Stefan Kappaun

Phosphorescent Organic Light-Emitting Devices: Working Principle and Iridium Based Emitter Materials

Even though organic light-emitting device (OLED) technology has evolved to a point where it is now an important competitor to liquid crystal displays (LCDs), further scientific efforts devoted to the design, engineering and fabrication of OLEDs are required for complete commercialization of this technology. Along these lines, the present work reviews the essentials of OLED technology putting special focus on the general working principle of single and multilayer OLEDs, fluorescent and phosphorescent emitter materials as well as transfer processes in host materials doped with phosphorescent dyes. Moreover, as a prototypical example of phosphorescent emitter materials, a brief discussion of homo- and heteroleptic iridium(III) complexes is enclosed concentrating on their synthesis, photophysical properties and approaches for realizing iridium based phosphorescent polymers.

WPLEDs prepared from main-chain fluorene–iridium(III) polymers

A novel heteroleptic iridium(III) complex containing 5,7-dibromoquinolinolate as co-ligand has been used to prepare main-chain iridium(III) polymers which produced white light emission when used as the active layer in polymeric light-emitting devices.

Molecular fluorescent pH-probes based on 8-hydroxyquinoline

Three 5,7-pi-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and 5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent pH-probes in nonaqueous solution. Absorption and photoluminescence spectra of the introduced compounds also including the starting material 8-benzyloxy-5,7-dibromoquinoline as well as their N-protonated counterparts were recorded and the results were rationalized by quantum-chemical calculations. A pronounced red shift of the emission occurred upon protonation of the non halogenated derivatives, while the dibromo-derivative is hardly emissive and is virtually not protonated under the conditions used. The diphenyl- and the bis(biphenyl)-derivative especially show promising photoluminescence quantum yields both in the parent and the protonated state making them candidates for the active component in pH sensing applications.

Optically Active Chemical Defects in Polyfluorene-Type Polymers and Devices

Polyfluorenes and polyfluorene-type polymers have emerged as a promising class of emitter materials for realizing blue polymeric light-emitting devices. However, during device operation material degradation leads to the occurrence of an unwanted green emission band at 2.2–2.3 eV. This chapter reviews the latest scientific investigations on the origin of this low energy emission band, putting special focus on chemical (i.e., keto defect sites, hydroxy-terminated polyfluorenes, etc.) and interface defects. Along this line, the formation of defect sites during polymer synthesis, their enhancement in the solid state upon, e.g., thermal stress, and the defect emission under device operation are discussed. Finally, novel approaches for realizing stable blue emitter materials are included, demonstrating that a thorough consideration of the herein presented results in materials design paves the way to polyfluorene-type polymers suitable for practical applications.

Investigation of new polymers with regard to the application in hybrid solar cells

Hybrid solar cells consist of electroactive polymers and semiconducting nanoparticles. They will become very important for various applications in the nearer future. Their big advantages are the cheap production, the low demand on material and their flexibility. In this work, the behaviour of CdSe-nanodots in combination with new polymers was investigated. For all experiments the promising bulk heterojunction technique was used. Copolymers of fluorene and dye molecules were synthesized and compared to the fluorene homopolymer. It was demonstrated that hybrid solar cells using polyfluorenes, whose absorption was tuned to higher wavelengths by copolymerisation, can achieve efficiencies which are five times higher than the efficiencies of hybrid solar cells containing the fluorene homopolymer