It is becoming increasingly clear that the way in which a conjugated polymer film is cast affects the interactions between polymer chains and thus the optical and electrical properties of the film. Given that conjugated polymer 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 properties of a conjugated polymer film? In this paper, we address this question using fluorescence near-field scanning optical microscopy (NSOM) to examine films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) that were prepared in different ways. The spatially resolved photoluminescence (SRPL) spectra collected on top of the nanometer-scale topographic features ("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-shift 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 together, these observations suggest that the bumps on the films correspond to regions in which the chains are packed more tightly: the red-shifted emission results from increased interchain interactions, while the decreased photooxidation rate results from the fact that oxygen cannot easily diffuse between the tightly packed polymer chains. We also find that the spatial homogeneity of MEH-PPV films can be greatly improved by annealing: heating the films above the glass transition temperature removes the topographic features and produces a uniform but weak and red-shifted SRPL due to increased interchain interactions. In contrast to spin-cast films, the SRPL of annealed films undergoes a blue-shift upon photooxidation. This result can be explained by considering the differences between the local chain packing in annealed and nonannealed films, combined with the fact that excitations in the film tend to migrate to low-energy aggregated-chain "traps". All of these results provide insight into how polymer film morphology can be controlled through film processing conditions to improve the optical properties and the performance of electroluminescent devices based on this class of materials.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry