Browsing by Author "Quevedo-Lopez, Manuel A."
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Item Charge Transport and Device Physics in Fullerene-based Organic Photovoltaics(2021-08-01T05:00:00.000Z) Mosur Saravana Murthy, Lakshmi Narayanan; Hsu, Julia W. P; O, Kenneth K.; Vandenberghe, William G.; Quevedo-Lopez, Manuel A.; Young, Chadwin D.Organic photovoltaics (OPV) has been one of the consistently researched photovoltaic (PV) technology for the past three decades. Efficient charge generation, charge transport, and collection process account for higher performance in OPV devices. Several donor and nonfullerene (NFA) acceptors are developed to enhance charge generation. However, charge transport and collection still need critical understanding to further boost the device performance. In this dissertation, we focus on characterizing the defect states for efficient charge collection and charge transport mechanism in fullerene-based OPV devices. Surface photovoltage spectroscopy (SPS) was applied to probe the defect states without fabricating the complete devices. The physical location and the energetics of defect states are determined by comparing two types of SPS and top layer deposition. Understanding the charge transport mechanism is highly important in fullerene-based OPVs, where donor concentration is too low to form a percolation path to the anode. The effect of device architecture was studied to gain insights into the charge transport in fullerene-based OPV devices. From experimental results combined with drift-diffusion simulations, we found the imbalance in carrier mobility between electrons and holes results in inferior performance in inverted devices. Thienothiophene (TT)-based small molecule donors was designed and synthesized to study the photocurrent generation in fullerenebased OPVs. The donor and acceptor must form type-II energy level alignment at its interface for efficient exciton dissociation. We showed that the hole back transfers from donor to acceptor and transports to the anode via fullerene matrix. The photocurrent generation results from the hole back transfer mechanism in fullerene-based OPVs.Item The Effect of Poling Conditions on the Performance of Piezoelectric Energy Harvesters Fabricated by Wet Chemistry(Royal Soc Chemistry) Fuentes-Fernandez, Erika M. A.; Gnade, Bruce E.; Quevedo-Lopez, Manuel A.; Shah, Pradeep; Alshareef, H. N.; 00049719 (Gnade, BE); 6602171886 (Quevedo-Lopez, MA); Fuentes-Fernandez, Erika M. A.; Gnade, Bruce E.; Quevedo-Lopez, Manuel A.The effect of poling conditions on the power output of piezoelectric energy harvesters using sol-gel based Pb(Zr{0.53}, Ti{0.47})O{3}3-Pb(Zn{1/3}, Nb{2/3})O{3} piezoelectric thin-films has been investigated. A strong correlation was established between the poling efficiency and harvester output. A method based on simple capacitance-voltage measurements is shown to be an effective approach to estimate the power output of harvesters poled under different conditions. The poling process was found to be thermally activated with an activation energy of 0.12 eV, and the optimum poling conditions were identified (200 kV cm⁻¹, 250 ⁰C for 50 min). The voltage output and power density obtained under optimum poling conditions were measured to be 558 V cm⁻² and 325 μW cm⁻², respectively.Item Effects of Titanium Oxide Surface Properties on Bone-Forming and Soft Tissue-Forming Cells(Wiley) Wheelis, Sutton E.; Montaño-Figueroa, Ana Gabriela; Quevedo-Lopez, Manuel A.; Rodrigues, Danieli C.; 0000-0003-4068-9896 (Wheelis, SE); 0000-0002-0389-0833 (Rodrigues, DC); Wheelis, Sutton E.; Montaño-Figueroa, Ana Gabriela; Quevedo-Lopez, Manuel A.; Rodrigues, Danieli C.Background: Previous studies have concluded that certain titanium oxide (TiO₂) surface properties promote bone-forming cell attachment. However, no comprehensive studies have investigated the effects of TiO₂ surface and film morphology on hard and soft tissues. Purpose: The aim of this study is to understand the effects of TiO₂ morphology on the proliferation and differentiation of murine preosteoblasts (MC3T3-E1) and proliferation of human gingival fibroblasts (HGF-1) using in vitro experiments. Materials and Methods: Samples were fabricated with several TiO₂ thickness and crystalline structure to mimic various dental implant surfaces. in vitro analysis was performed for 1, 3, and 7 days on these samples to assess the viability of MC3T3-E1 and HGF-1 cells in contact with the modified oxide surfaces. Results: Results showed