Wojciech Zajaczkowski

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

Free-Standing Monolayer Two-Dimensional Supramolecular Organic Framework with Good Internal Order

Utilizing dynamic self-assembly and self-sorting to obtain large-area, molecularly precise monolayered structures represents a promising approach toward two-dimensional supramolecular organic frameworks (2D SOF) or 2D supramolecular polymers. So far, related approaches suffer from small domain sizes, fragility and weak long-range internal order. Here we report on the self-assembly of a host–guest enhanced donor–acceptor interaction, consisting of a tris(methoxynaphthyl)-substituted truxene spacer, and a naphthalene diimide substituted with N-methyl viologenyl moieties as donor and acceptor monomers, respectively, in combination with cucurbit[8]uril as host monomer toward monolayers of an unprecedented 2D SOF. Featuring orthogonal solubility, the participating molecules self-assemble at a liquid–liquid interface, yielding exceptionally large-area, insoluble films, which were analyzed by transmission electron microscopy, atomic force microscopy and optical microscopy to be monolayers with a thickness of 1.8 nm, homogeneously covering areas up to 0.25 cm2, and featuring the ability to be free-standing over holes of 10 μm2. Characterization with ultraviolet–visible absorption spectroscopy, solid-state nuclear magnetic resonance spectroscopy, infrared spectroscopy, and grazing incidence wide-angle X-ray scattering allowed for confirmation of a successful complexation of all three monomers toward an internal long-range order and gave indications to an expected hexagonal superstructure. Our results extend the existing variety of two-dimensional soft nanomaterials by a versatile supramolecular approach, whereas the possibility of varying the functional monomers is supposed to open adaptability to different applications like membranes, sensors, molecular sieves, and optoelectronics.

Hierarchical Supramolecular Assembly of Sterically Demanding π‐Systems by Conjugation with Oligoprolines

Self-assembly from flexible worm-like threads via bundles of rigid fibers to nanosheets and nanotubes was achieved by covalent conjugation of perylene monoimide (PMI) chromophores with oligoprolines of increasing length. Whereas the chromophoric π-system and the peptidic building block do not self-aggregate, the covalent conjugates furnish well-ordered supramolecular structures with a common wall/fiber thickness. Their morphology is controlled by the number of repeat units and can be tuned by seemingly subtle structural modifications.

Thermal Evaporation versus Spin-Coating: Electrical Performance in Columnar Liquid Crystal OLEDs

The electrical responses of a columnar liquid crystal (a diimidodiester derivative of benzo[ghi]perylene) deposited either by spin-coating or by thermal evaporation into a typical OLED device are compared. For the spin-coated film, homeotropic alignment was induced by thermal annealing, which enhanced the charge carrier mobility significantly. For the evaporated films, homeotropic alignment could not be obtained by annealing. However, a degree of rectification higher than 3 orders of magnitude was achieved, even without annealing, with an electrical response similar to the response of the aligned spin-coated film. A trap-limited space-charge-limited current model was used to extract the charge carrier mobility directly from the current-voltage curves. Grazing incidence wide-angle X-ray scattering confirmed the homeotropic alignment of the annealed spin-coated film, whereas the columns are mostly oriented parallel to the surface in the evaporated case. In a field-effect transistor with bottom-gate bottom-contact geometry, the evaporated film exhibited a typical behavior of an n-type transistor. The degree of intermolecular order is thereby strongly dependent on the deposition method where vacuum deposition leads to a higher order. This higher order, however, impedes reorientation by annealing of the evaporated film but leads to improved charge transport between the electrodes even without homeotropic alignment of columnar liquid crystal.

Thermotropic liquid crystals from biomacromolecules

Significance Liquid crystals (LCs) found in biology are usually dispersed in a solvent, typically water, and are therefore classified as lyotropic. However, from a technological perspective, thermotropic LCs (TLCs), typically based on small rod- or disc-shaped organic molecules, have been of much greater importance. In this contribution, we show that thermotropic liquid crystal phases and materials can also be made from biomolecules, demonstrating a simple generic method to form thermotropic phases from biosystems ranging from nucleic acids and proteins to even whole viruses, spanning a size from only a few nanometers to 1 μm. Complexation of biomacromolecules (e.g., nucleic acids, proteins, or viruses) with surfactants containing flexible alkyl tails, followed by dehydration, is shown to be a simple generic method for the production of thermotropic liquid crystals. The anhydrous smectic phases that result exhibit biomacromolecular sublayers intercalated between aliphatic hydrocarbon sublayers at or near room temperature. Both this and low transition temperatures to other phases enable the study and application of thermotropic liquid crystal phase behavior without thermal degradation of the biomolecular components.

