Both Luminous Efficiency and Lifetime Enhancement in Blue Fluorescence OLEDs by Modifying Molecular Structure of Hole Tr
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Both Luminous Efficiency and Lifetime Enhancement in Blue Fluorescence OLEDs by Modifying Molecular Structure of Hole Transporting Material Yoonhyun Kwak 1, Sun-Young Lee1, Hye-In Jeong1, Dae-Yup Shin2, Young-Woo Song1, Jong Hyuk Lee1, Sung Chul Kim2 1 OLED Precede Technology Team, Samsung Mobile Display Co., Ltd., Gyunggi-Do 446-711, Korea, 2 OLED Development Department, Samsung Mobile Display Co., Ltd., Gyunggi-Do 446-711, Korea ABSTRACT Both luminous efficiency and lifetime in blue fluorescence organic light emitting devices (OLEDs) have been improved by modified HTMs with higher LUMO energy levels. The LUMO energy levels of HTM were increased by modifying substituent in HTM molecules. Two HTMs containing ortho and meta biphenyl substituent and one HTM containing thiophene substituent were synthesized via palladium catalyzed amine coupling reactions to compare with a para biphenyl substituent HTM-1 as a standard molecule. According to TDDFT calculations, these three modified HTMs showed 0.05-0.15 eV higher LUMO energy levels compared to the para biphenyl substituent HTM-1. The luminous efficiency and the lifetime (LT90) of OLEDs using HTM-2 at 500 cd/m2 have been enhanced up to 20 % and 52 %, respectively, compared to the standard device using HTM-1. INTRODUCTION Since the invention of multilayered OLEDs by Tang and VanSlyke, [1] organic electroluminescent materials have attracted many industrial and academic interests owing to their potential applications of flat panel displays and solid state lighting sources. [2] OLEDs have many advantages in terms of low power consumption, high brightness, high contrast, and potentially low cost. [3] In order to produce efficient OLEDs, the materials used in each device for three primary colors (red, green, blue) have to be developed. Among them, the blue EL performance is a bottleneck for the improvement of the overall performance of full-color OLED displays. The blue device shows relatively lower performance in both luminous efficiency and the lifetime than red and green devices. The multilayered OLEDs can be comprised of hole injection layer (HIL), hole transporting layer (HTL), emitting layer (EML), electron transporting layer (ETL), electron injection layer (EIL), and so on. Among them, HIL, HTL and ETL could be deposited as a common layer in each RGB device for convenience of full color device fabrication. It is known that the energy transfer processes between host and guest molecules in EML play an important role in emission efficiency in OLEDs. [4] In order to prevent the quenching of the guest emission, the singlet excited state of blue host molecules has to be higher than that of blue guest molecule which has larger band gab energy than red or green guest molecules. [5] However, a higher singlet excited state of host molecules implies larger band gab energy and it can result in two drawbacks. First it can act as energy barriers for charge transport from nearby HTL and/or ETL to EML to increase the operating voltages. [6] Second, it can make holes and electrons diffi
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