You-Hyun Kim

Energy transfer between host and dopant molecules in blue organic light-emitting devices

Blue organic light-emitting devices were fabricated with an activator of 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl doped into the various host materials such as N,N′-bis-(1-naphtyl)-N,N′-diphenyl-1,1-biphenyl-4,4′-diamine; 4,4′-bis(2,2′-diphenylyinyl)-1,1′-biphenyl; 2-methyl-9,10-di(2-naphthyl) anthracene; and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene to investigate optical properties of blue light emission in the host-dopant system. By spectroscopic analysis based on multi-peak fits to the emission spectra, we found that energy transfer between the host and dopant molecules have a strong correlation with key features; current density, luminous efficiency, and color index for the devices. Among the present dopant-host systems, the TPBi molecule was found to be the best molecule as a host material for our devices. In contrast, the DPVBi host induced a complex excimer (electromer) leading to a shoulder spectrum with a longer wavelength emission. It was found that the electromer significantly affects the optical and electrical properties of the device.

White Organic Light-Emitting Diodes With Different Order of RGB-Emitting Layers

To achieve the emissive layers in white organic light-emitting diode (WOLED) for the three primary colors, DCJTB doped in Alq3, C545T doped in Alq3, and BCzVBi doped in MADN were applied for red, green, and blue emissive layer, respectively. By stacking all the red, green, and blue emissive layers in a single device, WOLED devices with the color coordinates close to (0.33, 0.33) were successfully fabricated. The structure of the blue–green–red (BGR) type of the three primary color WOLED device was NPB(700 Å)/MADN:BCzVBi-13%(210 Å)/Alq3:C545T-2%(30 Å)/Alq3:DCJTB-2%(60 Å)/Alq3(300 Å)/Liq(20 Å)/Al(1000 Å). Its maximum luminance and luminous efficiency were 25,680 cd m−2 and 3.79 cd A−1, as well as its white color coordinate was (0.32, 0.31) at 7 V. The structure of blue–red–green (BRG) type of the three primary color WOLED device was NPB(700 Å)/MADN:BCzVB-13%(210 Å)/Alq3:DCJTB-2%(30 Å)/Alq3: C545T-2%(60 Å)/Alq3(300 Å)/Liq(20 Å)/Al(1000 Å). Its maximum luminance and luminous efficiency were 26,010 cd m−2 and 4.95 cd A−1, as well as its white color coordinate was (0.33, 0.33) at 7 V. Both devices were successfully induced closer to the ideal white emissive lighting source, and three different peaks at 480 nm, 512 nm, and 616 nm could be confirmed at red, green, and blue region in electroluminescence (EL) spectra.

Spectroscopic study of white organic light-emitting devices with various thicknesses of emissive layer

White light-emitting devices based on a donor-acceptor structure were fabricated in order to investigate the dependence of the optical properties of white light emission on the thickness variance (15, 20, 25, and 30 nm) of the emissive layer. The emissive layer has a donor-acceptor system with the host 4,4′,4′′-tris(carbasol-l-nyl)triphenylamine molecule doped with 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) and 5,6,11,12-tetraphenylnaphtacene (Rubrene) molecules for blue and yellowish-green light activators, respectively. The characteristics of current density were analyzed by using a power function of applied field, J=σlEl+1 and the characteristic exponential function, J=J0(e(V-Vd)/V0-1). Through spectroscopic analysis, we obtained three physical quantities governing the device performance: 1) an effective conductivity, 2) a threshold potential, and 3) a characteristic potential barrier, which are associated with the trap-charge limited concentration in the bulk layer, the energy gap of t...

Study of Deep Blue Organic Light-Emitting Diodes Using Doped BCzVBi with Various Blue Host Materials

Ever since the efficient and small molecular organic light-emitting diode (OLED) was reported for the first time by Tang and VanSlyke in 1987 [1], enormous interest has been shown in developing the emitting materials in order to realize high-resolution full-color flat panel displays that can provide a long lifetime. However, the performance of blue emitting material is still not sufficient for their applications. There are very few re-ports of OLEDs with a deep blue color, high efficiency and a long operational lifetime [2-5]. Among the three principal colors necessary for display ap-plications, or red, green and blue, blue-emitting materials and devices are particularly in need of improvement in terms of effi-ciency and color purity in comparison to green and red emitters. In recent years, developing a deep blue electroluminescence (EL) color with a CIEy coordinate value of 0.15 has been considered essential [6] because such emitters can effectively reduce the power consumption of a full-color OLED panel and can also be utilized to generate emission of other colors via energy transfer to a matching emissive dopant [2,7]. While high-efficiency green and red emitting colors can be readily obtained by doping the commonly used guest materials, such as tris(8-ydroxyquinolinato) aluminum(Alq

