[show abstract][hide abstract] ABSTRACT: Tri-isopropylsilylethynyl (TIPS)-functionalized polycyclic aromatic hydrocarbon (PAH) molecules incorporate structural components of graphene nanoribbons and represent a family of model molecules that form organic crystal semiconductors for electronic devices. Here, we report a series of TIPS-functionalized PAHs and discuss their electronic properties and crystal packing features. We observe that these soluble compounds easily form one-dimensional (1 D) packing arrangements and allow a direct evolution of the π stacking by varying the geometric shape. We find that the aspect ratio between length and width plays an important role on crystal packing. Our result indicates that when the PAH molecules have zigzag edges, these can provide enough volume for the molecules to rotate and reorient, alleviating the unfavorable electrostatic interactions found in perfectly cofacial π–π stacking. Density functional theory calculations were carried out to provide insights into how the molecular geometric shape influences the electronic structure and transport properties. The calculations indicate that, among the compounds studied here, “brick-layer” stacks provide the highest hole mobility.
[show abstract][hide abstract] ABSTRACT: A solution-based strategy for fabrication of high dielectric constant (κ) nanocomposites for flexible organic field effect transistors (OFETs) has been developed. The nanocomposite was composed of a high-κ polymer, cyanoethyl pullulan (CYELP), and a high-κ nanoparticle, zirconium dioxide (ZrO2). Organic field effect transistors (OFETs) based on neat CYELP exhibited anomalous behavior during device operation, such as large hysteresis and variable threshold voltages, which yielded inconsistent devices and poor electrical characteristics. To improve the stability of the OFET, we introduced ZrO2 nanoparticles that bind with residual functional groups on the high-κ polymer, which reduces the number of charge trapping sites. The nanoparticles, which serve as physical cross-links, reduce the hysteresis without decreasing the dielectric constant. The dielectric constant of the nanocomposites was tuned over the range of 15.6-21 by varying the ratio of the two components in the composite dielectrics, resulting in a high areal capacitance between 51 and 74 nF cm(-2) at 100 kHz and good insulating properties of a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3.5 V (0.25 MV cm(-1)). Bottom-gate, top-contact (BGTC) low operating voltage p-channel OFETs using these solution processable high-κ nanocomposites were fabricated by a contact film transfer (CFT) technique with poly(3-hexylthiophene) (P3HT) as the charge transport layer. Field effect mobilities as high as 0.08 cm(2) V(-1) s(-1) and on/off current ratio of 1.2 × 10(3) for P3HT were measured for devices using the high-κ dielectric ZrO2 nanocomposite. These materials are promising for generating solution coatable dielectrics for low cost, large area, low operating voltage flexible transistors.
[show abstract][hide abstract] ABSTRACT: We investigated the structure-morphology-performance relationship of diketopyrrolopyrrole (DPP)-based low band gap polymers with different donor cores in organic field effect transistors (OFETs) and organic photovoltaics (OPVs). The change in the chemical structure led to strong physical property differences, such as crystalline behavior, blend morphology and device performance. In addition, the choice of solvents and additives enabled one to fine-tune the properties of these materials in the condensed state. For instance, when thin films were processed from solvent mixtures, both in the pure polymer and in a blend, we observed an enhanced edge-on orientation and the formation of thinner and longer polymer fibrils. In the BHJ blends, processing from a solvent mixture reduced the size scale of the phase separation, promoted the formation of a fibrillar network morphology, having a polymer-PCBM mixture filling the inter-fibrillar regions. The characteristic length scale of the fibrillar network dictated the specific inner surface area, which directly correlated to the performance in the OPV devices. When the BHJ mixture was processed from a single solvent, a large-scale phase separated morphology was observed that was stratified, normal to the film surface. A strong scattering anisotropy was observed in the resonant soft x-ray scattering of the blends that provided insight into the packing of the polymer chains within the fibrils. The morphology and performance trend in OPVs paralleled the performance in an OFET, suggesting that similar processing conditions should be considered in OFET fabrication.
