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Devižis, Andrius. "Charge carrier transport in conjugated polymer films revealed by ultrafast optical probing." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2011. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2011~D_20110222_154904-88738.
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Conjugated polymers are promising candidates for applications in all kinds of organic optoelectronic devices: OLEDs, organic field-effect transistors (OFETs) and organic photovoltaic cells. The main goal of this work was to investigate transport features of photogenerated electrical charge in pi-conjugated polymers by means of novel technique based on time-resolved electric field-induced second harmonic generation (TREFISH). TREFISH measurement setup was implemented in the laboratory of Molecular compounds physics, and applicability of the method has been verified. Measurements were performed on three different model polymers: methyl substituted ladder-type poly(para-phenylene) (MeLPPP), poly(fluorene-co-benzothiadiazole) (F8BT) and poly(spirobifluorene-co-benzothiadiazole) (PSF-BT), having different morphological and chemical structure.
It has been found that motion of photogenerated charge carriers in π-conjugated polymer films experiences rapid dynamics after excitation. Different time domains of charge transport were distinguished. Initial fast transport of photogenerated charge carriers corresponds to the carrier motion along the single polymer chain or conjugated segment of the polymer chain. Slowest carrier motion phase is well described by the stochastic drift, which is attributed to interchain jumps and determines the macroscopic equilibrium mobility. Thus, the equilibrium mobility value is not applicable to the transport on nanometer scale up to tens of nanometers... [to full text]Konjuguotieji polimerai kaip funkcinės medžiagos gali būti panaudoti įvairiuose prietaisuose: organiniuose šviestukuose, organiniuose lauko tranzistoriuose, organiniuose saulės elementuose. Šio darbo tikslas - nustatyti fotogeneruotų krūvininkų pernašos dėsningumus π – konjuguotuose polimeruose panaudojant naują žadinimo-zondavimo metodą pagrįstą išoriniu elektriniu lauku indukuota antrosios optinės harmonikos generacija. Pagrindinis dėmesys buvo skiriamas pernašos dinamikai. Molekulinių darinių fizikos laboratorijoje buvo įrengta matavimų schema ir įvertintas metodo tinkamumas krūvio pernašos tyrimams. Buvo atlikti krūvio pernašos matavimai trijuose skirtinguose konjuguotuosiuose polimeruose. Nustatyta, kad fotogeneruotų krūvininkų judris tuoj po sužadinimo yra daug didesnis lyginant su stacionaria judrio verte, o krūvio pernašos dinamiką lemia konjuguoto polimero struktūrinė hierarchija, krūvininkų judėjimas yra daugialypis, susidedantis iš greito judėjimo viena polimero grandine ar konjuguotais polimero grandinės segmentais ir lėto šokavimo tarp atskirų polimero grandinių Pirmą kartą detaliai išnagrinėta šviesa sugeneruotų krūvininkų pernašos dinamika konjuguotuose polimeruose. Darbo rezultatai suteikia žinių apie fundamentalius krūvininkų pernašos mechanizmus konjuguotuose polimeruose, kurios gali būti panaudotos kuriant organinius elektronikos prietaisus.
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Fonari, Alexandr. "Theoretical description of charge-transport and charge-generation parameters in single-component and bimolecular charge-transfer organic semiconductors." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/54323.
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In this dissertation, we employ a number of computational methods, including Ab Initio, Density Functional Theory, and Molecular Dynamics simulations to investigate key microscopic parameters that govern charge-transport and charge-generation in single-component and bimolecular charge-transfer organic semiconductors.
First, electronic (transfer integrals, bandwidths, effective masses) and electron-phonon couplings of single-component organic semiconductors are discussed. In particular, we evaluate microscopic charge-transport parameters in a series of nonlinear acenes with extended pi-conjugated cores. Our studies suggest that high charge-carrier mobilities are expected in these materials, since large electronic couplings are obtained and the formation of self-localized polarons due to local and nonlocal electron-phonon couplings is unlikely. Next, we evaluate charge detrapping due to interaction with intra-molecular crystal vibrations in order to explain changes in experimentally measured electric conductivity generated by pulse excitations in the IR region of a photoresistor based on pentacene/C60 thin film. Here, we directly relate the nonlocal electron-phonon coupling constants with variations in photoconductivity.
In terms of charge-generation from an excited manifold, we evaluate the modulation of the state couplings between singlet and triplet excited states due to crystal vibrations, in order to understand the effect of lattice vibrations on singlet fission in tetracene crystal. We find that the state coupling between localized singlet and correlated triplet states is much more strongly affected by the dynamical disorder due to lattice vibrations than the coupling between the charge-transfer singlet and triplet states.
Next, the impact of Hartree-Fock exchange in the description of transport properties in crystalline organic semiconductors is discussed. Depending on the nature of the electronic coupling, transfer integrals and bandwidths can show a significant increase as a function of the amount of the Hartree-Fock exchange included in the functional. Similar trend is observed for lattice relaxation energy. It is also shown that the ratio between electronic coupling and lattice relaxation energy is practically independent of the amount of the Hartree-Fock exchange, making this quantity a good candidate for incorporation into tight-binding transport models. We also demonstrate that it is possible to find an amount of the Hartree-Fock exchange that recovers (quasi-particle) band structure obtained from a highly accurate G0W0 approach. Finally, a microscopic understanding of a phase transition in charge-carrier mobility from temperature independent to thermally activated in stilbene-tetrafluoro-tetracyanoquinodimethane crystal is provided.
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Pokhrel, Chandra Prasad. "Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors." [Kent, Ohio] : Kent State University, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=kent1254234736.
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Thesis (Ph.D.)--Kent State University, 2009.Title from PDF t.p. (viewed April 1, 2010). Advisor: Brett Ellman. Keywords: Laser; Charge generation; Charge transport; Mobility; Trapping; Space charge; Hopping; Tunneling; Lattice vibration; Exciton; Polaron; HUMO; LUMO; Action Spectrum; Quantum efficiency; Crystal Growth; Liquid crystal; Disordered medium. Includes bibliographical references.
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Ho, Carr Hoi Yi. "Toward better performing organic solar cells: impact of charge carrier transport and electronic interactions in bulk heterojunction blends /Ho Hoi Yi, Carr." HKBU Institutional Repository, 2017. https://repository.hkbu.edu.hk/etd_oa/359.
