Highly efficient polymer light-emitting devices using ambipolar phosphorescent polymers

Abstract

We report on highly efficient polymer light-emitting devices (PLEDs) achieved using a phosphorescent polymer, which is a copolymer that has bis(2-phenylpyridine)iridium (acetylacetonate) [Ir(ppy)2(acac)], N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as a side group. The phosphorescent polymer has an ambipolar charge-transport ability. An increase in PBD unit concentration allows an improvement in the efficiency of the PLEDs. Ba and Cs were used for electron-injection layers as well as Ca, to improve the electron injection. An external quantum efficiency of 11.8% and a power efficiency of 38.6lm∕W were obtained by using Cs. The results indicate that this can be attributed to an improvement in the charge balance of electrons and holes.

Synthesis of Polytriphenylamine with Emitting Part at Terminal for Light Emitting Application

“…In a 10-mL two-necked round-bottom flask under nitrogen atmosphere, initiator 1 (0.14 g, 0.22 mmol) which was prepared as described in Scheme 1, Pd(OAc) 2 (0.0098 g, 2 mol%), P(t-Bu) 3 (0.356 g, 8 mol%) and THF (2 mL) were added and the mixture was stirred to obtain a clear solution. In the other 10-mL two-necked round-bottom flask under nitrogen atmosphere, 4-(4'-bromophenyl) -4"-n-butyldiphenylamine (0.84 g, 2.2 mmol), sodium t-butoxide (0.25 g, 2.6 mmol) [9] were dissolved in 2 mL of THF, and to this solution was added the previous prepared solution of the initiator.…”

“…An approach to overcome this problem is the use of multifunctional polymers in which hole transporting, electron transporting, and emitting abilities are included by means of random or block copolymerization. [2][3][4][5] Recently we have developed multifunctional block copolymers which affords EL devices with microphase separated active layers and higher efficiency. [6][7] In this work we synthesized the polytriphenylamine with the phenothiazine unit at terminal as a multifunctional polymer.…”

Solution Processable Ionic p-i-n Organic Light-Emitting Diodes

“…For a representative example, a power conversion efficiency ( P ) of 24 lm/W at current efficiency ( C ) of 30 cd/A was reported for PHOLEDs with the 1 nm CsF interfacial layer and the Al cathode (Yang & Neher, 2004). Another method is to use a bilayer cathode that consists of an electron-injection layer, such as Ca, Ba, or Cs, and Al evaporated onto the light-emitting polymer layer (Suzuki et al, 2005). A peak  P of 38.6 lm/W was reported when the bilayer cathode was used (Suzuki et al, 2005).…”

“…For that purpose, several trials have been made. One of the simplest devices tested is a solution-processed PHOLED (Yang & Neher, 2004, Liu et al, 2005, Niu et al, 2005, Suzuki et al, 2005. Usually, solution-processed PHOLEDs are prepared by doping a low molecular weight phosphorescent dye, such as an iridium complex, into a proper polymer matrix that contains a large-band-gap polymer, such as poly(vinylcarbazole) (PVK) (Yang & Neher, 2004, Liu et al, 2005, Niu et al, 2005.…”

“…Another method is to use a bilayer cathode that consists of an electron-injection layer, such as Ca, Ba, or Cs, and Al evaporated onto the light-emitting polymer layer (Suzuki et al, 2005). A peak  P of 38.6 lm/W was reported when the bilayer cathode was used (Suzuki et al, 2005). Although such efficient singlelayered PHOLEDs with the interfacial layer have been demonstrated, the processes by which PHOLEDs are fabricated remain inadequate; the device structure is still complex and thus the fabrication process is complicated.…”