Browsing by Author "Hsu, Julia W. P."
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Item 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.Item Charge Dynamics and Device Physics in Bulk-Heterojunction Organic Photovoltaics(2017-05) Xu, Liang; Hsu, Julia W. P.Organic photovoltaic (OPV) technology is still advancing towards commercialization with its potential for low-cost and light-weight flexible electricity generating applications. However, the lack of thorough understanding on the charge dynamics including charge generation, recombination, and transport processes still hinders the further boost of OPV device performance. The aim of my research is applying advanced characterization techniques to gain comprehensive understandings on charge dynamics in various OPV systems. Particularly the impedance spectroscopy was extensively applied as a powerful and non-destructive tool to study charge carrier collection and recombination. Meanwhile, time-resolved charge extraction measurement was performed as a complementary method for recombination study. External quantum efficiency spectroscopy was also carefully adapted to probe the device energetics. On the other hand, to better understand the experimental results, numerical drift-diffusion simulation and transfer matrix method modeling were carried out to provide theoretical guidance. With the combination of these techniques morphology effects, including the donor-acceptor interface and structural order, on recombination in OPV devices were thoroughly investigated. The photocurrent loss mechanisms in a novel fullerene-based OPV system were systematically and quantitatively analyzed. Moreover, the defect characterization using capacitance-frequency analysis was discovered to have severe artifacts when applied to OPV devices. In general, this research not only carefully addressed the charge dynamics in several OPV systems, but the multifaceted approaches it demonstrated also provide insights for future photovoltaic research.Item 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.Item Intensity and Wavelength Dependence of Bimolecular Recombination in P3HT:PCBM Solar Cells: A White-Light Biased External Quantum Efficiency StudyCowan, 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.Item Laterally Diffused Metal-oxide-semiconductor Field-effect Transistors: Device Design and Optimization for Low-voltage and Mid-voltage Power Applications(2022-05-01T05:00:00.000Z) Saadat, Ali; Iungo, Giacomo Valerio; Vandenberghe, William; Edwards, Hal; Hsu, Julia W. P.; Quevedo-Lopez, ManuelElectronics have become a fundamental aspect of modern society and smaller, even more efficient, transistors form the backbone of the electronics industry of the future. Laterally Diffused Metal-Oxide-Semiconductor (LDMOS) field-effect transistors are a class of transis- tors commonly used in every electronic device we use in our daily life such as smart-phone chargers, kitchen appliances, and autonomous vehicles. In this thesis, we present a systematic investigation of device design optimization for LDMOS transistors suitable for low-voltage (< 30 V) and mid-voltage (30 V – 100 V) power applications. We perform a numerical study as well as an analytical investigation to find the theoretical limits of LDMOS tran- sistors. We target critical transistor characteristics such as subthreshold leakage current, breakdown voltage, on-resistance, and figure-of-merit to perform a fundamental analysis. For low-voltage applications, we perform a channel length study to find the optimum chan- nel length, resulting in the minimum on-resistance, to achieve the minimum device size with maximum efficiency. We use a Technology Computer-Aided Design (TCAD) commercial drift-diffusion simulation package to perform our numerical study. Moreover, we develop an in-house computer code to automate the process of device design and optimization. For mid-voltage applications, we analytically and numerically investigate the performance of LDMOS transistors with different field-oxide configurations. We derive a new analytical relation between breakdown voltage and on-resistance quantifying the fundamental limits of the trade-off between resistance and current associated with the drift region in an LDMOS with field oxides. We find the optimized device characteristics, such as drift doping concen- tration, which minimizes the on-resistance. We finally verify our analytical findings with a large number of numerical simulations modeled in TCAD. We also present a first-principles investigation to discover novel two-dimensional dielectric materials used in two-dimensional transistors. We calculate the critical properties of dielectric materials such as in-plane and out-of-plane dielectric constants, bandgap energy, exfoliation energy, and equivalent oxide thickness.Item 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.Item 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).Item 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, JiyoungMolecular-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.Item 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.Item Processing Oxide Thin Films with Light: a Path to High-Throughput Solution-Processed Oxide Electronics(2019-12) Daunis, Trey Benjamin; 0000-0002-1368-8639 (Daunis, TB); Hsu, Julia W. P.High-throughput roll-to-roll manufacturing of solution-deposited metal oxide electronics has the potential to enable the widespread and low-cost availability of thin film electronic devices including solar panels, large-area displays and lighting, and flexible medical devices and sensors. However, several challenges have prevented the realization of this potential: (1) The processing speed of solution-deposited oxides is severely limited by the length of the annealing process, usually greater than 20 minutes, which is typically necessary to convert solution-deposited precursor films into metal oxides, (2) the quality of the resultant metal oxide films is limited by the low upper operating temperatures of low-cost, roll-to-roll compatible, plastic substrates, typically much less than 250 °C, and (3) equilibrium heating of devices to these temperatures often results in mechanical failure of the thin films due to the different thermal responses of the films and substrates. In this dissertation, we address these challenges by using light, rather than heat, as a source of energy for curing solution-deposited metal oxide thin films. We use this method to develop solution-deposited oxide thin film transistors (TFTs) on shape memory polymer (SMP) substrates. We demonstrate the direct patterning of Al₂O₃ and In₂O₃ precursor films on SMP by exposure to UV light through a shadow mask as a method to overcome the mechanical failure of blanket-coverage films during thermal annealing. The patterned precursors are then converted to oxides for the gate dielectric and the channel semiconductor of the TFTs by thermal annealing without causing damage to the films. The unexpectedly high mobility displayed by these TFTs is studied and the absorption of water from the atmosphere by the gate dielectric is identified as the cause. Finally, we demonstrate the high-speed photonic curing of ZrO₂ dielectric films on polyethylene naphthalate substrates. Using intense pulsed light to both heat and cure the oxide film in as little as 100 milliseconds without significantly heating the substrate, we achieve a process for fabricating metal oxide electronic devices that is compatible with roll-to-roll processing speeds exceeding 30 m/min.Item 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.Item Sources of Delectric Loss in Polypropylene Based Nanocomposites(2017-12) Womble, Michael; Hsu, Julia W. P.As data-rate communication speeds continue to increase to hundreds of GHz, low-loss polymer nanocomposites have emerged as a promising material for producing interconnects and waveguides capable of operating at these high frequency speeds with minimal loss in signal transmission. A major roadblock to increasing data-rate communications is a lack of low-loss interconnects, similar to metallic waveguides at the microwave regime, at these submillimeter-wave frequencies. Current coaxial cables are capable of operating speeds of 15 Gbps, but require high power consumption due to their high signal loss. Polymer nanocomposites, materials consisting of metal oxide nanoparticles in a polymer matrix, offer unique advantages over current waveguides. Higher permittivities than the pure polymer can be obtained while maintaining low loss of the polymer matrix. Furthermore, the dielectric properties can be tuned, depending on the choice of materials and the loading fraction of inorganic nanoparticles while retaining the processibility of polymers. In this work, each constituent’s role and its contribution to the overall dielectric loss of the nanocomposite across two frequency ranges, 100 Hz – 300 kHz and 140 – 220 GHz, will be discussed. We have achieved an effective permittivity of 6.84 with loss tangent of 0.0049 at 220 GHz in a 21.5vol% TiO2/polypropylene nanocomposite. The role of polypropylene-graft-maleic anhydride as a compatibilizer, and its effects on nanoparticle distribution and as a source of dielectric loss will be presented. Furthermore, the effects of TiO2 nanoparticle size on the effective permittivity and loss in polypropylene is investigated. Introducing complex permittivity into Lichtenecker’s model, we are able to separate the loss contributions from either the polymer matrix or the TiO2 nanoparticles to the measured loss in the nanocomposites. Finally, future areas of study and routes for reducing loss in polymer nanocomposites are introduced.Item 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.Item 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.Item Synthesis and Characterization of Transition Metal Oxides and Dichalcogenides and Their Application in Organic Photovoltaics(2017-05) Barrera Mendez, Diego; Hsu, Julia W. P.This Ph.D. research focused on the development of new materials for alternative renewable energy using organic photovoltaics (OPVs). The first step was to established reliable fabrication and characterization methods of organic photovoltaic devices. The reproducibility of organic photovoltaic cell performance is one of the essential issues that must be achieved before engaging serious investigations of the applications of creative and challenging ideas. Secondly, we thoroughly studied the surface chemistry of the underlying layer and its critical role on the morphology of the BHJ active layer. We showed that when the active layer (which consists of blends of poly(3-hexylthiophene) (P3HT) and phenyl-C60-butyric acid methyl ester (PCBM)) is deposited and annealed over a sol-gel ZnO electron transport layer surface made from monoethanolamine (MEA) containing precursor, PCBM clusters form during annealing and this phase segregation leads to a drastic reduction of OPV parameters due to both low charge generation and high bimolecular recombination. Rinsing the pyrolyzed ZnO films with solvents or using a ZnO recipe without MEA significantly reduced the formation of PCBM clusters and produced devices with good performance. Third, we developed new materials suitable for low-temperature processing and large-area deposition methods to be used as transport layer on OPVs. We achieved the synthesis of MoOx suspensions suitable for large area deposition, with controlled size, stoichiometry, and electronic properties using controlled oxidative dissolution of organometallic powders with H2O2 in n-butanol. The small nanoparticle diameters of ~ 2 nm enabled solution processing of nanoparticle films on ITO with electronic properties comparable to solution processed and vacuum deposited counterparts, without the need for any post processing. We also accomplished the synthesis of transition metal dichalcogenides (TMDs) directly from precursors in solution using a versatile synthesis method. We demonstrate the ability to synthesize few-layer (~ 2 nm) MoS2, MoSe2, WS2, and WSe2 flakes with relatively large lateral sizes (> 2 µm) using a solvothermal method. We demonstrate that the reducing agent, 1,2-hexadecanediol is critical to ensure TMD formation and eliminate corresponding metal oxide. TEM, Raman, PESA, and Kelvin Probe measurements confirmed that all TMDs are p-type, highly crystalline, exhibit 2H phase and present hexagonal crystalline structure. In addition, thickness for all TMDs was consistent with a few-layer flakes. Finally, we studied the use of films spray casted from liquid-exfoliated MoS2 suspensions as hole transport layer for OPVs. Electrical measurements on the devices showed that FF achieved using MoS2 is identical to that using spin-coated polymeric reference material. Calculations showed that the lower Jsc observed in MoS2 devices is explained by reduced light absorption in the active layer region due to less back-reflected light in MoS2 devices. With all these results we have contributed to enable a route towards low-cost OPV and other electronics fabrication.Item 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.