Journal of the American Chemical Society 12/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Rubrene single crystals can serve as a model material platform for studying the intrinsic photophysical processes in organic semiconductors and advance our understanding of material functionality in organic photovoltaic applications. The high degrees of structural order and material purity of organic single crystals enable a level of study that is unattainable in materials of current practical importance. Here, the photovoltaic effect at the Schottky interface of rubrene single crystal–aluminum electrode is demonstrated in a lateral ITO–rubrene–Al device geometry. The mechanism of the effect formation is explained based on the reconstructed energy band diagram of the ITO–rubrene–Al heterostructure. In particular, the open circuit voltage (VOC) of the devices shows a strong dependency on the interfacial band bending and corresponding built-in potential at the rubrene–Al Schottky interface. Initially, the photovoltage is found to be equal to the built-in potential at the Schottky interface defined by the work function difference between the bulk of rubrene and the Al electrode, that is, following the Schottky–Mott model. A good agreement is found between the systematically varied built-in potential and the resulting photovoltage magnitude upon insertion of an ultrathin LiF interlayer between the rubrene and Al electrode.
[show abstract][hide abstract] ABSTRACT: A new organic semiconductor (BT-TTF) based on molecular moieties of benzothiadiazole and tetrathiafulvalene was designed and synthesized, and its structure, molecular packing and charge-transporting properties were determined. Thermal properties, electrochemical behaviors, and optical absorption of this molecule were studied by using differential scanning calorimetry/thermal gravimetric analysis, cyclic voltammetry, and ultraviolet-visible spectroscopy, respectively. Its bulk and nanowire single crystals were prepared and characterized by X-ray crystallography, scanning electron microscopy, transmission electron microscopy, and field-effect transistors. It is found that short intermolecular S···S (3.41 Å), S···C (3.49 Å), and S···N (3.05 Å) contacts define the solid-state structure of BT-TTF single crystals which π-stack along the  with interplanar distances of 3.49 Å. Solvent-cast single-crystal nanowire transistors showed mobilities as large as 0.36 cm(2)/(V s) with current on/off ratios of 1 × 10(6). This study further illustrates the impact of molecular design and a demonstration of high-performance single-crystal nanowire transistors from the resulting semiconductor.
[show abstract][hide abstract] ABSTRACT: Monodispersed conjugated oligothiophenes are receiving attention in fundamental and applied science due to their interesting optical, optoelectronic, and charge transport properties. These "low molecular weight" polymers serve as model structures for the corresponding polymer analogues, which are inherently polydispersed. Here we report the synthesis, electronic structure, molecular packing/morphology, and charge transport properties of monodispersed oligothiophenes with up to six didodecylquaterthiophene (DDQT) building block repeat units (i.e. 24 thiophene units). At the point where the effective conjugation length is reached, the electronic structure showed convergence behavior to the corresponding polymer, poly(3,3''-didodecyl-quaterthiophene) (PQT-12). X-ray crystal structure analysis of the dimer (DDQT-2) showed that terminal thiophenes exhibit syn-conformations, similar to the terminal syn-conformations observed in the trimer (DDQT-3). The dimer also exhibits a rare bending of the terminal alkyl side chains in order to prevent steric hindrance with neighboring hydrogens attached to core thiophenes. Grazing incidence X-ray scattering (GIXD) measurements reveal a morphology evolution from small molecule-like packing to polymer-like packing in thin films, with a morphology transition occurring near the effective conjugation length. Charge transport measurements showed a mobility increase with decreasing chain length. We correlated the molecular packing and morphology to charge transport and determined that carrier mobilities are most sensitive to crystallinity and crystal grain misorientation. This indicates that molecular weight is not a decisive factor for improved carrier mobility in the low molecular weight region, but rather the degree in crystallinity and in-plane crystal orientation. These results represent a fundamental advancement in understanding the relationship between conjugation length and carrier mobilities in oligothiophene semiconductors.
