Blue-hazard-free Candlelight OLED

A candlelight-style organic light emitting diode (OLED) is a human-friendly type of lighting because it is blue-hazard-free and has a low correlated color ...

National Tsing Hua University

A buckminsterfullerene (C(60)) ion beam was used for X-ray photoelectron spectrometry depth profiling of various organic thin films. Specimens representing different interfaces in organic light-emitting diode devices, including hole-conducting poly(ethylenedioxythiophene), poly(styrenesulfonic acid) (PEDOT:PSS) thin films on ITO with and without polysilicic acid doping, light-emitting Ir-containing 4,4'-bis(carbazol-9-yl)biphenyl (CBP) molecules on PEDOT:PSS, and electron-conducting 2,2',2' '(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) molecules on CBP, were studied. In all cases, a clear multilayer structure was observed. The chemical composition and elemental state were preserved after C(60+) ion sputtering. The sputter rate was found to decrease with sputtering time. This is due to the deposition of amorphous carbon on the surface, with the rate of implantation highly dependent on the surface interacting with the ion beam.

X-ray photoelectron spectrometry depth profiling of organic thin films using C60 sputtering.

By sputtering organic films with 10 kV, 10 nA C60+ and 0.2 kV, 300 nA Ar+ ion beams concurrently and analyzing the newly exposed surface with X-ray photoelectron spectroscopy, organic thin-film devices including an organic light-emitting diode and a polymer solar cell with an inverted structure are profiled. The chemical composition and the structure of each layer are preserved and clearly observable. Although C60+ sputtering is proven to be useful for analyzing organic thin-films, thick organic-devices cannot be profiled without the low-energy Ar+ beam co-sputtering due to the nonconstant sputtering rate of the C60+ beam. Various combinations of ion-beam doses are studied in this research. It is found that a high dosage of the Ar+ beam interferes with the C60+ ion beam, and the sputtering rate decreases with increasing the total ion current. The results suggest that the low-energy single-atom projectile can disrupt the atom deposition from the cluster ion beams and greatly extend the application of the cluster ion-sputtering. By achievement of a steady sputtering rate while minimizing the damage accumulation, this research paves the way to profiling soft matter and organic electronics.

Depth profiling of organic films with X-ray photoelectron spectroscopy using C60+ and Ar+ co-sputtering.

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.

Effect of fabrication parameters on three-dimensional nanostructures of bulk heterojunctions imaged by high-resolution scanning ToF-SIMS.

By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.

Effect of fabrication parameters on three-dimensional nanostructures and device efficiency of polymer light-emitting diodes.

The efficiency of highly efficient blue, green, red, and white organic light-emitting diodes (OLEDs) has been substantially advanced through the use of high surface-charge nanodots embedded in a nonemissive layer. For example, the blue OLED's markedly high initial power efficiency of 18.0 lm W(-1) at 100 cd m(-2) was doubled to 35.8 lm W(-1) when an amino-functionalized polymeric nanodot was employed. At high luminance, such as 1000 cd m(-2) used for illumination applications, the efficiency was improved from 12.4 to 21.2 lm W(-1), showing a significant enhancement of 71%. The incorporated highly charged nanodots are capable of effectively modulating the transportation of holes via a blocking or trapping mechanism, preventing excessive holes from entering the emissive layer and the resulting carrier-injection imbalance. Furthermore, in the presence of a high-repelling or dragging field arising from the highly charged nanodots, only those holes with sufficient energy are able to overcome the included barriers, causing them to penetrate deeper into the emissive layer. This penetration leads to carrier recombination over a wider region and results in a brighter emission and, therefore, higher efficiency.

Extraordinarily high efficiency improvement for OLEDs with high surface-charge polymeric nanodots.

The nanostructure of the light emissive layer (EL) of polymer light emitting diodes (PLEDs) was investigated using force modulation microscopy (FMM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) excited with focused Bi(3)(2+) primary beam. Three-dimensional nanostructures were reconstructed from high resolution ToF-SIMS images acquired with different C(60)(+) sputtering times. The observed nanostructure is related to the efficiency of the PLED. In poly(9-vinyl-carbazole) (PVK) based EL, a high processing temperature (60 °C) yielded less nanoscale phase separation than a low processing temperature (30 °C). This nanostructure can be further suppressed by replacing the host polymer with poly[oxy(3-(9H-9-carbazol-9-ilmethyl-2-methyltrimethylene)] (SL74) and poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (RS12), which have similar chemical structures and energy levels as PVK. The device efficiency increases when the phase separation inside the EL is suppressed. While the spontaneous formation of a bicontinuous nanostructure inside the active layer is known to provide a path for charge carrier transportation and to be the key to highly efficient polymeric solar cells, these nanostructures are less efficient for trapping the carrier inside the EL and thus lower the power conversion efficiency of the PLED devices.