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Organic photovoltaic (OPV) is an exciting energy harvesting technique. Although its power conversion efficiency (PCE) now exceeds 10% in a research laboratory, the processing window of an OPV cell is still narrow. A fundamental understanding of the OPV materials is desired. This thesis presents the charge carrier transport properties and electronic interactions in the bulk heterojunction (BHJ) active layer of OPV cells. They were found to be well correlated with OPV device performances. Space-charge-limited current (SCLC) measurements and admittance spectroscopy (AS) were employed to study the charge transports, while photothermal deflection spectroscopy (PDS) was used to probe the trap densities inside the materials. Beneficial effects of a common solvent additive, 1,8-diiodooctance (DIO), on PTB7:PC71BM OPV cells have been investigated. With DIO present in the casting solution, the resulting BHJ films have much enhanced electron mobilities, whereas the impact on the hole mobility is negligible. The origin of increased electron mobility is the reduced average electron hopping distance for those films prepared with DIO solvent additive. A balance of hole-electron mobility by tuning the DIO concentration was demonstrated to be the way to optimize the OPV device performance. In light of carrier transport measurement results, a "polymer-rich" strategy with preserved device performance was demonstrated. After understanding the importance of balanced hole-electron mobility, the impact of donor-acceptor weight ratio on the performance of PTB7 : PC71BM based OPV cells was explored. Early stage electronic donor-acceptor interactions were revealed using ultra-low dosages of fullerenes. Before electron transport pathways percolate, the unconnected fullerene domains act as traps and hinder electron transport. From PDS, the trap density observed inside BHJ films was found to be anti-correlated with the fill factor of OPV devices. The origin of low FFs is mainly due to electron traps and localized states from fullerenes. Based on the observations, it is proposed that PC71BM tends to intercalate with PTB7 backbone instead of forming self-aggregates before the electron pathway percolation. Apart from investigating the fundamentals in OPV devices, a solution to improve its processing window was proposed in this thesis. Thermally stable polymer : fullerene OPV cells were fabricated by employing fluorenone-based solid additives. A charge transfer interaction between the additives and donor moiety of polymer formed a locked network which freezes the BHJ morphology under thermal stress. The most promising result retains 90% of the origin efficiency, upon thermal aging at 100 °C for more than 20 hours in PTB7:PC71BM solar cells. Besides fullerene-based OPV, all-polymer photovoltaic solar cells (all-PSCs) were also investigated. Two new difluorobenzene-naphthalene diimide based polymer electron acceptors, one random (P1) and one regioregular (P2) structure, were compared. P2 exhibited a much better molecular packing, a higher electron mobility and more balanced hole-electron mobilities in its composite film with polymer donor, PTB7-Th. An optimized PTB7-Th:P2 device can achieve a respectably high PCE over 5% for all-PSC devices. These all-PSCs should open a new avenue for next generation OPVs.
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Lörtscher, Emanuel Marc. "Charge-carrier transport measurements through single molecules." Göttingen Cuvillier, 2006.
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Limketkai, Benjie 1982. "Charge-carrier transport in amorphous organic semiconductors." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/43063.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 101-106).
Since the first reports of efficient luminescence and absorption in organic semiconductors, organic light-emitting devices (OLEDs) and photovoltaics (OPVs) have attracted increasing interest. Organic semiconductors have proven to be a promising material set for novel optical and/or electrical devices. Not only do they have the advantage of tunable properties using chemistry, but organic semiconductors hold the potential of being fabricated cheaply with low temperature deposition on flexible plastic substrates, ink jet printing, or roll-to-roll manufacturing. These fabrication techniques are possible because organic semiconductors are composed of molecules weakly held together by van der Waals forces rather than covalent bonds. Van der Waals bonding eliminates the danger of dangling bond traps in amorphous or polycrystalline inorganic films, but results in narrower electronic bandwidths. Combined with spatial and energetic disorder due to weak intermolecular interactions, the small bandwidth leads to localization of charge carriers and electron-hole pairs, called excitons. Thus, the charge-carrier mobility in organic semiconductors is generally much smaller than in their covalently-bonded, highly-ordered crystalline semiconductor counterparts. Indeed, one major barrier to the use of organic semiconductors is their poor charge transport characteristics. Yet this major component of the operation of disordered organic semiconductor devices remains incompletely understood. This thesis analyzes charge transport and injection in organic semiconductor materials. A first-principles analytic theory that explains the current-voltage characteristics and charge-carrier mobility for different metal contacts and organic semiconductor materials over a wide range of temperatures, carrier densities, and electric field strengths will be developed.
(cont) Most significantly, the theory will enable predictive models of organic semiconductor devices based on physical material parameters that may be determined by experimental measurements or quantum chemical simulations. Understanding charge transport and injection through these materials is crucial to enable the rational design for organic device applications, and also contributes to the general knowledge of the physics of materials characterized by charge localization and energetic disorder.
by Benjie N. Limketkai.
Ph.D.
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Limketkai, Benjie 1982. "Charge carrier transport in amorphous organic semiconductors." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/87446.
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Davies, Andrew Wynn. "Charge carrier transport in THM grown CdTe." Thesis, University of Surrey, 2008. http://epubs.surrey.ac.uk/843369/.
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Cadmium telluride (CdTe) has long been regarded as a good material for the use as a solid state ionising chamber. Since the advent of well controlled liquid phase growing processes, such as the traveling heater method (THM), single crystal CdTe has been available. To improve detection processes further one must understand the parameters effecting charge transport, especially the role of crystal defects (such as the prevalent tellurium inclusion) in deviating the behavior from the 'ideal' crystal. This work focuses on induced charge and current pulses studied as a function of temperature and with spatial resolution in single crystal CdTe grown by EURORAD. The electron and hole mu product is individually derived for each piece of material as is the electron and hole mobility. These values are all within the range of accepted values for good quality spectroscopic grade material given in the literature. To complement the electrical characterisation several non destructive optical methods are carried out. These include Lang X-ray topography and infrared microscopy, both of which allow the structural quality of the material to be assessed. From these measurements it is clear that we are in possession of several samples with a high density of tellurium inclusions. The effect of these defects on charge transport is observed using time resolved ion beam induced charge (IBIC) imaging. Temperature dependent mobility is investigated and results are compared to a model of scattering mechanism in CdTe. Voltage dependent trapping/detrapping is also observed leading to the conclusion that a multiple trapping/emission process is taking place and finally electric field profiling is undertaken and the results compared to other techniques given in the literature.
