Anna Hayer

How do disorder, reorganization, and localization influence the hole mobility in conjugated copolymers?

In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.

Blue-to-green electrophosphorescence of iridium-based cyclometallated materials

The photo- and electroluminescence properties of a series of novel, heteroleptic, mer-cyclometallated iridium complexes have been fine-tuned from green to blue by changing the substituents on the pyridyl ring of the phenylpyridyl ligand. The X-ray crystal structures of two Ir-based triazolyl complexes are reported.

Concepts for solution-processable OLED materials at Merck

Drastic performance increase and accelerated progress in organic light-emitting diodes from solutions are shown using solution-processable small molecules based on the core structures of vacuum-evaporable materials. Systematic modification of small-molecule materials toward better processability from solutions at identical electro-optical properties is shown. This is demonstrated to lead to a significant improvement in the device performance.

The effect of delocalization on the exchange energy in meta- and para-linked Pt-containing carbazole polymers and monomers

A series of novel platinum-containing carbazole monomers and polymers was synthesized and fully characterized by UV-VIS absorption, luminescence, and photoinduced absorption studies. In these compounds, a carbazole unit is incorporated into the main chain via either a para- or a meta-linkage. We discuss the effects of linkage and polymerization on the energy levels of S1, T1, and Tn. The S1-T1 splitting observed for the meta-linked monomer (0.4 eV) is only half of that in the para-linked monomer (0.8 eV). Upon polymerization, the exchange energy in the para-linked compound reduces, yet still remains larger than in the meta-linked polymer. We attribute the difference in exchange energy to the difference in wave function overlap between electron and hole in these compounds.

Synthesis of triplet emitters and hosts for electrophosphorescence

Electroluminescence from conjugated polymers can be significantly improved by harnessing the energy of their nonemissive triplet states. Poly(2,7-dibenzosilole) has been prepared and its triplet energy has been measured as 2.14 eV, a figure that is slightly higher than that of polyfluorene (2.09 eV). Two new tris-cyclometalated iridium complexes with blue-to-green emission properties have been prepared and characterized.

New Light Emitting Polymers and High Energy Hosts for Triplet Emission

This paper describes two aspects of research aimed at harnessing the triplet energy generated in electron-hole recombination in polymer electroluminescent devices. The purpose is to design solution-processible phosphorescent organometallic triplet emitters and to design high triplet energy polymer hosts that can transfer triplet energy to the phosphorescent guests. The method employed Suzuki cross coupling reactions to incorporate either phosphorescent cores or high energy triplet monomers covalently into polymer hosts to evaluate their optoelectronic properties. The results showed (i) efficient energy transfer from polyfluorene hosts to red phosphorescent guests and (ii) that pyridine and carbazole monomers could raise triplet energies of hosts. It is concluded that these approaches offer promise in the design of solution processible electrophosphorescent materials for red and green light emitting devices.