ToF-SIMS imaging of the nanoscale phase separation in polymeric light emitting diodes: effect of nanostructure on device efficiency.

A high-efficiency pure white organic light-emitting diode was fabricated with lifetime approaching that of the low-excitation-energy (yellow) emitter containing counterpart, or six times that of the deep-blue counterpart. The white device was composed of two emission layers with mixed hosts of different compositions. They were respectively doped with yellow rubrene and deep-blue 4,4'-bis-[4-{N,N,N',N'- tetrakis-(4-fluoro-diphenylamino)-phenyl}-vinyl]-biphenyl. The resulting efficiency was 6.0 lm/W (12.4 cd/A) at 20 mA/cm(2). The long device lifetime may be attributed to the double mixed-host architecture employed that effectively dispersed the injected carriers into three different recombination zones and consequently diluted the damaging effect arising from the accumulated charge from un-recombined carriers, hence leading to a markedly improved lifespan.

Pure white organic light-emitting diode with lifetime approaching the longevity of yellow emitter.

New blue- to yellow-emitting materials have been developed by incorporating fluorene-based chromophores on pyrene core with acetylene linkage and using multifold palladium-catalyzed cross-coupling reactions. Both mono- and tetrasubstituted derivatives have been synthesized and characterized. The tetrasubstituted derivatives displayed red-shifted emission when compared to the monosubstituted derivative indicative of an extended conjugation in the former. End-capping with a diphenylamine unit further red-shifted the absorption and emission profiles and imparted a weak dipolar character to the molecules. Amine-containing derivatives displayed positive solvatochromism in the fluorescence spectra indicating a more polar excited state due to an efficient charge migration from the diphenylamine donor to the pyrene π-acceptor. All of the derivatives were tested as emitting dopants with host material 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP) in a multilayered OLED and found to exhibit bright blue or yellow electroluminescence. The device utilizing 1,3,6,8-tetrasubstituted pyrene derivative as a dopant emitter displayed highest maximum luminescence 4630 cd/m(2) with power efficiency 3.8 lm/W and current efficiency 7.1 cd/A at 100 cd/m(2) attributable to the proper alignment of energy levels that led to the efficient harvesting of excitons. All of the devices exhibited color purity over a wide range of operating voltages.

Pyrene-fluorene hybrids containing acetylene linkage as color-tunable emitting materials for organic light-emitting diodes.

A series of pyrenoimidazoles that contained various functional chromophores, such as anthracene, pyrene, triphenylamine, carbazole, and fluorene, were synthesized and characterized by optical, electrochemical, and theoretical studies. The absorption spectra of the dyes are dominated by electronic transitions that arise from the pyrenoimidazole core and the additional chromophore. All of the dyes exhibited blue-light photoluminescence with moderate-to-high quantum efficiencies. They also displayed high thermal stability and their thermal-decomposition temperatures fell within the range 462-512â€‰Â°C; the highest decomposition temperature was recorded for a carbazole-containing dye. The oxidation propensity of the dyes increased on the introduction of electron-rich chromophores, such as triphenylamine or carbazole. The application of selected dyes that featured additional chromophores such as pyrene, carbazole, and triphenylamine as blue-emissive dopants into multilayered organic light-emitting diodes with a 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP) host was investigated. Devices that were based on triphenylamine- and carbazole-containing dyes exhibited deep-blue emission (CIE 0.157, 0.054 and 0.163, 0.041), whereas a device that was based on a pyrene-containing dye showed a bright-blue emission (CIE 0.156, 0.135).

Pyrenoimidazole-Based Deep-Blue-Emitting Materials: Optical, Electrochemical, and Electroluminescent Characteristics.

