White Polymer LEDs Incorporate New Dopant

Daniel S. Burgess

Researchers at the Chinese Academy of Sciences in Beijing and at the University of California, Los Angeles, have demonstrated white LEDs that feature emission layers of the orange-emitting polymer poly[5-methoxy-2-(2'-ethyl-hexylthio)-p-phenylenevinylene] in the blue-emitting conjugated polymer poly(9,9-dioctylfluorene-2,7-diyl). The stability of the color coordinates of the new devices suggests their potential in large-area displays and lighting.

White LEDs fabricated using a new polymer dopant, poly[5-methoxy-2-(2'-ethyl-hexylthio)-p-phenylenevinylene], show promise for large-area display and lighting applications. Courtesy of Yongfang Li, Chinese Academy of Sciences.

Yongfang Li, a professor in the Key Laboratory of Organic Solids at the academy’s Institute of Chemistry, explained that polymers are of particular interest for such applications because they open the door to fabrication methods such as spin-coating and printing from solution, enabling the relatively simple and inexpensive production of arbitrarily large, flexible emitters. He noted that one promising material system for white polymer LEDs has been the blue-emitting polymer doped with the orange-emitting poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1 c4-phenylene vinylene], but that the color coordinates of the emission shift relatively significantly with increasing voltage, limiting its usefulness in high-brightness display applications.

The substitution of this dopant with poly[5-methoxy-2-(2'-ethyl-hexylthio)-p-phenylenevinylene], Li said, seems to resolve this problem, yielding purer and more color-stable white-light emission. In their work, the scientists produced single-layer LEDs using the materials, as well as two-layer devices that incorporated poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine], poly(N-vinyl-carbazole) or a mixture of the two as the additional hole-transport layer.

They fabricated the devices by spin-coating toluene solutions of the emission materials and chlorobenzene solutions of the hole-transport materials atop ITO electrodes coated with a 30-nm-thick layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). They then vacuum-deposited Ca/Al electrodes atop the stack to serve as a cathode.

They found that the bilayer LEDs that used a mixture of the hole-transport materials displayed the best performance. Devices with a 1:1 mixture of poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] and poly(N-vinyl-carbazole) and an emission layer of poly(9,9-dioctylfluorene-2,7-diyl) doped with 0.5 percent by weight poly[5-methoxy-2-(2'-ethyl-hexylthio)-p-phenylenevinylene] had CIE color coordinates of (0.32, 0.36) at 8 V and (0.29, 0.32) at 16 V.

The LEDs were found to exhibit a maximum brightness of approximately 5000 cd/m2 and a maximum electroluminescent efficiency of 3.15 cd/A, which Li noted must be improved before such devices could have practical applications. He suggested that using different luminescent polymers or alternative device structures could yield better devices.

To that end, Li said, the investigators are seeking to improve the electroluminescent efficiency by modifying the interface between the emission layer and the Ca/Al electrode.

Applied Physics Letters, Oct. 9, 2006, 153501.