Jwo-Huei Jou

Approaches for fabricating high efficiency organic light emitting diodes

Efficiency is crucial for organic light emitting diodes (OLEDs) to be energy-saving and to have a long lifetime for display and solid state lighting applications. Numerous approaches have been proposed to attain high efficiency OLEDs through the synthesis of novel organic materials, the design of light extraction structures and the design of efficiency-effective device architectures. In this report, we first summarise the efficiency records of OLED devices using fluorescent, phosphorescent, and thermally activated delay fluorescent materials. Importantly, we review all the available efficiency-effective device architectural approaches, which include using thin layer structures, low carrier injection barriers, high carrier mobility, balanced carrier injection, effective carrier confinement, effective host-to-guest energy transfer, effective recombination zone, effective exciton generation on the host, effective exciton confinement, p–i–n structures, and tandem structures. It is hoped that better device structures can therefore be devised upon suitable device engineering to achieve higher efficiency for OLED devices.

High-efficiency white organic light-emitting devices with dual doped structure

A dual doped structure was proposed for obtaining organic light-emitting devices with high-efficiency and full-spectrum white light emission. The device structure included indium tin oxide glass substrate/120 A copper phthalocyanine hole injection layer/500 A N, N′-bis-(1-naphthy)-N, N′diphenyl-1,1′-biphenyl-4-4′-diamine hole transport layer/110 A 1,4-bis(2,2-diphenylvinyl) biphenyl doped with 0.025% 4-Dicyanomethylene-2-methyl-6- [2-(2,3,6,7-tetra-hydro-1H,5Hbenzo[ij]quinolizin-8-yl)vinyl]-4H-pyran+tris- (8-hydroxyquinoline) (Alq3) doped with (0.1%) coumarin 6 emitting layer/200 A Alq3 electron transport layer/5 A lithium fluoride (LiF)/aluminum (Al). A stable white emission [Commission Internationale de l’Eclairage 1931 chromaticity coordinate (X=0.30, Y=0.36)] was obtained for luminance ranging from 100 to 1000 cd/m2. The device turned on at 2.5 V. Its luminance was 8800 cd/m2 at 9 V, and the maximum efficiency was 5 lm/W.

Sunlight-style color-temperature tunable organic light-emitting diode

We demonstrate a man-made lighting device of organic light-emitting diode (OLED) capable of yielding a sunlight-style illumination with various daylight chromaticities, whose color temperature ranges between 2300 and 8200 K, fully covering those of the entire daylight at different times and regions. The OLED employs a device architecture capable of simultaneously generating all the emissions required to form a series of daylight chromaticities. The wide color-temperature span may be attributed to that the recombination core therein can easily be shifted along the different emissive zones simply by varying the applied voltage via the use of a thin carrier-modulating layer.

High-efficiency blue organic light-emitting diodes using a 3,5-di(9H-carbazol-9-yl)tetraphenylsilane host via a solution-process

We present a solution-processed blue organic light-emitting diode (OLED) with markedly high current efficiency of 41.2 cd A−1 at 100 cd m−2 and 31.1 cd A−1 at 1000 cd m−2. The high efficiency was partly attributed to the use of a molecular host, 3,5-di(9H-carbazol-9-yl)tetraphenylsilane, which possesses a wide triplet band gap, high carrier mobility, ambipolar transport property and high glass transition temperature. Besides the intrinsically good physical properties, the solution-process also played an important role in fabricating the high-efficiency device, since it could make the molecular distribution of host and guest homogeneous in the emissive layer. Moreover, the device efficiency at higher brightness could be markedly enhanced by using an electron-blocking layer. As the microlens was introduced on the glass substrate to enhance the light outcoupling, the resultant device efficiency of the blue OLED further increased to 50.1 cd A−1 at 100 cd m−2 and 37.3 cd A−1 at 1000 cd m−2.

