Hsu, Julia W. P.

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Julia Hsu is a Professor in the Department of Materials Science and Engineering. She also holds the Texas Instruments Distinguished Chair in Nanoelectronics. Her research focuses on nanoscale materials physics. She has done extensive work on local characterization of electronic and photonic materials and devices using scanning probe techniques. The material systems she has studied include metals and alloys, group IV, III-V and II-VI semiconductors, and oxides. She is most interested in the relationship between microscopic organization and macroscopic properties of nanocomposites, including controlling assembly of inorganic nanomaterials in organic matrices and understanding electronic properties at the organic-inorganic interface, with applications towards energy applications, such as nanostructured solar cells. The research projects will include improving organic photovoltaics, nanomaterial synthesis, electrical and electronic studies of nanocomposites and heterojunctions, and nanofabrication.

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    Superior Low-Temperature NO Catalytic Performance of PrMn₂O₅ over SmMn₂O₅ Mullite-Type Catalysts
    (Royal Society of Chemistry, 2019) Thampy, Sampreetha; Ashburn, Nickolas; Liu, C.; Xiong, K.; Dillon, Sean; Zheng, Yongping; Chabal, Yves J.; Cho, Kyeongjae; Hsu, Julia W. P.; 0000-0002-7821-3001 (Hsu, JWP); 0000-0002-6435-0347 (Chabal, YJ); 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Thampy, Sampreetha; Ashburn, Nickolas; Dillon, Sean; Zheng, Yongping; Chabal, Yves J.; Cho, Kyeongjae; Hsu, Julia W. P.
    By studying their surface chemistry, metal-oxygen bond strength, and critical energy barrier heights, we elucidate the differences in the NO oxidation catalytic performance of PrMn₂O₅ and SmMn₂O₅ mullite-type oxides. The 50% conversion temperature is lower (230 °C vs. 275 °C) and the maximum conversion efficiency is higher (81% at 282 °C vs. 68% at 314 °C) for PrMn₂O₅ compared to SmMn₂O₅, despite having a ∼15% lower specific surface area. Furthermore, PrMn₂O₅ exhibits higher maximum efficiency compared to Pt/Al₂O₃. Combined experimental and theoretical findings indicate that the superior catalytic performance of PrMn₂O₅ at low temperatures arises from the presence of more labile and reactive surface lattice oxygen due to weaker Mn-O bond strength and lower thermal stability of surface NOₓ ad-species. ©2019 The Royal Society of Chemistry.
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    Probing Defect States in Organic Polymers and Bulk Heterojunctions Using Surface Photovoltage Spectroscopy
    (American Chemical Society, 2019-04-10) Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Cao, F. -Y; Tseng, C. -C; Cheng, Y. -J; Hsu, Julia W. P.; 0000-0002-7821-3001 (Hsu, JWP); Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Hsu, Julia W. P.
    We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC₆₁ BM, or PFBT₂Se₂Th:PC₇₁ BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC₆₁ BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT₂Se₂Th:PC₇₁ BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems. ©2019 American Chemical Society.
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    Solution-Processed Oxide Thin Film Transistors on Shape Memory Polymer Enabled by Photochemical Self-Patterning
    (Cambridge University Press) Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.; Hsu, Julia W. P.; 0000-0003-0135-0531 (Voit, WE); 0000-0002-7821-3001 (Hsu, JWP); Hsu, Julia W. P.; Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.
    Solution-processed metal oxide electronics on flexible substrates can enable applications from military to health care. Due to limited thermal budgets and mismatched coefficients of thermal expansion between oxides and substrates, achieving good performance in solution-processed oxide films remains a challenge. Additionally, the use of traditional photolithographic processes is incompatible with low-cost, high-throughput roll-to-roll processing. Here, we demonstrate solution-deposited oxide thin film transistors (TFTs) on a shape memory polymer substrate, which offers unique control of final device shape and modulus. The key enabling step is the exposure of the precursor film to UV-ozone through a shadow mask to perform patterning and photochemical conversion simultaneously. These TFTs exhibit mobility up to 160 cm2/(V s), subthreshold swing as low as 110 mV/dec, and threshold voltage between -2 and 0 V, while maintaining compatibility with a flexible form factor at processing temperatures below 250 °C. ©2018 Materials Research Society.
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    Minimizing Performance Degradation Induced by Interfacial Recombination in Perovskite Solar Cells through Tailoring of the Transport Layer Electronic Properties
    (Amer Inst Physics) Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.; 0000-0003-2710-5227 (Xu, L); 0000-0002-7821-3001 (Hsu, JWP); Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.
    The performance of hybrid organic-inorganic metal halide perovskite solar cells is investigated using one-dimensional drift-diffusion device simulations. We study the effects of interfacial defect density, doping concentration, and electronic level positions of the charge transport layer (CTL). Choosing CTLs with a favorable band alignment, rather than passivating CTL-perovskite interfacial defects, is shown to be beneficial for maintaining high power-conversion efficiency, due to reduced minority carrier density arising from a favorable local electric field profile. Insights from this study provide theoretical guidance on practical selection of CTL materials for achieving high-performance perovskite solar cells.
