polymer folding properties: polymers
TRANSCRIPT
RESEARCH NEWS
December 2004 25
Better understanding of chemical
structure/physical property correlations
in organic semiconducting materials,
particularly the interactions of
chromophores on a microscopic level,
could aid in designing improved
materials for light-emitting diodes
(LEDs), solar cells, photodiodes, and
other optoelectronic devices. Secondary
and tertiary structure, such as chain
folding, can be as important as the
primary chemical structure of the
polymer in determining their optical
properties. Now researchers from
Germany’s Ludwig-Maximilians-
Universität and Universität Wuppertal
have found that structure-property
correlations in conjugated polymers
derive mainly from chain morphology
rather than chromophoric properties
[Schindler et al., Proc. Natl. Acad. Sci.
USA (2004) 101, 14695].
John M. Lupton and coworkers used
single-molecule spectroscopy to study
comparatively ordered and highly
disordered polymers. The technique
enables the researchers to identify
single chromophores in highly ordered
polymers and demonstrate that the
efficiency of intramolecular excitation
energy transfer is dominated by the
temperature-dependent spectral
overlap of chromophores. Even though
the ordered and disordered polymers
exhibit virtually identical spectroscopic
features on the single-chromophore
level, the single-molecule properties of
the two conjugated polymers are quite
different. The energetic range of
spectral diffusion is small but can
influence the intramolecular excitation
energy transfer and the emission. The
fundamental difference between
ordered and disordered polymers is
that the latter can undergo coherent
interchromophoric coupling resulting in
strong spectral broadening.
John K. Borchardt
Polymer folding propertiesPOLYMERS
Patterning polymers is more efficientPOLYMERS
Light can be trapped in an organic light-emitting diode (LED)as a result of total internal reflection. As much as 70% ofthe photons produced by a light-emitting polymer (LEP) with arefractive index greater than two can be trapped in the glasssubstrate or in the polymer and indium tin oxide (ITO) anodelayers. Preventing this trapping would significantly increaseLED power efficiency. Current methods of accomplishing thisinclude the use of microlenses, aerogel layers, silicamicrospheres, or periodic corrugations fabricated by spin-coating an LEP on top of a sinusoidal photoresist layer andhot-embossing the layer with a patterned stamp. However,these methods are limited by excessive light absorption bythe photoresist and detrimental thermal effects. Researchers from the University of Cambridge, UK haveovercome these difficulties by developing a method thatharnesses the waveguide modes trapped in polymer-blendLEDs [Corcoran et al., Appl. Phys. Lett. (2004) 85 (14),2965]. Richard H. Friend and coworkers accomplished this byfabricating self-organized, two-dimensional micron-scalephotonic structures within the emissive layer of a polymer-blend LED. Fabrication involves phase separation of twosemiconducting polymers directed by a surface chemicalpattern to form a relief-and-phase grating. Phase separationin polymer blends is strongly dependent upon the substratesurface energy. On patterned surfaces, preferentialsegregation of one of the blend components to higher surfaceenergy regions results in periodic domain structures. TheCambridge team spin-cast a 100 nm thick film of poly(9,9'-dioctylfluorene-co-benzothiadiazole), or F8BT, and poly(9,9'-dioctylfluorene), or PFO, onto a two-dimensional patternedanode and allowed them to phase separate. Micro-contactprinting is then used to create a pattern of 2 µm dots with aperiodicity of 4 µm. “By utilizing surface patterning and self-organization, we were able to construct a photonic structurefrom the actual polymers generating the light, therebyimproving the device efficiency two-fold without changing any
of the device layout,” explains Wilhelm T. S. Huck. While this does not lead to efficient photonic gratingstructures for visible light, these phase-separated domainsdevelop selectively amplified Fourier harmonics locked todomain boundaries. The result of these high-order harmonicsis strong broadband outcoupling of the waveguide modeswithout causing spectral dispersion. This can double LEDexternal quantum and power efficiencies while maintainingspectral integrity with viewing angle. “Since the surface patterning is compatible with devicefabrication and self-organization in polymer blends is auniversal phenomena, we envisage that this strategy can beused to improve device efficiency in all polymer LEDs,” saysHuck. The researchers are now working on getting a betterunderstanding of how to control the patterns in morecomplex blends and extend the scope to the nanoscale.John K. Borchardt
Green and blue fluorescence microscopy images of 100 nm films clearly showing the
phase separation process. (Reproduced with permission from Adv. Mater. © 2004 Wiley-
VCH Verlag.)