Eric Verploegen

Tuning charge transport in solution-sheared organic semiconductors using lattice strain

Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33 Å to 3.08 Å. We believe that 3.08 Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04 Å has been achieved through intramolecular bonding). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from 0.8 cm2 V−1 s−1 for unstrained films to a high mobility of 4.6 cm2 V−1 s−1 for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.

Fabrication and Characterization of Ultrahigh‐Volume‐ Fraction Aligned Carbon Nanotube–Polymer Composites

Aligned CNT nanocomposites with variable volume fraction, up to 20%, are demonstrated. Biaxial mechanical densification of aligned CNT forests, followed by capillarity-driven wetting using unmodified aerospace-grade polymers, creates centimeter-scale specimens. Characterizations confirm CNT alignment and dispersion in the thermosets, providing a useful platform for controlled nanoscale interaction and nanocomposite property studies that emphasize anisotropy.

A path to ultranarrow patterns using self-assembled lithography.

The templated self-assembly of block copolymer (BCP) thin films can generate regular arrays of 10-50 nm scale features with good positional and orientational accuracy, but the ordering, registration and pattern transfer of sub-10-nm feature sizes is not well established. Here, we report solvent-annealing and templating methods that enable the formation of highly ordered grating patterns with a line width of 8 nm and period 17 nm from a self-assembled poly(styrene-b-dimethylsiloxane) (PS-PDMS) diblock copolymer. The BCP patterns can be registered hierarchically on a larger-period BCP pattern, which can potentially diversify the available pattern geometries and enables precise pattern registration at small feature sizes. Sub-10-nm-wide tungsten nanowires with excellent order and uniformity were fabricated from the self-assembled patterns using a reactive ion etching process.

Thiophene-Rich Fused-Aromatic Thienopyrazine Acceptor for Donor-Acceptor Low Band-Gap Polymers for Otft and Polymer Solar Cell Applications

Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to produce low band gap polymers (Eg = 1.0–1.4 eV) when copolymerized with fluorene and cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and organic photovoltaic applications. Molecular packing in the thin films of these polymers was investigated using Grazing incidence X-ray Scattering. Although both fluorene and cyclopentadithiophene polymers follow similar face to face π–π stacking, the latter polymers show much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared to fluorene polymers (up to 8.1 × 10−3 cm2/V.s). Power conversion efficiency of 1.4% was achieved using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the cyclopentadithiophene polymers show lower band gaps with higher absorption coefficients compared to fluorene copolymers, but the power conversion efficiencies in solar cells of these polymers are low due to their low ionization potentials.

Controlling Electric Dipoles in Nanodielectrics and Its Applications for Enabling Air-Stable n-Channel Organic Transistors

We present a new method to manipulate the channel charge density of field-effect transistors using dipole-generating self-assembled monolayers (SAMs) with different anchor groups. Our approach maintains an ideal interface between the dipole layers and the semiconductor while changing the built-in electric potential by 0.41-0.50 V. This potential difference can be used to change effectively the electrical properties of nanoelectronic devices. We further demonstrate the application of the SAM dipoles to enable air-stable operation of n-channel organic transistors.

3,4-Disubstituted Polyalkylthiophenes for High-Performance Thin-Film Transistors and Photovoltaics

We demonstrate that poly(3,4-dialkylterthiophenes) (P34ATs) have comparable transistor mobilities (0.17 cm(2) V(-1) s(-1)) and greater environmental stability (less degradation of on/off ratio) than regioregular poly(3-alkylthiophenes) (P3ATs). Unlike poly(3-hexylthiophene) (P3HT), P34ATs do not show a strong and distinct π-π stacking in X-ray diffraction. This suggests that a strong π-π stacking is not always necessary for high charge-carrier mobility and that other potential polymer packing motifs in addition to the edge-on structure (π-π stacking direction parallel to the substrate) can lead to a high carrier mobility. The high charge-carrier mobilities of the hexyl and octyl-substituted P34AT produce power conversion efficiencies of 4.2% in polymer:fullerene bulk heterojunction photovoltaic devices. An enhanced open-circuit voltage (0.716-0.771 eV) in P34AT solar cells relative to P3HT due to increased ionization potentials was observed.

Molecular design for improved photovoltaic efficiency: band gap and absorption coefficient engineering

Removing the adjacent thiophene groups around the acceptor core in low band gap polymers significantly enhances solar cell efficiency through increasing the optical absorption and raising the ionization potential of the polymer.