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    Nucleation and Growth of WSe₂: Enabling Large Grain Transition Metal Dichalcogenides
    (IOP Publishing Ltd, 2017-09-22) Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Cha, Pil-Ryung; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.; 0000-0002-2910-2938 (Liang, C); Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.
    The limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).
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    Understanding the Source of Dielectric Loss in Titania/Polypropylene Nanocomposites up to 220 GHz
    (2017-02-20) Womble, Michael D.; Herbsommer, Juan; Lee, Yun-Ju; Hsu, Julia W. P.; Schroder, H. Chen,RT; Hsu, Julia W. P.
    Nanocomposites are a promising new dielectric material for on-chip and chip-to-chip waveguides that operate at millimeter (mm)-wave frequencies because of their higher relative permittivity compared to neat polymers and their compatibility with printed circuit board processing. For dielectric waveguides, extremely low loss is critical; thus, understanding the origins of loss is an important step for these applications. In this paper, we investigate the sources of loss in TiO₂/polypropylene (PP) nanocomposites, in which polypropylene-graft-maleic anhydride (PP-g-MA) is added as a compatibilizer. Compared to nanocomposites made without PP-g-MA, we find that PP-g-MA improves the distribution of nanoparticles in the PP matrix and significantly lowers loss. We also examine the contribution to dielectric loss from PP-g-MA by measuring samples that contain no TiO2 nanoparticles, and find that while increasing the amount of PP-g-MA in PP results in a higher loss, it is small compared to the loss that comes from the addition of TiO₂ nanoparticles.
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    Sub-10 NM Copper Chromium Oxide Nanocrystals as a Solution Processed P-Type Hole Transport Layer for Organic Photovoltaics
    (Royal Society of Chemistry) Wang, Jian; Lee, Yun -Ju; Hsu, Julia W. P.; Wang, Jian; Lee, Yun -Ju; Hsu, Julia W. P.
    We report the synthesis of CuCrO₂ nanocrystals, a p-type transparent conducting oxide, and their application as an efficient hole transport layer (HTL) for organic photovoltaic (OPV) devices. A nanometer-sized mixture of Cu and Cr oxide/hydroxide is synthesized using microwave-assisted heating. With a 550 °C post-annealing treatment in N₂, <10 nm CuCrO₂ nanocrystals are successfully synthesized. XRD, XPS, EDAX, PESA, UV-vis spectrometry, and Kelvin probe technique are applied to confirm the delafossite phase, optical transmission, and p-type characteristics. Methanol is found to be a good solvent to disperse these nanocrystals for forming a smooth and transparent film. In comparison with the previously reported CuGaO₂ HTL, the reduced film roughness enables the CuCrO₂ HTL to produce highly efficient thin active layer OPV devices. UV-ozone treatment on the CuCrO₂ HTL is found to increase the fill factor. Drift-diffusion modeling, energy level measurements, and XPS results reveal that the device improvement is not due to the reduced injection barrier, but due to an improved CuCrO₂ conductivity arising from the formation of Cu²⁺ species.
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    Organic-Inorganic Hybrid Semiconductor Thin Films Deposited Using Molecular-Atomic Layer Deposition (MALD)
    (Royal Society of Chemistry) Huang, Jie; Zhang, Hengji; Lucero, Antonio; Cheng, Lanxia; KC, Santosh; Wang, Jian; Hsu, Julia W. P.; Cho, Kyeongjae; Kim, Jiyoung; 0000 0003 8600 0978 (Hsu, JWP); 0000-0003-2698-7774 (Cho, K); 0000-0003-2781-5149 (Kim, J); Huang, Jie; Zhang, Hengji; Lucero, Antonio; Cheng, Lanxia; KC, Santosh; Wang, Jian; Hsu, Julia W. P.; Cho, Kyeongjae; Kim, Jiyoung
    Molecular-atomic layer deposition (MALD) is employed to fabricate hydroquinone (HQ)/diethyl zinc (DEZ) organic-inorganic hybrid semiconductor thin films with accurate thickness control, sharp interfaces, and low deposition temperature. Self-limiting growth is observed for both HQ and DEZ precursors. The growth rate remains constant at approximately 2.8 Å per cycle at 150°C. The hybrid materials exhibit n-type semiconducting behavior with a field effect mobility of approximately 5.7 cm² V⁻¹ s⁻¹ and an on/off ratio of over 103 following post annealing at 200°C in nitrogen. The resulting films are characterized using ellipsometry, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), UV-Vis spectroscopy, transistor behavior, and Hall-effect measurements. Density functional theory (DFT) and many-body perturbation theory within the GW approximation are also performed to assist the explanation and understanding of the experimental results. This research offers n-channel materials as valuable candidates for efficient organic CMOS devices. © 2016.
