J. W. P. Hsu

University of Texas at Dallas, Richardson, Texas, United States

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Publications (12)44.34 Total impact

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    ABSTRACT: We demonstrate improved organic photovoltaic device performance using solution processed electron transport layers of ZnO nanoparticle (NP) films containing organic additives, poly(vinylpyrrolidone) (PVP) or diethanolamine (DEA), that do not require post processing after film deposition. Inclusion of PVP or DEA decreased ZnO work function by 0.4 eV through interfacial dipole formation. While PVP did not change ZnO NP shape or size, DEA modified ZnO shape from 5 nm x 15 nm nanorods to 5 nm nanoparticles. At optimized PVP concentration of 0.7 wt%, ZnO NP:PVP electron transport layers (ETLs) improved efficiency of inverted P3HT:PCBM devices by 37%, primarily through higher fill factor. ZnO NP:PVP and ZnO NP:DEA ETLs increased open circuit voltage of inverted P3HT:ICBA devices by 0.07 V due to decreasing ETL work function, leading to enhanced built-in field. The relationship between ZnO nanocomposite ETL work function, donor-acceptor energy offset, and device performance is discussed. The effects of the two additives are compared.
    ACS Applied Materials & Interfaces 08/2013; · 5.01 Impact Factor
  • Chemistry of Materials. 03/2011;
  • J W P Hsu, R A Vaia, A Trionfi
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    ABSTRACT: A Reply to the Comment by I. Balberg et al..
    Physical Review Letters 02/2011; 106(7):079702. · 7.73 Impact Factor
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    ABSTRACT: We present strategies to improve low open-circuit voltage (V<sub>oc</sub>) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting V<sub>oc</sub> variability is the ZnO bandgap (E<sub>g</sub>), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO E<sub>g</sub> by 0.14 eV and V<sub>oc</sub> of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in V<sub>oc</sub> by depositing conformal amorphous TiO<sub>x</sub> films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiO<sub>x</sub> coatings monotonically increased V<sub>oc</sub> from 0.4 to 0.8 V for devices with increasing TiO<sub>x</sub> thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiO<sub>x</sub> coating significantly decreases the reverse-bias current density, leading to an improvement in V<sub>oc</sub>, in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiO<sub>x</sub>. In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.
    IEEE Journal of Selected Topics in Quantum Electronics 01/2011; · 4.08 Impact Factor
  • J. W. P. Hsu, M. T. Lloyd
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    ABSTRACT: Organic and hybrid (organic/inorganic) solar cells are an attractive alternative to traditional silicon-based photovoltaics due to low-temperature, solution-based processing and the potential for rapid, easily scalable manufacturing. Using oxide semiconductors, instead of fullerenes, as the electron acceptor and transporter in hybrid solar cells has the added advantages of better environmental stability, higher electron mobility, and the ability to engineer interfacial band offsets and hence the photovoltage. Further improvements to this structure can be made by using metal oxide nanostructures to increase heterojunction areas, similar to bulk heterojunction organic photovoltaics. However, compared to all-organic solar cells, these hybrid devices produce far lower photocurrent, making improvement of the photocurrent the highest priority. This points to a less than optimized polymer/metal oxide interface for carrier separation. In this article, we summarize recent work on examining the polymer structure, electron transfer, and recombination at the polythiophene-ZnO interface in hybrid solar cells. Additionally, the impact of chemical modification at the donor-acceptor interface on the device characteristics is reviewed.
    MRS Bulletin. 06/2010; 35(6, June 2010).
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    ABSTRACT: Wurtzite ZnO nanorods were grown from solution onto coarse-grain bulk polycrystalline Ag substrates to explore the nature of preferred heteroepitaxial orientations. ZnO nanorods grow copiously on grains with <111> and <001> surface normals. Two epitaxial orientations were observed: {0001} ZnO ‖ {111} Ag with <20> ZnO ‖ <10> Ag and {0001} ZnO ‖ {001} Ag with <20> ZnO ‖ <10> Ag. Both feature ZnO basal plane growth, and the specific in-plane orientation relationships both feature alignment of close-packed directions in the interface. Nanorod growth was strongly suppressed on Ag grains in most other orientations. Although strain energy minimization is often invoked to explain the {0001} ZnO ‖ {111} Ag with <20> ZnO ‖ <10> Ag orientation, associated with an almost ideal near-coincidence site lattice matching, our data suggests that strain may not be the sole, or even the most important, determinant of the preferred orientations during solution-based epitaxial growth in this system.
    Journal of Materials Research 01/2010; 25:1352-1361. · 1.82 Impact Factor
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    ABSTRACT: We present the first experimental measurement of the geometric critical exponent beta associated with the percolation probability, the probability a metallic filler belongs to the conducting network, of an electrical composite. The technique employs conducting-tip atomic force microscopy to obtain a conducting areal density, and is demonstrated on polyimide nanocomposites containing different concentrations of carbon nanofibers. We find beta approximately 1 and t (the exponent for bulk conductivity) approximately 3. These values are consistent with the predictions for the Bethe lattice and larger than the values predicted in the 3D lattice percolation model. Hence, this electrical composite likely belongs to the same universality class as the Bethe lattice. The ability to measure geometric and transport critical exponents on the same material is critical to drawing this conclusion.