Long-lifetime, high-efficiency white organic light-emitting diodes with mixed host composing double emission layers

A long-lifetime, high-efficiency white organic light-emitting diode was fabricated with a mixed host in one of double emission layers. The first layer comprised yellow rubrene doped in a mixed host consisting of 50% N,N′diphenyl-N,N′-bis-(1-naphthyl)-1,1′-biphenyl-4-4′-diamine (NPB) and 50% 2-(t-butyl)-9,10-bis(2′-naphthyl)anthracene (TBADN). The second layer comprised blue 4,4′-bis[2-{4-(N,N-diphenylamino)phenyl}vinyl] biphenyl doped in TBADN. This device exhibited the longest lifetime, five times that of its pure NPB counterpart. The resulting efficiency was 6.0lm∕W (10.9cd∕A) at 10mA∕cm2, 33% better than that of the NPB counterpart. These improvements were attributable to the mixed-host structure, which effectively dispersed carriers and gave a good charge balance.

Efficient, color-stable fluorescent white organic light-emitting diodes with single emission layer by vapor deposition from solvent premixed deposition source

Efficient, color-stable fluorescent white organic light-emitting diodes (OLEDs) with single emission layer were fabricated by vapor deposition from solvent premixed mixtures of 1,4-bis(2,2-diphenylvinyl)biphenyl doped with 4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran and/or 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopy-rano(6,7,8-l, j)quinolizine-11-one. The power efficiencies at 100 cd/m2 were 4.6 lm/W for the two-spectrum pure white OLEDs and 7.2 lm/W m2 for the three-spectrum ones with white emission. By using a different host of 10,10′-bis(biphenyl-4-yl)-9,9′-bianthryl and a greenish-blue dye of di(triphenylamine)-1,4-divinylnaphthalene, the three-spectrum OLEDs with a power efficiency of 6.8 lm/W at 100 cd/m2 were obtained.

Depth Profiling of Organic Films with X-ray Photoelectron Spectroscopy Using C60+ and Ar+ Co-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.

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

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.

Efficient pure-white organic light-emitting diodes with a solution-processed, binary-host employing single emission layer

Efficient white light-emitting diodes were fabricated with a solution-processed single emission layer composing a molecular and polymeric materials mixed binary host. The main host used was a molecule of 4,4′-bis-(carbazol-9-yl) biphenyl and the assisting host used was a blue light-emitting polyfluorene-derived copolymer of poly[(9,9-dioctylfluo-renyl-2,7-diyl)-alt-co-(9-hexyl-3,6-carbazole)]. The hosts were doped via solution-mixing a green dye of tris(2-phenylpyridine) iridium (III) and a red dye of bis[2-(2′-benzo-thienyl)-pyridi-nato-N,C3,](acetylacetonate) iridium (III). One resultant device having a pure white emission of Commission Internationale de l’Eclairage (0.33, 0.33) has a maximum power efficiency of 4.2lm∕W at 802cd∕m2 and a maximum brightness of 11800cd∕m2. The better efficiency performance may be attributed to the addition of the assisting host, which halves the energy barrier for holes to inject into the light-emitting zone.

Highly efficient blue organic light-emitting diode with an oligomeric host having high triplet-energy and high electron mobility

We report a high-efficiency blue organic light-emitting diode (OLED) with a solution-processed emissive layer composed of an oligomeric host of poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (PCMO) that possesses high triplet-energy and high electron mobility. The device exhibited a current efficiency of 40.4 cd A−1 with an external quantum efficiency (EQE) of 21.6% and power efficiency of 28.2 lm W−1 at 230 cd m−2 or 24.7 cd A−1, 10.3%, and 15.5 lm W−1 at 1 000 cd m−2. The high efficiency may be attributed to the host possessing a high electron mobility and lower electron injection barrier, resulting in a more balanced carrier-injection. Moreover, the high electron-mobility favors the transport of electrons, resulting in a more balanced carrier-injection in the emissive layer. The device efficiency has been further enhanced to 42.6 cd A−1 (22.9%, 29.7 lm W−1) at 124 cd m−2 or 28.8 cd A−1 (15.4%, 17.8 lm W−1) at 1 000 cd m−2 by pre-heating the emissive solution at an elevated temperature before spin-coating.