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Schoefer, Sebastian. "High charge-carrier density transport studies in pBTTT." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610581.
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Smith, Christian W. "A study of charge carrier transport in graphene nanoribbons." Honors in the Major Thesis, University of Central Florida, 2010. http://digital.library.ucf.edu/cdm/ref/collection/ETH/id/1496.
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This item is only available in print in the UCF Libraries. If this is your Honors Thesis, you can help us make it available online for use by researchers around the world by following the instructions on the distribution consent form at http://library.ucf.edu/Systems/DigitalInitiatives/DigitalCollections/InternetDistributionConsentAgreementForm.pdf You may also contact the project coordinator, Kerri Bottorff, at kerri.bottorff@ucf.edu for more information.Bachelors
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Pahner, Paul. "Charge Carrier Trap Spectroscopy on Organic Hole Transport Materials." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-217882.
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Electronic circuits comprising organic semiconductor thin-films are part of promising technologies for a renewable power generation and an energy-efficient information technology. Whereas TV and mobile phone applications of organic light emitting diodes (OLEDs) got ready for the market awhile ago, organic photovoltaics still lack in power conversion efficiencies, especially in relation to their current fabrication costs. A major reason for the low efficiencies are losses due to the large number of charge carrier traps in organic semiconductors as compared to silicon. It is the aim of this thesis to identify and quantify charge carrier traps in vacuum-deposited organic semiconductor thin-films and comprehend the reasons for the trap formation. For that, the techniques impedance spectroscopy (IS), thermally stimulated currents (TSC), and photoelectron spectroscopy are utilized. In order to assess the absolute energy of charge carrier traps, the charge carrier transport levels are computed for various hole transport materials such as MeO-TPD, pentacene, and ZnPc. Unlike inorganics, organic semiconductors possess in first-order approximation Gaussian distributed densities of states and temperaturedependent transport levels. The latter shift by up to 300 meV towards the energy gap-mid when changing from room temperature to 10 K as it is done for TSC examinations. The frequency-dependent capacitance response of charge carrier traps in organic Schottky diodes of pentacene and ZnPc are studied via impedance spectroscopy. In undoped systems, deep traps with depths of approx. 0.6 eV and densities in the order of 1016...1017 cm−3 are prevailing. For pentacene, the deep trap density is reduced when the material undergoes an additional purification step. Utilizing p-doping, the Fermi level is tuned in a way that deep traps are saturated. Vice versa, the freeze-out of p-doped ZnPc provides further insight into the influence of trap-filling, impurity saturation and reserve on the Fermi level position in organic semiconductors. Furthermore, charge carrier traps are investigated via thermally stimulated currents. It is shown that the trap depths are obtained correctly only if the dispersive transport of the released charge carriers until their extraction is considered. For the first time, the polarity of charge carrier traps in MeO-TPD, ZnPc, and m-MTDATA is identified from TSC’s differences in release time when spacer layers are introduced in the TSC samples. Simultaneously, tiny hole mobilities in the order of 10−13 cm2 Vs−1 are detected for low-temperature thin-films of the hole transporter material Spiro-TTB. It is shown for Spiro-TTB co-evaporated with the acceptor molecule F6-TCNNQ and a p-doped ZnPc:C60 absorber blend that the doping process creates shallow trap levels. Finally, various organic hole transport materials are examined upon their stability in water and oxygen atmosphere during sample fabrication and storage of the organic electronics. In case of pentacene, ZnPc, MeO-TPD, and m-MTDATA, hole traps are already present in unexposed thin-films, which increase in trap density upon oxygen exposure. A global trap level caused by oxygen impurities is found at energies of 4.7...4.8 eV that is detrimental to hole transport in organic semiconductorsElektronische Bauelemente aus Dünnschichten organischer Halbleiter sind Teil möglicher Schlüsseltechnologien zur regenerativen Energiegewinnung und energieeffizienten Informationstechnik. Während Fernseh- und Mobilfunkanwendungen organischer Leuchtdioden (OLEDs) bereits vor einiger Zeit Marktreife erlangt haben, ist die organische Photovoltaik (OPV) noch durch zu hohe Fertigungskosten in Relation zu unzureichenden Effizienzen unrentabel. Ein wesentlicher Grund für die niedrigen Wirkungsgrade sind Verluste durch die im Vergleich zu Silizium hohe Zahl an Ladungsträgerfallen in organischen Halbleitern. Ziel dieser Arbeit ist es, mittels Impedanz-Spektroskopie (IS), thermisch stimulierten Strömen (TSC) und Photoelektronenspektroskopie methodenübergreifend Ladungsträgerfallen in vakuumverdampften organischen Dünnschichten zu identifizieren, zu quantifizieren und ihre Ursachen zu ergründen. Um die Energie von Ladungsträgerfallen absolut beziffern zu können, wird zunächst für verschiedene Lochtransportmaterialien wie z.B. MeO-TPD, Pentazen und ZnPc die Transportenergie aus den in erster Ordnung gaußförmigen Zustandsdichten berechnet. Im Gegensatz zu anorganischen Halbleitern ist die Transportenergie in organischen Halbleitern temperaturabhängig. Sie verschiebt sich beim Übergang von Raumtemperatur zu 10 K, wie für TSC Untersuchungen bedeutsam, um bis zu 300 meV in Richtung der Bandlückenmitte. Mittels Impedanz-Spektroskopie wird die frequenzabhängige Kapazitätsantwort von Ladungsträgerfallen in organischen Schottky-Dioden aus Pentazen und ZnPc untersucht. In undotierten Systemen dominieren Defekte mit Tiefen um 0.6 eV, deren Dichte in der Größenordnung von 1016...1017 cm−3 liegt, sich aber im Fall von Pentazen durch einen zusätzlichen Materialaufreinigungsschritt halbieren lässt. Über p-Dotierung wird das Fermi-Level so eingestellt, dass tiefe Fallen abgesättigt werden können. Umgekehrt liefert das Ausfrieren von p-dotiertem ZnPc weitere Belege für den Einfluss von Fallenzuständen, Störstellen-Erschöpfung und Reserve auf das Fermi-Level in dotierten organischen Halbleitern. Im Weiteren werden Ladungsträgerfallen über thermisch stimulierte Ströme untersucht. Es wird gezeigt, dass die Fallentiefen nur dann konsistent bestimmt werden, wenn der dispersive Transport von freigesetzten Ladungsträgern zur Extraktionsstelle berücksichtigt wird. Durch Einführung von ’Abstandshalterschichten’ werden erstmalig über TSC die Polaritäten von Ladungsträgerfallen in MeO-TPD, ZnPc und m-MTDATA per Laufzeitunterschied bestimmt. Gleichzeitig werden geringste Löcherbeweglichkeiten in der Größenordnung von 10−13 cm2 Vs−1 für stark gekühlte Dünnschichten des Lochtransporters Spiro-TTB gemessen. Wie für Spiro-TTB koverdampft mit dem Akzeptormolekül F6-TCNNQ und p-dotierte Mischschichten der Absorbermaterialien ZnPc und C60 gezeigt, erzeugt Dotierung relativ flache Störstellen. Abschließend werden verschiedene organische Lochtransporter-Materialien auf ihre Stabilität in Wasser- und Sauerstoffatmosphären während der Prozessierung und der Lagerung fertiger elektronischer Bauelemente untersucht. Für Pentazen, ZnPc, MeO-TPD und m-MTDATA werden Löcherfallen in intrinsischen Dünnschichten nachgewiesen. Bei Kontakt mit Sauerstoff nimmt deren Defektdichte zu. Es findet sich ein universales Fallenniveau bei rund 4.7...4.8 eV, verursacht durch Sauerstoffverunreinigungen, welches den Lochtransport in organischen Halbleitern limitiert