    Physical Review Letters 04/2009; 102(11):116601. · 7.73 Impact Factor
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    ABSTRACT: Using conducting-tip atomic force microscopy (C-AFM), we study the spatial distribution of current paths and local electrical properties in carbon nanofiber/polymer nanocomposites. Previous studies of similar systems were hindered by a polymer-rich skin layer that exists at the nanocomposite surfaces. We present an experimental technique using oxygen plasma etching to controllably remove this polymer skin layer. After this treatment, we can directly probe the microscopic transport characteristics of the nanocomposite using C-AFM. The C-AFM results show that the electrical transport is solely carried by the carbon nanofiber (CNF) networks in the nanocomposites. In addition, high-resolution C-AFM maps show nonuniform distribution of current along the length of some CNFs, suggesting the presence of a heterogeneously distributed adsorbed polymer layer around nanofibers. Finally, two probe conductivity measurements in which one electrode (the C-AFM tip) is contacting a single constituent conducting particle were performed to study local conductivity. Results indicate that Ohmic pathways exist in the conducting network of the nanocomposite to the lowest measured nanofiber concentrations. However, non-Ohmic behavior indicating tunneling transport may also be present, especially near the percolation threshold.
    Journal of Applied Physics 11/2008; · 2.21 Impact Factor
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    ABSTRACT: Electrical transport studies of multi-walled carbon nanotube (c-MWNT)/polymer nanocomposites have shown metallic behavior with conductivity sigma=sigma0( phi-phic )^t above the percolation threshold. The conductivity depends on three aspects of the conducting network (CN): the conductivity of the constituent c-MWNT, the number of c-MWNT making up the CN, and the detailed interconnectivity of the CN. Using conducting-tip atomic force microscopy (C-AFM), we have studied the density and conductivity of the c-MWNT CN as a function of c-MWNT loading between 0.5 - 5.0 wt % in a polyimide matrix. Using the Principle of Delesse, the volume fraction of the c-MWNT CN can be calculated from the conducting areal density measured in the C-AFM scans. The results of the C-AFM tests have shown localized areas of electrical transport associated with c-MWNT as well a clear dependence of conducting areal density and conductivity on the c-MWNT loading. This work was performed in part at the US Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos and Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.
    03/2008;
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    ABSTRACT: We present a study on hot electron transport through Au/ molecule/ n-GaAs001 diodes via ballistic electron emission microcopy BEEM. The molecules in the structure form a monolayer of either octanedithiol HS – CH 2 8 –SH or hexadecanethiol HS – CH 2 15 –CH 3 . For the dithiol case, the presence of the molecular interlayer leads to undetectable BEEM transmission. Whereas a small photoinduced collector current is detected at random locations at a forward reverse scanning tunneling microscopy STM tip voltage of −1.43± 0.01 V +1.50± 0.02 V. In comparison, with monothiol diodes, or diodes where the molecules are sandwiched between two Au films Au/ molecule/ Au/ GaAs, the BEEM transmission remains a significant fraction of the reference diode signal 30%–80% with a slight increase in the ballistic transport threshold voltage −1.0 to − 1.1 V from that of the reference Au/ GaAs diodes −0.89 V. Auger depth profiling and cross-sectional transmission electron microscopy show that Au-molecule intermixing occurs in Au/hexadecanethiol/GaAs but not in Au/octanedithiol/GaAs diodes. The suppression of BEEM signal and the detection of STM-induced photocurrent in the Au/octanedithiol/GaAs case are consistent with an insulating monolayer containing pinholes or recombination centers with densities of 1 every 25 25 nm 2 or 2000 m −2 . © 2007 American Institute of Physics.
    Journal of Applied Physics 07/2007; · 2.21 Impact Factor
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    ABSTRACT: The optical properties of solution-grown ZnO nanorods were investigated using photoluminescence and cathodoluminescence. The as-grown nanorods displayed a broad yellow-orange sub-band-gap luminescence and a small near-band-gap emission peak. The sub-band-gap luminescence can only be observed when exciting above band gap. Scanning cathodoluminescence experiments showed that the width of the sub-band-gap luminescence is not due to an ensemble effect. Upon reduction, the sub-band-gap luminescence disappeared and the near-band-gap emission increased. Compared to ZnO powders that are stoichiometric and oxygen deficient, we conclude that the yellow-orange sub-band-gap luminescence most likely arises from bulk defects that are associated with excess oxygen.
    Applied Physics Letters 06/2006; 88(25):252103-252103-3. · 3.79 Impact Factor
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    ABSTRACT: 335 Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low-temper- ature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of ori- ented nanorods and nanotubes. More recently, aqueous-phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded-growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologi- cally important materials, and therefore may play a key role in advancing complex nanomaterials into applications.
    Advanced Functional Materials 01/2006; 16(3):335 - 344. · 9.77 Impact Factor