Electrophosphorescence from a Polymer Guest-Host System with an Iridium Complex as Guest: Förster Energy Transfer and Charge Trapping

• Published in: Advanced functional materials 2003-06, Vol.13 (6), p.439-444
• Main Authors: Gong, X., Ostrowski, J.C., Moses, D., Bazan, G.C., Heeger, A.J.
• Format: Article
• Language: English
• Subjects: Charge trapping; Conjugated polymers; Electrophosphorescence; Energy transfer; Host-guest systems; Iridium complexes; Light-emitting diodes; Light‐emitting diodes, organic; organic
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200304334

We report high‐efficiency green electrophosphorescent light‐emitting diodes obtained by using tris[9,9‐dihexyl‐2‐(phenyl‐4′‐(‐pyridin‐2″‐yl))fluorene]iridium(III) (Ir(DPPF)3) as the guest, and a blend of poly(vinylcarbazole) (PVK) with 2‐tert‐butylphenyl‐5‐biphenyl‐1,3,4‐oxadiazol (PBD) as the host. The electrophosphorescent emission is characteristic of Ir(DPPF)3, with its maximum at 550 nm. An external quantum efficiency of 8 % photons per electron and luminous efficiency of 29 cd A–1, with maximum brightness of 3500 cd m–2, were achieved at 1 wt.‐% concentration of Ir(DPPF)3. The devices exhibited no emission from PVK or PBD, even at the lowest concentration of Ir(DPPF)3 (0.1 wt.‐%). The results indicate that Förster energy transfer plays a minor role in achieving high efficiencies in these devices. Direct charge trapping appears to be the main operating mechanism. Green electrophosphorescent light‐emitting diodes (LEDs) with a luminance of 3500 cd m–2 were fabricated from a PVK–PBD (poly(vinylcarbazole)–2‐tert‐butylphenyl‐5‐biphenyl‐1,3,4‐oxadiazol) polymer blend doped with an iridium complex. The devices exhibited an external quantum efficiency of 8 % ph/el and a luminous efficiency of 29 cd A–1. The results demonstrate that Förster energy transfer plays a minor role in achieving high efficiencies in these devices; direct charge trapping appears to be the main operating mechanism. (See also cover)