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Luber, David R. "Direct imaging of minority charge carrier transport in luminescent semiconductors." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2005. http://library.nps.navy.mil/uhtbin/hyperion/05Sep%5FLuber.pdf.
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Thesis (M.S. in Space Systems Operations and M.S. in Applied Physics)--Naval Postgraduate School, September 2005.Thesis Advisor(s): Nancy M. Haegel, Sherif Michael. Includes bibliographical references (p.55-56). Also available online.
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Hauff, Elizabeth von. "Field effect investigations of charge carrier transport in organic semiconductors." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=981450210.
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Leitch, James R. "The study of charge carrier transport in the semiconductor lattice." Thesis, University of Surrey, 2002. http://epubs.surrey.ac.uk/773032/.
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This thesis explores the physical origins of noise in semiconductors. A novel method of analysing the electron and hole carrier flow in the junction is developed, and the results are analysed with the objective of comparing the model against physical measurements made on actual semiconductor junctions. The current state of the art for commercial device fabrication is currently about 100nm device length, and new approaches to modelling device behaviour and noise mechanisms are required as the technology shrinks. In addition higher order transport models are needed due to smaller transistor sizes, and the need to account for current crowding effects in high current density situations. The various known forms of noise are reviewed, and prioritised in terms of their contribution to a pre-selected demonstration vehicle, in this case a BJT, and their contribution to the BJT's overall noise figure. The alternative methods of analysing noise are considered, and compared against the objective of reducing noise in this selected semiconductor structure. Many simulation techniques are available offering 1, 2 and pseudo three dimensional (21/2-D) approaches. The continuing trend to reduce the dimensions of the active devices requires more accurate models, and so more detailed physical correlation, especially at higher current densities. The models should be able to run as quickly and efficiently as possible with this increased complexity. The minority carrier based operation of the Bipolar Junction Transistor or BJT is selected, and this is used as an example in the model, with the intention of validating the model against a mature and very predictable technology. The model is also validated using well known and established simulation methods and then the model is applied to evaluate new structures. in order to propose a low noise BJT or LNBJT. The dominant noise in BJT devices is shown to be due to shot noise at mid to high frequencies. The noise mechanism known as shot noise. has been assumed in the past to be a fundamental noise form that limits the noise performance of bipolar structures. This work develops and demonstrates a method for modelling the carrier transport in theory of devices down to geometries of 15nm between the emitter and collector, using a true 3D simulation model called JAMES (Junction Atomistic Modelling of Extrinsic Semiconductors), based on a multi-carrier model, and able to model a large number of carriers in realistic simulation times. The thesis then goes on to develop the model for studying the nature of the individual carrier flow, and proposes a physically consistent explanation of electrical shot noise, and the carrier transport intrinsic in the BJT current flow. The work demonstrates the use of the model to redesign the bipolar junction transistor, and by modifying its doping structure and biasing, the spectral density of the shot noise Sv can be reduced by -26.4dB at frequencies approaching DC, and greater than -lO.75dB reduction at IGhz. The mechanism used to reduce the shot noise is to re-thermalise the minority carriers which have been grouped into "noise quanta", by introducing a semiconductor region to return the individual carriers to a classically chaotic state before they reach the collector. This new proposed structure is a dual base LNBJT capable of not only reducing the shot noise and total device noise, but also increasing the AC voltage gain, and increasing the transition frequency when it is used as a switch. A proposed new constant will be introduced, referred to as KSA, defined as the ratio of reduction in the spectral density as a result of adding a new structure. The work offers as one result, a design methodology to minimise the impact of noise in integrated and discrete BJT's. The noise attenuation mechanism proposed can also reduce other noise forms however, but to a smaller extent. The mechanism proposed here is effective in attenuating shot noise because the noise quanta given by Q = n.q is relatively small, where q is the carrier charge, and n is the number of charge carriers in the quanta. Low frequency noise forms can have a larger noise quanta, and so the attenuation of these forms is therefore smaller.
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Choi, Wing Hong. "Carrier transport characterization and divice applications of amorphous organic semiconductors." HKBU Institutional Repository, 2010. http://repository.hkbu.edu.hk/etd_ra/1093.
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Michel, Christoph. "Theoretical studies of spin dependent transport phenomena [transport in magnetic semiconductors ; spin dependent charge carrier recombination]." Göttingen Cuvillier, 2007.
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Schober, Matthias. "Charge Transport in Organic Light-Emitting Diodes." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-100071.
