Small organic molecules with strong intermolecular interactions have a wide range of desirable optical and electronic properties and rich phase behaviors. Incorporating them into block copolymer (BCP)-based supramolecules opens new routes to generate functional responsive materials. Using oligothiophene-containing supramolecules, we present systematic studies of critical thermodynamic parameters and kinetic pathway that govern the coassemblies of BCP and strongly interacting small molecules. A number of potentially useful morphologies for optoelectronic materials, including a nanoscopic network of oligothiophene and nanoscopic crystalline lamellae, were obtained by varying the assembly pathway. Hierarchical coassemblies of oligothiophene and BCP, rather than macrophase separation, can be obtained. Crystallization of the oligothiophene not only induces chain stretching of the BCP block the oligothiophene is hydrogen bonded to but also changes the conformation of the other BCP coil block. This leads to an over 70% change in the BCP periodicity (e.g., from 31 to 53 nm) as the oligothiophene changes from a melt to a crystalline state, which provides access to a large BCP periodicity using fairly low molecular weight BCP. The present studies have demonstrated the experimental feasibility of generating thermoresponsive materials that convert heat into mechanical energy. Incorporating strongly interacting small molecules into BCP supramolecules effectively increases the BCP periodicity and may also open new opportunities to tailor their optical properties without the need for high molecular weight BCP.

Polymer-fullerene based photovoltaic devices have attracted a great deal of attention based on the potential for realizing low-cost, solution-processable, and flexible solar cells. 1 Recently, power conversion efficiencies in excess of 5% have been reported for the poly(3-hexylthiophene)/[6,6]-phenyl-C 61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) solar cell. 2 The successful combination of P3HT and PCBM is based on the ability of the two components to mix homogenously in a pristine cast film and then, under the influence of thermal or solvent annealing, 3,4 undergo a controlled phase segregation yielding a nanometer length scale bicontinuous network of highly ordered donor and acceptor phases, suitable for charge transport. 2 Much effort has been dedicated to the optimization of these devices, including a focus on developing a deeper understanding of the role of polythiophene structure on device performance. [5][6][7][8][9][10] The ability of regioregular (RR) P3HT to form crystalline phases with strong interchain and intrachain π-π overlap is credited for the observed hole mobilites as high as 0.1 cm 2 V -1 s -1 measured in FETs 11 and for the enhanced visible light absorption properties of the polymer. 12 Polythiophene analogues that exhibit higher levels of crystallinity and higher hole mobilites than P3HT, such as the regiosymmetric polymers poly(3,3-didodecylquaterthiophene) (PQT-DD) 13,14 and poly(2,5-bis(3-tetradcylthiophen-2-yl)thieno[3,2b]thiophene) (PBTTT), 15 have been studied for use in FETs, displaying mobilities from 0.18 to 0.6 cm 2 V -1 s -1 . Such polymers achieve a greater overall degree of crystallinity than P3HT based on the length and distribution of alkyl side chains, which favors long-range three-dimensional ordering via π-π stacking and side-chain interdigitation. While such polymers define the state-of-the-art in solution-processed FETs, their photovoltaic performance has not yet been reported, so it is unclear whether the enhanced inherent crystallinity will be beneficial to composite solar cells. As a means of investigating such highly ordered polymers and directly assessing the influence of a high degree of crystallinity on solar cell performance, here we examine two thiophene-alkylthiophene copolymers with identical molecular weight, composition, and electronic structure, which are composed of equal parts of 3-dodecylthiophene and unsubstituted thiophene. The first polymer is the perfectly alternating copolymer PQT-DD, described above, and the other is the random copolymer poly(3-dodecylthiophene-co-thiophene) (P3DDT-co-T). The effect of substituent sequence distribution on polymer crystallinity and solar cell performance in PCBM based BHJ devices is examined.

Organic small molecule semiconductors have many advantages over their polymer analogues. However, to fabricate organic semiconductor-based devices using solution processing, it is requisite to eliminate dewetting to ensure film uniformity and desirable to assemble nanoscopic features with tailored macroscopic alignment without compromising their electronic properties. To this end, we present a modular supramolecular approach. A quaterthiophene organic semiconductor is attached to the side chains of poly(4-vinylpyridine) via noncovalent hydrogen bonds to form supramolecular assemblies that act as p-type semiconductors in field-effect transistors. In thin films, the quaterthiophenes can be readily assembled into microdomains, tens of nanometers in size, oriented normal to the surface. The supramolecules exhibited the same field-effect mobilities as that of the quaterthiophene alone (10−4 cm2/(V·s)). Since the organic semiconductors can be readily substituted, this modular supramolecular approach is a viable method for the fabrication of functional, nanostructured organic semiconductor films using solution processing.

