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Numerical Analysis of Multilayer Organic Light-Emitting Diodes
Park, Jongwoon,Kawakami, Yoichi,Park, Seoung-Hwan IEEE 2007 Journal of lightwave technology Vol.25 No.9
<P> We make a comparative analysis of two different multilayer organic light-emitting diodes (OLEDs), one of which has an emitting layer (EML), and the other has a carrier transport controlling layer (CTCL) embedded between a hole transport layer (HTL) and an electron transport layer (ETL). The key differences between them lie in the carrier mobilities (relatively low for EML but high for CTCL) and energy levels of the middle layer. An in-depth numerical analysis has been done to provide guidelines for the design of trilayer OLED structures, especially in the context of mobility and energy level offsets. Furthermore, we focus on the transient response and carrier charge and discharge dynamics of those devices. Other than the HTL/CTCL/ETL structure, the transient current balance of the HTL/EML/ETL structure is shown to be much affected by the energy level offsets at the organic/organic interfaces due to the slow carrier dynamics in the EML. It is also demonstrated that the electroluminescence (EL) delay upon turn-on is mainly determined by the electron transport passing through the ETL and further EML, while the fast EL decay upon turn-off is by the rapid discharge of the abrupt accumulation of carriers at the organic/organic interfaces. </P>
Dual Partial Dye Doping for Chromaticity Tuning and Performance Enhancement of White OLEDs
Jongwoon Park,Suganuma, N.,Kawakami, Y. IEEE 2008 Journal of display technology Vol.4 No.1
<P>In general, a guest dopant is doped into a single host matrix for white-light emission with two complementary colors. In this work, however, we have fabricated a white organic light-emitting diode (WOLED) based on dual partial dye doping in which a guest dopant is partially doped into two different host emitters; namely, orange-red emitting 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminos-tyryl)-4H-pyran (DCM) is partially doped into both blue-emitting 4, 4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) and green-emitting Tris-(8-hydroxyquinoline) aluminum (Alq<SUB>3</SUB>). We demonstrate that dual partial dye doping allows us to finely tune the Commission Internationale d'Eclairage (CIE) chromaticity coordinates to the equienergy white point (x = 0.33, y = 0.33). In addition, it enhances device performance further, compared to WOLEDs based on DCM partially doped into a single host matrix (either DPVBi or Alq<SUB>3</SUB>). Moreover, the dual partial doping scheme is shown to provide a way of suppressing the self-quenching effect (singlet-singlet annihilation). For a systematic study, we have implemented a comprehensive numerical model and performed simulations of the OLED structure, providing a clear understanding with regard to the underlying physics of OLEDs. We also carry out an investigation of the effects of key design parameters such as the doped layer position and thickness and dye.</P>
Luminance Uniformity of Large-Area OLEDs With an Auxiliary Metal Electrode
Jongwoon Park,Jongho Lee,Dongchan Shin,Seounghwan Park IEEE 2009 Journal of display technology Vol.5 No.8
<P>One of the key issues of a large-area organic light-emitting diode (OLED) for flat panel lighting applications is to enhance the uniformity of light emission. In this work, we have investigated the effect of an auxiliary metal (chrome) electrode in association with a device configuration on the luminance uniformity of a large-area (15 times 15 cm<SUP>2</SUP>) white OLED. We demonstrate that the ratio between the effective horizontal resistance of anode (indium-tin-oxide (ITO) with the grid patterned metal electrode) and the vertical resistance of the OLED device is the critical factor to determine the luminance uniformity. Moreover, the luminance uniformity is shown to be a function of the current density and degraded with increasing current density. Namely, the OLED panel with the 200-mu m-wide metal lines exhibits the luminance uniformity as high as 90% at 200 mA and 85% at 500 mA.</P>
Power Recycling of Large-Area OLEDs Using Solar Cells
Jongwoon Park,Jongho Lee,Dongchan Shin IEEE 2010 Journal of display technology Vol.6 No.7
<P>We demonstrate that power recycling is feasible by merging a large-area (30 ×120 mm<SUP>2</SUP>) OLED panel and a solar cell into each other. The power recycling efficiency of 0.152% is achieved under the illumination of one side-emitting white OLED at 2450 cd/m<SUP>2</SUP> when the conversion efficiency of a reference solar cell is 4% and the distance between the OLED and solar cell is 1.5 mm. We have found that the power recycling efficiency is decreased under high brightness due to a decrease in the power efficiency of OLED and a loss of current induced by the resistance of a transparent electrode. We have also shown that local heat generation of the large-area OLED panel would be an issue for power recycling.</P>
Electrical Properties of Trilayer Organic Light-Emitting Diodes With a Mixed Emitting Layer
Jongwoon Park,Seounghwan Park,Dongchan Shin IEEE 2009 Journal of Lightwave Technology Vol.27 No.13
<P>We investigate the electrical properties of three different trilayer organic light-emitting diodes (OLEDs), one of which is based on a conventional layered structure and the others on a blended structure where an emitting layer (EML) is uniformly or stepwise mixed with an electron transport layer (ETL), Tris-(8-hydroxyquinoline) aluminum ( Alq<SUB>3</SUB>). By way of simulations, we visualize the electrical behaviors that provide a clear understanding on why the uniformly mixed structure enhances further the longevity of OLEDs, compared to the other configurations. Namely, the uniformly mixed structure has the lowest concentration of positive charges in the ETL (thereby reducing oxidative degradation of Alq<SUB>3</SUB>) and weakest electric field (decreasing the probability of Joule heating), followed by the stepwise mixed one and then the layered one. However, such blended structures show lower recombination efficiency due to the delocalization of carriers (recombination), which has been demonstrated by simulations of the current balance.</P>
Organic LEDs Combined With Inorganic LEDs
Jongwoon Park,Taehyun Ban,Daehyuk Choi IEEE 2011 Journal of Lightwave Technology Vol.29 No.18
<P>An organic light-emitting device (OLED) is coupled with inorganic LEDs to strengthen the OLED's weak point (i.e., low luminous intensity). To this end, side-view LED bars are deployed on the sides of the glass substrate of OLED. To extract not only OLED light but also LED light from the glass substrate, a microlens array (MLA) film is attached to the outer surface of the glass substrate. It is found by experiments that the LED light intensity from the emission area of OLED is relatively low, compared with the OLED light intensity. Moreover, the LED light distribution is shown to be highly non-uniform. To tackle those problems, we introduce a scattering layer at the outer surface of the glass substrate. It is demonstrated by simulations that the luminous intensity and spatial uniformity of the LED light can be significantly enhanced with the aid of the scattering layer configuration where the scattering particle density is stepwisely varied.</P>