Glass transition temperature prediction of disordered molecular solids

Abstract Glass transition temperature, T g, is the key quantity for assessing morphological stability and molecular ordering of films of organic semiconductors. A reliable prediction of T g from the chemical structure is, however, challenging, as it is sensitive to both molecular interactions and analysis of the heating or cooling process. By combining a fitting protocol with an automated workflow for forcefield parameterization, we predict T g with a mean absolute error of ~20 °C for a set of organic compounds with T g in the 50–230 °C range. Our study establishes a reliable and automated prescreening procedure for the design of amorphous organic semiconductors, essential for the optimization and development of organic light-emitting diodes

Synthesis and photophysical studies of bipolar host materials for phosphorescent oleds

The remarkable progress of light-emitting organic semiconductors has led to the establishment of organic light-emitting diodes (OLEDs) as a major display technology. Furthermore, it has opened new directions for materials and devices, and exciting new applications. In the present work, three efficient organic green to yellow emitting bipolar materials, 3-(4-(diphenylamino)phenyl)-1-phenylprop-2-en-1-one (DPPO), 3-(4-(9H-carbazol-9-yl)phenyl)-1-phenylprop-2-en-1-one (CPPO) and 4-(bis(4-(diphenylamino)phenyl)amino) )-1-phenylprop-2-en-1-one (BDPPO) were synthesized. The bipolar materials were characterized by 1H NMR, 13C NMR, Mass, UV-Visible, TGA-DTA and PL spectroscopy techniques. The synthesized phosphor materials are emitting green to yellow color with good emission intensity and can be used in the white light emitting LEDs

Amorphous Metal-Free Organic Phosphors for Sensor Applications

Phosphorescent organic light-emitting diodes are promising for many applications such as display and solid-state lighting because they can reach 100% theoretical internal quantum efficiency. In order to realize bright purely organic phosphors, efficiently promoting intersystem crossing from singlet to triplet and suppressing vibrational dissipation of triplets must be achieved. In this dissertation, I systematically investigated the two critical processes and devised some strategies to achieve bright room temperature purely organic phosphorescence in amorphous films and optical ozone sensors based on phosphorescence phenomena. Embedding organic phosphors into amorphous glassy polymer was investigated as the first strategy to efficiently suppress the triplet vibration by restricting molecular motion of the embedded organic phosphors. This system showed temperature dependent phosphorescence attributed to changing vibrational characteristics of the matrix polymer. An optical temperature sensor integrated in a microfluidic device was devised and demonstrated. Incorporating strong hydrogen bonding between a newly devised purely organic phosphor and hydrogen bonding capable matrix polymer was the second strategy, resulting in much brighter phosphorescence. Modulation of hydrogen bonding by water showed unique reversible phosphorescence-to-fluorescence switching behavior, which was utilized to develop a ratiometric water sensor. Based on the finding that the phosphorescence intensity of the purely organic phosphors is sensitive to environmental ozone concentration, I revealed that the origin of the ozone sensitivity is oxidation of the aldehyde moiety of the organic phosphors and devised highly sensitive and convenient optical ozone sensors by utilizing the observed inverse linear correlation between the phosphorescence emission intensity and the ozone concentration. Since manipulating conjugation length of organic phosphors is a powerful tool to tune the emission color, establishing an understanding on the conjugation length effects on the phosphorescent emission intensity is important. The effects of the conjugation length of the purely organic phosphors on their phosphorescence intensity were systematically studied using a combined experimental and computational approach. The obtained knowledge regarding the role of intermolecular interactions for vibration suppression was adapted to achieve high thermal conductivity in amorphous polymers by designing hydrogen bonding donating and accepting polymer pairs, providing uniformly distributed strong interpolymer linkage, and leading to high thermal conductivity of 1.5 Wm-1K-1.PHDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111449/1/dongwook_1.pd

The Investigation of Phosphorescent Dopants and Novel Blue Fluorescent Polymer Hosts for PLED Devices

