White organic light‐emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid‐state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid‐crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid‐state lighting sources are also noted.

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We demonstrate efficient, deep-blue organic electrophosphorescence using a charge-trapping phosphorescent guest, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) doped in the wide-energy-gap hosts, diphenyldi(o-tolyl)silane (UGH1) and p-bis(triphenylsilyly)benzene (UGH2), where exciton formation occurs directly on the guest molecules. Charge trapping on the guest is confirmed by the dependence of the drive voltage and electroluminescence spectrum on guest concentration. Ultraviolet photoemission spectroscopy measurements establish the relative highest occupied molecular orbital positions of FIr6 in UGH1 and UGH2. Peak quantum and power efficiencies of (8.8±0.9)% and (11.0±1.1) lm/W in UGH1 and (11.6±1.2)% and (13.9±1.4) lm/W in UGH2 are obtained, while the emission in both cases is from FIr6 and is characterized by Commission Internationale de l’Eclairage coordinates of (x=0.16, y=0.26) in UGH2.

The use of organic light-emitting devices (OLEDs) for general purpose illumination is being given careful consideration, since OLED power efficiency is now approaching that of incandescent bulbs.[1] The ultimate goal is to demonstrate a device that has an efficiency exceeding that of fluorescent bulbs, which are among the most power-efficient illumination sources available. Here, we discuss three principle means for achieving a high external power efficiency [2] (g p ) white OLED (WOLED): using thin layers for low voltage operation, efficiently confining charge and excitons within the emissive layer (EML), and using direct triplet exciton formation on a blue dopant with a high quantum yield. Additionally, we examine the energy transfer process between dopants, and investigate the dependence of outcoupling efficiency on absorption and reflection losses. A WOLED that combines these strategies, resulting in power efficiencies equal to or exceeding those of the best incandescent light sources, is also described. The devices have a peak total power efficiency of (42 ± 4) lm W ±1 at low intensities, falling to (14 ±2) lm W ±1 at a drive current of 10 mA cm ±2 (corresponding to 0.8 lm cm ±2 for an isotropic illumination source). The Commission Internationale de L'Eclairage [3] (CIE) coordinates shift from (0.43,45) at 0.1 mA cm ±2 to (0.38,0.45) at 10 mA cm ±2 , and a color rendering index [4] (CRI) = 80 is obtained.The electrophosphorescent device employs an EML containing three metallorganic phosphors: 2 wt.-% iridium(III) bis(2-phenyl quinolyl-N,C 2¢ ) acetylacetonate (PQIr) providing red emission, 0.5 wt.-% fac-tris(2-phenylpyridine) iridium (Ir(ppy) 3 ) for green emission, and 20 wt.-% bis(4¢,6¢-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) for blue emission, all simultaneously co-doped into a wide energy gap p-bis(triphenylsilyly)benzene (UGH2) host. It has previously been shown that blue OLEDs employing FIr6 in the inert host UGH2 results in direct charge injection and triplet exciton formation on FIr6. Additionally, FIr6 transports both holes and electrons.[5] The process of direct triplet exciton formation results in high luminance efficiency blue and white electrophosphorescence, since the elimination of host±guest energy transfer avoids exchange energy losses common to earlier red, green, and blue electrophosphorescent OLEDs.[6]The external quantum efficiency of a WOLED [2] (g ext ) issignificantly affected by the thicknesses of the EML and electron transport layer (ETL). [7] Inefficient charge and exciton confinement severely reduces g ext when the EML thickness is < 5 nm, [7] and additional reductions in efficiency are incurred when the ETL thickness is < 25 nm due to exciton quenching at the metal cathode. The optimized device in Figure 1 (fabricated as described in the Experimental section) effectively balances the competing effects of lower operating voltage and reduced quantum efficiency of thin devices. The current density versus voltage (J±V) characteristics are shown in the inset of Fi...

Fully electrophosphorescent organic LEDs can take advantage of the diffusion of triplets to produce bright white devices with high power and quantum efficiencies. It is shown that the device color can be tuned by varying the thickness and the dopant concentrations in each layer, and by introducing exciton blocking layers between emissive layers. The Figure depicts the device structure used to probe the triplet exciton concentration.

Efficient white electrophosphorescence has been achieved with a single emissive dopant. The dopant in these white organic light emitting diodes (WOLEDs) emits simultaneously from monomer and aggregate states, leading to a broad spectrum and high quality white emission. The dopant molecules are based on a series of platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C 2 0 ] b-diketonates. All of the dopant complexes described herein have identical photophysics in dilute solution with structured blue monomer emission (l max ¼ 468, 500, 540 nm). A broad orange aggregate emission (l max % 580 nm) is also observed, when doped into OLED host materials. The intensity of the orange band increases relative to the blue monomer emission, as the doping level is increased. The ratio of monomer to aggregate emission can be controlled by the doping concentration, the degree of steric bulk on the dopant and by the choice of the host material. A doping concentration for which the monomer and excimer bands are approximately equal gives an emission spectrum closest to standard white illumination sources. WOLEDs have been fabricated with doped CBP and mCP luminescent layers (CBP ¼ N,N 0-dicarbazolyl-4,4 0-biphenyl, mCP ¼ N,N 0-dicarbazolyl-3,5-benzene). The best efficiencies and color stabilities were achieved when an electron/exciton blocking layer (EBL) is inserted into the structure, between the hole transporting layer and doped CBP or mCP layer. The material used for an EBL in these devices was fac-tris(1-phenylpyrazolato-N,C 2 0)iridium(III). The EBL material effectively prevents electrons and excitons from passing through the emissive layer into the hole transporting NPD layer. CBP based devices gave a peak external quantum efficiency of 3.3 AE 0.3% (7.3 AE 0.7 lm W À1) at 1 cd m À2 , and 2.3 AE 0.2% (5.2 AE 0.3 lm W À1) at 500 cd m À2. mCP based devices gave a peak external quantum efficiency of 6.4% (12.2 lm W À1 , 17.0 cd A À1), CIE coordinates of 0.36, 0.44 and a CRI of 67 at 1 cd m À2 (CIE ¼ Commission Internationale de l'Eclairage, CRI ¼ color rendering index). The efficiency of the mCP based device drops to 4.3 AE 0.5% (8.1 AE 0.6 lm W À1 , 11.3 cd A À1) at 500 cd m À2 , however, the CIE coordinates and CRI remain unchanged.

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Operational degradation of blue electrophosphorescent organic light emitting devices (OLEDs) is studied by examining the luminance loss, voltage rise, and emissive layer photoluminescence quenching that occur in electrically aged devices. Using a model where defect sites act as deep charge traps, nonradiative recombination centers, and luminescence quenchers, we show that the luminance loss and voltage rise dependence on time and current density are consistent with defect formation due primarily to exciton-polaron annihilation reactions. Defect densities ∼1018cm−3 result in &gt;50% luminance loss. Implications for the design of electrophosphorescent OLEDs with improved lifetime are discussed.