that HGF-1 cells exhibited no significant difference in viability on modified oxide surfaces versus a titanium control across experiments. MC3T3-E1 cells exhibited a significantly higher viability for the modified oxide surface in 1 day experiments, but not in 3 or 7 day experiments. Alkaline phosphatase expression in MC3T3-E1 was not significantly different on modified oxide surfaces versus the control across all experiments. A slight positive trend in viability was observed for cells in contact with rougher modified oxide surfaces versus a titanium control in both cell types. Conclusions: These observations suggest that crystallinity and thickness do not affect the long-term viability of hard or soft tissue cells when compared to a cpTi surface. Therefore, treatments like anodization on implant components may not directly affect the attachment of hard or soft tissue cells in vivo. © 2018 Wiley Periodicals, Inc.Item Evaluation of CdS Interfacial Layers in ZnO Nanowire/Poly(3-Hexylthiophene) Solar CellsMurphy, John Winter; Mejia, Israel; Gnade, Bruce E.; Quevedo-Lopez, Manuel A.; 0000 0003 8371 1336 (Gnade, BE); 00049719 (Gnade, BE)We prepare ZnO:poly(3-hexylthiophene) (P3HT) thin-film solar cells and ZnO nanowire:P3HT nanostructured solar cells and evaluate the effect of adding an interfacial layer between the ZnO and P3HT as a function of the nanowire height. We evaluate several different interlayers of CdS deposited, using two different chemical bath deposition (CBD) recipes. The height of the nanowire array is varied from a bilayer device with no nanowires up to arrays with a height of 2 mu m. We find that achieving a conformal coating of the ZnO with the interfacial layer is critical to improve device performance and that CBD can be used to grow conformal films on nonuniform surfaces.Item Layered Metal Dichalcogenides Thin Films Deposited by Pulse Laser Deposition(2017-12) Serna Villacis, Martha Isabel; Quevedo-Lopez, Manuel A.; Minary-Jolandan, MajidLayered metal dichalcogenides (LMDs) have outstanding intrinsic properties and are considered potential candidate electronic materials for emerging devices in the fields of optoelectronics, piezoelectronics, and lately in spintronics. However, for the implementation of these materials on large scale applications, new synthesis methods that enable deposition with controlled area and geometry are needed. This work explores Pulsed Laser Deposition (PLD), as a new technique for the development of layered dichalcogenides materials. In particular, MoS2, SnSe2, and MoSe2 deposition parameters are explored, using materials characterization techniques such as X-ray photoelectron spectroscopy, Atomic Force Microscopy, Transmission Electron Microscopy, Raman Spectroscopy, Rutherford Backscattering Spectrometry, X-Ray diffraction, and in-situ Residual Gas Analysis – Mass Spectrometry. The structure, morphology and chemistry of each of the thin films is used to estimate the quality of the thin films. In addition, electrical characterization such as Hall effect measurements and the first SnSe2 PLD-grown/Silicon junction diodes are demonstrated.Item New Applications for CDTE/CDS Heterojunctions: the Prospects of the Thin-Film JFET(2017-12) Avila Avendano, Jesus A.; 6602171886 (Quevedo-Lopez, MA); Young, Chadwin D.; Quevedo-Lopez, Manuel A.Junction Field Effect Transistors (JFETs) based on II-VI polycrystalline materials are expected to be faster than metal-insulator-semiconductor field-effect transistors (MISFETs) that use the same materials. JFET operation is not based on MIS capacitor theory of operation where generated traps at the oxide-polycrystalline material interface are known to adversely affect the performance of MIS transistors. Pulsed Laser Deposition (PLD) is a physical deposition technique that can transfer precise chemical stoichiometry of the targets onto the substrates. PLD has been demonstrated as an appropriate deposition method for JFET fabrication based in ZnO films. The control of the doping concentration and low leakage currents in PN junction are prerequisites for JFET fabrication. In this dissertation, CdTe/CdS diodes demonstrated low leakage current devices - on the order of nA. Also, CdTe and ZnTe have been doped with Cu, decreasing the resistivity. However, only a Cu doped ZnTe deposited by co-deposition method by PLD modulated the resistivity from 10⁶ to 10² Ω·cm. Simulation results showed that the leakage current values and the doping concentration achieves by PLD are suitable values for