Publications

Control of Molecular Orientation in Polydiketopyrrolopyrrole Copolymers via Diffusive Noncovalent Interactions

In thin films of semiconductor polymers, the polymer chains often exhibit distinct orientation with respect to the substrate. The planar π-face of the backbone typically orients either in an edge-on or face-on manner. Generally, an edge-on alignment is thought to be favorable for transport in thin film transistors, whereas face-on alignment is considered to improve vertical transport as desired in solar cells. However, molecular orientation is among the very few parameters that usually cannot be controlled when tailoring new semiconducting polymers. Here we show for an important class of semiconducting polymer that both the mode of orientation as well as the degree of alignment can be well-controlled by exploiting diffusive noncovalent interactions along the backbone. Studying polydiketopyrrolopyrroles (PDPPs) as a case study, by strategically varying chemical structure, we demonstrate systematic variation in molecular orientation with degree of chain planarization resulting from different degrees of diffusive noncovalent interactions. This observation opens the possibility of controlling and optimizing the orientation of semiconducting polymer chains in thin films by rational design. We anticipate our findings to open the door to new high-performance organic semiconductors with the additional benefit of tailored orientation that fits the desired application.

Understanding building blocks of photonic logic gates: Reversible, read-write-erase cycling using photoswitchable beads in micropatterned arrays

Using surface-templated electrophoretic deposition, we have created arrays of polymer beads (photonic units) incorporating photo-switchable DAE molecules, which can be reversibly and individually switched between high and low emission states by direct photo-excitation, without any energy or electron transfer processes within the molecular system. The micropatterned array of these photonic units is spectroscopically characterized in detail and optimized with respect to both signal contrast and cross-talk. The optimum optical parameters including laser intensity, wavelength and duration of irradiation are elucidated and ideal conditions for creating reversible on/off cycles in a micropatterned array are determined. 500 such cycles are demonstrated with no obvious on/off contrast attenuation. The ability to process binary information is demonstrated by selectively writing information to the given photonic unit, reading the resultant emissive signal pattern and finally erasing the information again, which in turn demonstrates the possibility of continuous recording. This basic study paves the way for building complex circuits using spatially well-arranged photonic units.

Emission modulation of fluorescent turn-on mode dibenzothienyl sulfonyl ethene photoswitches embedded in a polymer film

For decades photochromic molecules have attracted attention for their potential in using light as an external stimulus to change their photophysical properties. Here we report the spectroscopic characterization of two emissive photochromic molecules that are intrinsically fluorescent and that undergo a photocyclization/cycloreversion reaction upon illumination with light in the UV and VIS spectral ranges. For appropriately adjusted illumination intensities the emission can be modulated between the high- and the low-level with a contrast ratio exceeding 80%. The data are in reasonable agreement with the predictions from a simple kinetic model.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

Adv. Energy Mater. 2017, 7, 1700264 In the originally published article, the equal contribution statement was incorrectly reported. The equal contribution statement is corrected to: M.L.P., J.S., and C.L. contributed equally to this work.

The influence of π–π-stacking on the light-harvesting properties of perylene bisimide antennas that are covalently linked to a [60]fullerene

A flexible organic triad consisting of two perylene bisimide antennas covalently linked to a [60]fullerene by flexible spacers has been synthesized and studied by electrochemistry, steady-state and time-resolved spectroscopy. We found that π-π-stacking of the two antenna subunits has considerable impact on the photophysics of the system and leads to a reduction of the effective light-harvesting efficiency of the assembly. This finding is important for light harvesting in flexible materials based on the dye antenna-fullerene concept.

Thermal Degradation Studies of Polythiophenes Containing Hetero Aromatic Side Chains

Conjugated Polymers: π‐Conjugated Donor Polymers: Structure Formation and Morphology in Solution, Bulk and Photovoltaic Blends (Adv. Energy Mater. 16/2017)

In article number 1700314, Mukundan Thelakkat and co-workers focus on the correlation of structures, morphology and chain orientation as a function of molecular weight, dispersity, intramolecular and intermolecular interactions and processing techniques of conjugated polymers and their photovoltaic blends. The tools for elucidating fundamental information of structure formation and orientation consist of optical spectroscopy and scattering techniques.

A comparative study of a polyene-diphenylaniline dye and Ru(dcbpy)2(NCS)2 in electrolyte-based and solid-state dye-sensitized solar cells

Fluorescence Quenching of Substituted Polyperylene with Functionalized Polythiophenes

Tuning of Composition and Morphology of LiFePO4 Cathode for Applications in All Solid-State Lithium Metal Batteries

