Unified approach to the DC and AC conductivity and optical absorption of amorphous semiconductors below the fundamental edge

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Abstract
A microscopic theory of the AC conductivity and optical absorption on localized states in the mobility gap of amorphous semiconductors is developed. The resulting formulae are exact to all orders in the electron-phonon coupling constant and to second order in overlaps of electronic localized orbitals. In the DC limit, previous results for the phonon-assisted hopping conductivity which are in agreement with assumptions of Mott's theory are obtained. The role of higher-order effects, including both the extended and possible localized phonon modes is discussed.