Saturated and efficient blue phosphorescent organic light emitting devices with Lambertian angular emission
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(a) Structure of the strong microcavity OLED compared to that of a conventional or weak microcavity OLED. In the strong microcavity the anode is a thin, semitransparent layer of Ag. The cathode is defined by a diameter shadow mask. The electron transport layer is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine or BCP). To aid hole injection from the silver anode, the first of the hole transport layer -diphenyl--bis(3-methylphenyl)-[-biphenyl]--diamine (TPD) is doped with 3% by mass of the acceptor tetrafluorotetracyanoquinodimethane. The emissive layer consists of 6% by mass iridium(III)bis[(4,6-difluorophenyl)-pyridinato-]picolinate (FIrpic) in -dicarbazolyl-3,5-benzene (mCP). The devices were grown directly on the smooth back surface of frosted glass and opal glass diffusers. The holographic diffuser was employed external to devices grown on regular glass. The weak microcavity OLED has an anode precoated with indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT-PSS). All other layers were deposited by thermal evaporation at a base pressure of less than . Each layer is subject to 20% uncertainty in the interferometric measure of thickness. (b) The calculated distribution of energy dissipation within the OLEDs. In the strong microcavity OLED, energy lost to the cathode, anode, and waveguide modes is labeled, aluminum, silver, and glass, respectively. The remaining energy is outcoupled to air. The modeled layers are Ag . In the conventional, or weak microcavity OLED, some energy is dissipated in the aluminum cathode, but most energy is lost to waveguided modes. Roughly 20% of the energy is coupled to waveguide modes in the organic films. These modes are absorbed by the PEDOT and ITO layers. Another is waveguided within the glass substrate. The modeled layers are ITO /PEDOT-PSS .
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(Color online) (a) External quantum efficiency of the strong microcavity FIrpic OLED compared to the performance of a conventional weak microcavity device built on ITO/PEDOT:PSS rather than silver. The comparison demonstrates that the strong microcavity increases the efficiency of the OLED. All devices were measured in a nitrogen environment to minimize degradation. (b) The optical transmission efficiencies of our glass substrates compared to the three diffusing filters, frosted glass, opal glass, and the holographic diffuser. Of the scattering filters, the holographic diffuser exhibits the highest optical transparency.
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(Color online) Electroluminescent spectra of the strong microcavity FIrpic OLED as a function of angle from the surface normal (a) without and (b) with the holographic diffuser. A solid angle cone of 0.6° was collected at each rotational position. With the holographic diffuser the color shift is barely perceptible. For comparison in (a) we plot the intrinsic photoluminescent spectrum of FIrpic and in (b) we plot the modeled electroluminescent spectrum of the strong microcavity after transmission through an ideal scattering filter. (c) The color coordinates of the strong microcavity devices with holographic diffusers are deep blue with . The intrinsic FIrpic photoluminescence spectrum is sky blue with . Inset: the full CIE diagram identifying the expanded blue region.
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(Color online) (a) Angular emission profile of the strong microcavity FIrpic OLED as a function of angle from the surface normal, together with its modification by the three diffusing filters. Opal glass and the holographic diffuser both yield nearly ideal Lambertian emission patterns, but the holographic diffuser has superior optical transparency. A solid angle cone of was collected at each rotational position. [(b) and (c)] Scanning electron micrographs of the surface and cross section, respectively, of the holographic diffuser.
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