Effects of bimolecular recombination and charge-trapping on the transient photoconductivity of poly(p-phenylene vinylene)

We report on fast transient photoconductivity measurements on highly oriented films of poly(p-phenylene vinylene) at various excitation densities and temperatures via linear (400 nm) or two-photon (800 nm) excitation. The photocurrent waveform is found to be independent of the applied electric field (up to F ∼ 3 × 105 V/cm) but strongly dependent on excitation density. A significant reduction of the photocurrent lifetime is observed at higher light intensities, clearly indicating carrier bimolecular recombination. A transport mechanism mediated by traps is also evident by the long-lived (up to 1 μs) photocurrent 'tail' that diminishes at low temperatures. We have fitted the transient photocurrent waveforms to a kinetic model from which the dynamics of carrier recombination, trapping and de-trapping were deduced.