Mark Harrison

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

M. G. Harrison, J. Gruner and G. C. W. Spencer.

Physical Review B 55:12 (1997) p7831-7849

We have measured the photocurrent action spectra of the conjugated polymers, poly(2-methoxy, 5 ethyl (2' hexyloxy) para-phenylene vinylene) [MEHPPV] and poly(phenylene vinylene), [PPV] in sandwich cells between indium tin oxide [ITO] and aluminium electrodes. Under forward bias and illumination through ITO, the photocurrent spectrum is broad and has a maximum at high energy, where the absorption co-efficient is greatest (the symbatic response). Under reverse bias and illumination through ITO, the photocurrent spectrum consists of a very narrow peak (f.w.h.m. = 0.1eV), located in the low energy tail of the absorption profile (the antibatic response). Several established models attempt to explain this behaviour 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 which we observe in both MEH-PPV and PPV. Upon exposure to air, we observe 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 red-shift of the absorption edge, although the photocurrent onset undergoes a much smaller red-shift. 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 neighbouring conjugated segments which are more extended and more planar.

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Mark Harrison       mgh@who.net,       Marburg, May 3, 1998