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This thesis is about the development and validation of a numerical model for the simulation of the current-voltage characteristics of organic thin-film devices. The focus is on the analysis of a white organic light-emitting diode (OLED) with fluorescent blue and phosphorescent red and green emitters. The simulation model describes the charge transport as a one-dimensional drift-diffusion current and is developed on the basis of the Scharfetter-Gummel method. It incorporates modern theories for the charge transport in disordered organic materials, which are considered by means of special functions for the diffusion coefficient and the charge-carrier mobility. The algorithm is designed such that it can switch between different models for mobility and calculates both transient and steady-state solutions. In the analysis of the OLED, electron and hole transport are investigated separately in series of single-carrier devices. These test devices incorporate parts of the layers in the OLED between symmetrically arranged injection layers that are electrically doped. Thereby, the OLED layer sequence is reconstructed step by step. The analysis of the test devices allows to obtain the numerous parameters which are required for the simulation of the complete OLED and reveals many interesting features of the OLED.
For instance, it is shown how the accumulation of charge carriers in front of an interface barrier increases the mobility and the transfer rate across the interface. Furthermore, it is demonstrated how to identify charge-trapping states. This leads to the detection of deep trap states in the emission zone of the OLED -- an interesting aspect, since these states can function as recombination centers and may cause non-radiative losses. Moreover, various other effects such as interface dipoles and a slight freeze-out of active electric dopants in the injection layers are observed. In the simulations of the numerous test devices, the parameters are consistently applied. Thereby, the agreement between simulation and experiment is excellent, which demonstrates the correctness and applicability of the developed model. Finally, the complete OLED is successfully simulated on the basis of the parameters that have been obtained in the analysis of the single-carrier devices. The simulation of the OLED illustrates the transport levels of electrons and holes, and proofs that the OLED efficiency is low because of non-radiative recombination in the interlayer between the phosphorescent and fluorescent emission zones. In this context, many interesting issues are discussed, e.g. the applicability of the Langevin model in combination with the mobility models for the description of recombination and the relevance of interactions between free charge carriers and excitons.
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Tackley, Daniel Roger. "A Raman, infrared and computational study of electrophotographic charge generation and charge transport materials." Thesis, University of Strathclyde, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.248309.
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Mills, Ted Jonathan. "Direct imaging of minority charge carrier transport in triple junction solar cell layers." Thesis, Monterey, Calif. : Naval Postgraduate School, 2006. http://bosun.nps.edu/uhtbin/hyperion.exe/06Dec%5FMills.pdf.
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Thesis (M.S. in Applied Physics)--Naval Postgraduate School, December 2006.Thesis Advisor(s): Nancy M. Haegel, Sherif Michael. "December 2006." Includes bibliographical references (p. 63-64). Also available in print.
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Fischer, Janine. "Density of States and Charge Carrier Transport in Organic Donor-Acceptor Blend Layers." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-184493.
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In the last 25 years, organic or "plastic" solar cells have gained commercial interest as a light-weight, flexible, colorful, and potentially low-cost technology for direct solar energy conversion into electrical power. Currently, organic solar cells with a maximum power conversion effciency (PCE) of 12% can compete with classical silicon technology under certain conditions. In particular, a variety of strongly absorbing organic molecules is available, enabling custom-built organic solar cells for versatile applications. In order to improve the PCE, the charge carrier mobility in organic thin films must be improved. The transport characterization of the relevant materials is usually done in neat layers for simplicity. However, the active layer of highly efficient organic solar cells comprises a bulk heterojunction (BHJ) of a donor and an acceptor component necessary for effective charge carrier generation from photo-generated excitons. In the literature, the transport properties of such blend layers are hardly studied. In this work, the transport properties of typical BHJ layers are investigated using space-charge limited currents (SCLC), conductivity, impedance spectroscopy (IS), and thermally stimulated currents (TSC) in order to model the transport with numerical drift-diffusion simulations. Firstly, the influence of an exponential density of trap states on the thickness dependence of SCLCs in devices with Ohmic injection contacts is investigated by simulations. Then, the results are applied to SCLC and conductivity measurements of electron- and hole-only devices of ZnPc:C60 at different mixing ratios. Particularly, the field and charge carrier density dependence of the mobility is evaluated, suggesting that the hole transport is dominated by exponential tail states acting as trapping sites. For comparison, transport in DCV5T-Me33:C60, which shows better PCEs in solar cells, is shown not to be dominated by traps. Furthermore, a temperature-dependent IS analysis of weakly p-doped ZnPc:C60 (1:1) blend reveals the energy-resolved distribution of occupied states, containing a Gaussian trap state as well as exponential tail states. The obtained results can be considered a basis for the characterization of trap states in organic solar cells. Moreover, the precise knowledge of the transport-relevant trap states is shown to facilitate modeling of complete devices, constituting a basis for predictive simulations of optimized device structuresOrganische oder "Plastik"-Solarzellen haben in den letzten 25 Jahren eine rasante Entwicklung durchlaufen. Kommerziell sind sie vor allem wegen ihres geringen Gewichts, Biegsamkeit, Farbigkeit und potentiell geringen Herstellungskosten interessant, was zukünftig auf spezielle Anwendungen zugeschnittene Solarzellen ermöglichen wird. Die Leistungseffzienz von 12% ist dabei unter günstigen Bedingungen bereits mit klassischer Siliziumtechnologie konkurrenzfähig. Um die Effzienz weiter zu steigern und damit die Wirtschaftlichkeit zu erhöhen, muss vor allem die Ladungsträgerbeweglichkeit verbessert werden. In organischen Solarzellen werden typischerweise Donator-Akzeptor-Mischschichten verwendet, die für die effziente Generation freier Ladungsträger aus photo-induzierten Exzitonen verantwortlich sind. Obwohl solche Mischschichten typisch für organische Solarzellen sind, werden Transportuntersuchungen der relevanten Materialien der Einfachheit halber meist in ungemischten Schichten durchgeführt. In der vorliegenden Arbeit wird der Ladungstransport in Donator-Akzeptor-Mischschichten