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. By changing the concentration of fluorescent emitter, we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80% decay, initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes, helping to overcome the limitations of thermally activated delayed fluorescence.

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. molecular aggregation between neighboring host molecules. The resulting aggregates, which our calculations identify as carbazolyl dimers, lead to decreased PhOLED efficiency. Statistical assessment of some host-only morphologies reveals a structure-dependent propensity for molecular aggregation corroborating the identified EPIA mechanism. Our findings provide a fresh look at established molecular design rules and will help to improve blue PhOLED host materials to enhance blue PhOLED device lifetimes.

Focus-Induced Photoresponse: a novel way to measure distances with photodetectors.

We present the Focus-Induced Photoresponse (FIP) technique, a novel approach to optical distance measurement. It takes advantage of a universally-observed phenomenon in photodetector devices, an irradiance-dependent responsivity. This means that the output from a sensor is not only dependent on the total flux of incident photons, but also on the size of the area in which they fall. If probe light from an object is cast on the detector through a lens, the sensor response depends on how far in or out of focus the object is. We call this the FIP effect. Here we demonstrate how to use the FIP effect to measure the distance to that object. We show that the FIP technique works with different sensor types and materials, as well as visible and near infrared light. The FIP technique operates on a working principle, which is fundamentally different from all established distance measurement methods and hence offers a way to overcome some of their limitations. FIP enables fast optical distance measurements with a simple single-pixel detector layout and minimal computational power. It allows for measurements that are robust to ambient light even outside the wavelength range accessible with silicon.

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. All calculations are carried out using density functional theory (DFT) while the environmental effects in solution are accounted for by the COSMO-RS model. The simulated results clearly reveal the outstanding importance of the complex interplay between acid strength, coordinating power of the corresponding base and the steric influence of the ligand system in contrast to the plain calculation of minimum energy pathways for selected complexes. Eventually, general rules to enhance the material-specific reaction yields are provided.

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. The results reveal a subtle balance between the strength of the acid, the coordinating power of the corresponding base and steric effects of the ligand sphere. Based on these findings, general rules are given for a systematic and material-specific modification of the reaction conditions for the mer-fac isomerization of homoleptic carbenic Ir(iii) complexes.

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. We show that this approach is limited by the fixed range of applicability of analytic expressions as well as approximations inherent to lattice models. To overcome these limitations we propose a scheme which first tabulates simulation results performed on small-scale off-lattice models, corrects for finite size effects, and then uses the tabulated mobility values to solve the drift-diffusion equations. The scheme is tested on DPBIC, a state of the art hole conductor for organic light emitting diodes. We find a good agreement between simulated and experimentally measured current-voltage characteristics for different film thicknesses and temperatures.

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. We have applied this protocol to systems comprising argon, buckminsterfullerene, N,N-Di(naphthalene-1-yl)-N,N'-diphenyl-benzidine, mer-tris(8-hydroxy-quinoline)aluminum(III), and phenyl-C61-butyric acid methyl ester, with and without postdeposition relaxation of the individually deposited molecules. The proposed single molecule deposition protocol leads to formation of highly ordered morphologies in argon and buckminsterfullerene systems when postdeposition relaxation is used to locally anneal the configuration in the vicinity of the newly deposited molecule. The other systems formed disordered amorphous morphologies and the postdeposition local relaxation step has only a small effect on the characteristics of the disordered morphology in comparison to the materials forming crystals.

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. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.

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. The difference in electron and hole mobility originates from the different localization of charge density in the radical anion (more delocalized) compared to the radical cation (more confined). This results in higher diagonal disorder for holes and less favourable overlap properties for the hole transfer integrals leading to an overall higher electron mobility.

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. Conceptual frameworks that facilitate the understanding of this relationship are sparse commodities. This paper seeks to develop a theoretical framework that can be used to shed light on the conditions, processes, and effects of the new relation between the two rental tenures from an economic competition viewpoint. Therefore, this paper adapts the structure-conduct-performance paradigm to rented housing and discusses the framework's applicability and value on a theoretical level.

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. We find that the wide site energy distribution governs Poole-Frenkel-type behavior of charge mobility against electric field, while the spatially correlated site energy is not a dominant mechanism of Poole-Frenkel behavior in the range from 2x10(5) to 1.4x10(6) V/cm. Also we reveal that randomly meshed connectivities are, in principle, required to account for the Poole-Frenkel mechanism. Charge carriers find a zigzag pathway at low electric field, while they find a straight pathway along electric field when a high electric field is applied. In the space-charge-limited current scheme, the charge-carrier density increases with electric field strength so that the nonlinear behavior of charge mobility is enhanced through the strong charge-carrier density dependence of charge mobility.

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. Charge-transfer integrals based on the new definition are compared with those from previous models; significant differences are found. The authors' results indicate that, within a simple Marcus theory approach, the charge-transport parameters of the BCP and Bphen crystals are expected to be similar.

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 R1 at the alkyne terminus.

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 R1 at the alkyne terminus. Attachment of a hydrogen atom (R1 =H) leads to C2 -C7 cyclization and formation of biradical 2, whereas C2 -C6 cyclization to provide biradical 3 is observed with R1 =Me3 Si or Ph.