Near-field scanning optical microscopy (NSOM) studies of the relationship between interchain interactions, morphology, photodamage, and energy transport in conjugated polymer films

It is becoming increasingly clear that the way in which a conjugated polyme r film is cast affects the interactions between polymer chains and thus the optical and electrical properties of the film. Given that conjugated polym er films cast in different ways also show different nanometer-scale surface topographies, the question that arises is: What is the correlation between surface topography, local chain packing, and the local electronic properti es of a conjugated polymer film? In this paper, we address this question us ing fluorescence near-field scanning optical microscopy (NSOM) to examine f ilms of poly(2-melhoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PP V) that were prepared in different ways. The spatially resolved photolumine scence (SRPL) spectra collected on top of the nanometer-scale topographic f eatures ("bumps") exhibited by spin-cast MEH-PPV films show an enhancement of the red portion of the emission relative to spectra collected from flat regions of the film. Moreover, photooxidative damage (signified by a red-sh ift and drop in quantum yield of the SRPL) occurs much more quickly in the flat regions of the MEH-PPV films than on the topographic bumps. Taken toge ther, these observations suggest that the bumps on the films correspond to regions in which the chains are packed more tightly: the red-shifted emissi on results from increased interchain interactions, while the decreased phot ooxidation rate results from the fact that oxygen cannot easily diffuse bet ween the tightly packed polymer chains. We also find that the spatial homog eneity of MEH-PPV films can be greatly improved by annealing: heating the f ilms above the glass transition temperature removes the topographic feature s and produces a uniform but weak and red-shifted SRPL due to increased int erchain interactions. In contrast to spin-cast films, the SRPL of annealed films undergoes a blue-shift upon photooxidation. This result can be explai ned by considering the differences between the local chain packing in annea led and nonannealed films, combined with the fact that excitations in the f ilm tend to migrate to low-energy aggregated-chain "traps". All of these re sults provide insight into how polymer film morphology can be controlled th rough film processing conditions to improve the optical properties and the performance of electroluminescent devices based on this class of materials.