High‐Efficiency, Saturated Red‐Phosphorescent Polymer Light‐Emitting Diodes Based on Conjugated and Non‐Conjugated Polymers Doped with an Ir Complex

Abstract

Recently, organic light-emitting diodes (OLEDs) based on phosphorescent dyes have attracted increasing attention due to their potential application in full-color flat panel displays. Owing to the strong spin±orbital mixing of heavy-metal ions in phosphorescent complexes, both singlet and triplet excitons can be fully utilized, [1±5] creating the possibility for electrophosphorescent dye doped devices to reach an internal quantum efficiency of 100 %. Baldo et al.[5] first demonstrated extremely high-efficiency green phosphorescence OLEDs with fac-tris(2-phenylpyridine)iridium, [Ir(ppy) 3 ]. Devices with Ir(ppy) 3 doped into 4,4¢-N,N¢-dicarbazole-biphenyl (CBP) and 3-phenyl-4-(1¢-naphthyl)-5-phenyl-1,2,4-triazole (TAZ), show a high external quantum efficiency of 15 % ph/ el (photon/electron) and a power efficiency of 40 lm/W.[5±7]By modifying the ligand structure of phosphorescent dyes, emission can be tuned over the entire visible region.[8±13]The use of a conjugated polymer as the host material is attractive, since it allows light-emitting diodes (LEDs) to be made with a spin-coating or printing technique. Despite extensive work in many research groups, [14±16] the external quantum efficiency of polymer light-emitting diodes (PLEDs) based on phosphorescent dye doped into a polymer host is still much lower than that of small-molecule-based OLEDs. To date, the highest efficiency (external quantum efficiency (QE ext ) = 10 % ph/el, luminance efficiency (LE) = 32 cd/A, k max = 550 nm) for green phosphorescent PLEDs has been reported by Gong et al., [17] with tris[9,9-dihexyl-2-(pyridinyl- ]iridium(acetylacetonate) (BtpIr) into PVK, Chen et al. [18] attained an external quantum efficiency of 3.3 % ph/el and luminance efficiency (LE) of 2.6 cd/A with a peak emission at 614 nm. For the same system, a similarÐbut slightly lowerÐefficiency was also reported by Kawamura et al. [19] Gong et al. reported a high efficiency of QE ext = 5 % ph/el and LE = 7.2 cd/A with an emission maximum at 600 nm by doping tris(2,5-bis-2¢-(9¢,9¢-dihexylfluorene) pyridine) iridium(III) [Ir(HFP) 3 ] into PVK:PBD (40 %). [20] However, the same authors found that by replacing the PVK(PBD) host with a statistic conjugated co-polymer, poly(9,9-dihexylfluorene-co-2,5-dicyanophenylene) (PF3CNP1), the external quantum and luminous efficiencies of the [Ir(HFP) 3 ]/PF3CNP1 device were significantly reduced, to QE ext = 1.5 % ph/el and LE = 3 cd/A at 142 cd/m 2 .2¢[21] O'Brien and co-workers [22,23] achieved QE ext = 3.5 % ph/el with LE < 1 cd/A by doping PtOEP into poly(9,9-dioctylfluorene) (PFO), with an emission maximum at 646 nm. It seems that conjugated polymer (like PFO and CNPPP) [21±24] hosted phosphorescent PLEDs generally yield lower quantum efficiencies than the non-conjugated polymer, PVK. Sudhakar et al. attributed this phenomenon to phosphorescence quenching by conjugated polymers with a low-energy triplet state. [25] From a Stern±Volmer analysis of the quenching of phosphorescent emission by fluorene oligomers, they concluded that phosp...

Highly Efficient Nanoporous TiO2‐Polythiophene Hybrid Solar Cells Based on Interfacial Modification Using a Metal‐Free Organic Dye

“…Moreover, photogenerated exciton in D102 may be relaxed by transferring its energy to P3HT, since the energy levels of P3HT are contained within those of D102, which disfavors good device performance. [42] Figure 4D schemes the energy levels after treating the dyemodified TiO 2 film with Li salt and TBP. For N719, the treatment leads to a slight increase in the LUMO energy level (3.21 eV), while the HOMO level (5.06 eV) is retained.…”

Stable and Efficient White Electroluminescent Devices Based on a Single Emitting Layer of Polymer Blends

“…[20,21] Moreover, PFs can be used as host materials to generate other colors through energy transfer to lowerenergy emitters in blends with other conjugated polymers, fluorescent dyes, and organometallic triplet emitters. [5][6][7][8][9][10][22][23][24][25] Consequently, PFs can function as both the host and the blue emitter in white-light-emitting PLEDs.…”

Organic Semiconductor Electroluminescent Materials

“…The difference between the two Ir complexes is the homoleptic one for Ir(piq) 3 O/Al), it was confirmed that the electron-transporting host BAlq outperforms the rest of the host materials [156]. (piq) 2 Ir(acac) can also be employed in solution processed [157] device ITO/PEDOT:PSS/PVK/blends/Ba/Al, where the active layer contains a blend of electron-transporting 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) or 3-phenyl-4-(1′-naphyl-5-phenyl-1,2,4-triazole) (TAZ), and one of three kinds of polymeric host material: poly-9,9-dioctylfluorene (PFO), polyvinylcarbazole (PVK), and polyfluorene-p-substituted triphenylamine (PFTA). Among them, PFTA blended with PBD is the best device, having EQE near 12 %, which is higher than that of (piq) 2 Ir(acac) OLED with the thermal evaporation process.…”