Controlling interchain interactions in conjugated polymers: The effects ofchain morphology on exciton-exciton annihilation and aggregation in MEH-PPV films

The presence of interchain species in the photophysics of conjugated polyme r films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated p olymer films, and that the degree of interchain interactions can be control led by varying the solvent and polymer concentration of the solution from w hich the films are cast. Thus, much of the controversy in the literature ca n be resolved by noting that the polymer samples in different studies had d ifferent side groups or were prepared in different ways and thus have diffe rent degrees of interchain interaction. The photoluminescence (PL) of poly( 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes bo th its spectral shape and quantum yield when the films are prepared from di fferent solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film 's FL, a result assigned to a combination of changes in the vibronic struct ure of the PL of the exciton and increased numbers of weakly emissive inter chain species. Photoluminescence excitation spectroscopy shows that excitat ion to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a dist inct ground state absorption. Scanning force microscopy shows topographic f eatures that correlate with the degree of interchain interactions, verifyin g that the morphology of conjugated polymer films changes with polymer conc entration, choice of solvent, and spin-casting speed. Even at low excitatio n intensities, photooxidative damage occurs quickly in MEH-PPV films excite d in air, and the rate at which damage occurs is sensitive to the packing o f the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excited-state absorption d ynamics that do not match those of the emissive species, provide direct evi dence for the production of interchain aggregates. Annealing an MEH-PPV fil m produces a photophysical signature similar to photooxidation, implying th at defects in conjugated polymer films are intrinsic and depend on the deta ils of how the chains are packed. At higher excitation intensities, we find that exciton-exciton annihilation occurs, and that the probability for ann ihilation can vary by an order of magnitude depending on the degree of inte rchain contact in the film. Finally, we show that changing the film morphol ogy has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of ca rriers at the expense of lower quantum efficiencies for electroluminescence Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer film s for particular device applications.