Alan Sellinger

Evaporation-Induced Self-Assembly: Nanostructures Made Easy

As we look toward the next millennium, we envision new technologies based on nanoscale machines and devices. Key to the realization of this nanotech world are simple, efficient methods of organizing materials (molecules, molecular clusters, polymers, or, generally speaking, building blocks) into precise, predetermined nanostructures that can be preserved in a robust engineering form. Marine organisms like diatoms and radiolaria provide us with many examples of intricately organized architectures preserved in silica or calcium carbonate. Such natural microstructures are formed by biomineralization, a templated self-assembly process in which preorganized organic surfaces regulate the nucleation, growth, morphology and orientation of inorganic crystals. To date, a variety of synthetic pathways that mimic aspects of biomineralization have been explored to prepare patterned ceramic materials. In an early attempt to achieve antigen/ antibodyselectivity inaporousadsorbent,Dickey prepared silicagels inthepresenceofthetargetmoleculetobeadsorbed (in this case methyl orange). After methyl orange extraction, the resulting templated silicas showed preferential selectivity for methyl orange over its alkyl orange homologues. In the 1960s researchers at the Mobil Oil Corporation used alkylammonium ions as templates to control the pore size, shape and periodicity of zeolites, crystalline solids that define 1-, 2-, or 3-dimensional (1-, 2-, or 3-D, respectively) networks of microporous channels. More recently Kresge and colleagues at Mobil used longer-chain alkylammonium ions in an attempt to increase the maximum pore size of zeolites beyond ~1.2 nm. They observed honeycomb-like arrays of ~4 nm pores and, based on analogies with hexagonal liquidcrystalline systems, proposed a supramolecular liquid-crystalline templating mechanism. Although excellent progress has been made in the preparationofawidevarietyofpatternedceramicmaterials, current synthetic methods have several inherent drawbacks fromthestandpointofnanotechnology:First,mosttemplating procedures are conducted in time-consuming batch operations often employing hydrothermal processing conditions. Second, the resultant products are typically ill-defined powders, precluding their general use in thin film technologies. Third, procedures developed to date are often limited to forming patterns of pores. For many envisioned nanotechnologies, it would be desirable to create patterned nanocomposites consisting of periodic arrangements of two or more dissimilar materials. This article summarizes a simple evaporation-induced self-assembly (EISA) process, that enables the rapid production of patterned porous or nanocomposite materials in the form of films, fibers, or powders.

Continuous self-assembly of organic-inorganic nanocomposite coatings that mimic nacre

Nanocomposite materials are widespread in biological systems. Perhaps the most studied is the nacre of abalone shell, an orientated coating composed of alternating layers of aragonite (CaCO3) and a biopolymer. Its laminated structure simultaneously provides strength, hardness and toughness: containing about 1 vol. % polymer, nacre is twice as hard and 1,000 times as tough as its constituent phases. Such remarkable properties have inspired chemists and materials scientists to develop synthetic, ‘biomimetic’ nanocomposite assemblies. Nonetheless, the efficient processing of layered organic–inorganic composites remains an elusive goal. Here we report a rapid, efficient self-assembly process for preparing nanolaminated coatings that mimic the structure of nacre. Beginning with a solution of silica, surfactant and organic monomers, we rely on evaporation during dip-coating to induce the formation of micelles and partitioning of the organic constituents into the micellar interiors. Subsequent self-assembly of the silica–surfactant–monomer micellar species into lyotropic mesophases simultaneously organizes the organic and inorganic precursors into the desired nanolaminated form. Polymerization fixes this structure, completing the nanocomposite assembly process. This approach may be generalized both to other composite architectures and to other materials combinations.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Hole Transport Materials with Low Glass Transition Temperatures and High Solubility for Application in Solid-State Dye-Sensitized Solar Cells

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs.