mithilfe raumladungsbegrenzter Ströme (space-charge limited currents, SCLCs), Leitfähigkeit, Impedanzspektroskopie (IS) und thermisch-generierter Ströme (thermally stimulated currents, TSC) untersucht und mit numerischen Drift-Diffusions-Simulationen modelliert. Zunächst wird mittels Simulation der Einfluss exponentiell verteilter Fallenzustände auf das schichtdickenabhängige SCLC-Verhalten unipolarer Bauelemente mit Ohmschen Kontakten untersucht. Die Erkenntnisse werden dann auf Elektronen- und Lochtransport in ZnPc:C60-Mischschichten mit verschiedenen Mischverhältnissen angewendet. Dabei wird die Beweglichkeit als Funktion von elektrischem Feld und Ladungsträgerdichte dargestellt, um SCLC- und Leitfähigkeitsmessungen zu erklären, was mit einer exponentiellen Fallenverteilung gelingt. Zum Vergleich werden dieselben Untersuchungen in DCV2-5T-Me33:C60, dem effizientesten der bekannten Solarzellenmaterialien dieser Art, wiederholt, ohne Anzeichen für fallendominierten Transport. Des weiteren werden erstmals schwach p-dotierte ZnPc:C60-Mischschichten mit temperaturabhängiger IS untersucht, um direkt die Dichte besetzter Lochfallenzustände zu bestimmen. Dabei werden wiederum exponentielle Fallenzustände sowie eine Gaußförmige Falle beobachtet. Insgesamt tragen die über Fallenzustände in Mischschichten gewonnenen Erkenntnisse zum Verständnis von Transportprozessen bei und bilden damit eine Grundlage für die systematische Identifizierung von Fallenzuständen in Solarzellen. Außerdem wird gezeigt, dass die genaue Beschreibung der transportrelevanten Fallenzustände die Modellierung von Bauelementen ermöglicht, auf deren Grundlage zukünftig optimierte Probenstrukturen vorhergesagt werden können
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Gieseking, Björn [Verfasser], and Vladimir [Gutachter] Dyakonov. "Excitation Dynamics and Charge Carrier Generation in Organic Semiconductors / Björn Gieseking. Gutachter: Vladimir Dyakonov." Würzburg : Universität Würzburg, 2014. http://d-nb.info/1109750048/34.
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Pattanapanishsawat, Piyapong. "Study of Surface Modification and Effect of Temperature on Charge Carrier Generation and Recombination." University of Akron / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=akron1281364113.
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Mauer, Ralf [Verfasser]. "Charge generation, transport and recombination in organic solar cells / Ralf Mauer." Mainz : Universitätsbibliothek Mainz, 2012. http://d-nb.info/1023189275/34.
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Ibaceta, Jaña Josefa Fernanda. "Thermal instabilities of charge carrier transport in solar cells based on GaAs PN Junctions." Tesis, Universidad de Chile, 2017. http://repositorio.uchile.cl/handle/2250/145405.
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Magíster en Ciencias de la Ingeniería, Mención Mecánica.
Ingeniera Civil MecánicaDentro de los factores que afectan negativamente una celda solar fotovoltaica se destaca la temperatura. Ya sea por imperfecciones del material o a condiciones de operación no uniformes, es posible que se concentre calor en una zona debido a la disminución de la resistencia local y su consecuente aumento de corriente eléctrica. Estas zonas de concentración de calor pueden estabilizarse, generando gradualmente degradación de la celda, disminución de su vida útil y eficiencia. En caso contrario, puede ocurrir un fenómeno de descontrol térmico que resulta catastrófico para la celda, inhabilitando su correcto funcionamiento. Estudios en módulos de película delgada revelan que esta condición ocurre incluso cuando la radiación está uniformemente distribuida y con ello, el perfil de temperatura inicial es constante. La evolución temporal, bajo radiación, induce zonas de calor que incrementan exponencialmente la temperatura, contrayendo su área; por otra parte, la temperatura de las zonas más alejadas disminuye simultáneamente mientras disipan pequeñas corrientes. Para evitar este fenómeno se pueden escalar propiedades del dispositivo, como aumentar la conductividad térmica y disminuir el espesor. Actualmente, estos análisis se realizan a partir de modelos numéricos y analíticos basados en el comportamiento de diodos y mediciones experimentales del perfil de temperatura en la capa superficial de la celda y en la juntura. El propósito de esta Tesis es determinar criterios de estabilidad electro-térmico que pueden ser utilizados para evitar el descontrol de temperatura a partir de aplicar un análisis a un modelo hidrodinámico de mayor complejidad que uno basado en diodos; más aún, considerar un estado fuera del equilibro entre la temperatura de la red y los portadores de carga. Se determinó que la inestabilidad ocurre en la juntura PN y depende fuertemente la temperatura de la juntura en los bordes. Además, aumentar la temperatura de los portadores, disminuir el largo y aumentar el voltaje aplicado pueden estabilizar el sistema, aumentando el tiempo en que el sistema duplica su temperatura.
Este trabajo ha sido parcialmente financiado por CONICYT-PCHA/Magíster Nacional/2016 - 22160729
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Lee, Yung-Huei. "Dual-carrier charge transport and damage formation of LPCVD nitride for nonvolatile memory devices /." The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487322984316841.
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Gabrysch, Markus. "Charge Transport in Single-crystalline CVD Diamond." Doctoral thesis, Uppsala universitet, Elektricitetslära, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-122794.
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Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties, as compared to other (wide bandgap) semiconductors, make it desirable to develop single-crystalline epitaxial diamond films for electronic device and detector applications. Future diamond devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond's excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material. Many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. This problem is largely due to incomplete knowledge of the defects in the material and due to a lack of understanding of how these defects influence transport properties. Since diamond lacks a shallow dopant that is fully thermally activated at room temperature, the conventional silicon semiconductor technology cannot be transferred to diamond devices; instead, new concepts have to be developed. Some of the more promising device concepts contain thin delta-doped layers with a very high dopant concentration, which are fully activated in conjunction with undoped (intrinsic) layers where charges are transported. Thus, it is crucial to better understand transport in high-quality undoped layers with high carrier mobilities. The focus of this doctoral thesis is therefore the study of charge transport and related electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples, in order to improve knowledge on charge creation and transport mechanisms. Fundamental characteristics such as drift mobilities, compensation ratios and average pair-creation energy were measured. Comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond deposition process.
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Fischer, Janine [Verfasser]. "Density of States and Charge Carrier Transport in Organic Donor-Acceptor Blend Layers / Janine Fischer." München : Verlag Dr. Hut, 2015. http://d-nb.info/1079768858/34.
