Photochemical Sciences Ph.D. Dissertations

Synthesis of High Triplet Hosts for Phosphorescent Organic Light Emitting Diodes

Date of Award

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Pavel Anzenbacher, Jr.

Second Advisor

Felix Castellano (Committee Member)

Third Advisor

Thomas Kinstle (Committee Member)

Fourth Advisor

Craig Zirbel (Committee Member)

Abstract

Dr. Pavel Anzenbacher, Jr., Advisor Organic light-emitting diodes (OLEDs) have attracted significant attention for the application in next generation display technologies and solid state lightning due to their advantages, such as, the large viewing angle, range of colors, high brightness, low power consumption etc. In OLEDs, based on the type of emitter fluorescence or phosphorescence can be observed. If the emitter is florescent then internal quantum efficiency of OLEDs is limited to 25%. Introducing phosphorescent emitters to the OLEDs allows us to take advantage of spin statistics. The presence of heavy metal facilitates the intersystem crossing due to the spin-orbit coupling consequently the internal quantum efficiency can reach up to 100%.6 These type of OLEDs are classified as Phosphorescent OLED (PhOLED). Because the transition metal complexes are not the best conductors, in PhOLEDs, the phosphorescent emitter is often dispersed in a host material, which is usually a better conductor and is capable of energy transfer to phosphorescent emitters. In order to achieve efficient energy transfer; it is essential that the triplet energy of the host should be larger than that of guest to prevent reverse energy transfer from guest to the host. Moreover, efficient charge injection to active layer requires compatible energy levels of frontier orbitals for adjacent layer since efficiency of an OLED is correlated to recombination efficiency of electrons and holes. However, in order to obtain high triplet energy π-conjugation of molecule should be confined which would inversely effect charge transport properties of molecule. Another requirement for an efficient OLED is materials should be thermally stable to prevent the degradation of devices.

In this present work, proposed compounds have been designed to display limited conjugation as a result of large steric congestion and corresponding limited rotations. Introducing bulky substituents also have improved the morphological properties of molecules, as expected. For charge transport issue, electron withdrawing and/or electron donating moieties have been incorporated for possible charge transporting hosts with high triplet energies. For blue PhOLEDs, new compounds with high triplet energy levels have been synthesized and their electronic, optical and thermal properties have been studied. Most of these compounds have revealed that they are possible hosts for blue PhOLEDs. Furthermore, computational results of some compunds have suggested that they can reveal ambipolar character which benefits charge transport.

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