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From semiconductors to solar cells

Similar to gas, the electrons in the outer atomic shell of metals – the socalled valence electrons – can move freely within the metal lattice. If an electric field is applied, they will follow it. This is why metals have a high electrical conductivity. With non-conductors, on the other hand, the valence electrons are firmly bound to the atom or molecule. Semiconductors take up an intermediate position. Here the valence electrons are very loosely bound to the atom. Even small amounts of energy, such as from light quanta, are enough to loosen them. They are said to be raised from the valence band into the conductivity band. The minimum energy that the light quantum requires for this is determined by the energetic distance between the conductivity and valence bands, the so-called band gap. In the conductivity band, the electrons then contribute to the conductivity, as do the positively charged atoms that remain behind, the so-called holes. The electronsmostly remain in the conductivity band for just a short time, release their energy as heat, and fall back in the closest hole; this is known as recombination.

This is where doping comes into play. Foreign atoms are introduced to the semiconductor lattice that have either less valence electrons (p-doping with acceptors) or more valence electrons (n-doping with donors). With p-doping using acceptors, e.g. boron in silicon, the missing electron generates a hole in the crystal lattice. With n-doping using donors, e.g. phosphorus in silicon, the surplus electron can be easily separated and is available in the conductivity band. This creates a change in electric potential (voltage), which occurs at an interface between the p- and the n-doped material. The thermal movement causes free electrons to move from the n-area into the p-area and free holes move from the p-area into the n-area. This causes recombination at the interface and the movable charge carriers at the interface vanish. The fixed positive charge carriers in the n-semiconductor and the negative charges in the p-semiconductor generate an internal electric field. If now photons in this zone generate electron hole pairs, the electrons in the p-area are accelerated and the holesmove into the n-area. The resulting voltage depends on the material and is around 0.5 volts for silicon. Corresponding front and rear-side contacts can be used to tap a voltage.

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