Transient photoconductivity current-time profiles in low carrier mobility solids

A recent explanation by Silver et al. of the shape of transient photoconductivity current-time traces observed in thin films of such markedly different materials as anthracene, selenium, arsenic triselenide, and poly(n-vinyl-carbazole) is questioned. It is suggested that a carrier-trap distribution proposed by Spear and Le Comber to account for the time variation of transient currents flowing in thin amorphous silicon films exposed to electron-beam pulses is unlikely to be found in many other materials. Calculations are performed which show that there is no particular widely occurring trapped space-charge distribution which would yield the experimentally observed photocurrent traces. An explanation of the shape of such traces involving radiative recombination of mobile positive holes and trapped electrons within a limited volume close to the cathode, followed by photogeneration of additional carriers through the sample bulk, is also rejected. Finally a charge-transport mechanism involving phonon-assisted tunnelling of carriers between localized states in the mobility gap is proposed, the carriers tunnelling not to the nearest state but to one further away which is energetically more favorable.