Gyeong Heon Kim

High efficiency red top-emitting micro-cavity organic light emitting diodes

In this study, we present optical simulation versus real fabricated device results in the micro-cavity red top-emitting organic light emitting diodes (TEOLEDs). The optical simulation results indicate that the two kinds of possible emissive layer (EML) positions exist in the second order micro-cavity effect and each EMLs could emit the similar radiance with near National Television System Committee (NTSC) color coordinate. Expected current efficiency and external quantum efficiency by the optical simulation toward the surface normal in the red tandem TEOLED are 98.8 cd/A and 22.6% for two EMLs, while fabricated device shows 95.8 cd/A and 26.5%, respectively.

New bipolar host materials for high performance of phosphorescent green organic light-emitting diodes

We report a series of four bipolar host materials with indenocarbozole and biphenyl pyrimidine moieties for efficient phosphorescent green organic light-emitting diodes (OLEDs). All four host materials possess excellent electro-optical properties as well as good thermal and morphological stability due to their high glass transition temperature of 129 °C and decomposition temperature of 363–401 °C. The new materials also show good triplet energy of 2.76 eV and excellent charge carrier transport properties. The fabricated OLED device with 11-[3-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-dimethyl-11,12-dihydro-11-aza-indeno[2,1-a]fluorene (mDPPICz1) as green host shows maximum current and power efficiency as high as 75.8 cd A−1 and 87.1 lm W−1 at 1000 cd m−2, and an almost ideal external quantum efficiency of 21.7%.

Optimized structure of silane-core containing host materials for highly efficient blue TADF OLEDs

Three new derivatives containing silane cores, viz. 9,9′,9′′-(((4-(pyridin-3-yl)phenyl)silanetriyl)tris(benzene-4,1-diyl))tris(9H-carbazole) (SiCz3Py1), bis(4-(9H-carbazol-9-yl)phenyl)bis(4-(pyridin-3-yl)phenyl)silane (SiCz2Py2), and 9-(4-(tris(4-(pyridin-3-yl)phenyl)silyl)phenyl)-9H-carbazole (SiCz1Py3), were designed and synthesized. Carbazole as a donor and pyridine as an acceptor were tethered to tetraphenylsilane at different mole ratios. All three host materials showed high glass transition temperatures between 118 and 164 °C, which are different from those of the previous silane-based host materials (e.g., diphenyldi-o-tolylsilane (UGH-1), 1,4-bis(triphenylsilyl)benzene (UGH-2), and 1,3-bis(triphenylsilyl)benzene (UGH-3)). The triplet energies of these three hosts are observed at 2.85–2.90 eV, which is high enough for them to act as blue host materials in thermally activated delayed fluorescence organic light emitting diodes (TADF OLEDs). In particular, SiCz2Py2 and SiCz1Py3 hosted-TADF OLEDs demonstrated excellent performances, with a maximum external quantum efficiency (EQEmax) of 18.7 and 18.8%, respectively. Such good performances of SiCz2Py2 and SiCz1Py3 are originated by suppressing the non-radiative triplet decay and high reverse intersystem crossing (RISC) rate constant for efficient triplet to singlet up-conversion. This work demonstrates that tetraphenylsilane is a promising non-conjugate (i.e., sp3-hybridized) linkage core for developing a variety of high Tg host materials, particularly for blue TADF OLEDs.

A new rigid diindolocarbazole donor moiety for high quantum efficiency thermally activated delayed fluorescence emitter

In this communication, we report an excellent new highly conjugated and rigid electron donor (diindolocarbazole) for thermally activated delayed fluorescence (TADF) emitters. The donor–acceptor structure with diindolocarbazole donor induces high oscillator strength as well as high probability for the radiative transition, suitable frontier molecular overlap with proper intramolecular charge transfer and a large dihedral angle with out-of-plane structure between electron donor and acceptor; this results in high photoluminescence quantum yield, small singlet–triplet energy splitting, and extremely short delayed fluorescence exciton lifetime. The fabricated organic light emitting diode with new donor applied green emitter exhibits a remarkably high external quantum efficiency of 31.4% without any out-coupling technique. It also shows extremely low efficiency roll-off characteristics with the highest luminance of 71 160 cd m−2 as well as good operational device stability.

Blue thermally activated delayed fluorescence emitters with a δ-pyridoindole donor moiety

Three blue thermally activated delayed fluorescence (TADF) emitters containing a δ-pyridoindole donor moiety and diphenyltriazine acceptor moiety were designed, synthesized and characterized using photophysical and electroluminescence studies. Their photophysical properties and device performances were compared with those of control materials comprising a carbazole donor moiety. The functional properties were altered by different numbers and linking position of the δ-pyridoindole donor moiety. The δ-pyridoindole-substituted materials exhibited blue-shifted emissions and low-lying HOMO levels when compared to their control materials. The as-fabricated OLED devices based on these TADF emitters with a δ-pyridoindole donor demonstrated the blue electroluminescence property with maximum external quantum efficiency of 22.1% and CIE color coordinates of (0.16, 0.23). This result showed that heteroatom engineering in the molecular skeleton is an effective way towards designing highly efficient blue TADF emitters.

High Efficiency Red Phosphorescent Organic Light Emitting Diodes with Single Quantum Well Structure

We report highly efficient red phosphorescent organic light-emitting diodes with a triplet p-i-n single quantum well (QW) structure. This p-i-n single QW structure is realized using p-doped and n-doped wide band-gap hole and electron injection and transporting layers with narrow band-gap host and dopant materials. The maximum current and power efficiencies of 11.1 cd/A and 14.4 lm/W, respectively, are demonstrated by a good carrier confinement effect of QW. Very low driving voltage characteristics of 3.7 ∼ 3.9 V at 1000 cd/m2 are realized by almost no injection barrier with high conductivity configuration in our p-i-n structure.

9.3: Active‐Matrix Organic Light Emitting Diode Using Inverse Staggered Poly‐Si TFT with a Center‐Offset Gated Structure

We have developed a low cost AM backplane using non-laser polycrystalline silicon (poly-Si) having inverse staggered TFT. The thin-film transistors (TFTs) have a center-offset gated structure to reduce the leakage current without scarifying the on-currents. A center-offset length of the TFTs has 3 μm for both switching and driving TFTs. Finally, we have made a 2.2 inch QQVGA (160 × 120) active-matrix organic light emitting diode (AMOLED) display with conventional 2T 1C pixel circuits.