Development of Fluorescent Materials for Non-doped Organic Light-emitting Diodes and Color Tunable Devices

Abstract

Abstract

Development of Fluorescent Materials for Non-doped Organic Light-emitting Diodes and Color-tunable Devices

Seongjin Jeong Major in Organic Chemistry Department of Chemistry Graduate School Seoul National University

Organic light-emitting diodes (OLEDs) have been attracting great attention as next-generation displays due to advantages such as self-emitting, high brightness, high contrast ratio, fast response and ultra-thin thickness. The materials used for the emitting layer (EML) are the key part of the OLEDs and demand high color purity, high quantum efficiency and easy reproducibility of production. Among them, since it is particularly difficult to obtain high color purity and quantum efficiency for blue luminescent materials, research on them is further required than other primary colors. In this study, the development of fluorescent materials for the blue emission and the delayed fluorescence (DF) is discussed, and followed by the structure of color-tunable OLEDs (CTOLEDs) fabrication. The blue fluorescent materials exhibit better color purity and quantum efficiency than conventional blue materials, and the delayed fluorescent materials show higher quantum efficiency in a non-doped condition without a host material in the EML than those of doped condition. The CTOLED using an exciton blocking layer (EBL) is applied to the development of ultra-thin carbon nanotube (CNT)-based electrocardiogram (ECG) monitors and shows sensitive color change according to heart rate. Part I summarizes basic theories for the understanding of the overall photophysical phenomena involved in the OLEDs. Understanding the mechanism of absorption and emission processes is necessary to analyze the principle of electroluminescence, its quantum efficiency and lifetime in an actual device. Also, this understanding is helpful to devise novel blue fluorescent materials and delayed fluorescent materials for use in OLEDs. Part II is a study on the development of the highly efficient blue fluorescent emitters without a host material in the EML. It has been known to be difficult to develop blue fluorescent materials with the standard blue color coordinates corresponding to CIE (0.15, 0.07) and high quantum efficiency, because of the inherent high band-gap energy characteristic and the imbalanced charge transporting properties. In this study, donor-chromophore-acceptor type blue fluorescent materials are developed where hole transport and/or electron transport functional groups are twisted with respect to the plane of the chromophore. These materials show high charge balance and prevent the bathochromic shift of emission in the solid state. In the first section, four bipolar luminescent molecules (3PAA, IQAA, 3QAA, and PTAA) with anisole-anthracene (AA) cores attached to the electron withdrawing group such as pyridine, isoquinoline, quinoline and phenanthroline, respectively, are designed and synthesized for use as non-doped blue fluorescent materials. Because of highly twisted molecular structure, each molecule shows deep-blue to sky-blue emission in solution state. The emission of the quinoline-attached 3QAA is shifted to the longer wavelength region (bathochromic shift) compared to those of 3PAA and IQAA, whereas phenanthroline-attached PTAA emitted relatively longer wavelength region than that of 3QAA. These results come from the difference in the conjugation length and position of the electron accepting group. In the second section, blue fluorescent emitters named T1B and T2B are synthesized by direct coupling of triphenylamine with good hole mobility and benzimidazole with good electron mobility, for use as deep-blue emitters for efficient non-doped fluorescent OLEDs, where the correlation between the device performance and the molecular packing density is investigated. Fluorescent OLEDs using sterically bulky T2B as an emitter without a host material show deep-blue emissions and higher external quantum efficiency (EQE) than that of T1B owing to more efficient inhibition of exciton quenching. In the third section, three blue fluorescent materials, named QT, XT, and ZT, are developed by combining triphenylamine for good hole mobility, quinoline, quinoxaline and quinazoline for good electron mobility, respectively. Non-doped OLEDs based on QT, XT, and ZT generate saturated blue to sky-blue EL emission. Among them, OLEDs fabricated using QT exhibit the highest efficiency and color coordinates close to the standard blue color, and is also used as a host material for XT and ZT. In the fourth section, a new indenophenanthrene core structure is developed to induce a deep blue emission by hybridizing a phenanthrene moiety with fluorene. TIP and DIP are synthesized by coupling triphenylamine and diphenylamine with good hole mobility to the indenophenanthrene core, respectively. DIP-based non-doped OLED devices exhibit higher efficiency compared to TIP-based devices, as they have well-matched hole and electron density at the same driving voltage. Part III is a study on the development of high-efficiency materials that produce delayed fluorescence without a host material. Thermally activated delayed fluorescent (TADF) molecules tend to aggregate easily, through π-π interactions arising from their innate hydrophobicity and rigid-planar structures. This phenomenon leads to aggregation-caused quenching (ACQ) and exciton concentration quenching in the solid state, which is the fatal weakness in OLEDs using solid-state emission. To overcome this problem, two materials named AmT and AmmT are designed. These two emitters are composed of electron donor (acridine) and acceptor (triphenyltriazine) moieties, which are connected at the meta positions so that intra- and intermolecular charge transfer (CT) states can be induced through exciplex-like interactions between the same emitters. These materials also show DF characteristic. AmT-and AmmT-based non-doped OLEDs devices exhibit higher external quantum efficiencies than those of doped devices, due to not only the formation of excitons by intramolecular interaction, but also by intermolecular interaction. In part IV, a method for manufacturing wearable ECG monitors is developed using a CNT based ultra-thin electronic device and a CTOLEDs based display. A p-type metal-oxide semiconductor (p-MOS) inverter with four CNT transistors leads to successful acquisition of the ECG signals by allowing high amplification. In the CTOLEDs, an ultrathin EBL of bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) is used to manipulate the balance of charges between two adjacent emission layers

blue EML by bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato) iridium(III) (FIrpic) and red EML by bis(2-phenylquinolyl-N,C(2′))-iridium(acetylacetonate) (pq2Ir(acac)), which thereby produces different colors with respect to applied voltages. These ultrathin fabricated devices support superior wearability yielding the conformal integration for the on-skin sensor, with high resilience verified through repetitive bending/folding fatigue tests. The wearable CTOLEDs integrated with CNT electronics are used to display human ECG changes in real-time using tunable colors.

Keywords: Fluorescent organic light emitting device, Bipolar, Blue fluorescent material, Delayed fluorescent material, Intermolecular interaction, Carbon nanotube, Color tunable organic light emitting device, Wearable sensor

Student Number: 2011-30103