High contrast blue organic light-emitting diodes using inorganic multilayers of Al and ZnSe

High contrast blue organic light-emitting diodes were fabricated using an inorganic multilayer of NPB (700 Å)/MADN (200 Å)/Alq3 (300 Å)/LiF (10 Å)/Al (70 Å)/ZnSe (300 Å)/Al (1000 Å). The optical and electrical characteristics were measured and compared to conventional organic light-emitting devices (OLEDs) and OLEDs with polarizers. OLEDs with the metal multilayer cathodes had an improved contrast ratio of 135∶1 compared to 104∶1 for OLEDs with polarizers. In addition, the multilayer OLEDs had a low turn-on voltage of 3.5 V due to energy band fitting of ZnSe with Al and the electron transport layer.

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes (OLEDs) are developed. A concept of using multiple-emissive layer (EML) configuration is adopted. In this letter, bis(2-methyl-8-quinolinolate)-4(phenylphenolato)Al (BAlq) and 9,10-di(naphtha-2-yl)anthracene (ADN), which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL’eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN. OCIS codes: 230.0230, 230.3670, 230.0250, 160.4890, 130.5990. doi: 10.3788/COL201412.012302.

Destructive Optical Interference of Ambient Light for High Contrast Organic Light-Emitting Diode Using Inorganic Metal Multi-Layer

We fabricated high contrast red, green, blue organic light-emitting diodes using inorganic metal multi layer (IMML) composed of thin Al, KBr, and thick Al. High contrast OLEDs using IMML were observed to compare with conventional OLEDs with circular polarizer and bare metal-electrode normal OLEDs. Average ambient reflectance of OLEDs using inorganic multi layer, polarizer OLEDs, and normal OLEDs were 18.2, 31.1, and 82.5% respectively. Contrast ratio of inorganic metal multi-layer OLED with red emission and polarizer attached OLED were same as 186:1 especially.

Fabrication of White Organic Light-Emitting Diodes Using Two Complementary Color Methods

White organic light-emitting diodes (WOLEDs) were fabricated using two complementary color methods with two emissive layer structure such as NPB (500 Å)/DPVBi (65 Å)/MADN:DCM2-0.5% (170 Å)/Bphen (300 Å)/Liq (20 Å)/Al (1000 Å). A deep blue and orange emissions were obtained from DPVBi layer and MADN host doped with a red fluorescent DCM2 dopant each. White emission was achieved through controlling hole-electron recombination by optimization of emissive layers thicknesses and DCM2 concentrations. Optimized WOLED device shows emission efficiency of 5.04 cd/A, current density of 2.64 mA/cm2 and luminance of 938 cd/m2 at 6 V with CIEx,y color coordinate of (0.338, 0.330). These results indicate that DPVBi layer effects on hole blocking and the exciton generation due to difference of energy level in highest occupied molecular orbital. The CIEx,y coordinates of device was slightly changed change from (0.338, 0.330) at 6 V to (0.341, 0.333) at 12 V, which is almost independent on driving voltages.

The novel organic photovoltaic cell using inter-layer for hole-electron separation

We investigated organic photovoltaic (OPV) cells using hole-electron separation interlayer for improving conversion efficiency. The novel structure of OPV cell was composed of ITO / PEDOT:PSS / DPASN / P3HT:PC71BM / Liq / Al. OPV device inserting DPASN has 2.28% of energy conversion efficiency higher than 1.99% of conventional OPV cell. DPASN layer effected on blocking electron transfer to anode electrode by their LUMO and HOMO energy levels of DPASN whereas DPASN mixing layer with P3HT:PC71BM restricted holes and electrons separation in the active layer when light absorption occurred due to changing Voc.

Study on Optical Characteristics of Organic Light-emitting Diodes Using Two Fluorescence Dopants in Single Emissive Layer

Organic light-emitting diodes (OLEDs) with single emissive layer structures using two fluorescent dopants were fabricated and the device was composed of ITO / NPB () / MADN : C545T - 1.0% : DCJTB - 0.3% () / Bphen () / LiF () /Al (). C545T and DCJTB were functioned as green fluorescent dye and red fluorescent dye under MADN as host material. Concentrations of C545T and DCJTB was changed in emissive layer of MADN. Optimized OLED device using two fluorescence dopants shows emission efficiency of 8.42 cd/A and luminescence of 3169 cd/at 6 V with CIE color coordinate, (0.43, 0.50). Electroluminescence of optimized OLED showed two peak at 500 and 564 nm according to C545T and DCJTB. These results indicate that Fster energy transfer energy transfer was from MADN to C545T and rather than to DCJTB continuously.