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Paradisi, Andrea. "Ultra-high carrier modulation in two dimensions through space charge doping : graphene and zinc oxide." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066297/document.
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La modulation de la densité de charge est un aspect important de l'étude de les transitions de phase électroniques ainsi que des propriétés électroniques des matériaux et il est à la base de plusieurs applications dans la micro-électronique. L'ajustement de la densité des porteurs de charge (dopage) peut être fait par voie chimique, en ajoutant des atomes étrangers au réseau cristallin du matériau ou électrostatiquement, en créant un accumulation de charge comme dans un Transistor é Effet de Champ. Cette dernier m ethode est réversible et particuliérement appropriée pour les matériaux bidimensionnels (2D) ou pour des couches ultra-minces. Le Dopage par Charge d'Espace est une nouvelle technique inventée et développée au cours de ce travail de thèse pour le dopage electrostatique de matériaux déposés sur la surface du verre. Une charge d'espace est créée à la surface en provoquant le mouvement des ions sodium présents dans le verre sous l'effet de la chaleur et d'un champ électrique extérieur. Cette espace de charge induit une accumulation de charge dans le matériau déposé sur la surface du verre, ce qui peut être supérieure à 10^14/cm^2. Une caractérisation détaillée faite avec mesures de transport, effet Hall, mesures Raman et mesures de Microscopie a Force Atomique (AFM) montrent que le dopage est réversible, bipolaire et il ne provoque pas des modifications chimiques. Cette technique peut être appliquée a des grandes surfaces, comme il est montré pour le cas du graph ene CVD. Dans une deuxiéme partie le dopage par espace de charge est appliqué à des couches ultra-minces (< 40 nm) de ZnO_(1-x). Le résultat est un abaissement de la résistance par carré de 5 ordres de grandeur. Les mesures de magnéto-transport faites à basse température montrent que les électrons dop es sont confinés en deux dimensions. Une transition remarquable de la localisation faible à l'anti-localisation est observée en fonction du dopage et de la température et des conclusions sont tirées à propos des phénoménes de diffusion qui gouverne le transport électronique dans des diff erentes conditions dans ce matériauCarrier modulation is an important parameter in the study of the electronic phase transitions and the electronic properties of materials and at the basis for many applications in microelectronics. The tuning of charge carrier density (doping) can be achieved chemically, by adding foreign atoms to the crystal structure of the material or electrostatically, by inducing a charge accumulation like in a Field Eect Transistor device. The latter method is reversible and particularly indicated for use in two dimensional (2D) materials or ultra-thin films. Space Charge Doping is a new technique invented and developed during this thesis for the electrostatic doping of such materials deposited on a glass surface. A space charge is created at the surface by causing sodium ions contained in glass to drift under the Eect of heat and an external electric field. This space charge in turn induces a charge accumulation in the material deposited on the glass surface which can be higher than 10^14/cm^2. Detailed characterization using transport, Hall effect, Raman and AFM measurements shows that the doping is reversible, ambipolar and does not induce chemical changes. It can be applied to large areas as shown with CVD graphene. In a second phase the space charge doping method is applied to polycrystalline ultra-thin films (< 40 nm) of ZnO_(1-x). A lowering of sheet resistance over 5 orders of magnitude is obtained. Low temperature magneto-transport measurements reveal that doped electrons are confined in two dimensions. A remarkable transition between weak localization and anti-localization isobserved as a function of doping and temperature and conclusions are drawn concerning the scattering phenomena governing electronic transport under different conditions in this material
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Strunk, Karl-Philipp [Verfasser], and Christian [Akademischer Betreuer] Melzer. "Charge Carrier Transport in Solution Processed Organic Semiconductor Thin Films / Karl-Philipp Strunk ; Betreuer: Christian Melzer." Heidelberg : Universitätsbibliothek Heidelberg, 2019. http://d-nb.info/1195143788/34.
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Majdi, Saman. "Experimental Studies of Charge Transport in Single Crystal Diamond Devices." Doctoral thesis, Uppsala universitet, Elektricitetslära, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-173599.
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Diamond is a promising material for high-power, high-frequency and high- temperature electronics applications, where its outstanding physical properties can be fully exploited. It exhibits an extremely high bandgap, very high carrier mobilities, high breakdown field strength, and the highest thermal conductivity of any wide bandgap material. It is therefore an outstanding candidate for the fastest switching, the highest power density, and the most efficient electronic devices obtainable, with applications in the RF power, automotive and aerospace industries. Lightweight diamond devices, capable of high temperature operation in harsh environments, could also be used in radiation detectors and particle physics applications where no other semiconductor devices would survive. The high defect and impurity concentration in natural diamond or high-pressure-high-temperature (HPHT) diamond substrates has made it difficult to obtain reliable results when studying the electronic properties of diamond. However, progress in the growth of high purity Single Crystal Chemical Vapor Deposited (SC-CVD) diamond has opened the perspective of applications under such extreme conditions based on this type of synthetic diamond. Despite the improvements, there are still many open questions. This work will focus on the electrical characterization of SC-CVD diamond by different measurement techniques such as internal photo-emission, I-V, C-V, Hall measurements and in particular, Time-of-Flight (ToF) carrier drift velocity measurements. With these mentioned techniques, some important properties of diamond such as drift mobilities, lateral carrier transit velocities, compensation ratio and Schottky barrier heights have been investigated. Low compensation ratios (ND/NA) < 10-4 have been achieved in boron-doped diamond and a drift mobility of about 860 cm2/Vs for the hole transit near the surface in a lateral ToF configuration could be measured. The carrier drift velocity was studied for electrons and holes at the temperature interval of 80-460 K. The study is performed in the low-injection regime and includes low-field drift mobilities. The hole mobility was further investigated at low temperatures (10-80 K) and as expected a very high mobility was observed. In the case of electrons, a negative differential mobility was seen in the temperature interval of 100-150K. An explanation for this phenomenon is given by the intervally scattering and the relation between hot and cold conduction band valleys. This was observed in direct bandgap semiconductors with non-equivalent valleys such as GaAs but has not been seen in diamond before. Furthermore, first steps have been taken to utilize diamond for infrared (IR) radiation detection. To understand the fundamentals of the thermal response of diamond, Temperature Coefficient of Resistance (TCR) measurements were performed on diamond Schottky diodes which are a candidate for high temperature sensors. As a result, very high TCR values in combination with a low noise constant (K1/f) was observed.
