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 electrode s. Under different bias conditions, the photocurrent spectra are eithe r symbatic or antibatic to the absorption spectrum. We have investigat ed several established models which attempt to explain this behaviour and to relate the photocurrent action spectrum to the absorption co-ef ficient, considering penetration depth of the light and diffusion of e xcitons or directly photogenerated charges. At a qualitative level, ma ny of these seem to provide an adequate description, although quantita tively none of these models can reproduce the very narrow antibatic ph otocurrent peak (f.w.h.m. = 0.1eV) which we observe in both MEH-PPV an d PPV, corresponding to the low energy onset of absorption. Enhancemen t of the photocurrent in air is further evidence for the role of excit ons. As the temperature decreases, we observe a progressive red-shift of the absorption edge, although the photocurrent onset undergoes a mu ch smaller red-shift. We propose that the particularly sharp onset of photocurrent at low energy may be due to enhanced intermolecular charg e separation within crystallite grains between neighbouring conjugated segments which are more extended and more planar.