low-weight polymers improve efficiency: electronic materials

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.)