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Hybrid Inorganic/Organic Light Emitting Materials and Devicesfor Displays and Lighting

A. J. Steckl, J. Heikenfeld and S. C. AllenUniversity of Cincinnati, Nanoelectronics Laboratory and Extreme Photonix LLC

Cincinnati, Ohio, USA

AbstractWe present a review of hybrid inorganic/organiclight emitting materials and devices. The essenceof the Hybrid I/O™ approach is to combinematerials and structures from each category insuch a way as to obtain best-of-both-worldsperformance. Examples of Hybrid I/O™applications to displays and solid state lightingare discussed.

IntroductionA great variety of light emitting materials andexcitation mechanisms have been developed fordisplays and lighting applications. Two majorfactors are changing the landscape of lightemission and causing a reevaluation of existingoptions as well as a search for new alternatives. Inthe past decade, the advent of flat panel displaysfor computer and television applications hasgreatly increased the investigation of manymethods of light emission and control, includingthe light valve approach (as in LCDs), the gasplasma emission approach, and the solid stateemissive approaches (such as inorganic andorganic EL devices). Display applications haveput a premium on development of lumophoreswith high brightness, efficiency, and contrast, andaccurate RGB chromaticity. In addition, devicetechnology requirements for displays include athin form factor, the ability to scale up to largeoverall dimensions for TV-type applications andto scale down to small pixel dimensions formicrodisplay applications. More recently, thesolid state lighting initiative has also broughtattention to the need for high power sources withexcellent color rendering index, operating withvery high efficiency, and at very low cost. Giventhis complex web of requirements, it is not likelythat a single type of lumophore or a single devicetechnology can emerge as the single dominantsolution.

In this paper, we present a review of combinationsof inorganic and organic materials for applications

in displays and solid state lighting. The HybridI/O™ approach combines high power, longlifetime inorganic LED pumps with organiclumophores. The organic lumophores exhibit veryhigh photon conversion efficiency, adjustableemission color, can be easily deposited over largeareas, and are very low cost. This review coversoptical properties of the organic lumophores, lightwave coupling of the inorganic pump source to theorganic lumophores, and preliminary results onHybrid I/O™ displays and solid-state lightingdevices.

The basic Hybrid I/O™ is shown schematically inthe diagram of Fig. 1. Photons from UV-emittinginorganic LEDs are absorbed by organiclumophores, which in turn emit RGB photonsbased on their specific chemical makeup. Thebasis of many organic lumophores is the benzenemolecule (C6H6) whose structure of conjugatedbonds results in molecular orbitals that tend toabsorb UV light and emit in the visible range.Many organic color conversion materials (CCMs)can be viewed as combinations of benzene ringswith functional groups that tailor their opticalproperties by modifying the electron distributionthrough groups that either accept or donateelectrons. Examples of CCM color conversionunder optical pumping with inorganic (InGaN)LEDs are shown in Fig. 2. The internal colorconversion efficiency ranges from 50 to 90% andcan reach as high as 98%.

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UV (or blue)emitter LEDsRGB colorconversion material

transparent substrate

Fig. 1 Hybrid Inorganic/Organic (I/O™)approach wherein UV light from inorganicLEDs is converted to visible light by organiccolor conversion materials (CCM).

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HYBRID I/OTM White LampsSolid state Hybrid I/O™ lamp prototypes based onthe light wave coupling [1] have been fabricated.In LWC devices, pump (UV, violet or blue) lightis distributed through a transparent waveguide toan index-matched CCM which converts the pumpwavelength to the desired color. LWC HybridI/O™ lamps with cylindrical geometry (designedto match the form factor of fluorescent tubes) areshown in Fig. 3.

Emission spectra from UV (400 nm) pumpedRGB CCMs are shown in Fig. 4. The colorcoordinates of these CCMs are shown in Fig. 5a.The color temperature of several white-emittingCCMs (red dots) are shown in Fig. 5b along thePlanckian blackbody locus. Approximatelocations for common lighting conditions areshown for comparison. As an example,preliminary results with a 7500 K white lampindicate [2] a luminous efficiency of ~ 7 lm/W anda wall-plug efficiency of ~ 3%. One of the mainpresent limiters of the white Hybrid I/O™ lampsis actually the efficiency of the violet LED beingutilized of ~ 7% wall-plug efficiency. Usinghigher performance violet LEDs (which arebecoming available) will directly translate intoimprovements of the white CCM lamps.

