Light-induced defects

The founding member of the TRP channel family, TRP, was identified as the product of a gene locus, which was referred to a transient receptor potential (TRP), because TRP mutant flies display a defect in light induced Ca2+ influx. 

Illumination of solar cells causes a reduction of efficiency and fill factor, as a result of light-induced creation of defects (Staebler-Wronski effect. Section 1.1.2.5). This reduction is halted after several hundred hours of illumination. The reduction is correlated with solar cell thickness. A large intrinsic layer thickness leads to a large reduction of efficiency and fill factor compared to a small intrinsic layer thickness. The solar cell properties can be completely recovered by annealing at about 150°C. The open circuit voltage and short circuit current decrease only slightly. 

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

This model has a number of limitations, including the fact that only point defects are considered. As discussed below, there is some evidence that more complex defect pairs or clusters may occur in BaTi03. In addition, the model requires that photocarriers are associated with only a single partially filled level in thermal equilibrium and cannot account for light-induced charge redistribution among multiple levels. Several studies (13-15) have suggested the possible importance of the latter effect, based on observations of intensity-dependent absorption and the sublinear intensity-dependence of the response time in many crystals. 

The optical absorption arising from the defect transitions is weak because of the low defect densities and in a thin film cannot be measured by optical transmission. The techniques of PDS, CPM and photoemission yield, described in Section 3.3, have sufficient sensitivity. Photocapacitance, which measures the light-induced change in the depletion layer capacitance, is similarly sensitive to weak absorption (Johnson and Biegelsen 1985). PDS measures the heat absorbed in the sample and detects all of the possible optical transitions. At room temperature virtually all the recombination is non-radiative and generates heat by phonon emission. CPM detects photocarriers and so is primarily sensitive to the optical transitions which excite electrons to... 

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. 

Fig. 6.27. The time dependence of light-induced defect density at different temperatures, showing that a steady state is reached. The inset shows the steady state defect density (Stutzmann et at. 1985).

The aimealing kinetics of the light-induced defects are shown in Fig. 6.29. Several hours at 130 °C are needed to anneal the defects completely, but only a few minutes at 200 C. The relaxation is nonexponential, and in the initial measurements of the decay the results were analyzed in terms of a distribution of time constants, Eq. (6.78) (Stutzmann, Jackson and Tsai 1986). The distribution is centered close to 1 eV with a width of about 0.2 eV. Subsequently it was found that the decay fits a stretched exponential, as is shown in Fig. 6.29. The parameters of the decay-the dispersion, p, and the temperature dependence of the decay time, t - are similar to those found for the thermal relaxation data and so are consistent with the same mechanism of hydrogen diffusion. The data are included in Fig. 6.23 which describes the general relation between x and D,. The annealing is therefore the process of relaxation to the equilibrium state with a low defect density. 

The light-induced and thermal equilibrium defect reactions are aspects of the same general process. Indeed, the structural models proposed are virtually identical (compare Figs. 6.13 and 6.30). In the two-well description of Fig. 6.1, excitation over the barrier in either direction can, in principle, be thermal or by an external excitation. The... 

Fig. 6. 29. The decay of the normalized light-induced defect density at different annealing temperatures, showing the stretched exponential behavior (Jackson and Kakalios 1988).

Fig. 6.30. One of the models proposed to explain the light-induced defect creation, in which the broken Si—Si bond is stabiUzed by hydrogen motion (Stutzmann et al. 1985).

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