A triaxial supramolecular weave

Despite recent advances in the synthesis of increasingly complex topologies at the molecular level, nano- and microscopic weaves have remained difficult to achieve. Only a few diaxial molecular weaves exist-these were achieved by templation with metals. Here, we present an extended triaxial supramolecular weave that consists of self-assembled organic threads. Each thread is formed by the self-assembly of a building block comprising a rigid oligoproline segment with two perylene-monoimide chromophores spaced at 18 Å. Upon π stacking of the chromophores, threads form that feature alternating up- and down-facing voids at regular distances. These voids accommodate incoming building blocks and establish crossing points through CH-π interactions on further assembly of the threads into a triaxial woven superstructure. The resulting micrometre-scale supramolecular weave proved to be more robust than non-woven self-assemblies of the same building block. The uniform hexagonal pores of the interwoven network were able to host iridium nanoparticles, which may be of interest for practical applications.

Controlling the volatility of the written optical state in electrochromic DNA liquid crystals

Liquid crystals are widely used in displays for portable electronic information display. To broaden their scope for other applications like smart windows and tags, new material properties such as polarizer-free operation and tunable memory of a written state become important. Here, we describe an anhydrous nanoDNA–surfactant thermotropic liquid crystal system, which exhibits distinctive electrically controlled optical absorption, and temperature-dependent memory. In the liquid crystal isotropic phase, electric field-induced colouration and bleaching have a switching time of seconds. Upon transition to the smectic liquid crystal phase, optical memory of the written state is observed for many hours without applied voltage. The reorientation of the DNA–surfactant lamellar layers plays an important role in preventing colour decay. Thereby, the volatility of optoelectronic state can be controlled simply by changing the phase of the material. This research may pave the way for developing a new generation of DNA-based, phase-modulated, photoelectronic devices.

Improved charge carrier transport in ultrathin poly(3-hexylthiophene) films via solution aggregation

Field-effect transistors based on poly(3-hexylthiophene) (P3HT) bulk films exhibit maximum charge carrier mobilities of up to 0.1 cm2 V−1 s−1. However, reducing the thickness of the polymer film beyond 10 nm results in a significant deterioration of the charge transporting properties. In our work, we demonstrate a strategy towards ultrathin (i.e. thinner than 10 nm) polymer layers with charge carrier mobilities identical to bulk films. The improvement in conduction is related to aggregation of P3HT in solution allowing the formation of fibrils in the ultrathin films. Changing the molar mass of P3HT as well as varying the solvent type, aging time, and spin coating parameters resulted in layers with different thicknesses and fibrillar microstructures. The crystal packing and microstructure of the P3HT films, studied by atomic force microscopy and X-ray diffraction, were correlated with the transistor performance. It has been found that P3HT nanofibrils serve in the ultrathin films as pathways for charge carriers. Films of 8 nm thickness revealing a high density and a sufficient length of nanofibrils, along with pronounced internal crystallinity and long π-stacking coherence length, yield a mobility of 0.1 cm2 V−1 s−1. In this way, we demonstrated that controlling the microstructure of the active film in the ultrathin regime does not have to be at the expense of charge carrier mobility.

Solvent-free Liquid Crystals and Liquids from DNA

As DNA exhibits persistent structures with dimensions that exceed the range of their intermolecular forces, solid-state DNA undergoes thermal degradation at elevated temperatures. Therefore, the realization of solvent-free DNA fluids, including liquid crystals and liquids, still remains a significant challenge. To address this intriguing issue, we demonstrate that combining DNA with suitable cationic surfactants, followed by dehydration, can be a simple generic scheme for producing these solvent-free DNA fluid systems. In the anhydrous smectic liquid crystalline phase, DNA sublayers are intercalated between aliphatic hydrocarbon sublayers. The lengths of the DNA and surfactant are found to be extremely important in tuning the physical properties of the fluids. Stable liquid-crystalline and liquid phases are obtained in the -20 °C to 200 °C temperature range without thermal degradation of the DNA. Thus, a new type of DNA-based soft biomaterial has been achieved, which will promote the study and application of DNA in a much broader context.

Solvent-Free Liquid Crystals and Liquids Based on Genetically Engineered Supercharged Polypeptides with High Elasticity

A series of solvent-free elastin-like polypeptide liquid crystals and liquids are developed by electrostatic complexation of supercharged elastin-like polypeptides with surfactants. The smectic mesophases exhibit a high elasticity and the values can be easily tuned by varying the alkyl chain lengths of the surfactants or the lengths of the elastin-like polypeptides.