Une s&eacute;rie de pyrenoimidazoles qui contenait des chromophores fonctionnels diff&eacute;rents, tels que l&#39;anthrac&egrave;ne, pyr&egrave;ne, triphenylamine, carbazole et le fluor&egrave;ne, ont &eacute;t&eacute; synth&eacute;tis&eacute;s et caract&eacute;ris&eacute; par des &eacute;tudes d&#39;optiques, &eacute;lectrochimiques et th&eacute;oriques. Les spectres d&#39;absorption des colorants sont domin&eacute;es par des transitions &eacute;lectroniques qui proviennent de la base de pyrenoimidazole et le chromophore suppl&eacute;mentaire. Tous les colorants expos&eacute; photoluminescence lumi&egrave;re bleue avec une efficacit&eacute; quantique de mod&eacute;r&eacute; &agrave; &eacute;lev&eacute;. Ils affichent &eacute;galement une stabilit&eacute; thermique &eacute;lev&eacute;e et leur temp&eacute;rature de d&eacute;composition thermique relevait de la gamme 462-512 &deg; C ; la temp&eacute;rature de d&eacute;composition plus &eacute;lev&eacute;e a &eacute;t&eacute; enregistr&eacute;e pour un colorant contenant de carbazole. La propension de l&#39;oxydation des colorants a augment&eacute; sur l&#39;introduction des chromophores riches en &eacute;lectrons, tels que triphenylamine ou carbazole. L&#39;application de certains colorants qui a comport&eacute; des chromophores suppl&eacute;mentaires comme le pyr&egrave;ne, carbazole et triphenylamine comme dopants bleus-&eacute;missifs en multicouches organiques light - emitting diodes avec une foule de biph&eacute;nyle (CBP) 4, 4&#39;-bis(9H-carbazol-9-yl) a &eacute;t&eacute; &eacute;tudi&eacute;e. P&eacute;riph&eacute;riques qui reposaient sur les teintures contenant triphenylamine et carbazole expos&eacute; bleu profond d&#39;&eacute;mission (CIE 0,157, 0,054 et 0,163, 0,041), alors qu&#39;un dispositif qui s&#39;inspire d&#39;un colorant contenant de pyr&egrave;ne a montr&eacute; une &eacute;mission bleu vif (CIE 0,156, 0,135).

Mat&eacute;riaux de base de Pyrenoimidazole en &eacute;mettant Deep-Blue : Optiques, &eacute;lectrochimiques et ELECTROLUMINESCENTS caract&eacute;ristiques.

A series of thienylphenothiazine decorated carbazoles were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations. Absorption spectra of the compounds are influenced by the substitution pattern and chromophore number density. Compounds containing 2,7-substitution exhibited red-shifted absorption, while the chromophore loading on the other positions led to the increment in molar extinction coefficients due to the increase in the chromophore density. Multiple substitutions resulted in twisting of chromophores and affected the conjugative delocalization of the π-electrons, which produced shorter wavelength absorption for the 2,3,6,7-tetrasubstituted derivative. Interestingly, the compounds exhibited excited-state solvatochromism attributable to the structural reorganization-induced electronic perturbations. The solvatochromic data are supportive of a general solvent effect, which is further confirmed by Lippert-Mataga correlation. End-capping with butterfly shaped phenothiazine restrained the formation of molecular aggregates in the solid state. All of the compounds displayed exceptional thermal stability attributable to the rigid carbazole building block. Solution processed OLED fabricated using the new materials as emitting dopants in 4,4'-bis(9H-carbazol-9-yl)biphenyl host exhibited bluish green electroluminescence.

Phenothiazine decorated carbazoles: effect of substitution pattern on the optical and electroluminescent characteristics.

Increasing studies report blue light to possess a potential hazard to the retina of human eyes, secretion of melatonin and artworks. To devise a human- and artwork-friendly light source and to also trigger a "Lighting Renaissance", we demonstrate here how to enable a quality, blue-hazard free general lighting source on the basis of low color-temperature organic light emitting diodes. With the use of multiple candlelight complementary emitters, the sensationally warm candle light-style emission is proven to be also drivable by electricity. To be energy-saving, highly efficient candle-light emission is demanded. The device shows, at 100 cd m<sup>-2</sup> for example, an efficacy of 85.4 lm W<sup>-1</sup>, an external quantum efficiency of 27.4%, with a 79 spectrum resemblance index and 2,279 K color temperature. The high efficiency may be attributed to the candlelight emitting dyes with a high quantum yield, and the host molecules facilitating an effective host-to-guest energy transfer, as well as effective carrier injection balance.

