Below-gap primary photocurrent associated with correlated defects in hydrogenated amorphous silicon

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Abstract
The statistics for correlated defects under non-equilibrium conditions are derived and applied to formulate the d.c. and a.c. below-gap primary photocurrents (PPC) of undoped a-Si:H. Through a detailed theoretical examination, it has been found that the a.c. PPC originating from different optical transitions associated with the correlated defects exhibit their own specific phase shifts. The theoretically predicted characteristics of the d.c. and a.c. PPCs have been verified by experimental results on an a-Si:H p-i-n junction photocell, leading to the conclusion that the optical transition from the valence band to the doubly occupied dangling bond state makes a predominant contribution to the below-gap PPC in undoped a-Si:H. By means of a vector analysis of the a.c. PPC spectrum, the positions of the dangling bond states have been evaluated to be about 0–90 eV (singly occupied state) and 0–56 eV (doubly occupied state) below the conduction band, and the magnitude of the correlation energy U to be 0·34 eV.