Alejandro L Briseno

University of Massachusetts Amherst, Amherst Center, Massachusetts, United States

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Publications (75)596.77 Total impact

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    ABSTRACT: We demonstrate the use of poly(sulfobetaine methacrylate) (PSBMA), and its pyrene-containing copolymer, as solution-processable work function reducers for inverted organic electronic devices. A notable feature of PSBMA is its orthogonal solubility relative to solvents typically employed in the processing of organic semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calculated by density functional theory. PSBMA interlayers reduced the work function of metals, graphene, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced the electron injection barrier between indium tin oxide (ITO) and C70 by 0.67 eV. As a result, the performance of organic photovoltaic devices with PSBMA interlayers is significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS, and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive polymer interlayers with demonstrated effectiveness in inverted devices.
    Journal of the American Chemical Society 12/2014; · 11.44 Impact Factor
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    ABSTRACT: Conspectus Polycyclic aromatic hydrocarbons (PAHs), consisting of laterally fused benzene rings, are among the most widely studied small-molecule organic semiconductors, with potential applications in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Linear acenes, including tetracene, pentacene, and their derivatives, have received particular attention due to the synthetic flexibility in tuning their chemical structure and properties and to their high device performance. Unfortunately, longer acenes, which could exhibit even better performance, are susceptible to oxidation, photodegradation, and, in solar cells which contain fullerenes, Diels-Alder reactions. This Account highlights recent advances in the molecular design of two-dimensional (2-D) PAHs that combine device performance with environmental stability. New synthetic techniques have been developed to create stable PAHs that extend conjugation in two dimensions. The stability of these novel compounds is consistent with Clar's sextet rule as the 2-D PAHs have greater numbers of sextets in their ground-state configuration than their linear analogues. The ionization potentials (IPs) of nonlinear acenes decrease more slowly with annellation in comparison to their linear counterparts. As a result, 2-D bistetracene derivatives that are composed of eight fused benzene rings are measured to be about 200 times more stable in chlorinated organic solvents than pentacene derivatives with only five fused rings. Single crystals of the bistetracene derivatives have hole mobilities, measured in OFET configuration, up to 6.1 cm(2) V(-1) s(-1), with remarkable Ion/Ioff ratios of 10(7). The density functional theory (DFT) calculations can provide insight into the electronic structures at both molecular and material levels and to evaluate the main charge-transport parameters. The 2-D acenes with large aspect ratios and appropriate substituents have the potential to provide favorable interstack electronic interactions, and correspondingly high carrier mobilities. In stark contrast to the 1-D acenes that form mono- and bis-adducts with fullerenes, 2-D PAHs show less reactivity with fullerenes. The geometry of 2-D PAHs plays a crucial role in determining both the barrier and the adduct stability. The reactivity and stability of the 2-D PAHs with regard to Diels-Alder reactions at different reactive sites were explained via DFT calculations of the reaction kinetics and of thermodynamics of reactions and simple Hückel molecular orbital considerations. Also, because of their increased stability in the presence of fullerenes, these compounds have been successfully used in OPVs. The small-molecule semiconductors highlighted in this Account exhibit good charge-transport properties, comparable to those of traditional linear acenes, while being much more environmentally stable. These features have made these 2-D PAHs excellent molecules for fundamental research and device applications.
    Accounts of Chemical Research 12/2014; · 24.35 Impact Factor
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    ABSTRACT: Due to the unique crystallinity of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), it is an excellent model polymer to study the structure-property relationship in organic devices, especially those relying on junctions of electron and hole transporting materials. Here, we report the synthesis and characterization of a series of monodisperse PBTTT oligothiophenes (n = 1-5) and systematically examine the evolution of crystalline behavior, morphology, and interaction with PC71BM as the molecular conjugation length increases. We discovered that fullerene intercalation occurs when there is enough free volume between the side-chains to accommodate the fullerene molecule. The intercalation of PC71BM is observed beyond BTTT-2 and longer oligomers, likely similar to that of PBTTT. Interestingly, both experiments and molecular simulations show that the PC71BM intercalation also appears to "catalyze" a more efficient packing of the BTTT-2 dimers. Crystal structure analysis revealed that the straight BTTT-2 side chains form 1D channels that could perfectly host PC71BM but, in the pure material, accommodate the interdigitated side chains from adjacent layers. In the blend with PC71BM, these channels are maintained and enable the co-crystallization and intercalation of PC71BM. This is the first time the actual sub-lattice cell of PC71BM has been determined from the X-ray data and we feel that this work sets a fine example of the utility of a material's oligomers to behave as model systems for their polymer counterparts and other conjugated polymers. Among the organic photovoltaic devices (OPVs) made from the BTTT oligomers and PC71BM blends, the ones using the BTTT-2 dimer exhibit the highest performance.
