WEBREVIEW

Modelling of transport and recombination of photocarriers in un-doped hydrogenated amorphous silicon (a-Si:H)

In this paper, we report on the simulation of steady state photoconductivity in un-doped a-
Si:H at temperatures from 30 to 500 K. The model is based on recombination at dangling bond states
and band tail states. It takes also into account the hopping transitions in the conduction hand tail
states to describe the conduction in localized states at low temperatures. At high temperatures, the
multiple trapping process is considered to describe the conduction in extended states. The density of
states includes the exponential density of conduction band tail states and valence band tail slates and
the density of dangling bond states. This later is determined by the Defect Pool Model ‘DPM’. The
experimental features observed on the temperature dependence of the photoconductivity ( p σ ) are
generally the thermal quenching, the low activated region and the temperature independent
photoconductivity at very low temperatures. All these observations are well reproduced by the model
in un-doped a-Si:H. By the examination of the relative contributions of two processes of conduction :
(i) the multiple trapping and (ii) the multiple trapping associated with the hopping, the model results
show that the multiple trapping process of electrons where the conduction is assured by free carriers
in the thermal quenching region above 140 K is important while the hopping process of electrons is
negligible. At 140 K and below, the hopping transport of electrons in the conduction band tail states
makes an important contribution in the photoconductivity. It explains successfully the low activated
region and the temperature independent photoconductivity at very low temperatures.


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