Metastable light-induced effects

Other metastable centers appear to be associated with microstructural imperfections since light-induced effects are influenced by annealing treatments (Staebler and Pankove, 1980) and by variations in deposition conditions (Carlson, 1982b Hirabayashi et al., 1982). For a more thorough discussion of metastable effects see Chapter 11 by Schade of Volume 2 IB. 

The first observation of a metastable component to the optically induced ESR in a-Si H was reported by Street ct a/., (1981) in heavily P- and B-doped samples and in doped and compensated samples. [For a recent review of metastable, optically induced effects in a-Si H see Pankove (1982).] As near as can be determined, this metastable signal is a fraction ( 20%) of the transient optically induced response, and the dependences of the line shapes on doping appear to be similar for the two eflFects. These ESR centers can be generated with light intensities of 150 mW cm , and they anneal in a few minutes at 80 ° K. It is particularly surprising that a subset of the photoexcited electrons are trapped in band-tail states or in a Tj configuration in p-type material in which there exists a large density of holes to facilitate rapid recombination. 

Experimentally, it is clear that different centers are metastable at diflferent temperatures depending upon the initial ESR spin densities and the type of sample, the temperature of irradiation, the power density of the exciting light, and perhaps several other variables. To complicate this situation further, there is once again disagreement as to the interpretation of these metastable, optically induced effects. One model suggests that there is an increase... 

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

In 1984, Decurtins et al. discovered that the compound [Fe(ptz)6](BF4)2 (ptz=l-propyltetrazole) can be converted from the stable LS state to the metastable HS state by irradiation with green light at sufficiently low temperatures [14]. This phenomenon has become known as light-induced excited spin state trapping (LIESST) and is dealt with in detail by A. Hauser in a separate chapter in this series. Later, Hauser reported the reverse-LIESST effect, whereby red light is used to convert the compound back into the LS state [15]. 

Staebler and Wronski (1977) were the first to observe light-induced changes in the properties of a-Si H. The Staebler-Wronski effect is now known to affect the performance of a-Si H solar cells through the creation of recombination centers and charged traps (Carlson et al., 1983b). These light-induced centers are metastable and can be annealed out at temperatures of 150 - 200,C. 

Some metastable centers may be associated with doped layers or with interface states. The light-induced generation of these centers appears to increase the surface recombination velocity in some cells, causing a decrease in the spectral response at short wavelengths. This effect and the others mentioned above are reversible annealing the a-Si H cells at 200°C for several minutes restores the cells to their initial conditions. 

The role of impurities in the defect creation is also controversial. There is no doubt that the density of metastable defects increases when the concentration of oxygen or nitrogen is above about 1 at% (Stutzmann, Jackson and Tsai 1985). However, the likely explanation is that alloying changes the network disorder to allow easier defect creation, rather than the impurity being associated directly with the light-induced defect. Samples of a-Si H still show the effect even when the impurity density is greatly reduced. 

Optical excitation of the LS( Aig) ground state at very low temperature in some thermally driven spin-crossover compounds may result in the formation of a trapped HS( T2g) metastable state. This effect, termed light-induced excited spin-state trapping (LIESST), has been discovered and described by Giitlich and co-workers... 

In the original studies of the S-W effect on undoped and doped a-Si H films, the principal characteristics of the S-W effect and the presence of metastable defects in a-Si H were established (Staebler and Wronski, 1977 Wronski, 1978 Staebler and Wronski, 1980). It was found that the large light-induced conductivity changes are a bulk phenomenon that occurs between what may be considered a thermally stable state A and a new metastable conductivity state B. State A is perfectly reproducible and is independent of previous exposures to light. It is obtained after the a-Si H film is annealed (in the dark) at temperatures above 150 C and then cooled to room temperature. The annealing time required for state A depends on the... 

In a limited number of systems which stay in the HS state down to low temperatures but sufficiently close to the crossover point, it is possible to populate the LS state as metastable state by irradiation in the near infrared. " Cooperative effects can result in a true light-induced bistability for such systems, which can persist to quite high temperatures. " In exchange-coupled binuclear iron spin-crossover systems, which are in the LS state at low temperatures, it is also possible to induce the HS state by irradiation. Although the individual iron centers are... 

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