    Journal of the American Chemical Society 11/2014; · 11.44 Impact Factor
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    ABSTRACT: In polymer-based photovoltaic (PV) devices optimizing and controlling the active layer morphology is important to enhance the device efficiency. Using poly(3-hexylthiophene) (P3HT) with well-defined molecular weights, synthesized by the Grignard Metathesis method (GRIM), we show that the morphology of the PV active layer and the absorption and crystal structure of the P3HT are dependent on the molecular weight. Differential scanning calorimetry (DSC) showed that the crystallinity of P3HT reached a maximum for intermediate molecular weights. Grazing incidence wide-angle x-ray diffraction (GIXD) showed that the spacing of the (100) planes of P3HT increased with increasing molecular weight, while the crystal size decreased. Nonlinear crystal lattice expansions were found for both the (100) and (020) lattice planes, with an unusual π-π stacking enhancement observed between 50 to 100 °C. The melting point depression for P3HT, when mixed with [6,6]-phenyl C61- butyric acid methyl ester (PCBM), and, hence, the Flory-Huggins interaction parameter, depended on molecular weight. PCBM was found to perturb the ordering of P3HT chains. In PV devices, P3HT with a molecular weight of ~20k showed the best device performance. The morphologies of these blends were studied by grazing incidence small angle x-ray scattering (GISAXS) and resonant soft x-ray scattering (RSoXS). In GISAXS, we observed that the low molecular weight P3HT more readily crystallizes, promoting a phase-separated morphology.
    ACS Applied Materials & Interfaces 10/2014; · 5.90 Impact Factor
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    ABSTRACT: Poly(3-hexyl thiophene) (P3HT)-block-poly(3-(3-aminopropyl)oxymethyl thiophene) (P3AmT) diblock copolymers were synthesized and assembled into nanowires by solvent-induced crystallization. Bis(4-[1,6-hexyldiisocyanate]benzylpyrrolidine)-C60 was synthesized and used to covalently cross-link the structures, affording robust p-type/n-type nanowires. These cross-linked nanowires proved stable to solvents and temperatures that would disrupt conventional P3HT-nanowires, as characterized by transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy. Photoluminescence spectroscopy showed quenching of the PL signal of the fullerene-crosslinked material, suggesting electronic communication between the polymer and fullerene in these novel donor/acceptor assemblies. Grazing incidence X-ray diffraction (GIXD) showed a similar crystal structure for nanowires before and after cross-linking, while field effect transistor transfer measurements of the cross-linked nanowires showed hole and electron mobilities of 3.5 × 10−5 cm2 V−1 s−1 and 4.6 × 10−5 cm2 V−1 s−1, respectively.
    J. Mater. Chem. C. 10/2014;
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    ABSTRACT: The most efficient architecture for achieving high donor/acceptor interfacial area in organic photovoltaic (OPV) devices would employ arrays of vertically interdigitated p- and n- type semiconductor nanopillars (NPs). Such morphology could have an advantage in bulk heterojunction systems; however, precise control of the dimension morphology in a crystalline, interpenetrating architecture has not yet been realized. Here we present a simple, yet facile, crystallization technique for the growth of vertically oriented NPs utilizing a modified thermal evaporation technique that hinges on fast deposition rate, short substrate-source distance and ballistic mass transport. A broad range of organic semiconductor materials is beneficial from the technique to generate NP geometries. Moreover, this technique can also be generalized to various substrates, namely, graphene, PEDOT, ZnO, CuI, MoO3, and MoS2. The advantage of the NP architecture over the conventional thin film counterpart is demonstrated with an increase of power conversion efficiency of 32% in solar cell devices. This technique will advance the knowledge of organic semiconductor crystallization, and create opportunities for the fabrication and processing of NPs for applications that include solar cells, charge storage devices, sensors and vertical transistors.
