Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films

Abstract

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 aggregatedchain "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.

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

“…We chose to study annealed MEH-PPV films because they contain a much higher degree of interchain interactions than as-cast films. 9,44 We find that even though the interfacial PL from annealed MEH-PPV films is spatially homogeneous in the absence of applied solvent, 44 the emission becomes spatially inhomogeneous in the presence of polar solvents. The data show that the majority of the detected interfacial emission from the annealed film shifts to the blue with increasing solvent polarity, indicating that most of the interchain species in the annealed film are excimer-like in nature, with an excited state that is less-polar than the ground state.…”

“…This packing of the chains into low-energy structures also greatly increases the concentration of interchain species, as evidenced by a large redshift of the luminescence spectrum and a decrease of the PL quantum yield. 9,44 Our selection of annealed MEH-PPV films for this solvatochromism study provides two major advantages. First, the majority of the emission from annealed films is interchain, facilitating solvatochromic determination of the change in dipole associated with production of the interchain species.…”

Dispositivos poliméricos eletroluminescentes

“…Quanto maior o tamanho efetivo da conjugação, mais deslocados para valores menores de energias estarão os espectros eletrônicos de absorção e de emissão. O tamanho efetivo de conjugação é definido pelo valor médio de uma distribuição aleatória de segmentos com diferentes tamanhos de unidades adjacentes conjugadas 108,[111][112][113] . Esta distribuição estatística segue o modelo gaussiano e é conseqüência de defeitos na cadeia principal, gerados pela presença de ligações cis, de sítios oxidados, de grupos carbonila, de ligações alifáticas, ou por torções conformacionais geradas por influência de solventes, por pressão ou mesmo pela temperatura 114 .…”

Development of a Controlled Environment Near‐Field Optical Microscope for Organic Thin Film Studies

“…Efforts have been made to correlate PL and topography on the nanometer scale making use of near-field scanning optical microscopy (NSOM) and shear force microscopy. In thin films of MEH-PPV, Nguyen and coworkers 5 observed that topographic features (bumps) and local PL spectra were generally correlated for thin films of MEH-PPV which had been prepared from polymer dissolved in chlorobenzene (CB) and tetrahydrofuran (THF). A similar set of experiments performed by Huser and coworkers 6 with toluene as the solvent did not find any correlation between topography and PL intensity variations.…”