Solar cells convert the energy of the sunlight into electrical energy. They don’t do this directly. In a first step, which determines the efficiency of the conversion process, chemical energy of electron-hole pairs is produced. This happens in atoms, molecules and semiconductors. The special structure of the solar cell is required for the second step, the conversion of chemical energy into electrical energy. Contrary to a widespread belief, there is no driving force, which would drive electrons and holes into different directions, as an electric field acting on the different charge of electrons and holes would do. The existing driving force (gradient of the electro-chemical potential) drives electrons and holes into the same direction and towards both contacts. An electrical current is produced by manipulating the conductivities of electrons and holes on their way to different contacts. The only function of the structure of a solar cell is to provide the necessary differences of the conductivities. Possible structures are discussed. A pn-junction is one of them, but not the best.
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