Diffusional Self-Organization in Exponential Layer-By-Layer Films with Micro- and Nanoscale Periodicity

The layer-by-layer (LBL) assembly technique is currently one of the most widely utilized methods for the preparation of nanostructured, multilayered thin films. The structure of LBL films is typically controlled by varying the deposition sequence of adsorbed layers, leading to stratified assemblies. For specific, non-spherical inorganic LBL components, such as sheets, or axial nanocolloids, such as nanotubes, nanowires, nanowiskers, or nanorods, the structure of the films can also be controlled by their orientation. As such, clay nanosheets spontaneously adsorb almost exclusively in the orientation parallel to the substrate, whilst assembly of axial nanocolloids under conditions of shear or dewetting results in partial alignment of the fibrous components. Morphological or structural control of the multilayers can also be imparted by the choice of the assembly method (e.g. spin coating versus dip coating), the assembly conditions, or post-assembly processing of the assembly. The shape and surface morphology of the assemblies can also be tailored by the structure or shape of the substrate, as has been shown in the preparation of hollow capsules or sculptured/perforated membranes. Both polymers and nanoparticles exhibit strong tendencies toward self-organization. This effect has not been utilized in the LBL assemblies, except for the recent observation by Yoo et al. of the organization of rod-shaped viruses on the surface of a film consisting of a few bilayers. Overall, the need for more sophisticated degrees of structural organization is quite extensive and commensurate with the increasingly complex applications for which they are being prepared. Importantly, this control must be possible on a nanometer and a micrometer scale. In principle, the LBL approach does allow such a broad-scale control, but microscale films require deposition of a great number of layers in traditional LBL. It would be exceptionally advantageous to design a method that can lead to well-organized materials combining fast deposition and hierarchical nano-, micro-, and macroscopic levels of organization. To achieve this aim, a degree of smartness and the presence of elements of selforganization in the film will most likely be required. Layered systems with alternating microand nanostrata of a stiff and an elastic nature might be particularly interesting because of mechanical properties associated with the distribution of stress in hierarchical structures and predicted theoretically unique mechanical properties. Exponentially grown LBL (e-LBL) films are multilayers in which polymer chains retain their mobility and diffuse through the deposited strata. The degree of mobility makes possible to observe self-organization phenomena in such structures. Herein, we show that a system with alternating nanometerand micrometer-scale layers of predominantly inorganic (stiff) and polymeric (elastic) layers forms upon LBL deposition of poly(diallyldimethylammonium chloride) (PDDA), poly(acrylic acid) (PAA), and sodium montmorillonite clay nanosheet (MTM) multilayers. Despite the expectations of fairly homogeneous coatings in the framework of both traditional and exponential LBL deposition, the deposition sequence (PDDA/PAA/PDDA/MTM)n (n is the number of deposition cycles), results in well-defined indexing of the films after the first few cycles, with a periodicity of (1.7 0.4) mm for 10 min deposition, and superimposed organization of MTM sheets at the interfaces with 0–10 nm spacing (Figure 1). The indexing can be further controlled by varying the deposition times for polyelectrolytes (Supporting Information, Figure S1). A typical assembly included alternate immersion of a glass slide into solutions of the polycation PDDA and an [*] Prof. N. A. Kotov Departments of Chemical Engineering, Materials Science and Engineering, and Biomedical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) Fax: (+1)734-764-7453 E-mail: kotov@umich.edu

Controlling the Morphology of Side Chain Liquid Crystalline Block Copolymer Thin Films through Variations in Liquid Crystalline Content

In this paper, we describe methods for manipulating the morphology of side-chain liquid crystalline block copolymers through variations in the liquid crystalline content. By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer (BCP) backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase-segregated BCP microstructures can be precisely manipulated. Increases in LC functionalization lead to stronger preferences for the anchoring of the LC mesophase relative to the substrate and the intermaterial dividing surface. By manipulating the strength of these interactions, the arrangement and ordering of the ultrathin film block copolymer nanostructures can be controlled, yielding a range of morphologies that includes perpendicular and parallel cylinders, as well as both perpendicular and parallel lamellae. Additionally, we demonstrate the utilization of selective etching to create a nanoporous liquid crystalline polymer thin film. The unique control over the orientation and order of the self-assembled morphologies with respect to the substrate will allow for the custom design of thin films for specific nanopatterning applications without manipulation of the surface chemistry or the application of external fields.

Automated spin-assisted layer-by-layer assembly of nanocomposites

We present the design and verification of a desktop system for the automated production of nanostructured thin films via spin-assisted layer-by-layer (spin-LBL) assembly. The utility of this system is demonstrated by fabricating polyvinyl alcohol/clay nanocomposites. Ellipsometry measurements demonstrate that the automated spin-LBL method creates composites with bilayer thickness and growth rate comparable to traditional dip-LBL; however, the cycle time of the spin-LBL method is an order of magnitude faster. Small angle X-ray scattering analysis shows that the clay platelets in spin-LBL nanocomposites are more highly aligned than in dip-LBL composites. This method can significantly increase the throughput of laboratory-scale LBL discovery and processing, can enable testing of functional properties of LBL nanocomposites over wafer-scale areas, and can be scaled to larger substrates for commercial production.