We report the synthesis and characterization of boron(subphthalocyanine) derivatives with bithiophene and quaterthiophene as axial ligands, i.e., thiophene−subphthalocyanine dyads (nT-SubPcs), and their application in organic photovoltaic cells (OPVs). Thin films of nT-SubPcs prepared via solution processing can act as the electron donor in bilayer OPVs with evaporated C60 as the electron acceptor. The photophyscial and morphological properties of the nT-SubPcs are studied to rationalize OPV device parameters. The single-crystal X-ray structure is solved for two dyads to show the molecular structures in the solid state, and UV−vis spectroscopy and fluorescence spectroscopy are used to characterize the effect of conjugated thiophene ligands on the photophysical properties, i.e., absorption and photoluminescence quantum yield. Cyclic voltammetry, density functional theory (DFT) calculations, and low-temperature photoluminescence spectra show that photoluminescence yields depend on the overall flexibility of the SubPc derivatives and not on the oxidation potential or electronic relationship of the ligand and macrocycle molecular orbitals. We show with grazing-incidence X-ray scattering and atomic force microscopy (AFM) that careful choice of ligand structure can improve the crystallinity of thin films that leads to a relative increase in short-circuit current in OPV device. Our work clearly demonstrates that SubPcs can be used as light-harvesting chromophores in a matrix of a crystalline organic semiconductor for OPVs.

The patterning of a surface with more than one type of functionality in spatially resolved fashion is described. A scanning probe was used to create patterns composed of two orthogonal types of functionality within a dense, electrochemically active mixed monolayer via a simple modulation of the applied surface bias. One reductive pathway produces surface-bound amine moieties while the other creates an oxidized surface. The two newly created surface functionalities can each be used independently to locally deposit complementary materials such as electron- or hole-transporting molecules via self-assembly.

We describe the rational design of oligothiophene semiconductors to facilitate solution-based fabrication of environmentally stable organic field-effect transistors (OFETs). Ultrathin films of α,ω-distyryl quaterthiophene (DS4T), pentathiophene (DS5T), and sexithiophene (DS6T) were prepared via solution processing to probe the effect of styryl end groups, oligomer length, and thin film structure on air stability. These films were prepared via solution deposition and thermal annealing of precursors featuring thermally labile ester solubilizing groups. A detailed study of the thin film structure was performed using atomic force microscopy (AFM), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and grazing incidence X-ray diffraction (GIXD). Functional OFETs were obtained for DS5T and DS6T and have, respectively, hole mobilities of 0.051 and 0.043 cm2/(V s) and on/off ratios of 1 × 105 to 1 × 106, whereas DS4T OFETs failed to function because of poor film continuity. The effect of both short-term and long-term exposure to air is tracked in OFETs revealing remarkable stability for both DS5T and DS6T. This stability is attributed to the elimination of reactive sites in α,ω-distyryl oligothiophenes and suggests that careful choice of end-group structure can stabilize these molecules against oxidative degradation.

We report on the development of novel intrinsic conducting polymer two terminal surge protection devices. These resettable current limiting devices consist of polyaniline nanofibres doped with methane sulphonic acid electrochemically deposited between two 55 µm spaced gold electrodes. At normal applied voltages the low resistance devices act as passive circuit elements, not affecting the current flow. However during a current surge the devices switch from ohmic to non-ohmic behaviour, limiting current through the device. After the current surge has passed the devices reset back to their original state. Our studies show that a partial de-doping/redoping process caused by the rapid diffusion of moisture out of or into the polymer film during joule heating/cooling is the underlying mechanism responsible.

Surface interface engineering using superhydrophobic gold electrodes made with 1-dodecanethiol self-assembled monolayer (SAM) has been used to enhance the current limiting properties of novel surge protection devices based on the intrinsic conducting polymer, polyaniline doped with methanesulfonic acid. The resulting devices show significantly enhanced current limiting characteristics, including current saturation, foldback, and negative differential effects. We show how SAM modification changes the morphology of the polymer film directly adjacent to the electrodes, leading to the formation of an interfacial compact thin film that lowers the contact resistance at the Au-polymer interface. We attribute the enhanced current limiting properties of the devices to a combination of lower contact resistance and increased Joule heating within this interface region which during a current surge produces a current blocking resistive barrier due to a thermally induced dedoping effect caused by the rapid diffusion of moisture away from this region. The effect is exacerbated at higher applied voltages as the higher temperature leads to stronger depletion of charge carriers in this region, resulting in a negative differential resistance effect.