▪ Abstract The electronic structure of conjugated polymers is of current interest because of the wide range of potential applications for such materials in optoelectronic devices. It is increasingly clear that the electronic properties of conjugated polymers depend sensitively on the physical conformation of the polymer chains and the way the chains pack together in films. This article reviews the evidence that interchain electronic species do form in conjugated polymer films, and that their number and chemical nature depend on processing conditions; the chain conformation, degree of interchain contact, and rate of energy transfer can be controlled by factors such as choice of solvent, polymer concentration, thermal annealing, presence of electrically charged side groups, and encapsulation of the polymer chains in mesoporous silica. Taken together, the results reconcile many contradictions in the literature and provide a prescription for the optimization of conjugated polymer film morphology for device applications.

Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium. The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.

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.

Nanoscale architecture was used to control energy transfer in semiconducting polymers embedded in the channels of oriented, hexagonal nanoporous silica. Polarized femtosecond spectroscopies show that excitations migrate unidirectionally from aggregated, randomly oriented polymer segments outside the pores to isolated, aligned polymer chains within the pores. Energy migration along the conjugated polymer backbone occurred more slowly than Förster energy transfer between polymer chains. The different intrachain and interchain energy transfer time scales explain the behavior of conjugated polymers in a range of solution environments. The results provide insights for optimizing nanostructured materials for use in optoelectronic devices.