ANALYSIS OF THE PHOTOCURRENT ACTION SPECTRA OF MEH-PPV POLYMER PHOTODIODES

We have measured the photocurrent action spectra of the conjugated pol ymers poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (ME H-PPV) and poly(phenylenevinylene) (PPV) in sandwich cells between ind ium tin oxide (ITO) and aluminum electrodes. Under forward bias and il lumination through ITO, the photocurrent spectrum is broad and has a m aximum at high energy, where the absorption coefficient is greatest (t he symbatic response). Under reverse bias and illumination through ITO , the photocurrent spectrum consists of a very narrow peak (the full w idth at half maximum is 0.1 eV), located in the low-energy tail of the absorption profile (the antibatic response). Several established mode ls attempt to explain this behavior and to relate the photocurrent act ion spectrum to the absorption coefficient, considering penetration de pth of the light and diffusion of excitons or directly photogenerated charges. At a qualitative level many of these seem to provide an adequ ate description. In this paper, we undertake a quantitative examinatio n of these models and we find that none of them can reproduce the very narrow antibatic response that we observe in both MEH-PPV and PPV. Up on exposure to air, we observe an enhancement of the photocurrent by a much greater factor than the dark current, from which we conclude tha t charge generation is mediated by exciton dissociation. As the temper ature decreases we observe a progressive redshift of the absorption ed ge, although the photocurrent onset undergoes a much smaller redshift. We therefore conclude that the narrow antibatic peak is due to a spec ific enhancement of dissociation upon excitation at low energy. We pro pose that the particularly sharp onset of photocurrent at low energy m ay be due to enhanced intermolecular charge separation within crystall ite grains between those neighboring conjugated segments that are more extended and more planar.