Efficient light extraction and beam shaping from flexible, optically integrated organic light-emitting diodes

Melpignano, P.; Biondo, Viviana; Sinesi, S.; Gale, M. D.; Westenhofer, S.; Murgia, Mauro; Caria, S.; Zamboni, R.

doi:10.1063/1.2196065

Cited by 34 publications

(21 citation statements)

References 23 publications

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“…In particular, the origin of the efficiency roll-off at high voltages, [8][9][10] the light outcoupling, [11,12] the long-term stability [13,14] and the generation of white light with an all-phosphor device [6,15] are subjects under intense investigation. White light generation is a key issue because of the wide range of applications involving full-color displays and lighting.…”

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confidence: 99%

Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

et al. 2008

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Recently, solid-state lighting has received considerable attention in both academic and industrial research. [1,2] Of particular interest, for the replacement of the existing light sources, are organic light-emitting diodes (OLEDs) based on phosphorescent molecules. [3][4][5][6] The advantage of using these materials lies in the possibility to internally convert all the spin uncorrelated injected charges into light. Indeed, an internal quantum efficiency of nearly 100% has been achieved in devices based on the green-emitting organometallic complex Ir(ppy) 3 .[7]However, many unresolved issues are the subject of current research in order to implement efficient white light sources and expand the number of applications. In particular, the origin of the efficiency roll-off at high voltages, [8][9][10] the light outcoupling, [11,12] the long-term stability [13,14] and the generation of white light with an all-phosphor device [6,15] are subjects under intense investigation. White light generation is a key issue because of the wide range of applications involving full-color displays and lighting. [1,2] Among the different approaches, solution processed devices based on white light emitting molecules [16] have been demonstrated as well as thermally evaporated red, green and blue (RGB) blends [15] or stacks. To date, white light OLEDs (WOLEDs) with long term operational lifetimes have been obtained mainly with a combination of a blue fluorescent emitter [6] and phosphors for the other colors. Such an elegant approach relies on a well engineered harvesting of singlet and triplet excitons and requires therefore a precise doping of the RGB emitting dyes in the transporting hosts. In contrast, efficient WOLEDs based on blue phosphors can be obtained with all the emitters in one single layer, [17] simplifying the processing. Generally, however, blue phosphors have in the past turned out to be rather unstable. While a physical explanation for blue phosphor based device instability is still lacking, a shorter phosphorescence lifetime, eventually approaching the sub-microsecond time regime, would decrease the residence time of potentially unstable excited states. Moreover, processes detrimental to the efficiency, such as exciton charge-carrier quenching [8] or triplet-triplet annihilation, [9,10] could be strongly reduced with a faster exciton recombination. A shorter phosphorescence lifetime while maintaining high quantum efficiencies requires a large radiative rate. For organometallic complexes this rate is directly proportional to the spin-orbit coupling (SOC) matrix element involving the emitting triplet and the perturbing singlet state and inversely proportional to the degree of mixing between them, i.e., the singlet-triplet splitting (DE ST ). [18][19][20] Photophysical studies of the role of SOC and DE ST in tuning the radiative rate are still sparse, mainly because the large intersystem crossing (ISC) rates ($10 13 s À1 ) of such phosphors, [21] which makes detection (and therefore direct measurement of DE ST ) rather cha...

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Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

et al. 2008

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“…3,4 Another possibility to increase the light out coupling is lenses on the surface of OLEDs. [5][6][7][8][9][10][11][12] Hollow spheres embedded in the layers 13 or glass substrates blast with sand 14 are other possibilities for scattering the light. However, these methods are very time-consuming or very costly.…”

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confidence: 99%

Development and characterization of adjustable refractive index scattering epoxy acrylate polymer layers

Eiselt

Preinfalk

Gleißner³

et al. 2016

SPIE Proceedings

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Abstract. Several polymer films for improved optical properties in optoelectronic devices are presented. In such optical applications, it is sometimes important to have a film with an adjusted refractive index, scattering properties, and a low surface roughness. These diffusing films can be used to increase the efficiency of optoelectronic components, such as organic light-emitting diodes. Three different epoxy acrylate mixtures containing Syntholux 291 EA, bisphenol A glycerolate dimethacrylate, and Sartomer SR 348 L are characterized and optimized with different additives. The adjustable refractive index of the material is achieved by chemical doping using 9-vinylcarbazole. Titanium nanoparticles in the mixtures generate light scattering and increase the refractive index additionally. A high-power stirrer is used to mix and disperse all chemical substances together to a homogenous mixture. The viscosity behavior of the mixtures is an important property for the selection of the production method and, therefore, the viscosity measurement results are presented. After the mixing, the monomer mixture is applied on glass substrates by screen printing. To initiate polymerization, the produced films are irradiated for 10 min with ultraviolet radiation and heat. Transmission measurements of the polymer matrix and roughness measurements complement the characterization.

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“…7 They reported an improvement of 70% ͑an enhancement factor of 1.7͒ in the out-coupled emission for an OLED fabricated on such a foil. Miura et al fabricated periodic nanostructures at the air-glass interface by nanoimprint lithography using sol-gel material.…”

Section: Introductionmentioning

confidence: 98%

Comparative study of organic light emitting diode efficiency enhancement by the use of optical films

2007

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An optical film with some microstructures can enhance external quantum efficiency of a bottom-emission type organic light emitting diode ͑OLED͒ by out-coupling the light trapped inside its substrate. We measure the luminous intensity of two OLEDs coupled to several optical films. The films we try are four types of prism films and three kinds of diffuser films. A CCD-based measurement system rapidly evaluates the angular dependency of luminous intensity for each film and OLED combination. Integration of these distributions over the azimuth and polar angles gives luminous flux. We define an enhancement factor as the ratio of the luminous flux of an OLED-film combination to that of an OLED alone. In all the cases, the largest factor is 1.60 and the second largest is 1.54. The OLED with a higher reflectance always has a larger enhancement factor. A highly reflective OLED recycles the light reflected or scattered by the microstructure of the film more efficiently. Ray tracing analysis, assuming a Lambertian source, more or less reproduces the enhancement factors and their dependency on the OLED reflectance. It also indicates that an OLED with 100% reflectance coupled to a film with some spherical surface would give the highest efficiency of 1.68.

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Efficient light extraction and beam shaping from flexible, optically integrated organic light-emitting diodes

Cited by 34 publications

References 23 publications

Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

Development and characterization of adjustable refractive index scattering epoxy acrylate polymer layers

Comparative study of organic light emitting diode efficiency enhancement by the use of optical films

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