Light-induced metastable changes

Light-Induced Metastable Changes of the Surface Charge. 330... 

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... 

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. 

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. 

Both transient and metastable changes in the ESR are observed with optical excitation by light with energy greater than the band gap. Explanations analogous to those proposed to explain the ESR in doped samples have also been suggested to account for the optically induced changes. 

For calculating the space-charge conductance (j(F,) we need to know the density of states g E) and the surface potential F,. Under favorable conditions, F, of a free surface can be obtained by the Kelvin probe method or from the saturation value of the surface photovoltage. These methods will be illustrated with some preliminary results obtained on a-Si H. We then shall discuss metastable light-induced changes of the surface potential and conductance. 

The reversible photoinduced changes in dark conductivities are consistent with metastable changes in the gap states of a-Si H, but it is not yet clear whether a unique type of light-induced defect can account for the wide range of conductivity changes that have been reported for a-Si H fUms. This question can be answered only after a more detailed characterization has been carried out on both the different photoinduced conductivity changes and the gap states. 

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