Picosecond transient photoconductivity in poly(p-phenylenevinylene)

Abstract

We report the results of transient-photoconductivity measurements on films of poly(p-phenylenevinylene) (PPV) in the subnanosecond-time regime. The initial fast transient photocurrent decays exponentially with a decay time of about 100 ps, followed by a slower component with a decay time of about 600 ps. The magnitude of the fast component is proportional to the light intensity and independent of temperature, while the magnitude of the slower component is proportional to the square root of the light intensity and decreases as temperature decreases with a thermal-activation energy of about 100 meV. We attribute the initial response to photogenerated electrons and holes (polarons) and the following slower decay to bipolarons in the nondegenerate ground-state conducting polymer. Measurements of the spectral response of both transient and steady-state photoconductivity demonstrate that the onset of the photoconductivity coincides with that of photoabsorption, consistent with the photogeneration of free carriers via an interband transition. The interpretation of the initial temperature-independent transient photocurrent as the displacement current from the field-induced polarization of neutral excitons is ruled out by careful analysis of the transient-photocurrent data and by direct measurements on polydiacetylene-(toluene sulfonate), a conjugated polymer with a known exciton binding energy (${\mathit{E}}_{\mathrm{exc}}$), ${\mathit{E}}_{\mathrm{exc}}$≊0.4 eV. A variety of measurements has enabled us to set an upper limit on ${\mathit{E}}_{\mathrm{exc}}$ in PPV and several of its alkoxy derivatives; ${\mathit{E}}_{\mathrm{exc}}$ is comparable to, or less than, ${\mathit{k}}_{\mathit{B}}$T at room temperature.