Wojciech Pisula

Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics

Discotic liquid crystals are a promising class of materials for molecular electronics thanks to their self-organization and charge transporting properties. The best discotics so far are built around the coronene unit and possess six-fold symmetry. In the discotic phase six-fold-symmetric molecules stack with an average twist of 30 degrees, whereas the angle that would lead to the greatest electronic coupling is 60 degrees. Here, a molecule with three-fold symmetry and alternating hydrophilic/hydrophobic side chains is synthesized and X-ray scattering is used to prove the formation of the desired helical microstructure. Time-resolved microwave-conductivity measurements show that the material has indeed a very high mobility, 0.2 cm(2) V(-1) s(-1). The assemblies of molecules are simulated using molecular dynamics, confirming the model deduced from X-ray scattering. The simulated structures, together with quantum-chemical techniques, prove that mobility is still limited by structural defects and that a defect-free assembly could lead to mobilities in excess of 10 cm(2) V(-1) s(-1).

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.

Liquid Crystalline Ordering and Charge Transport in Semiconducting Materials

Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0.7 cm(2)  · V(-1)  · s(-1) . In this context, the significance of cross-linking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed.

Tailoring Structure−Property Relationships in Dithienosilole−Benzothiadiazole Donor−Acceptor Copolymers

Four new DTS-BTD copolymers (P1-P4) differing by the concentration of electron-donating and -withdrawing substituents along the backbone have been synthesized and characterized by 2D-WAXS and in bottom-contact FETs. While all copolymers can self-assemble into lamellar superstructures, only P2 and P4 show a propensity to pi-stack. P4 exhibits a hole mobility as high as 0.02 cm(2) V(-1) s(-1) in excellent agreement with the close pi-stacking and lamellar distances found by structural analysis (0.36 and 1.84 nm, respectively) and absorbs homogenously across the entire visible spectrum as solar cell applications require.

Organic Field-Effect Transistors based on Highly Ordered Single Polymer Fibers

Ultrahigh-mobility organic field-effect transistors (OFETs) based on a CDT-BTZ donor-acceptor copolymer are realized by reaching high molecular order and pronounced alignment in single fibers within a short OFET channel via solution processing. The macromolecules directionally self-assemble in a quasi crystal-like order in the fibers providing in this way an unhindered charge carrier pathway with mobilities as high as 5.5 cm(2) V(-1) s(-1).

Large polycyclic aromatic hydrocarbons: Synthesis and discotic organization

Polycyclic aromatic hydrocarbons (PAHs) have attracted enormous interest due to their unique electronic and optoelectronic properties as well as the potential applications in organic electronics. This article reviews the progress in the modern synthesis of large PAHs with different sizes, shapes, edge structures, and substituents. Due to their outstanding self-organization characteristics, the discotic liquid-crystalline properties, self-assembled nanostructures on the surfaces, as well as the application in electronic devices will be discussed.

Tuning the Columnar Organization of Discotic Polycyclic Aromatic Hydrocarbons

The spontaneous self-organization of small molecular entities to form soft materials with well-defi ned supramolecular assemblies is currently a topic of great interest in areas that range from chemistry and biology to materials science. [ 1 ] Over the past decade, non-covalent forces such as hydrogen-bonds, metal-ion-to-ligand coordination, electrostatic, π – π stacking, dipole-dipole, or hydrophilic-hydrophobic interactions have been identifi ed as enabling the construction of a broad range of complex superstructures from specifi cally engineered small molecular building blocks. [ 2 ] Incorporation of molecules into larger entities by non-covalent forces has enormous potential for materials science due to the possibility of bridging the gap between the molecular and the macroscopic scale in terms of structural order, when precise control of such a self-assembly process is achieved. [ 3 ]

High Mobility, Air Stable, Organic Single Crystal Transistors of an n‐Type Diperylene Bisimide

Recently, some impressive progress has been made by functionalization of (hetero-)acenes, thiophenes, and arylenes with electron-defi cient constituents. [ 3–5 ] However, the development of air-stable, high mobility, n-type organic semiconductors for organic electronics is still highly emergent. The mobility of organic semiconductors depends on the effi ciency of charge transport from one molecule to another. Hence, some organic semiconductors with dense molecule packing always give high mobility. [ 6 ] As to the stability of organic compounds, it is believed that the highest occupied molecular orbital (HOMO) of p-type organic semiconductors should be more negative than –5.0 eV, e.g., locating at –5.0 to –6.0 eV, and the lowest unoccupied molecular orbitals (LUMO) of n-type organic semiconductors are best located between –4.0 and –4.5 eV, for anti-oxidation in air. [ 2 , 7 ] We have acknowledged these requirements and believe that perylene bisimides (PBIs) will fi t as candidates because of their reasonable electron acceptor ability, [ 8 ] and have been focusing on the expansion of the chemistry of perylene bisimides (PBIs) by a combination of Ullmann coupling and C–H transformation for some time, and have developed a facile strategy to synthesize fully conjugated, triply linked, diperylene bisimides, [ 8 ] conferring the expanded

Synthesis and Controlled Self-Assembly of Covalently Linked Hexa-peri-hexabenzocoronene/Perylene Diimide Dyads as Models To Study Fundamental Energy and Electron Transfer Processes

We report the synthesis and photophysical characterization of a series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide (PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the two components and the conjugation of the bridging element are systematically modified to study the influence on self-assembly and energy and electron transfer between electron donor HBC and acceptor PDI. STM and 2D-WAXS experiments reveal that both in solution and in bulk solid state the dyads assemble into well-ordered two-dimensional supramolecular structures with controllable mutual orientations and distances between donor and acceptor at a nanoscopic scale. Depending on the symmetry of the dyads, either columns with nanosegregated stacks of HBC and PDI or interdigitating networks with alternating HBC and PDI moieties are observed. UV-vis, photoluminescence, transient photoluminescence, and transient absorption spectroscopy confirm that after photoexcitation of the donor HBC a photoinduced electron transfer between HBC and PDI can only compete with the dominant Förster resonance energy transfer, if facilitated by an intimate stacking of HBC and PDI with sufficient orbital overlap. However, while the alternating stacks allow efficient electron transfer, only the nanosegregated stacks provide charge transport channels in bulk state that are a prerequisite for application as active components in thin film electronic devices. These results have important implications for the further design of functional donor-acceptor dyads, being promising materials for organic bulk heterojunction solar cells and field-effect transistors.

Microstructure Evolution and Device Performance in Solution-Processed Polymeric Field-Effect Transistors: The Key Role of the First Monolayer

Probing the role of the first monolayer in the evolution of the film polymer microstructure is essential for the fundamental understanding of the charge carrier transport in polymeric field-effect transistors (FETs). The monolayer and its subsequent microstructure of a conjugated polymer [poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT] film were fabricated via solution deposition by tuning the dip-coating speed and were then studied as accumulation and transporting layers in FETs. Investigation of the microstructure of the layers prepared at different coating velocities revealed that the monolayer serves as an important base for further development of the film. Significant improvement of the charge carrier transport occurs only at a critical multilayer network density that establishes the required percolation pathways for the charge carriers. Finally, at a low dip-coating speed, the polymer chains are uniaxially oriented, yielding pronounced structural anisotropy and high charge carrier mobilities of 1.3 cm(2) V(-1) s(-1) in the alignment direction.