Electrophosphorescent polymer

Most reported PPLEDs were fabricated by doping a polymer with a phosphorescent dye. However, aggregation and phase separation effects may cause serious problems for device performance and aging. In this section, we describe the very recent progress in intrinsically electrophosphorescent polymers containing triplet-emitting complexes either as pendant substituents or as a part of a backbone. 

A Nakamura, T Tada, M Mizukami, and S Yagyu, Efficient electrophosphorescent polymer light-emitting devices using a Cs/Al cathode, Appl. Phys. Lett., 84 130-132, 2004. 

TF Guo, SC Chang, Y Yang, RC Kwong, and ME Thompson, Highly efficient electrophosphorescent polymer light-emitting devices, Org. Electron., 1 15-20, 2000. 

AJ Sandee, CK Williams, NR Evans, JE Davies, CE Boothby, A Kohler, RH Friend, and AB Holmes, Solution-processible conjugated electrophosphorescent polymers, J. Am. Chem. Soc., 126 7041-7048, 2004. 

F.C. Chen, G. He, and Y. Yang, Triple exciton confinement in electrophosphorescent polymer light-emitting diodes, Appl. Phys. Lett., 82 1006-1008, 2003. 

Although electrophosphorescent polymer devices of the latter-type are attractive candidates for development of low-cost large-area solid-state lighting sources, they are still of a lower quality to that of their small-molecule-based PHOLEDs analogs. 

FIGURE 2.6 (A) Chemical structures of white light-emitting polymers 93-97. (B) Chemical structures of electrophosphorescent polymers 98-103. 

While the debate about whether the 25% EL to PL rule applies to polymers continues, some researchers have begun to fabricate electrophosphorescent polymer devices (PPHOLEDs). A comprehensive review of these devices is beyond the scope of this chapter, but it is worth mentioning a few of the different approaches being taken by researchers. In the simplest format, phosphorescent complexes... 

To make available electrophosphorescent polymers, fluorinated poly(arylene ether phosphine oxide)-type macromolecules were synthesized (Scheme 64). °... 

W Zhu, Y Mo, M Yuan, W Yang, and Y Cao, Highly efficient electrophosphorescent devices based on conjugated polymers doped with iridium complexes, Appl. Phys. Lett., 80 2045-2047, 2002. 

X Gong, JC Ostrowski, MR Robinson, D Moses, GC Bazan, and AJ Heeger, High-efficiency polymer-based electrophosphorescent devices, Adv. Mater., 14 581-585, 2002. 

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

Following the encouraging results demonstrated by metal complex-based phosphorescent OLEDs [5,6,26], several groups started investigating a possibility to attain electrophosphorescence in solution-processible polymer-based LEDs. The first report on using... 

CHART 4.25 Chemical structure of electrophosphorescent fluorene copolymers where the holetransporting carbazole units have been introduced in the polymer backbone. 

Later, Yang and coworkers [84] reported similar electrophosphorescent fluorene copolymers 71 and 72, where the hole-transporting carbazole units have been introduced in the polymer backbone (Chart 4.25). Optimizing the polymer structure (comonomer ratio) and the device structure (blending with electron-transporting material PBD 8), the EQE of 4.9% has been achieved. 

X Yang, D Neher, D Hertel, and TK Daubler, Highly efficient single-layer polymer electrophosphorescent devices, Adv. Mater., 16 161-166, 2004. 

Y Kawamura, S Yanagida, and SR Forrest, Energy transfer in polymer electrophosphorescent light emitting devices with single and multiple doped luminescent layers, J. Appl. Phys., 92 87-93,... 

PA Lane, LC Palilis, DF O Brien, C Giebeler, AJ Cadby, DG Lidzey, AJ Campbell, W Blau, and DDC Bradley, Origin of electrophosphorescence from a doped polymer light emitting diode, Phys. Rev. B, 63 235206, 2001. 

FC Chen, SC Chang, G He, S Pyo, Y Yang, M Kurotaki, and J Kido, Energy transfer and triplet exciton confinement in polymeric electrophosphorescence devices, J. Polym. Sci. B Polym. Phys., 41 2681-2690, 2003. 

W Zhu, C Liu, L Su, W Yang, M Yuan, and Y Cao, Synthesis of new iridium complexes and their electrophosphorescent properties in polymer light-emitting diodes, J. Mater. Chem., 13 50-55, 2003. 

JM Lupton, A Pogantsch, T Piok, EJW List, S Patil, and U Scherf, Intrinsic room-temperature electrophosphorescence from a TT-conjugated polymer, Phys. Rev. Lett., 89 167401, 2002. 

---