Exciton-exciton annihilation and the production of interchain species in conjugated polymer films: Comparing the ultrafast stimulated emission and photoluminescence dynamics of MEH-PPV

Despite the enormous versatility of conjugated polymers for use in optoelectronic devices and the correspondingly large number of research studies on their photophysics, understanding of the fundamental nature of the primary photoexcitations remains highly controversial. Part of the reason for this controversy stems from the fact that the photophysics of conjugated polymer films depends sensitively on the excitation intensity, making it difficult to compare the results of different experiments such as pump-probe stimulated emission (SE) and time-resolved photoluminescence (PL), which usually are performed at excitation intensities that differ by orders of magnitude. In this paper, we present a detailed exploration of the pump-probe SE and time-resolved PL dynamics of poly(2-methoxy 5-[2'-ethylhexyloxy]-p-phenylene vinylene) (MEH-PPV) films under identical excitation conditions. Using an optically triggered streak camera, we are able to simultaneously measure the PL and SE dynamics as a function of both excitation intensity and emission wavelength. Although the SE and PL dynamics of dilute MEH-PPV solutions are identical, we find that even at relatively low excitation intensities, the PL and SE dynamics of MEH-PPV films are different, uncovering the presence of an interchain excited-state absorbing species that has dynamics distinct from the intrachain exciton. The number of interchain absorbing species increases nonlinearly with excitation intensity, suggesting that interchain species are formed both directly upon photoexcitation and as a by-product of exciton-exciton annihilation (E-EA). A comparison of the emission dynamics of pristine and intentionally oxidized MEH-PPV films leads us to conclude that there are multiple types of interchain species; we propose that the directly excited interchain species are likely aggregates or excimers, while the interchain species produced by E-EA are best assigned to polaron pairs. We also find that the wavelength dependence of the SE and PL from MEH-PPV films is complex, both because energy migration leads to a dynamic redshift of the exciton emission and because the rate of E-EA is higher for hot (blue-emitting) excitons than for thermalized (red-emitting) excitons. All the results are compared in detail to previous work, providing a means to resolve many of the apparently contradictory ideas in the literature concerning the photophysics of conjugated polymer films.