    Nano Letters 09/2014; · 12.94 Impact Factor
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    ABSTRACT: We report the morphological characterization of triisopropylsilylethynyl-dibenzochrysene (TIPS-DBC:PCBM) blends, a bulk heterojunction (BHJ) solar cell system based on a highly crystalline small molecule donor. We found that processing the blends from a volatile solvent such as chloroform is beneficial in controlling the crystal size and phase separation of the donor-acceptor phases. When a less-volatile solvent such as chlorobenzene is used, large crystalline domains formed, exceeding the length scale suitable for BHJ solar cells. When the BHJ films are thermally annealed, enhanced domain purity is observed for the chloroform processed thin films, which led to an increased short circuit current in the devices.
    J. Mater. Chem. C. 09/2014;
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    ABSTRACT: High molecular weight PBTTT-C12 is blended with the pure trimer, BTTT-3, to enhance intergrain connectivity and charge transport. Analysis of the morphology and crystallinity of the blends shows that the polymer and oligomer are well-integrated, leading to high hole mobilities, greater than 0.1 cm2 V-1 s-1, in films that contain as much as 83% oligomer.
    Physical Chemistry Chemical Physics 09/2014; · 4.20 Impact Factor
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    ABSTRACT: The primary role of substituted side chains in organic semiconductors is to increase their solubility in common organic solvents. In the recent past, many literature reports have suggested that the side chains play a critical role in molecular packing and strongly impact the charge transport properties of conjugated polymers. In this work, we have investigated the influence of side-chains on the charge transport behavior of a novel class of diketopyrrolopyrrole () based alternating copolymers. To investigate the role of side-chains, we prepared four diketopyrrolopyrrole-diketopyrrolopyrrole () conjugated polymers with varied side-chains and carried out a systematic study of thin film microstructure and charge transport properties in polymer thin-film transistors (PTFTs). Combining results obtained from grazing incidence X-ray diffraction (GIXD) and charge transport properties in PTFTs, we conclude side-chains have a strong influence on molecular packing, thin film microstructure, and the charge carrier mobility of copolymers. However, the influence of side-chains on optical properties was moderate. The preferential "edge-on" packing and dominant n-channel behavior with exceptionally high field-effect electron mobility values of >1 cm(2) V(-1) s(-1) were observed by incorporating hydrophilic (triethylene glycol) and hydrophobic side-chains of alternate DPP units. In contrast, moderate electron and hole mobilities were observed by incorporation of branched hydrophobic side-chains. This work clearly demonstrates that the subtle balance between hydrophobicity and hydrophilicity induced by side-chains is a powerful strategy to alter the molecular packing and improve the ambipolar charge transport properties in based conjugated polymers. Theoretical analysis supports the conclusion that the side-chains influence polymer properties through morphology changes, as there is no effect on the electronic properties in the gas phase. The exceptional electron mobility is at least partially a result of the strong intramolecular conjugation of the donor and acceptor as evidenced by the unusually wide conduction band of the polymer.
    Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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    ABSTRACT: The Diels-Alder (DA) reactions of pentacene (PT), 6,13-bis(2-trimethylsilylethynyl)pentacene (TMS-PT), bistetracene (BT), and 8,17-bis(2-trimethylsilylethynyl)bistetracene (TMS-BT) with the [6,6] double bond of [60]fullerene have been investigated by density functional theory (DFT) calculations. Reaction barriers and free energies have been obtained to assess the effects of frameworks and substituent groups on the Diels-Alder reactivity and product stability. Calculations indicate that TMS-BT is about 5 orders of magnitude less reactive than TMS-PT in the reactions with [60]fullerene. This accounts for the observed much higher stability of TIPS-BT than TIPS-PT when mixed with PCBM. Surprisingly, calculations predict that the bulky silylethynyl substituents of TMS-PT and TMS-BT have only a small influence on reaction barriers. However, the silylethynyl substituents significantly destabilize the corresponding products due to steric repulsions in the adducts. This is confirmed by experimental results here. Architectures of the polycyclic aromatic hydrocarbons (PAHs) play a crucial role in determining both the Diels-Alder barrier and the adduct stability. The reactivities of different sites in various PAHs are related to the loss of aromaticity, which can be predicted using the simple Hückel molecular orbital (HMO) localization energy calculations.
