Publications

Lithium−Quinolate Complexes as Emitter and Interface Materials in Organic Light-Emitting Diodes

We synthesized lithium−quinolate complexes, 8-hydroxyquinolinolatolithium (Liq) and 2-methyl-8-hydroxyquinolinolatolithium (LiMeq), as emitter and electron injection/transport materials to be used in conventional two-layer organic light-emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenylbenzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8-hydroxyquinolinolato-Al(III) (Alq3) as emitter material. The device efficiency with these complexes was optimized by combinatorial methods. We also compared the electron injection, transport, and emission properties of Li complexes with the well-known emitter Alq3 in the same experiment by taking advantage of the combinatorial approach. The Li quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq3 devices. Contrary to the Alq3 emission, the Li quinolates exhibit a bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence of exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li quinolate and DMeOTPD with the observed electroluminescence spectrum. However, the lithium complexes increase the efficiency of an optimized indium−tin oxide (ITO)/DMeOTPD/Alq3/Al device considerably when used as a thin interface layer between Alq3 and aluminum. The improvement of device characteristics with lithium quinolates is similar to that obtained with LiF salt.

High Extinction Coefficient “Antenna” Dye in Solid-State Dye-Sensitized Solar Cells: A Photophysical and Electronic Study

We present a photophysical and device-based investigation of a new bipyridyl−NCS ruthenium complex sensitizer with an extended π system, in both sensitized TiO2 and incorporated into solid-state dye-sensitized solar cells. We compare this new sensitizer to an analog dye without the extended π system. We observe very similar excited-state absorption spectra and charge recombination kinetics for the two systems. However, the π-extended senstizer has a phenomenally enhanced molar extinction coefficient which translates into far greater light harvesting and current collection in solid-state dye-sensitized solar cells. We also infer from transient photovoltage measurements that positioning the pendent extended π system away from the TiO2 surface has induced a favorable dipole shift, generating enhanced open-circuit voltage. The resulting power conversion efficiency for the solar cell has been increased from 2.4% to 3.2% when comparing the new sensitizer to an analogy with no pendent group.

<title>Organic photorefractive systems based on triphenyldiamine derivatives</title>

Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.

Principles of Structural Design of Conjugated Polymers Showing Excellent Charge Transport toward Thermoelectrics and Bioelectronics Applications

Conjugated polymers, especially their second generation with a donor-acceptor alternating structure, have promising properties. These are suitable for two emerging fields, thermoelectrics and bioelectronics, if appropriate structural designs are implemented. This review aims to give a perspective for the potential and challenges of novel conjugated polymers in such applications. In particular, the aspects of synthetic design and the consequences of modifications of the chemical structure on the charge transport in selected second-generation conjugated polymers are reviewed. By understanding the effects of structural motifs on the overall material properties, polymers can be specifically tailored for the respective application. The basics of charge transport measurements are briefly summarized, as the charge transport plays an important role for thermoelectrics as well as for bioelectronics. In particular, the correlation between the reported charge carrier mobility values and the structural design of the polymers is reviewed. Examples of the application of second-generation conducting polymers in thermoelectrics and bioelectronics are shown to demonstrate the current state of research. Finally, the prospect of a purposeful design of new materials for these two emerging fields is discussed.

Determination of charge carrier mobility of hole transporting polytriarylamine-based diodes

Perylenediimides with electron transport moieties for electroluminescent devices

<title>Temperature dependent measurements on a low-molecular-weight photorefractive glass</title>

The erasing dynamics of holographic gratings in a low molecular weight photorefractive glass depending on the sample temperature were investigated. Changes in the overall speed of the material by three orders of magnitude over a temperature range of 13 K were observed. We identified two distinct processes below the glass transition temperature T<SUB>g</SUB>, a fast one on time scales of seconds and a slower one with lifetimes around 10<SUP>3</SUP> s. We attribute the fast process to the electro-optic effect and the slower one to orientational diffusion processes of the glass molecules. Above T<SUB>g</SUB>, the fast process vanishes, whereas the diffusional processes accelerate up to time constants in the range of seconds. This study shows, that an accurate temperature control is indispensable when measuring photorefractive dynamics, especially in the temperature range around T<SUB>g</SUB>.

Macromol. Rapid Commun. 14/2009

Back Cover: A schematic illustration of the hybrid block copolymer [G2]-b-PPerAcr is shown. Also indicated is the energy transfer between the triphenylamine moieties and the perylene bisimide upon irradiation. Further details can be found in the article by A. S. Lang, F. R. Kogler, M. Sommer, U. Wiesner, and M. Thelakkat* on page 1243.

