An example of how such defects can be created locally was suggested by Henry and Lang (1977), who have shown that the recharging of a deep center after trapping a carrier causes violent oscillations of the defect before relaxation by dissipation of phonons. Such oscillations can be regarded as strong local heating and can lead to a jump of the defect atom into a new metastable position with a different electronic character, i. e., the creation of a photochemical reaction product. These F – and R-centers can be formed at temperatures far below room temperature, and require thermal activation energies for relaxation between 0.02 and 0.2 eV, depending on the specific alkali halide where the first experiments where performed (Itoh 1982).
In this example the exciton in an alkali halide has a very large electron-lattice coupling. The exciton can be trapped at a halogen lattice site, which in turn can relax to form a halogen molecule. contracting and moving it to the adjacent single halogen site where it forms an R-center. The electron from the original exciton becomes attached to the newly created adjacent vacancy and forms an F-center. This sequence of processes is shown in Fig. 5.3. Evidence of such simultaneous formation is provided by kinetic experiments (see Sect. 5.3.3), as reviewed by Itoh (1982). See also Stoneham (1979).
Photochemical reactions were observed very early by Poole in the 40’s with a strong change of the optical absorption spectrum of alkali halides.
Direct optical evidence in typical solar cell materials of relaxation-induced giant oscillations is more difficult to obtain because of the smaller band gap. But a wide variety of defects can form photochemical-reaction products in other semiconductors. Here recombination centers can be formed with an initiation energy much below a threshold at which an impacting electron can cause the ballistic creation of such lattice defects. It can also take place at relatively low temperatures, depending on the shape of the configuration diagram.
The creation of other photochemical reaction centers can be eased within high – fleld regions, for instance in space charge regions of hetero-junctions. Here with optical excitation new centers can be formed that will influence the potential distributions and thereby the cell characteristics.