    Journal of the American Chemical Society 07/2014; · 11.44 Impact Factor
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    ABSTRACT: Transport of charge carriers through conjugated polymers is strongly influenced by the presence and distribution of structural disorders. In the present work, structural defects caused by the presence of torsional angle were investigated in a diketopyrrolopyrrole (DPP)-based conjugated polymer. Two new copolymers of DPP were synthesized with varying torsional angles to trace the role of structural disorder. The optical properties of these copolymers in solution and thin film reveal the strong influence of torsional angle on their photophysical properties. A strong influence was observed on carrier transport properties of polymers in organic field-effect transistors (OFET) device geometry. The polymers based on phenyl DPP with higher torsional angle (PPTDPP-OD-TEG) resulted in high threshold voltage with less charge carrier mobility as compared to the polymer based on thiophene DPP (2DPP-OD-TEG) bearing a lower torsional angle. Carrier mobility and the molecular orientation of the conjugated polymers were correlated on the basis of grazing incidence X-ray scattering measurements showing the strong role of torsional angle introduced in the form of structural disorder. The results presented in this Article provide a deep insight into the sensitivity of structural disorder and its impact on the device performance of DPP-based conjugated polymers.
    The Journal of Physical Chemistry C 05/2014; 118(22):11536–11544. · 4.84 Impact Factor
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    ABSTRACT: We report the synthesis and characterization of "bistetracene," an unconventional, linearly extended conjugated core with eight fused rings. The annellation mode of the system allows for increased stability of the conjugated system relative to linear analogues due to the increased number of Clar aromatic sextets. By attaching the appropriate solubilizing substituents, efficient molecular packing with large transfer integrals can be obtained. The electronic structure calculations suggest these large polycyclic aromatic hydrocarbons exhibit excellent intrinsic charge transport properties. Charge carrier mobilities as large as 6.1 cm^2 V-1s-1 and current on/off ratios of 10^7 were determined experimentally for one of our compounds. Our study provides valuable insight into the design of unconventional semiconductor compounds based on higher polycyclic aromatic hydrocarbons (PAHs) for use in high performance devices.
    Journal of the American Chemical Society 05/2014; · 11.44 Impact Factor
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    ABSTRACT: Poly(3-hexylthiophene)-block-poly(3-(3-thioacetylpropyl) oxymethylthiophene) (P3HT)-b-(P3TT) diblock copolymers were synthesized and manipulated by solvent-induced crystallization to afford reversibly cross-linked semiconductor nanowires. To cross-link the nanowires, the thioacetate groups were deprotected to thiols and subsequently oxidized to disulfides. Cross-linked nanowires maintained their structural integrity in solvents that normally dissolve the polymers. These robust nanowires could be reduced to the fully solvated polymer, representing a novel, reversible cross-linking procedure for functional P3HT-based nanowire fibrils. Field-effect transistor measurements were carried out to determine the charge transport properties of these nanostructures.
    ACS Applied Materials & Interfaces 04/2014; · 5.90 Impact Factor
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    ABSTRACT: Rubicene, a molecular fragment of C70, is a promising organic semiconductor material that displays excellent electronic characteristics for use in organic field-effect transistors (OFETs). Bottom-gate/bottom-contact polycrystalline thin-film OFETs using rubicene exhibit a saturation hole mobility of 0.20 cm2 V−1 s−1 and a current on/off ratio (Ion/Ioff) of 1.0 × 104. In addition, the device performance can be improved with a mobility of 0.32 cm2 V−1 s−1 and Ion/Ioff of 2.5 × 104 with pentafluorobenzenethiol (PFBT) self-assembled monolayer (SAM) treatment on Au electrodes. To characterize the interfacial electronic structure and morphology of rubicene on Au and PFBT/Au, ultraviolet photoelectron spectroscopy (UPS), theoretical calculation with density functional theory (DFT) and grazing incidence X-ray diffraction (GIXD) were performed. With PFBT SAM treatment, the hole injection barrier from Au to rubicene is significantly decreased from 1.15 to 0.48 eV due to the formation of a large interface dipole on Au that increased its work function from 4.33 to 5.67 eV. Furthermore, PFBT SAM treatment also induces an “edge-on” configuration of rubicene, which can lead to the increase in carrier mobility. These results indicate that rubicene can serve as a benchmark organic semiconductor for model charge transport studies and in various organic electronic devices.