This thesis has focused on using experimental and simulation based techniques in an attempt to understand the interactions between polymer hosts and phosphorescent dopants in Organic Light Emitting Devices (OLEDs). The viability of the SEmiconducting Thin Film Optics Simulation (SETFOS) software as a modelling tool has been established using the well documented material poly(3,4-ethylenedioxythiophene) (PEDOT). Parameters including resistivity and work function were extracted using SETFOS and the trends observed compared favourably to the commercially provided values, despite some limitations. SETFOS was then used, along with steady state and transient electroluminescence characterisation, to investigate the effects of both phosphorescent dopant colour and concentration on device performance and extract important device parameters, such as the density of states and carrier mobilities. Different device behaviours were observed depending upon the dopant colour and concentration, highlighting the importance of both to device performance. SETFOS was again found to be able to produce quantitative values for a number of device parameters, but several more limitations within the models were identified, which makes further analysis and investigation necessary. Having gained an understanding of host and dopant interactions in OLED devices, the information gathered was used in the characterization of novel high triplet host polymers for OLED applications. Seven polyfluorene based copolymers were investigated in devices with a range of different coloured phosphorescent dopants and charge transport molecules. Unfortunately, they were found to be unsuitable for use as host materials in OLEDs, acting instead as charge traps. These polymers, along with four others, were alternatively assessed on their ability to perform as deep blue, or violet, fluorescent materials in undoped Polymer LED (PLED) devices. These devices were found to have some of the highest device characteristics currently detailed in the literature, and represent a variety of new ways of achieving efficient deep blue emission using PLED devices

NEMS/MEMSによる次世代有機ELシステム技術に関する研究

早大学位記番号:新7978早稲田大

Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices

Organic optoelectronic devices have been widely used in display, energy-storage, and consumer electronics. Insightful understanding on material properties, device architecture, and fabrication processes is inevitable to improve the performance of organic optoelectronic devices. My PhD research focuses on improving the performance of organic photovoltaics (OPV) and organic light-emitting diode (OLED) through the systematic processing and material design. The first part of the dissertation describes how to construct a highly conductive morphology of mixed donor:acceptor heterojunction. Organic vapor phase deposition (OVPD) was utilized to enhance crystallinity of C70 acceptor in the mixed tetraphenyldibenzoperiflanthen (DBP):C70 thin-film. Forming the face-center-cubic (fcc) structure of C70 facilitated charge extraction, thereby improving fill factor (FF) of the corresponding OPVs. The second part presents the study on the morphological stability and reliability of OPVs. The cathode buffer, bathophenanthroline (BCP), undergoes significant morphological degradation. This morphological degradation was successfully suppressed by making the underlying DBP:C70 layer rougher via the moving N2 carrier gas in OVPD. The open-circuit voltage (Voc) of the obtained heterojunction OPVs of DBP:C70 grown by OVPD experienced a negligible drop (< 3 % change) while the equivalent OPVs grown by VTE showed a significant decrease in Voc from 0.91±0.01 V to 0.74±0.01 after 1 Sun illumination for 250 h. The third part explains a more precise way to control the morphology of organic mixed layer. It was found that increase in the growth pressure of OVPD induced reorganization of molecules to form the equilibrium morphology. The morphology of the electron-filtering buffer layer of 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy):C60 was optimized to achieve the highest electron mobility by means of the control of the growth pressure. Consequently, the resulting OPVs with optimized BP4mPy:C60 buffer showed FF = 0.65±0.01 and a much higher PCE = 8.0±0.2 % compared to PCE = 6.6±0.2 % of the equivalent OPVs with the same composition buffer layer grown by VTE. The fourth part summarizes the effects of the inclusion of novel block-copolymers on the performance of the polymer bulk-heterojunction photovoltaic cells. The block-copolymers were composed of thiophene units with and without a dangling phenyl-C61-butyric acid methyl ester (PCBM) side chain. The added copolymer into the poly(3-hexylthiophene) (P3HT): PCBM active layer resulted in greatly improved thermal stability of P3HT:PCBM. Furthermore, electron conductivity also increased since the fullerene units of the copolymers contribute to the formation of a percolation pathway for electron transport. While PCE of conventional P3HT:PCBM bulk-heterojunction solar cells decreases significantly from 2.6±0.2 to 1.2±0.2% after 90-min of thermal annealing, the equivalent OPVs with the copolymer shows a much smaller decrease in PCE from 3.1±0.2% to 2.7±0.2%. The last section of this dissertation covers the design of phosphorescent OLED employing a metal-free purely organic phosphor. Owing to their much longer triplet lifetime in the millisecond regime compared to microseconds of organometallics, a more careful consideration should be given in the device design. The requirements for the host materials in metal-free purely organic phosphor OLEDs are identified to be a high triplet energy, suitable HOMO and LUMO energy levels, and large spectral overlap with the absorption of the phosphors. Systematic investigation on various host molecules, electron transporting molecules, and the layer thickness of each layer allows us to demonstrate an optimized phosphorescent OLED having an external quantum efficiency (EQE) of 2.5 % at 1 mA/cm2.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144195/1/bssong_1.pd