Further evidence for the role of excitons in photogeneration of charges in poly(phenylene vinylene)s

We have measured the photocurrent action spectra of MEH-PPV and PPV in sandwich cells between indium tin oxide [ITO] and aluminium electrodes. Under different bias conditions, the photocurrent spectra are either symbatic or antibatic to the absorption spectrum. We have investigated several established models which attempt to explain this behaviour and to relate the photocurrent action spectrum to the absorption co-efficient, considering penetration depth of the light and diffusion of excitons or directly photogenerated charges. At a qualitative level, many of these seem to provide an adequate description, although quantitatively none of these models can reproduce the very narrow antibatic photocurrent peak (f.w.h.m. = 0.1 eV) which we observe in both MEH-PPV and PPV, corresponding to the low energy onset of absorption. Enhancement of the photocurrent in air is further evidence for the role of excitons. As the temperature decreases, we observe a progressive red-shift of the absorption edge, although the photocurrent onset undergoes a much smaller red-shift. We propose that the particularly sharp onset of photocurrent at low energy may be due to enhanced intermolecular charge separation within crystallite grains between neighbouring conjugated segments which are more extended and more planar.