Abstract All solid-state rechargeable lithium metal batteries (SS-LMBs) are gaining more and more importance because of their higher safety and higher energy densities in comparison to their liquid-based counterparts. In spite of this potential, their low discharge capacities and poor rate performances limit them to be used as state-of-the-art SS-LMBs. This arise due to the low intrinsic ionic and electronic transport pathways within the solid components in the cathode during the fast charge/discharge processes. Therefore, it is necessary to have a cathode with good electron conducting channels to increase the active material utilization without blocking the movement of lithium ions. Since SS-LMBs require a different morphology and composition of the cathode, we selected LiFePO 4 (LFP) as a prototype and, we have systematically studied the influence of the cathode composition by varying the contents of active material LFP, conductive additives (super C65 conductive carbon black and conductive graphite), ion conducting components (PEO and LiTFSI) in order to elucidate the best ion as well as electron conduction morphology in the cathode. In addition, a comparative study on different cathode slurry preparation methods was made, wherein ball milling was found to reduce the particle size and increase the homogeneity of LFP which further aids fast Li ion transport throughout the electrode. The SEM analysis of the resulting calendered electrode shows the formation of non-porous and crack-free structures with the presence of conductive graphite throughout the electrode. As a result, the optimum LFP cathode composition with solid polymer nanocomposite electrolyte (SPNE) delivered higher initial discharge capacities of 114 mAh g-1 at 0.2C rate at 30 ᴼC and 141 mAh g-1 at 1C rate at 70 ᴼC. When the current rate was increased to 2C, the electrode still delivered high discharge capacity of 82 mAh g-1 even after 500 cycle, which indicates that the optimum cathode formulation is one of the important parameters in building high rate and long cycle performing SS-LMBs.

Fast and stable photorefractive systems with compatible photoconductors and bifunctional NLO-dyes

Semiconductor amphiphilic block copolymers for hybrid donor–acceptor nanocomposites

Block copolymers feature unique properties for organizing in a well-defined pattern on length scales of several tenths of nanometers. This special attribute enables the formation of ideal donor and acceptor domains for photovoltaic devices in the size of the exciton diffusion length. Thus we designed an amphiphilic block copolymer, able to act as a hole conductor and to coordinate inorganic semiconductor nanoparticles as electron acceptors. Utilizing controlled radical polymerization techniques, defined polymers were synthesized consisting of triphenylamine pendant groups in the hole conductor block and a hydrophilic polystyrene sulfonate block. This particular combination creates narrowly distributed micelles in aqueous solution exhibiting domain sizes suitable for photovoltaic applications. The strong anionic sulfonate groups offer high loading capacities for modified cationic nanoparticles. To guarantee a broad absorption and good conductivity, we synthesized cationic CdSe nanorods and combined them with our hole conductor micelles. The advantage of high loading combined with the processability from aqueous dispersions promises a novel “green” alternative for preparation of hybrid solar cells with controlled domain sizes in the desired length scale.

High electron mobility and tuneable glass transition temperature in fullerene-functionalized polynorbornenes

Ring-opening metathesis polymerization is employed to copolymerize a C 60 -functionalized norbornene monomer and its alkyl substituted analog, yielding a range of fullerene-grafted polynorbornene copolymers with varying fullerene-contents.

Different mesoporous titania films for solid-state dye sensitised solar cells

Reversible Laser‐Induced Amplified Spontaneous Emission from Coexisting Tetragonal and Orthorhombic Phases in Hybrid Lead Halide Perovskites

The photoluminescence in a lead halide perovskite is measured for different temperatures (5–300 K) and excitation fluences (21–1615 μJ cm −2 ). It is found that amplified spontaneous emission (ASE) is observed for an excitation density larger than about 1 × 10 18 cm −3 for both the tetragonal phase above 163 K and the orthorhombic phase below about 163 K. The fluence that is required to obtain this excitation density depends on temperature and phase since the nonradiative decay of excitations is temperature activated with different activation energies of and for the tetragonal and orthorhombic phase, respectively. The ASE from the tetragonal phase—usually prevailing at temperatures above about 163 K—can also be observed at 5 K, in addition to the ASE from the orthorhombic phase, when the sample is previously exposed to a fluence exceeding 630 μJ cm −2 at a photon energy of 3.68 eV. This additional ASE can be removed by mild heating to 35 K or optically, by exposing the sample by typically a few seconds with a fluence around 630 μJ cm −2 . The physical mechanism underlying this optically induced phase transition process is discussed. It is demonstrated that this phase change can, in principle, be used for an all‐optical “write–read–erase” memory device.

Synthesis and Characterization of Donor−Bridge−Acceptor Molecule Containing Tetraphenylbenzidine and Perylene Bisimide

A novel donor acceptor dyad consisting of tetraphenylbenzidine (TPD) and perylene bisimide was synthesized by linking these moieties using a dodecyl spacer. HOMO and LUMO values of the dyad were aquired by cyclic voltammetry. Photophysical properties were studied by steady-state UV−vis and fluorescence spectroscopy. After selective excitation of the TPD moiety, a quenching of donor fluorescence and the appearance of acceptor fluorescence was observed. This proves nonradiative energy transfer from the donor to the acceptor group. The energy transfer was 4 times more efficient in DBA than in a mixture of D and A. The energy transfer efficiency in the dyad is also independent of the concentration, indicating intramolecular transfer mechanism. However, the direct excitation of the acceptor in the dyad exhibited reduced fluorescence emission of the acceptor, indicating electron transfer between the moieties. Thus, this DBA is an excellent model system for the study of energy- and electron-transfer processes in organic semiconductors.

Key aspects of individual layers in solid-state dye-sensitized solar cells and novel concepts to improve their performance

Combinatorial Methods for Screening and Optimization of Materials and Device Parameters in OLEDs