Light soaking effect on defect states distribution of Hydrogenated amorphous silicon investigated By means of constant photocurrent technique

In the present paper we have investigated, by using the constant photocurrent
method in dc-mode (dc-CPM), the effect of the light soaking (LS) on the deep defect
density (Nd) and the slope of the Urbach tail (E0) of a slightly phosphorus-doped
hydrogenated amorphous silicon (a-Si:H) film prepared by Plasma-Enhanced Chemical
Vapour Deposition (PECVD). By applying the derivative method, we have converted the
measured data into a density of states (DOS) distribution in the lower part of the energy
gap. The evolution of the sub-band-gap absorption coefficient σ (h ν) and the CPMdetermined
density of gap-states distribution within the gap versus the illumination time
leads to : (i) an increase in the deep defect absorption without any significant changes in
the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as
predicted by the defect pool model, and (iii) a saturation point of the degradation of both
optical absorption coefficient and density of deep states of slightly P-doped sample
measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy
method for the defect distribution determination is, therefore, most reliable to study the
light soaking effect on the stability of hydrogenated amorphous silicon layers used in
solar cells manufacturing.

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