    J. Mater. Chem. C. 04/2014; 2(17).
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    ABSTRACT: The structure–property relationships of PTB7-phenyl-C61-butyric acid methyl ester (PCBM)-based organic photovoltaics are investigated. The morphology is investigated in an active layer setting where a multi-length-scale morphology is observed using a solvent additive-assisted film processing. This multi-length-scale structure consists of a phase separated morphology with a characteristic length scale of ≈30 nm, which is critical for producing large currents in devices; a second length scale of ≈130 nm, arises from face-on PTB7 crystalline aggregates. This latter morphological feature is also observed in films prepared without the use of an additive. By observing the structure formation in situ during solvent evaporation for blade coated thin films, the additive is found to promote the formation of ordered domains of the PTB7 at an earlier stage during the solvent evaporation, which is critical in the development of the final morphology. In studies on PTB7/PCBM bilayers, PCBM is found to diffuse into the PTB7 layer. However, the performance of devices prepared in this manner is low. This diffusion leads to a swelling of the PTB7 and a reduction in the crystallinity of the PTB7, reflecting the strong miscibility of PCBM with PTB7. The morphology resulting from the interdiffusion is single-length-scale with slightly large phase separation. This leads to devices with poor performance.
    Advanced Energy Materials 04/2014; · 14.39 Impact Factor
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    ABSTRACT: Recent synthetic work has realized a novel (n-type) small-molecule acceptor, 7,8,15,16-tetra-aza-terrylene (TAT), single-crystals of which can be grown oriented along the c-axis crystallographic direction, and over-coated with pentacene to form a highly ordered donor/acceptor interface for use in organic photovoltaic devices. However, characterization of single TAT crystals reveals highly variable emission spectra and excited state dynamics - properties which strongly influence photovoltaic performance. Through the use of single-crystal widefield imaging, photoluminescence spectroscopy, time correlated single photon counting, and resonant Raman studies, we conclude that this variability is a result of long-lived low-energy trap-emission from packing defects. Interestingly, we also discovered that TAT crystals whose width exceeds ∼200 nm begin acting as waveguides and optical microcavity resonators for their own photoluminescence. Several strategies are proposed for leveraging the size-dependant optical properties of TAT pillars to further enhance device performance using this active layer design.
    Physical Chemistry Chemical Physics 03/2014; · 4.20 Impact Factor
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    ABSTRACT: Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials which can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.
    ACS Applied Materials & Interfaces 03/2014; · 5.90 Impact Factor
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    ABSTRACT: The use of Rozen's reagent (HOF⋅CH3 CN) to convert polythiophenes to polymers containing thiophene-1,1-dioxide (TDO) is described. The oxidation of polythiophenes can be controlled with this potent, yet orthogonal reagent under mild conditions. The oxidation of poly(3-alkylthiophenes) proceeds at room temperature in a matter of minutes, introducing up to 60 % TDO moieties in the polymer backbone. The resulting polymers have a markedly low-lying lowest unoccupied molecular orbital (LUMO), consequently exhibiting a small bandgap. This approach demonstrates that modulating the backbone electronic structure of well-defined polymers, rather than varying the monomers, is an efficient means of tuning the electronic properties of conjugated polymers.
    Angewandte Chemie International Edition 02/2014; 53(7):1832-6. · 11.34 Impact Factor
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    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.
    Chemistry - A European Journal 12/2013; 19(52). · 5.93 Impact Factor
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    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.
    ACS Applied Materials & Interfaces 12/2013; · 5.90 Impact Factor

Publication Stats

2k Citations
596.77 Total Impact Points


  • 2010–2014
    • University of Massachusetts Amherst
      • Department of Polymer Science and Engineering
      Amherst Center, Massachusetts, United States
    • Northeast Institute of Geography and Agroecology
      • Institute of Chemistry
      Beijing, Beijing Shi, China
  • 2007–2011
    • University of Washington Seattle
      • Department of Chemistry
      Seattle, Washington, United States
  • 2007–2008
    • Stanford University
      • Department of Chemical Engineering
      Stanford, CA, United States
  • 2005–2007
    • University of California, Los Angeles
      • Department of Chemistry and Biochemistry
      Los Angeles, California, United States
  • 2001–2004
    • California State University, Los Angeles
      • Department of Chemistry and Biochemistry
      Los Angeles, CA, United States