Steric control of the donor/acceptor interface: implications in organic photovoltaic charge generation

The performance of organic photovoltaic (OPV) devices is currently limited by modest short-circuit current densities. Approaches toward improving this output parameter may provide new avenues to advance OPV technologies and the basic science of charge transfer in organic semiconductors. This work highlights how steric control of the charge separation interface can be effectively tuned in OPV devices. By introducing an octylphenyl substituent onto the investigated polymer backbones, the thermally relaxed charge-transfer state, and potentially excited charge-transfer states, can be raised in energy. This decreases the barrier to charge separation and results in increased photocurrent generation. This finding is of particular significance for nonfullerene OPVs, which have many potential advantages such as tunable energy levels and spectral breadth, but are prone to poor exciton separation efficiencies. Computational, spectroscopic, and synthetic methods were combined to develop a structure-property relationship that correlates polymer substituents with charge-transfer state energies and, ultimately, device efficiencies.

Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.

Polyfunctional cubic silsesquioxanes as building blocks for organic/inorganic hybrids

Cubic silsesquioxanes, [RSiO1.5]x, potentially offer access to organic/inorganic hybrids wherein the exact shape, size and mechanical properties of the inorganic component are perfectly defined. Furthermore, by tailoring the organic functionality bound to silicon, the inorganic/ organic interface can also be perfectly defined. Finally, careful selection of the polymerizable groups in the organic moieties can provide goodto-excellent control of the crosslinked density or degree of polymerization of the resulting hybrid materials. Thus, cubic silsesquioxanes may be exceptional model materials for inorganic/organic hybrids. Methods of synthesizing cubes with liquid-crystalline and/or polymerizable organic moieties are described. Some thermal properties are discussed. The catalytic copolymerization of the octavinyldimethylsiloxy-functionalized cube with the octahydridodimethylsiloxy-functionalized cube to produce a material with well-defined microporosity and high surface area is described. # 1998 John Wiley & Sons, Ltd.

Energy and Hole Transfer between Dyes Attached to Titania in Cosensitized Dye-Sensitized Solar Cells

Cosensitization of broadly absorbing ruthenium metal complex dyes with highly absorptive near-infrared (NIR) organic dyes is a clear pathway to increase near-infrared light harvesting in liquid-based dye-sensitized solar cells (DSCs). In cosensitized DSCs, dyes are intimately mixed, and intermolecular charge and energy transfer processes play an important role in device performance. Here, we demonstrate that an organic NIR dye incapable of hole regeneration is able to produce photocurrent via intermolecular energy transfer with an average excitation transfer efficiency of over 25% when cosensitized with a metal complex sensitizing dye (SD). We also show that intermolecular hole transfer from the SD to NIR dye is a competitive process with dye regeneration, reducing the internal quantum efficiency and the electron lifetime of the DSC. This work demonstrates the general feasibility of using energy transfer to boost light harvesting from 700 to 800 nm and also highlights a key challenge for developing highly efficient cosensitized dye-sensitized solar cells.

Electron-Accepting Conjugated Materials Based on 2-Vinyl-4,5-dicyanoimidazoles for Application in Organic Electronics

We report the Heck coupling of 2-vinyl-4,5-dicyanoimidazole (vinazene) with selected di- and trihalo aromatics in an effort to prepare linear and branched electron-accepting conjugated materials for application in organic electronics. By selecting the suitable halo-aromatic moiety, it is possible to tune the HOMO-LUMO energy levels, absorption, and emission properties for a specific application. In this regard, materials with strong photoluminescence from blue --> green --> red are reported that may have potential application in organic light-emitting diodes (OLEDs). Furthermore, derivatives with strong absorption in the visible spectrum, coupled with favorable HOMO-LUMO levels, have been used to prepare promising organic photovoltaic devices (OPVs) when combined with commercially available semiconducting donor polymers.

1,3,6,8-Tetrasubstituted Pyrenes: Solution-Processable Materials for Application in Organic Electronics

A series of star-shaped organic semiconductors have been synthesized from 1,3,6,8-tetrabromopyrene. The materials are soluble in common organic solvents allowing for solution processing of devices such as light-emitting diodes (OLEDs). One of the materials, 1,3,6,8-tetrakis(4-butoxyphenyl)pyrene, has been used as the active emitting layer in simple solution-processed OLEDs with deep blue emission (CIE = 0.15, 0.18) and maximum efficiencies and brightness levels of 2.56 cd/A and >5000 cd/m(2), respectively.