InGaNBlue LED428 nm

GreenCCM 490 nm

WhiteCCM428, 550 nm

YellowCCM585 nm

RedCCM630 nm

Fig. 2 OrganicCCMs excitedb y I n G a NLEDs: (a) RGBemission fromCCM-coatedglass disks; (b)conversion toGWYR(below).

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Fig. 4 Emission spectra from RGBCCMs pumped with violet (400nm) InGaN LED.

Fig. 3 Light wave coupled (LWC)Hybrid I/O™ lighting devices: (a)basic LWC concept; (b) emission fromviolet LED only in LWC rod (left) andfrom rods coated with color andwhite-emitting CCMs.

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HYBRID I/OTM DISPLAYSMany of the advantages of Hybrid I/OTM forlighting are also applicable to displays. To reapthe full benefits of Hybrid I/OTM for displays, wehave also chosen to utilize the light wave coupling(LWC) method. By using LWC, a display panelcan be created which has the potential to exhibitthe efficiency of Hybrid I/OTM lamps for lightingapplications. This level of efficiency is an orderof magnitude higher than that achievable withexisting display technologies. This overall highefficiency is achieved by implementation of a highefficiency light valve compatible with LWC.Previously, the best candidate light valvesavailable were liquid crystals, which actuallyabsorb ~90 to 95% of light even in the fully ONstate (maximum light transmission). We have,therefore, sought development of a novel and nearlossless light valve approach compatible with theLWC platform.

Currently, we have developed a light valveswitching approach based on electrowetting [3].EW switching functions by effectively modifying

the refractive index at the surface of a large planarwaveguide plate that is edge lit with high-efficiency violet LEDs. A basic representation ofthis switching mechanism is shown in Fig. 6a. Bymodulating refractive index, waveguided violetlight can be selectively coupled from thewaveguide plate to fluorescent lumophores. Red,green, and blue, lumophores are then spatiallyarranged to then create large arrays of color pixels.The electrowetting approach achieves thismodulation of refractive index throughmicrofluidic manipulation of low refractive indexwater and high refractive index oil. The oil isdoped with lumophores that fluoresce brightlywhen the oil is brought into a state of opticalcoupling with the waveguide plate. The process isreversible, extremely fast (~10 ms), and requiresvery little power consumption (low capacitance).Demonstration of these fluorescent electrowettinglight valves for light wave coupling displays isreported elsewhere [3]. As shown in Fig. 7, proofof concept for coupling light to lumophores ondisplay sized waveguide plates has also beendemonstrated. The powerful combination of

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Fig. 5 Color coordinates of (a) RGBCCMs; (b) white light emitting CCMs– red dots.

Red120 cd/m2

(0.64,0.35)n=1.47

Green440 cd/m2

(0.25,0.53)n=1.51

Blue55 cd/m2

(0.15,0.07)n=1.51

Planar Waveguide Configuration

Light Storage PlateLight Storage Plate

RGB Organic CCM Pixels UV-Violet Inorganic LEDs

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Fig. 6 Light wave coupled (LWC)Hybrid I/O display devices: (a) basicLWC concept; (b) emission from RGBCCM pixels under violet LED pumpingin planar waveguide.

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Hybrid I/OTM with near-lossless switchable pixels,results in a very compelling and innovativealternative display technology.

Future DevelopmentSignificant research and development is requiredfor Hybrid I/O™ devices to realize their fullpotential in solid-state lighting and displayapplications. However, early device results arevery promising. In Hybrid I/O™ lamps, newoptical designs of the LWC lamps are beinginvestigated, along with the design of the optimalCCM white emitting blend. In Hybrid I/O™displays, new cell designs and improved materials(fluorescent CCMs and hydrophobic insulatorcoatings) are currently being explored. We believethat the Hybrid I/O™ LWC concept will produce

a compelling new technology platform in the nearfuture.

References[1] J. Heikenfeld and A. J. Steckl, “A NovelFluorescent Display Using Light Wave CouplingTechnology”, Proc. Soc. Inf. Display, Vol. 35, pp.470-473, Seattle, May 2004.

[2] S. C. Allen, J. Heikenfeld, and A. J. Steckl,“Hybrid Inorganic/Organic Devices for Solid StateWhite Lighting Applications”,Electroluminescence 2004, Toronto, Sept. 2004.

[3] J. Heikenfeld and A. J. Steckl, “Demonstrationof Fluorescent RGB Electrowetting Devices forLight Wave Coupling Displays”,Electroluminescence 2004, Toronto, Sept. 2004.

Fig. 7 Hybrid I/O™ LWC displayprototype for full color signageapplications.