and the possible impact of nano-structures

The generation of electrons and holes by illumination and their subsequent thermalisation establishes different Fermi-distributions for the electrons in different energy ranges of a semiconducting absorber. As a consequence, chemical energy showing up as the difference of Fermi-energies is produced per electron-hole pair. The transformation of chemical into electrical energy requires a structure, the solar cell, in which semi-permeable membranes allow for selective transport of electrons and holes to different electrodes. With nano-structures as in the dye-cell, absorbers with very poor transport properties can be employed. While two-band systems are bounded by the Shockley-Queisser efficiency limit, higher efficiencies are possible for systems with more than 2 different Fermi-distributions. Examples like intermediate band structures, up- and down-converters or hot electron cells may heavily rely on the properties of nano-structures.