Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs.

Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge, we propose a unicolored phosphor-sensitized fluorescence approach, with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach, we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime.

Meltdown! Local Heating by Decaying Excited Host Positive Polarons Triggers Aggregation Quenching in Blue PhOLEDs.

Exciton-polaron induced aggregation (EPIA) in organic host materials for blue Phosphorescent Organic Light Emitting Diodes (PhOLEDs) is driven by a non-radiative decay of electronically excited positive polarons resulting in a local heating of the amourphous host matrix. The released heat triggers morphological changes, i. e.

A theoretical study on the mechanistic highlights behind the Brønsted-acid dependent mer-fac isomerization of homoleptic carbenic iridium complexes for PhOLEDs.

Recently, a successful Brønsted-acid mediated geometric isomerization of the meridional homoleptic carbenic iridium(iii) complexes tris-(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(iii) (1) and tris-(N-phenyl,N-benzyl-benzimidazol-2-yl)iridium(iii) (2) into their facial form has been reported. In the present work the pronounced acid-dependency of this particular isomerization procedure is revisited and additional mechanistic pathways are taken into account. Moreover, the acid-induced material decomposition is addressed.

Twist it! The acid-dependent isomerization of homoleptic carbenic iridium(iii) complexes.

The first successful meridional to facial isomerization of homoleptic carbenic iridium(iii) complexes is presented. The Brønsted-acid-mediated procedure allows the conversion of large amounts of material and additionally provides an in situ purification because of precipitation of the target material during the reaction. The pronounced acid-dependency of the reaction yield observed for tris(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(iii) and tris(N-phenyl,N-benzyl-benzimidazol-2-yl)iridium(iii) was investigated by labelling experiments and quantum chemical calculations.

Parameter-free continuous drift-diffusion models of amorphous organic semiconductors.

Continuous drift-diffusion models are routinely used to optimize organic semiconducting devices. Material properties are incorporated into these models via dependencies of diffusion constants, mobilities, and injection barriers on temperature, charge density, and external field. The respective expressions are often provided by the generic Gaussian disorder models, parametrized on experimental data.

Modeling disordered morphologies in organic semiconductors.

Organic thin film devices are investigated for many diverse applications, including light emitting diodes, organic photovoltaic and organic field effect transistors. Modeling of their properties on the basis of their detailed molecular structure requires generation of representative morphologies, many of which are amorphous. Because time-scales for the formation of the molecular structure are slow, we have developed a linear-scaling single molecule deposition protocol which generates morphologies by simulation of vapor deposition of molecular films.

Can lattice models predict the density of states of amorphous organic semiconductors?

We extend existing lattice models of small-molecule amorphous semiconductors by accounting for changes in molecular polarizability upon charging or excitation. A compact expression of this contribution to the density of states is provided. Although the lattice model and the description based on a microscopic morphology both qualitatively predict an additional broadening, shift, and an exponential tail (traps) of the density of states, a quantitative agreement between the two cannot be achieved.

Design rules for charge-transport efficient host materials for phosphorescent organic light-emitting diodes.

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents.

Molecular origin of differences in hole and electron mobility in amorphous Alq3--a multiscale simulation study.

In order to determine the molecular origin of the difference in electron and hole mobilities of amorphous thin films of Alq(3) (meridional Alq(3) (tris(8-hydroxyquinoline) aluminium)) we performed multiscale simulations covering quantum mechanics, molecular mechanics and lattice models. The study includes realistic disordered morphologies, polarized site energies to describe diagonal disorder, quantum chemically calculated transfer integrals for the off-diagonal disorder, inner sphere reorganization energies and an approximative scheme for outer sphere reorganization energies. Intermolecular transfer rates were calculated via Marcus-theory and mobilities were simulated via kinetic Monte Carlo simulations and by a Master Equation approach.

Competition between social and market renting: a theoretical application of the structure-conduct-performance paradigm.

Housing policies in many countries have become more market orientated as the role of governments has shifted from the direct supply and funding of non-market housing towards the role of a regulator and facilitator. Central to this development is the notion that providers of social housing have to become more competitive. Arguably, these social housing changes have important implications for the relationship between social and market rented housing and thus the rental market as a whole.

Atomistic simulation on charge mobility of amorphous tris(8-hydroxyquinoline) aluminum (Alq3): origin of Poole-Frenkel-type behavior.

The atomistic simulation of charge transfer process for an amorphous Alq(3) system is reported. By employing electrostatic potential charges, we calculate site energies and find that the standard deviation of site energy distribution is about twice as large as predicted in previous research. The charge mobility is calculated via the Miller-Abrahams formalism and the master equation approach.

First-principles theoretical investigation of the electronic couplings in single crystals of phenanthroline-based organic semiconductors.

The electronic couplings between adjacent molecules in the phenanthroline-based bathocuproine (BCP) and bathophenanthroline (Bphen) crystals have been studied using density functional theory on model dimers. Within the frame of the "two-state model" of charge-transfer theory, a generalized definition of the "effective transfer integral" is proposed. This definition addresses the issue arising when the lowest unoccupied molecular orbital (LUMO) [highest occupied molecular orbital (HOMO)] and LUMO+1 (HOMO-1) of the single molecules both have significant contributions to the dimer LUMO (HOMO) level.

Two Novel Thermal Biradical Cyclizations in Theory and Experiment: New Synthetic Routes to 6H-Indolo[2,3-b]quinolines and 2-Aminoquinolines from Enyne-Carbodiimides.

The regioselectivity of the biradical cyclization of enyne-carbodiimides 1 can easily be controlled by variation of R at the alkyne terminus. Attachment of a hydrogen atom (R =H) leads to C -C cyclization and formation of biradical 2, whereas C -C cyclization to provide biradical 3 is observed with R =Me Si or Ph.

Two Novel Thermal Biradical Cyclizations of Enyne-Ketenimines: Theory, Experiment, and Synthetic Potential.

Both benzocarbazoles and quinolines can be synthesized from enyne ketenimines 1 generated in situ via biradical intermediates (see reaction below). Which of the heterocyclic ring systems is formed depends on the choice of the substituent R at the alkyne terminus.