Analysis of the photocurrent action spectra of MEH-PPV polymer photodiodes

Abstract

We have measured the photocurrent action spectra of the conjugated polymers poly[2-methoxy, 5 ethyl (2\ensuremath{'} hexyloxy) paraphenylenevinylene] (MEH-PPV) and poly(phenylenevinylene) (PPV) in sandwich cells between indium tin oxide (ITO) and aluminum electrodes. Under forward bias and illumination through ITO, the photocurrent spectrum is broad and has a maximum at high energy, where the absorption coefficient is greatest (the symbatic response). Under reverse bias and illumination through ITO, the photocurrent spectrum consists of a very narrow peak (the full width at half maximum is 0.1 eV), located in the low-energy tail of the absorption profile (the antibatic response). Several established models attempt to explain this behavior and to relate the photocurrent action spectrum to the absorption coefficient, 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. In this paper, we undertake a quantitative examination 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. Upon exposure to air, we observe an enhancement of the photocurrent by a much greater factor than the dark current, from which we conclude that charge generation is mediated by exciton dissociation. As the temperature decreases we observe a progressive redshift of the absorption edge, although the photocurrent onset undergoes a much smaller redshift. We therefore conclude that the narrow antibatic peak is due to a specific enhancement of dissociation upon excitation at low energy. We propose that the particularly sharp onset of photocurrent at low energy may be due to enhanced intermolecular charge separation within crystallite grains between those neighboring conjugated segments that are more extended and more planar.