Thermalstudies on polythiophene containing mesogenic side chains

Characterization of perovskite solar cells: Towards a reliable measurement protocol

Lead halide perovskite solar cells have shown a tremendous rise in power conversion efficiency with reported record efficiencies of over 20% making this material very promising as a low cost alternative to conventional inorganic solar cells. However, due to a differently severe “hysteretic” behaviour during current density-voltage measurements, which strongly depends on scan rate, device and measurement history, preparation method, device architecture, etc., commonly used solar cell measurements do not give reliable or even reproducible results. For the aspect of commercialization and the possibility to compare results of different devices among different laboratories, it is necessary to establish a measurement protocol which gives reproducible results. Therefore, we compare device characteristics derived from standard current density-voltage measurements with stabilized values obtained from an adaptive tracking of the maximum power point and the open circuit voltage as well as characteristics extracted from time resolved current density-voltage measurements. Our results provide insight into the challenges of a correct determination of device performance and propose a measurement protocol for a reliable characterisation which is easy to implement and has been tested on varying perovskite solar cells fabricated in different laboratories.

Impact of excess PbI₂ on the structure and the temperature dependent optical properties of methylammonium lead iodide perovskites

Excess PbI₂ in MAPbI₃ thin films has no direct impact on the electronic structure but changes the temperature dependence of the exciton binding energy and the phase transition behaviour.

Crystalline–Crystalline Donor–Acceptor Block Copolymers

Efficient combination of two polymerization reactions allowed various complex issues in photovoltaic devices, such as light absorption, the presence of a donor–acceptor heterojunction, photoluminescence quenching, crystallinity, and microphase separation, to all be addressed in a single block copolymer (see picture).

Materials Screening and Combinatorial Development of Thin Film Multilayer Electro‐Optical Devices

Abstract Summary: Rapid materials screening and combinatorial development of thin film multilayer electro‐optical devices is essential for fast research and development progress and the implementation of device structures into commercial products. A well‐established and reliable film preparation technique within the required nanometer film thickness range is vapor deposition. The combinatorial approach underlying this feature article is based on the preparation of linear or step gradient and the preparation of sectors of material combinations or device structures by using mask movements in combination with a rotation of the substrate. Both of the two principles are combined to obtain an infinite number of possible libraries with different complexity, which may differ in each sector by the layer thickness, the sequence of layers, and the material selection. In addition by simultaneous evaporation using two or more sources and varying the deposition rate, material compositions can also be created. In this feature article we review our work on combinatorial material screening and combinatorial optimization of multilayer thin film organic electro‐optical devices prepared by vapor deposition. This article covers results on organic light emitting devices (OLEDs), organic solar cells, the orientation behavior of formanisotropic functional molecules on alignment layers and the in situ preparation and orientation of rodlike and thermally stable aromatic polyimides. Example of a combinatorial vapor deposition technique, which is a powerful tool to screen efficiently new materials and configurations in thin film multilayer electro‐optical devices. magnified image Example of a combinatorial vapor deposition technique, which is a powerful tool to screen efficiently new materials and configurations in thin film multilayer electro‐optical devices.

Low molecular weight and polymeric heterocyclics as electron transport/hole-blocking materials in organic light-emitting diodes

The use of low molecular weight, oligomeric and polymeric heterocyclics as electron transport/hole-blocking layers in organic light-emitting diodes is reviewed. The most widely applied materials are π-electron deficient heterocyclics carrying imine nitrogen atoms in the aromatic ring, such as 1,3,4-oxadiazoles, 1,2,4-triazoles, 1,3,5-triazines, and 1,4-quinoxalines. Properties such as redox potentials, ionization potential, electron affinity and charge transport mobility of the materials, if known, are taken into consideration to support the electron injection/transport and hole-blocking effectiveness. It can be generalized that heterocyclic moieties with high reduction potential reduce the interface barriers caused by the band offset between organic material and cathode and are most suitable materials for electron injection in organic electroluminescent devices. These materials are generally characterized by high ionization potential values that contribute towards the hole-blocking property. A general comparison of devices and materials is only possible with limitations owing to the variations in device structure, fabrication, electrode materials, emitter materials, etc. © 1998 John Wiley & Sons, Ltd.

Tunable Charge Transport Using Supramolecular Self-Assembly of Nanostructured Crystalline Block Copolymers

Electronically functionalized block copolymers, combining covalently linked p-type and n-type blocks, show switching behavior of charge transport in organic field effect transistors (OFETs). The electronically active subunits self-assemble into continuous microdomains in a nanoscale regime, thereby forming percolation channels for holes or electrons or both depending on the composition and processing conditions. Here, we establish a charge transport-morphology relation for donor-acceptor block copolymers with two crystalline blocks. The n-type and p-type blocks self-assemble into two-dimensional lattices of π-π stacks and main chain polymer lamellae, respectively, over a broad composition range. Controlling the crystallization preferences of the two blocks by thermal annealing allows controlling the OFET polarity. Depending on the block ratio, the charge transport can be tuned from p-type to n-type or p-type to ambipolar, respectively. The impact of nanostructured phase separation is further delineated by X-ray diffraction, time-resolved spectroscopy, and scanning electron microscopy studies.

