Green's Functions Approach to the Theory of Hopping Conductivity in Disordered Semiconductors

It is shown that care should be taken when the phonon‐assisted hopping conductivity is evaluated using the formal expansion in powers of the electron–phonon coupling constant g . This expansion appears to be divergent in the low‐frequency range, and the divergent terms should be partially summed up to give a correct lowest‐order contribution to the conductivity. The resulting expressions can be obtained as well by using a standard decoupling procedure in the equations for the higher‐order Green's functions.