Conjugated polymer aggregates in solution: Control of interchain interactions

We present evidence that the degree of interchain interactions and morphology in conjugated polymer films can be controlled by altering the chain conformation in the solution from which the film is cast. Light scattering experiments show that the physical size of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) chains can vary by a factor of 2 in different solvents such as chlorobenzene (CB) or tetrahydrofuran (THF). Photoluminescence and wavelength-dependent excitation indicate that MEH-PPV forms aggregate species with an absorption and luminescence spectra that are distinctly red-shifted from the intrachain exciton. The degree of aggregation is both concentration and solvent dependent; for solutions with concentrations typical of those used in spin casting, aggregates comprise a significant fraction of the total number of excited state species. The overall photoluminescence quantum yield is found to depend both on how restricted the polymer conformation is due to the choice of solvent and on aggregation due to polymer concentration. The excited state aggregates have a longer lifetime than their intrachain exciton counterparts, as evidenced by a near-infrared transient absorption in femtosecond pump-probe and anisotropy measurements. Memory of the chain conformation and the extent of aggregation of MEH-PPV in solution is carried into cast films. Thus, many conflicting results presented on the degree of interchain interactions can be explained by noting that the film samples in different studies were cast from precursor solutions with different solvents and concentrations. Overall, a careful choice of the solution (both solvent and concentration) can be used to produce MEH-PPV films with desired interchain interactions for particular device applications. (C) 1999 American Institute of Physics. [S0021-9606(99)50408-X].