&lt;title&gt;Novel hole transporting poly(triphenyldiamine)s for application in hybrid solar cells&lt;/title&gt;

Dye-sensitized hybrid solar cells with a nanoporous TiO<SUB>2</SUB> layer and different organic hole conducting polymers have been investigated. These hole transport materials (HTM) with low glass temperature (T<SUB>g</SUB>) are based on triphenyldiamines (TPD). To enhance the power conversion efficiency, the HTMs were doped. New low T<SUB>g</SUB> TPD-based polymers were designed and synthesized with long alkoxyl groups as sidechains or in the backbone, respectively, to investigate the influence of the soft alkoxyl chains on the penetration behavior into the nanoporous layer which has been studied by SEM. The effect of the penetration depth and wetting of the dye sensitized TiO<SUB>2</SUB> layer by HTM on the solar cell efficiencies have been investigated by I-V- characteristics and steady-state measurements. To improve the penetration, the polymers were heated above T<SUB>g</SUB>. The performance of the cell is decreasing probably due to degradation of the dye during thermal treatment. To enhance the conductivity of the hole transport materials, a Li salt has been used and this doped system was compared to a doped standard Gratzel cell with OMeTAD as HTM. Crystallization on the surface could be seen after storing the standard cell for some weeks.

Dynamic Distortions of Quasi-2D Ruddlesden-Popper Perovskites at Elevated Temperatures: Influence on Thermal and Electronic Properties

Ruddlesden-Popper hybrid halide perovskite are quasi-two-dimensional materials with a layered structure and structural dynamics that are determined by the interplay between the organic and inorganic layers. While their optical properties are governed by confinement effects, the atomistic origin of thermal and electronic properties of these materials is yet to be fully established. Here we combine computational and experimental techniques to study A$_2$PbI$_4$ (A=butylammonium (BA), phenethylammonium (PEA)) Ruddlesden-Popper perovskites and compare them with the quintessential perovskite CH$_3$NH$_3$PbI$_3$. We use first-principles density functional theory, molecular dynamics simulations based on machine-learned interatomic potentials, thermal measurements, temperature-dependent Raman spectroscopy, and ultraviolet photoelectron spectroscopy, to probe the thermal and electronic properties of these materials at elevated temperatures. Our molecular dynamics simulations demonstrate that dynamic fluctuations in the organic sublattice determine the bulk-average distortions of these materials at room-temperature, explaining significant differences in their electronic density of states close to the Fermi level. Furthermore, by analysing the organic layer dynamics in BA$_2$PbI$_4$ we provide a mechanistic explanation for the phase transition of this material at 274 K and observations from Raman measurements. Our results highlight the role of the organic interlayer for the electronic and thermal transport properties of Ruddlesden-Popper perovskites, paving the way for the design of new hybrid materials for tailored applications.

Crystalline vs Liquid Crystalline Perylene Bisimides: Improved Electron Mobility via Substituent Alteration

Three solution processable n-type semiconducting perylene bisimides (PBI) with an unsymmetrical substitution pattern are evaluated in terms of their charge transport properties, morphology, and crystal structure. The nature of the substituents is varied from hydrophobic alkyl chains to hydrophilic oligoethyleneglycol (OEG) chains to control intermolecular interactions and to tune self-assembly properties of the compounds. A correlation of structure and morphology with charge transport properties is attempted. Bulk X-ray diffraction (XRD) data are indicative for a lamello-columnar packing motif in the case of PBI 1 and PBI 3 and a columnar hexagonal packing for PBI 2. Further, OEG chains induce liquid crystalline phases while the alkyl substituted compound is crystalline. In the amorphous state after film formation all three materials have a low electron mobility in the range of 10–5 cm2 V–1 s–1. After annealing in the ordered state the mobility of the liquid crystalline compounds increases by 2 orders of magnitude up to 7 × 10–3 cm2 V–1 s–1, while the mobility of the crystalline material decreases by a factor of 4.

Highly Reproducible and Efficient Perovskite Solar Cells with Extraordinary Stability from Robust CH₃NH₃PbI₃: Towards Large‐Area Devices

Abstract Organic–inorganic hybrid solar cells based on air‐stable CH 3 NH 3 PbI 3 perovskites are reported. The perovskite layer was synthesized by CH 3 NH 3 I (methylammonium iodide, MAI) vapor‐assisted growth on a PbI 2 layer. This process is optimized to achieve uniform, pore‐free, compact, and highly stable crystalline perovskite layers. The extraordinary stability of this pristine perovskite layer for about 3 months in air was compared with films obtained by other crystallization methods using X‐ray diffraction (XRD) studies. The effect of the perovskite layer thickness on the photo‐conversion efficiency (PCE) and external quantum efficiency are evaluated. The solar cell optimization reveals that the 370 nm thick perovskite layer delivers highly reproducible devices with a PCE value of 14.8 % (average 13.4 %) and negligible parameter fluctuation as well as high stability (of more than one month) with negligible loss. Additionally, we demonstrate that this MAI vapor‐assisted method is suitable to fabricate devices having an area of 1.05 cm 2 and device efficiency of 8.7 %.