low-weight polymers improve efficiency: electronic materials
TRANSCRIPT
RESEARCH NEWS
July/August 2004 27
The production of the three basic colors
red, green, and blue is essential to the
realization of full-color polymer organic
light-emitting diodes (OLEDs). Many
methods have been developed for
tuning emission colors, such as
confining the conjugation length by
changing the polymer main chain
molecular structure, blending an
electroluminescent polymer with a
second active polymer or with low
molecular weight fluorescence dyes,
doping, and employing multilayer device
structures and alternating copolymer
systems. Alternating conjugated
polymer systems have many
advantages, such as the versatility
provided by the incorporation of two
monomers, and the easy tuning of their
emission color through control of
intramolecular energy transfer.
Using the Heck coupling reaction,
Sung-Ho Jin at Pusan National
University and coworkers at Kyungil,
Kyungsung, Hongik, and Dong-A
Universities in South Korea have
synthesized a new series of alternating
copolymers composed of
1,4-phenylenevinylene and
2,5-thienylenevinylene repeat units with
different side chain substituents (Eur.
Polym. J. (2004), doi:10.1016/
j.eurpolymj.2004.03.028). They
characterized these copolymers using
nuclear magnetic resonance
spectroscopy, differential scanning
calorimetry, and thermogravimetric
analysis. The copolymers exhibit
excellent thermal stability up to 300°C.
Introducing urethane linkages into the
thienylenevinylene units improves the
copolymer solubility in common organic
solvents such as tetrahydrofuran,
chloroform, and chlorobenzene. The
copolymers can, therefore, be easily
spin-coated onto glass substrates,
producing high optical quality thin films
without defects.
The electro-optical properties of the
copolymers can be tuned by changing
substituents on the thienylenevinylene
unit. In the case of systems containing
3-alkylthiophene units, using different
functional groups and controlling the
steric interactions of the thiophene
units via different sidechain
substituents produces light emission in
the color range blue to red. The
poly(1,4-phenylenevinylene) emission
color can be tuned from green to
orange-red by incorporating various
substituents into the phenylene ring or
varying the comonomer feed ratio.
John K. Borchardt
Tunable copolymers for displaysELECTRONIC MATERIALS
Low-weight polymers improve efficiencyELECTRONIC MATERIALS
Polymer organic light-emitting diode (OLED) technology canhave significant advantages in manufacturing and scalabilitycompared to small-molecule-based OLEDs. Improving efficiencywould reduce power consumption, enhancing the advantagesof polymer OLEDs for displays requiring high brightness andcontrast, wide viewing angle, and video capabilities. Researchers led by Klemens Brunner at Philips Research andBea M. W. Langeveld at TNO Industrial Technology in theNetherlands report two significant developments that resultin higher polymer OLED quantum efficiency, opening the wayto large-scale manufacturing and lower unit product costs(Brunner et al., J. Am. Chem. Soc. (2004) 126, 6035). The researchers first developed a novel anode layer thatsignificantly reduces quantum losses arising from imbalancesin hole and electron partial currents. With present anodelayers, hole current can far exceed electron current resultingin significant energy wastage. The new anode layer introducesa barrier to hole injection, permitting the number of excessholes to be reduced. It increases quantum efficiencies toaround 12% compared with typical quantum efficiencies of 2-4%. This translates into a luminous efficacy of 35 cd/A for a yellow light-emitting polymer and 20 cd/A for blue,breaking the world record for luminous efficacy for blue lightpolymer emitters. The second development further increases polymer OLEDefficiency by using both fluorescence and phosphorescence.By dispersing phosphorescent ‘guest’ material into a light-emitting polymer ‘host’, it is possible to use all excited statesfor the emission of light, provided the triplet energy gap ofthe host is higher than that of the guest. This is difficult forhigher-energy green and blue light emitters and, until now,the only polymers capable of hosting blue and green
phosphorescent emitters have proven impractical forcommercial use. The researchers discovered, however, thatlow molecular weight carbazole copolymers provide excellenthosts. By identifying the longest poly(p-phenyl) chain, they canpredict whether a compound would be a suitable host for aguest high-energy triplet emitter.John K. Borchardt
Luminance and efficiency measurement of a bright, blue polymer light emitting diode.
(Credit: Royal Philips Electronics.)