Journal of the American Chemical Society 12/2012; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The intrinsic mechanical properties of rubrene single crystals are investigated. The in-plane elastic constants are obtained by inducing the wrinkling instability in crystals laminated on elastomeric substrates. The results demonstrate a dependence of wrinkling wavelength on crystallographic direction. The observed elastic anisotropy suggests a non-linear coupling between mechanical and electrical properties.
[show abstract][hide abstract] ABSTRACT: Solar cell performance and morphology characterization of a diketopyrrolopyrrole-based low bandgap polymer is reported. The polymer adopts an H-type aggregation and solvent mixture processing gives a better morphology. The morphology evolution is characterized by combined GIXD and GISAXS experiments and a four step morphology development mechanism is proposed.
[show abstract][hide abstract] ABSTRACT: The fabrication of low-voltage flexible organic thin film transistors using zirconia (ZrO(2)) dielectric layers prepared via supercritical fluid deposition was studied. Continuous, single-phase films of approximately 30 nm thick ZrO(2) were grown on polyimide (PI)/aluminum (Al) substrates at 250 °C via hydrolysis of tetrakis(2,2,6,6-tetramethyl-3,5-heptane-dionato) zirconium in supercritical carbon dioxide. This dielectric layer showed a high areal capacitance of 317 nF cm(-2) at 1 kHz and a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3 V. By using poly(3-hexylthiophene) (P3HT) as a semiconductor, we have fabricated flexible thin film transistors operating at V(DS) = -0.5 V and V(G) in a range from 0.5 V to -4 V, with on/off ratios on the order of 1 × 10(3) and mobility values higher than 0.1 cm(2)/(V s).
[show abstract][hide abstract] ABSTRACT: Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO(2) as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared via self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-b-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices via roll-to-roll processing.
[show abstract][hide abstract] ABSTRACT: A facile three-step synthesis of 7,8,15,16-tetraazaterrylene (TAT) from phenalene-1,3-dione, with potential application in organic electronics, is reported. Single crystal structure analysis shows one-dimensional columnar stacks with an interplanar distance of 3.352 Å.
[show abstract][hide abstract] ABSTRACT: This communication reports the synthesis of a new polycyclic aromatic hydrocarbon and its unique packing motif. This molecule is shown to be an efficient electron donor in organic bulk heterojunction solar cells, exhibiting a power conversion efficiency of 2.0%.
Journal of Materials Chemistry 02/2012; 22(10):4266-4268. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Organic single crystals have opened the doors to a new generation of
high-performance organic electronic devices. Exceptional
charge-transport properties combined with the advent of large-area
patterning techniques make organic single crystals excellent candidates
for flexible electronics applications. However, in order to effectively
employ organic single crystals on mechanically flexible architectures,
their mechanical properties need to be understood and characterized. In
this presentation, the mechanical properties of rubrene single-crystals
are investigated. Given the limited dimensions of as-grown crystals and
associated handling difficulty, the elastic buckling instability is
chosen as a metrology tool for determining the in-plane elastic
constants. Our results show that ultrathin (200nm - 1000nm) rubrene
crystals exhibit anisotropic wrinkling wavelengths as a function of
crystallographic direction, which can be correlated to the anisotropic
nature of its molecular packing. An adaptive intermolecular reactive
bond order potential (AIREBO) is employed to calculate the nine elastic
constants corresponding to orthorhombic rubrene.
