New materials for organic light emitting diodes

This dissertation describes the development of interesting materials for different organic layers, which compose the standard organic light emitting diode (OLED) architecture.;Chapter one introduces the OLEDs architecture, energy transfer mechanisms within the OLED and current -- voltage behavior.;The first part of chapter two deals with the photoinitiated cationic radical polymerization of cross-linkable monomeric compounds for solution processable hole-transporting layers. These solutions where subjected to UV irradiation and the polymerization process monitored by the disappearance of characteristic IR-Bands using FTIR spectroscopy. The second part of chapter two deals with thermally cross-linkable hole-transporting polymers based on 4-vinyl-benzocyclobutene and, 4-[N-(4-vinylphenyl)- N-(4-methylphenyl)amino]-4-[N-phenyl-N-(4 methylphenyl)-amino]-biphenyl or 3-vinyl-triphenylamine. These polymer films were thermally annealed for 2 h at 170 °C followed by cross-linking at 210 °C for 5 h. OLED devices were made by vacuum deposition of aluminum tris(8-hydroxyquinoline) acting as both the emitting- and electron injection layer on top of spin-coated cross-linked, non-cross-linked, and homopolymer hole-transporting layers. The results obtained were compared with small molecule vacuum deposited TPD, a common hole transporting material used in devices today.;Chapter three deals with phosphorescent complexes employed as dopants in nIR emitting OLEDs. A family of metal complexes that have shown intense absorption and emission in the red-to-near infrared region of the electromagnetic spectrum are metalloporphyrins. In this chapter we studied the photophysical and electroluminescence properties of two Pt-metalloporphyrins; Pt(II)- tetraphenyltetrabenzoporphyrin and Pt(II)-tetraphenyltetranaphthoporphyrin.;Chapter four describes the rational large scale synthesis of asymmetric porphyrins to provide a phosphorescence dopant for the application in organic light emitting diode. These asymmetric porphyrins are expected to have a phosphorescence emission maximum between those reported for Pt(II)(tetraphenylbenzoporphyrin) and Pt(II)(tetraphenylnaphthoporphyrin). Do to scrambling during the porphyrin formation reaction, systematic synthesis of different asymmetric porphyrins was not possible on large scale without extensive work on improving the reaction conditions in order to minimize scrambling and product purification efforts. Thus a statistical synthetic approach was chosen along with HPLC purification to identify specific asymmetric compounds and to investigate their corresponding phosphorescence emission energies.;Chapter 5 describes host materials for OLEDs. The most common design for phosphorescence-based OLEDs involves a doped emissive region, where the emissive dopant is either an Ir or a Pt complex. While high-efficiency green and red emitting colors could be obtained readily by doping in the commonly used host materials, such as tris(8-hydroxyquinolinato)aluminum (Alq 3), a wider band gap host is essential for the efficient generation of blue dopant emission. Cyclooctatetraene (COT) is a highly interesting class of organic molecules and despite having the same (CH)n formulation, benzene and COT have strikingly different properties for example, COT is a non-planar tub-shaped molecule. But, the reduction of COT is accompanied by a structural change of the tub shaped neutral molecule to a planar ring, which has been both studied computationally and spectroscopically. For this reason, the geometry of the COT has to be locked into place in order to keep the high triplet energy need for efficient energy transfer to the blue phosphorescent dopant. This may be accomplished through the addition of bulky substituents to the COT scaffold.