Peter B. Mackenzie

Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions

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 >50% luminance loss. Implications for the design of electrophosphorescent OLEDs with improved lifetime are discussed.

19.3: Efficient White Phosphorescent Organic Light‐Emitting Devices

We demonstrate two phosphorescent white OLEDs. At 1,000 cd/m2, one has EQE = 20% at CIE = (0.38, 0.39), and another has 25 lm/W with CIE (0.39, 0.44). At 100 cd/m2 and with outcoupling enhancements, devices either have total EQE = 37% or total efficacy = 51 lm/W.

47.4: Blue Phosphorescent Organic Light Emitting Device Stability Analysis

based on defect generation by exciton-polaron annihilation interactions between the emitter and host molecules, in a blue phosphorescent OLED, is shown to fit well with experimental data. A blue PHOLED with (0.15, 0.25) chromaticity is shown to have a half- life, from 1,000 nits, of 690 hrs. 1. Introduction operational stability of blue phosphorescent organic light emitting devices (PHOLED TM s) (1) presents a challenge to their wide-spread acceptance for use in large-area displays and solid- state lighting (2). However, the steady progress in green and red PHOLED efficiencies and lifetime are testament to the strengths of phosphorescent emitters to provide long operational lifetime with high efficiency. Table I shows the state of the art for PHOLED device performance. Green and red PHOLEDs have high efficacies and long lifetime to meet the specifications for display applications, and these characteristics have been engineered through the development of synthetic and fabrication processes, materials design and device architectures. A deep understanding of device failure is still needed to achieve similar performance characteristics for saturated blue PHOLEDs. The intrinsic luminance loss and voltage rise accompanying long term device operation are not well understood (3), and various hypotheses have been offered to explain the basis for intrinsic degradation in device efficiency, with the most widely accepted advocating chemical degradation of a fraction of the emissive molecules (2). Presumably, bond cleavage produces radical fragments, which then participate in further radical addition reactions to form more degradation products. These products act as non-radiative recombination centers, luminescence quenchers, and deep charge traps. For example, evidence has recently been presented that the excited states themselves may form reaction centers in the case of the common host material 4,4-bis(9-carbazolyl)-2,2-biphenyl (CBP) (4).

Phosphorescent OLEDs with saturated colors

In this paper, two approaches are demonstrated to narrow phosphorescent OLED (PHOLED) emission lineshapes to increase color saturation while keeping device high efficiency performance, which is critical for large area flat panel displays. One approach uses bottom-emissive microcavity structure in green and blue devices to achieve 22 nm full width half maximum (FWHM) emissions. The other approach is to reduce the natural width of the emission as exemplifying in a red device. A new NTSC red with 64 nm FWHM emission is reported. In a standard device, it has a luminous efficiency of 18.3 cd/A at 10 mA/cm2.