JFET fabrication. Masks were designed to fabricate JFETs using a photolithographic process flow. JFETs were fabricated using a Cu:ZnTe(p+)/CdTe(p)/CdS(n+) structure. However, the fabricated JFETs did not show proper behavior because the channel resistivity was not modulated by the gate voltage. Some experiments are proposed as possible solutions to fabricate a fully functioning thin-film JFET with Cu:ZnTe(p+)/CdTe(p)/CdS(n+) structure deposited by PLD.Item Optical Properties of P-Type SnOₓ Thin Films Deposited by DC Reactive Sputtering(Springer, 2018-11-23) Guzman-Caballero, D. E.; Quevedo-Lopez, Manuel A.; Ramirez-Bon, R.; Quevedo-Lopez, Manuel A.Refractive index (n), extinction coefficient (k), effective complex dielectric function (epsilon) and band gap energy (Eg) of p-type SnOₓ thin films from 0.75 to 4eV are studied. 25 nm thick films were deposited by direct current (DC) magnetron sputtering in reactive argon and oxygen atmosphere at different relative oxygen partial pressure (OPP) followed by a post annealing treatment at 250 °C in air atmosphere for 30 min. The relative high Hall effect mobility (μ) of the SnOₓ was attributed to the dominant SnO phase in films grown at 15% OPP. Films deposited at 5 and 11% OPP showed incomplete Sn oxidation resulting in a mixture of Sn and SnO phases with lower hole mobility. Optical transmittance (T) and reflectance (R) are described by assuming a model where the p-type SnOₓ films are defined by a dispersion formula based on a generalization of the Lorentz oscillator model. The roughness of the films (r) was modeled by a Bruggeman effective medium approximation (BEMA). From the optical analysis, k in the visible spectral region show high values for films with phase mixture, while films with single SnO phase presented negligible values. Films with single SnO phase have low n, this latter result from the lower compact microstructure of these films. Also, energies associated to direct and indirect transitions of the Brillouin zone of the SnOₓ films were identified from the evaluated epsilon. Finally, the increase in the values of Eg energy was related to the increase in the SnO phase.Item Performance and Stability of Solution-Based Cadmium Sulfide Thin Film Transistors: Role of CdS Cluster Size and Film CompositionSalas-Villasenor, A. L.; Mejia, I.; Sotelo-Lerma, M.; Gnade, Bruce E.; Quevedo-Lopez, Manuel A.; 0000 0003 8371 1336 (Gnade, BE); 00049719 (Gnade, BE)Improved carrier mobility and threshold voltage (VT) stability in cadmium sulfide (CdS) thin film transistors (TFTs) were studied and attributed to larger grain clusters in thicker CdS films rather than individual crystallite size. Non-zero VT shifts (∼200 mV) in thicker films are attributed to the presence of cadmium hydroxide [Cd(OH)2] at the dielectric/CdS interface resulting from the chemical bath deposition process used to deposit the CdS films. VT and mobility analyses indicate that clusters of CdS grains have a larger impact on TFT performance and stability than the presence of impurities in the bulk of the CdS. TFTs using this fabrication method achieved mobilities of ∼22 cm2/Vs with V T of 7 V and ΔVT of <200 mV after testing. The maximum processing temperature is 100°C which makes this process compatible with flexible substrates.Item Thin Film Cadmium Telluride Charged Particle Sensors for Large Area Neutron Detectors(Amer Inst Physics) Murphy, John W.; Smith, Lindsey; Calkins, J.; Mejia, Israel; Cantley, Kurtis D.; Chapman, Richard A.; Quevedo-Lopez, Manuel A.; Gnade, Bruce E.; 0000 0003 8371 1336 (Gnade, BE); 00049719 (Gnade, BE); C-2327-2008 (Gnade, BE)Thin film semiconductor neutron detectors are an attractive candidate to replace ³He neutron detectors, due to the possibility of low cost manufacturing and the potential for large areas. Polycrystalline CdTe is found to be an excellent material for thin film charged particle detectors−an integral component of a thin film neutron detector. The devices presented here are characterized in terms of their response to alpha and gamma radiation. Individual alpha particles are detected with an intrinsic efficiency of >80%, while the devices are largely insensitive to gamma rays, which is desirable so that the detector does not give false positive counts from gamma rays. The capacitance-voltage behavior of the devices is studied and correlated to the response due to alpha radiation. When coupled with a boron-based neutron converting material, the CdTe detectors are capable of detecting thermal neutrons.