Enabling a blue-hazard free general lighting based on candle light-style OLED.

Organic light-emitting diode (OLED)-based display products have already emerged in the market and their efficiencies and lifetimes are sound at the comparatively low required luminance. To realize OLED for lighting application sooner, higher light quality and better power efficiency at elevated luminance are still demanded. This review reveals the advantages of incorporating a nano-scale carrier modulation layer (CML), also known as a spacer, carrier-regulating layer, or interlayer, among other terms, to tune the chromaticity and color temperature as well as to markedly improve the device efficiency and color rendering index (CRI) for numerous OLED devices. The functions of the CML can be enhanced as multiple layers and blend structures are employed. At proper thickness, the employment of CML enables the device to balance the distribution of carriers in the two emissive zones and achieve high device efficiencies and long operational lifetime while maintaining very high CRI. Moreover, we have also reviewed the effect of using CML on the most significant characteristics of OLEDs, namely: efficiency, luminance, life-time, CRI, SRI, chromaticity, and the color temperature, and see how the thickness tuning and selection of proper CML are crucial to effectively control the OLED device performance.

Carrier modulation layer-enhanced organic light-emitting diodes.

Dependences of gas-barrier performance on the deposition temperature of atomic-layer-deposited (ALD) Al2O3, HfO2, and ZnO films were studied to establish low-temperature ALD processes for encapsulating organic light-emitting diodes (OLEDs). By identifying and controlling the key factors, i.e. using H2O2 as an oxidant, laminating Al2O3 with HfO2 or ZnO layers into AHO or AZO nanolaminates, and extending purge steps, OLED-acceptable gas-barrier performance (water vapor transmission rates ∼ 10(-6) g m(-2) d(-1)) was achieved for the first time at a low deposition temperature of 50 °C in a thermal ALD mode. The compatibility of the low-temperature ALD process with OLEDs was confirmed by applying the process to encapsulate different types of OLED devices, which were degradation-free upon encapsulation and showed adequate lifetime during accelerated aging tests (pixel shrinkage <5% after 240 h at 60 °C/90% RH).

Article	Year
X-ray photoelectron spectrometry depth profiling of organic thin films using C60 sputtering. Analytical chemistry\| PubMed ID: 18081326	2008
Depth profiling of organic films with X-ray photoelectron spectroscopy using C60+ and Ar+ co-sputtering. Analytical chemistry\| PubMed ID: 18355087	2008
Effect of fabrication parameters on three-dimensional nanostructures of bulk heterojunctions imaged by high-resolution scanning ToF-SIMS. ACS nano\| PubMed ID: 20099877	2010
Effect of fabrication parameters on three-dimensional nanostructures and device efficiency of polymer light-emitting diodes. ACS nano\| PubMed ID: 20426427	2010
Extraordinarily high efficiency improvement for OLEDs with high surface-charge polymeric nanodots. ACS nano\| PubMed ID: 20575503	2010
ToF-SIMS imaging of the nanoscale phase separation in polymeric light emitting diodes: effect of nanostructure on device efficiency. The Analyst\| PubMed ID: 20938503	2011
Pure white organic light-emitting diode with lifetime approaching the longevity of yellow emitter. ACS applied materials & interfaces\| PubMed ID: 21732700	2011
Pyrene-fluorene hybrids containing acetylene linkage as color-tunable emitting materials for organic light-emitting diodes. The Journal of organic chemistry\| PubMed ID: 22443338	2012
Pyrenoimidazole-Based Deep-Blue-Emitting Materials: Optical, Electrochemical, and Electroluminescent Characteristics. Chemistry, an Asian journal\| PubMed ID: 23740829	2013
Phenothiazine decorated carbazoles: effect of substitution pattern on the optical and electroluminescent characteristics. The Journal of organic chemistry\| PubMed ID: 25951296	2015
Enabling a blue-hazard free general lighting based on candle light-style OLED. Optics express\| PubMed ID: 26072882	2015
Carrier modulation layer-enhanced organic light-emitting diodes. Molecules (Basel, Switzerland)\| PubMed ID: 26193252	2015
Low-temperature gas-barrier films by atomic layer deposition for encapsulating organic light-emitting diodes. Nanotechnology\| PubMed ID: 27299660	2016

Jwo-Huei Jou

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