Each atom is composed of a positively charged nucleus and a shell comprising negatively charged electrons (charge e — —1.602 ■ 10—19 A s). When a photon collides with an atom, the photon’s energy, E — h ■ n, can be transferred to an electron. In this case the photon is absorbed.
In the external photoelectric effect (e. g. in the case of highly ionizable alkali metals such as Li, Cs and so on), the electron can be liberated from the material (e. g. photoemission in a photocathode) if the energy, E — h ■ n, carried by the photon is higher than the energy EA needed by an electron to leave the material.
On the other hand, in solar cells made of semiconductor materials the internal photoelectric effect comes into play. A photon that carries sufficient energy, E — h ■ n, can liberate an electron from the crystal lattice or lift it out of the valence band into the conduction band.
In semiconductors, electrons are normally bonded to the outermost shell in the crystal lattice; these are known as valence electrons. In order for an electron to be liberated from its lattice, a minimum amount of additional energy is needed; this is known as band gap energy EG. This situation is illustrated by the semiconductor band model (Figure 3.1).
Strictly speaking, the aforementioned situation applies solely to temperatures close to the absolute zero point. When semiconductor temperature rises, the crystal-lattice atoms begin oscillating around their respective steady positions, with the result that some of the valence bonds are broken and the electrons thus liberated migrate to the conduction band; this phenomenon is known as intrinsic conductivity. The stronger the band gap energy, the fewer electrons migrate to the conduction band, i. e. the lower the electrical conductivity of the material at a given temperature. On the other hand, the higher the temperature of a given semiconductor material, the more electrons migrate to the conduction band and electrical conductivity increases accordingly.
Liberation of an electron from a valence bond creates a hole in the crystal lattice. An electron from an adjoining atom’s valence bond can fall into a crystal-lattice hole, in which case the original hole disappears but a new one is created elsewhere. Hence, like a free electron, a hole can move freely within a semiconductor and can also promote conductivity. If a free electron happens to collide with a hole, it will fall into it, i. e. the electron and hole recombine.
In the case of radiation onto semiconductor materials, photons with sufficient energy, h ■ n > EG, can lift an electron out of the valence band into the conduction band, whereupon the photon is absorbed, a
Photovoltaics: System Design and Practice. Heinrich Haberlin.
© 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.
Figure 3.1 Semiconductor band model. The allowable energy levels in a solid are no longer discrete as in atoms, but are instead distributed owing to the proximity of other atoms to the energy bands. The width of the prohibited zone, band gap energy Eg, is determined by the semiconductor material used. The lower threshold of the valence band and the upper threshold of the conduction band are often omitted
hole appears in the valence band and a free electron appears in the conduction band (see Figure 3.2). With directly absorbent semiconductors, micro-thin material (around 1 pm) can fully absorb all photons with sufficient energy, whereas with indirectly absorbent semiconductors such as crystalline silicon, the optical path in the semiconductor material needs to be at least 100 pm in order for all low-energy photons (red light and near-infrared light) to be reliably absorbed. Hence, directly absorbent semiconductors are mainly used to make thin-film solar cells entailing low material use, whereas more materials are needed for indirectly absorbent semiconductors due to the requisite minimum optical path; or, if thinner material is used, the effective optical path is extended using special techniques (see Figure 3.26).
The electrons generated by absorption of a photon and the consequent hole are in very close proximity to each other. Hence in semiconductors with no electric field that separates electrons and holes from each other, the electrons soon fall back into their respective holes, with the result that the photon energy deflagrates to no avail and does only heat up the semiconductor. Photons where h ■ n < Eg
do not allow an electron to be lifted out of the valence band into the conduction band and are thus not absorbed.
If an external voltage source in an irradiated semiconductor generates an electric field, this field separates the holes from the electrons generated by the absorbed photons, thus engendering a photoconductor or photoresistance whose conductivity is proportional to irradiance but which is nonetheless a passive element that unfortunately cannot produce electricity.
However, under certain circumstances (e. g. at a junction between a p-type and n-type semiconductor), strong internal electric fields can also be generated without an external voltage source. Hence these fields are used to separate photon-engendered electrons from their respective holes and thus leverage the energy resulting from these electrons and holes being separated. This is the mechanism that forms the basis for and is realized in solar cells. Hence to understand how a solar cell works, it is necessary briefly to discuss semiconductor doping and the conditions that obtain at the p-n junction.