Mg. Harrison et J. Gruner, FURTHER EVIDENCE FOR THE ROLE OF EXCITONS IN PHOTOGENERATION OF CHARGES IN POLY(PHENYLENE VINYLENE)S, Synthetic metals, 84(1-3), 1997, pp. 653-654
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.