[show abstract][hide abstract] ABSTRACT: We report the cooperative self-assembly of functionalized fullerenes and all conjugated block copolymers (BCPs) containing polythiophene derivatives in both segments to yield solar cells with well-defined nanostructures and enhanced morphological stability. Favorable hydrogen bonding interactions between the COOH-functionalized fullerene, bis-[6, 6]-phenyl C61-butyric acid (bis-PCBA), and the tetraethyleneglycol side chains of poly(3-hexylthiophene)-block-poly[3-(2,5,8,11-tetraoxadodecane)thiophene] (P3HT-b-P3TODT) allows for high loading of bis-PCBA (up to 40 wt % to the blend) within the P3TODT domains, while preserving the lamellar morphology. Characterization by grazing incidence small-angle X-ray scattering, electron microscopy, and atomic force microscopy indicates that the periods of the structures range between 24 and 29 nm depending on the bis-PCBA loading. The hydrogen bond interactions between bis-PCBA and P3TODT segments further suppress crystallization and macrophase separation of the fullerenes, even under harsh annealing conditions (150 °C for 12 h). Bulk heterojunction solar cells prepared using P3HT-b-P3TODT/bis-PCBA exhibit a photoconversion efficiency of 2.04%, which is greater than that of a reference system, P3HT-b-P3TODT/bis-PCBM. Accelerated aging experiments reveal enhanced thermal stability as a result of the limited translational mobility of COOH-functionalized fullerene in P3HT-b-P3TODT relative to devices prepared using bis-PCBM in P3HT-b-P3TODT or P3HT. We believe that cooperative assembly using strong noncovalent interactions is a general approach that can be used to improve the processing, morphological stability, and aging of organic and hybrid photovoltaic devices.Keywords: diblock copolymer; self-assembly; morphology; photovoltaic device; hydrogen bonding; supramolecular assembly
[show abstract][hide abstract] ABSTRACT: We report an investigation of the stability and durability of p- and n-channel polymer thin film transistors in air over a 4-year period. All-polymer p/n complementary inverters fabricated from an n-channel poly(benzobisimidazobenzophenanthroline) (BBL) transistor and a p-channel poly(3-hexylthiophene) (P3HT) transistor showed excellent switching characteristics and a large voltage gain. The electrical parameters (electron mobility, on/off current ratio, and threshold voltage) of the n-channel BBL transistors in air were found to be constant over the 4 years. The performance of the p-channel P3HT transistors deteriorated dramatically after only 2 weeks in air. The excellent stability/durability of the BBL transistors in air is explained by the closely-packed crystalline morphology which creates a kinetic barrier against diffusion of extrinsic molecules and its high electron affinity that provides energetic stability against chemical/electrochemical reactions. The results demonstrate the longest air-stability and durability of non-encapsulated organic electronic devices to date while offering insights for the design of more environmentally rugged organic semiconductors.
Journal of Materials Chemistry 10/2011; 21(41):16461-16466. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The current work provides a general approach to obtain reliable donor acceptor morphologies by H-bonding cooperative assembly and to achieve efficient photovoltaic devices with enhanced device stability. Herein, we utilize P3HT-based block copolymer (BCP), in which one block is P3HT and the other block is a P3HT derivative containing a poly(ethylene oxide) (PEO) oligomer side chain. This design both enables self-assembly of the devices via microphase segregation into lamellar, cylindrical or spherical morphologies depending on the relative volume fractions of the blocks and provides a means for establishing strong preferential interaction between fullerene derivatives containing hydrogen bond donating groups (such as COOH groups) and the PEO side chain. One advantage of this approach is excellent device stability due to the suppression of macrophase separation resulting from fullerene crystallization under harsh annealing condition.
[show abstract][hide abstract] ABSTRACT: There are certain aspects of the electronic and packing behavior of planar aromatic molecules containing exocyclic chalcogen atoms (i.e., sulfur, selenium, tellurium) which need considerable re-enlightenment. This class of semiconductors was once regarded as next-generation π-donors for applications in charge-transfer complexes. With the advent of new device technologies such as light-emitting diodes, solar cells, and organic transistors, the interest in charge-transfer complexes eventually tapered off. However, significant progress in the use of this class of materials in modern organic devices has been reported over the last five years. In this article, we review the exocyclic arenes with chalcogen atoms in peri-positions, summarize synthetic routes to these compounds and take a close look at their basic properties. Particular emphasis is placed upon their packing arrangements and the effect of exocyclic chalcogen atoms on the crystal packing motifs. Selected example applications from this class of materials in different fields will be highlighted. As a final note, we provide a prediction for their use in mainstream applications such as energy and fundamental charge transport/generation.
Journal of Materials Chemistry 01/2011; 21(5):1329-1337. · 5.97 Impact Factor