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    Charge Collection in Bulk Heterojunction Organic Photovoltaic Devices: An Impedance Spectroscopy Study
    (Amer Inst Physics) Xu, Liang; Lee, Yun-Ju; Hsu, Julia W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)
    Through thickness and applied bias variation, charge collection in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaic (OPV) devices was investigated with impedance spectroscopy. An equivalent circuit model incorporating chemical capacitance (Cμ), recombination resistance (R₂), and transport resistance (R₁) was used to analyze the results. Insufficient carrier extraction, exhibiting diffusion transport characteristics at high frequencies, was found in devices with a thick active layer. These devices also display a higher chemical capacitance, indicating greater carrier accumulation, and a lower recombination resistance, signaling increased bimolecular recombination. Increasing internal field with negative applied bias enhances carrier collection by reducing carrier accumulation and recombination. Moreover, we showed explicitly that charge collection can be quantified by (R₂/R₁)½, which is proportional to device fill factor. These results demonstrate that impedance spectroscopy is an effective tool for investigating charge collection in OPV devices.
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    Surface Photovoltage Characterization of Organic Photovoltaic Devices
    Lee, Y. -J; Wang, J.; Hsu, J. W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)
    Surface photovoltage response in bulk heterojunction organic solar cells is determined using a Kelvin probe with variable illumination intensity and wavelength. The effect of device architecture, carrier transport layers, donor:acceptor combinations, and device processing conditions are studied. We observe a positive (negative) surface photovoltage response, corresponding to efficient accumulation of electrons (holes) at the top electrode in conventional (inverted) devices. The linear relationship between surface photovoltage and log(intensity) and the agreement with open circuit voltage indicate that surface photovoltage magnitude quantifies the separation of photogenerated carriers in organic solar cells at open circuit condition.
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    Intensity and Wavelength Dependence of Bimolecular Recombination in P3HT:PCBM Solar Cells: A White-Light Biased External Quantum Efficiency Study
    Cowan, S. R.; Wang, J.; Yi, J.; Lee, Y. -J; Olson, D. C.; Hsu, Julia W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)
    Bimolecular recombination is often a major photogenerated charge carrier loss mechanism in organic photovoltaic (OPV) devices, resulting in lower fill factor (FF) compared to inorganic devices. The recombination parameter α can be obtained from the power law fitting of short-circuit current (J sc) on illumination intensity (I), J s c ∝ I , with α values less than unity taken as an indication of reduced photon-to-electron extraction efficiency and the presence of bimolecular recombination in OPV. Here, we show that this intensity-averaged measurement is inadequate. An external quantum efficiency (EQE) apparatus under constant white-light bias can be used to measure the recombination parameter (αEQE*) as a function of wavelength and carrier density (white-light intensity). Examining the dependence of α on background white-light bias intensity and excitation wavelength provides further understanding of photon-to-electron conversion loss mechanisms in P3HT:PCBM bulk heterojunction devices in standard and inverted architectures. In order to compare EQE and current-voltage (JV) measurements, we discuss the special case of devices exhibiting sub-linear intensity response (α <1). Furthermore, we demonstrate several important advantages of the white-light biased EQE method of measuring bimolecular recombination compared to existing methods, including sensitivity in probing intensity-dependent recombination compared to steady-state JV measurements, the correlation of αEQE and FF in devices, elucidation of recombination mechanisms through spectral dependence of carrier loss, and the robustness of αEQE obtained via integration over the entire absorption region. Furthermore, this technique for measuring recombination is immediately accessible to the vast majority of researchers as the EQE apparatus is ubiquitous in PV research laboratories.
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    In Situ Chemical Oxidation of Ultrasmall MoOx Nanoparticles in Suspensions
    (2012-07-23) Lee, Yun-Ju; Barrera, Diego; Luo, Kaiyuan; Hsu, Julia W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)
    Nanoparticle suspensions represent a promising route toward low cost, large area solution deposition of functional thin films for applications in energy conversion, flexible electronics, and sensors. However, parameters such size, stoichiometry, and electronic properties must be controlled to achieve best results for the target application. In this report, we demonstrate that such control can be achieved via in situ chemical oxidation of M o O 𝑥 nanoparticles in suspensions. Starting from a microwave-synthesized suspension of ultrasmall ( 𝑑 ∼ 2  nm) M o O 𝑥 nanoparticles in n-butanol, we added H2O2 at room temperature to chemically oxidize the nanoparticles. We systematically varied H2O2 concentration and reaction time and found that they significantly affected oxidation state and work function of MoO𝑥 nanoparticle films. In particular, we achieved a continuous tuning of MoO𝑥 work function from 4.4 to 5.0 eV, corresponding to oxidation of as-synthesized MoO𝑥 nanoparticle (20% Mo6+) to essentially pure MoO3. This was achieved without significantly modifying nanoparticle size or stability. Such precise control of MoO𝑥 stoichiometry and work function is critical for the optimization of MoO𝑥 nanoparticles for applications in organic optoelectronics. Moreover, the simplicity of the chemical oxidation procedure should be applicable for the development of other transition oxide nanomaterials with tunable composition and properties.