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Sánchez-Carrera, Roel S. "Theoretical characterization of charge transport in organic molecular crystals." Diss., Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26579.
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In this thesis, a first-principles methodology to investigate the impact of electron-phonon interactions on the charge-carrier mobilities in organic molecular crystals has been developed. Well-known organic materials such as oligoacene and oligothienoacene derivatives were studied in detail. The nature of the intramolecular vibronic coupling in oligoacenes and oligothienoacenes was studied using an approach that combines high-resolution gas-phase photo-electron spectroscopy measurements with first-principles quantum-mechanical calculations. The electron interactions with optical phonons in oligoacene single crystals were investigated using both density functional theory and empirical force field methods. The low-frequency optical modes are found to play a significant role in dictating the temperature dependence of the charge-transport properties in the oligoacene crystals. The microscopic charge-transport parameters in the pentathienoacene, 1,4-diiodobenzene, and 2,6-diiodo-dithieno[3,2-b:2',3'-d]thiophene crystals were also investigated. It was found that the intrinsic charge transport properties in the pentathienoacene crystal might be higher than that in two benchmark high-mobility organic crystals, i.e., pentacene and sexithienyl. For 1,4-diiodobenzene crystal, a detailed quantum-mechanical study indicated that its high mobility is primarily associated with the iodine atoms. In the 2,6-diiododithieno[3,2-b:2',3'-d]thiophene crystal, the main source of electronic interactions were found along the π-stacking direction. For negatively charged carriers, the halogen-functionalized molecular crystals show a very large polaron binding energy, which suggests significantly low charge-transport mobility for electrons.
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Kern, Julia [Verfasser], and Vladimir [Gutachter] Dyakonov. "Field Dependence of Charge Carrier Generation in Organic Bulk Heterojunction Solar Cells / Julia Kern. Gutachter: Vladimir Dyakonov." Würzburg : Universität Würzburg, 2013. http://d-nb.info/1108780598/34.
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Kraus, Michael [Verfasser], and Wolfgang [Akademischer Betreuer] Brütting. "Charge carrier transport in organic field-effect devices based on copper-phthalocyanine / Michael Kraus. Betreuer: Wolfgang Brütting." Augsburg : Universität Augsburg, 2011. http://d-nb.info/1077700180/34.
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Siedl, Nicolas [Verfasser], and Oliver [Akademischer Betreuer] Diwald. "Synthesis and Processing of Metal Oxide Nanoparticle Ensembles: Impact on Charge Carrier Generation / Nicolas Siedl. Betreuer: Oliver Diwald." Erlangen : Universitätsbibliothek der Universität Erlangen-Nürnberg, 2013. http://d-nb.info/1031558551/34.
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Kurita, Daisuke, Shingo Ohta, kenji Sugiura, Hiromichi Ohta, and Kunihito Koumoto. "Carrier generation and transport properties of heavily Nb-doped anatase TiO2 epitaxial films at high temperatures." American Institute of Physics, 2006. http://hdl.handle.net/2237/8777.
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Olson, Benjamin Varberg. "Time-resolved measurements of charge carrier dynamics and optical nonlinearities in narrow-bandgap semiconductors." Diss., University of Iowa, 2013. https://ir.uiowa.edu/etd/2596.
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All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as a next generation source of these materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential transmission measurements are used here to demonstrate that Ga-free InAs/InAsSb T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements at 77 K provided minority carrier lifetimes of 9 μs and 3 μs for an unintentionally doped mid-wave infrared (MWIR) InAs/InAsSb T2SL and InAsSb alloy, respectively; a two order of magnitude increase compared to the 90 ns minority carrier lifetime measured in a comparable MWIR InAs/GaSb T2SL. Through temperature-dependent lifetime measurements, the various carrier recombination processes are differentiated and the dominant mechanisms identified for each material. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs for potentially improved infrared photodetectors.
In addition to carrier lifetimes, the drift and diffusion of excited charge carriers through the superlattice growth layers (i.e. vertical transport) directly affects the performance of photodetectors and emitters. Unfortunately, there is a lack of information pertaining to vertical transport, primarily due to difficulties in making measurements on thin growth layers and the need for non-standard measurement techniques. However, all-optical ultrafast techniques are successfully used here to directly measure vertical diffusion in MWIR InAs/GaSb T2SLs. By optically generating excess carriers near one end of a MWIR T2SL and measuring the transit time to a thin, 2 lower-bandgap superlattice placed at the other end, the time-of-flight of vertically diffusing carriers is determined. Through investigation of both unintentionally doped and p-type superlattices at 77 K, the vertical hole and electron diffusion coefficients are determined to be 0.04±0.03 cm2/s and 4.7±0.5 cm2/s, corresponding to vertical mobilities of 6±5 cm2/Vs and 700±80 cm2/Vs, respectively. These measurements are, to my knowledge, the first direct measurements of vertical transport properties in narrow-bandgap superlattices.
Lastly, the widely tunable two-color ultrafast laser system used in this research allowed for the investigation of nonlinear optical properties in narrow-bandgap semiconductors. Time-resolved measurements taken at 77 K of the nondegenerate two-photon absorption spectrum of bulk n-type GaSb have provided new information about the nonresonant change in absorption and two-photon absorption coefficients in this material. Furthermore, as the nondegenerate spectrum was measured over a wide range of optical frequencies, a Kramers-Kronig transformation allowed the dispersion of the nondegenerate nonlinear refractive index to be calculated.
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Tiwari, Vandana [Verfasser]. "Unravelling the photophysics of charge generation and transport processes by ultrafast spectroscopic methods / Vandana Tiwari." Hamburg : Staats- und Universitätsbibliothek Hamburg Carl von Ossietzky, 2020. http://d-nb.info/122962550X/34.
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Giovanardi, Fabio <1984>. "Analysis of charge-transport properties in GST materials for next generation phase-change memory devices." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5583/.
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The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power.
The memory subsystem accounts for a significant cost and power budget of a computer system. Current DRAM-based main memory systems are starting to hit the power and cost limit. To resolve this issue the industry is improving existing technologies such as Flash and exploring new ones. Among those new technologies is the Phase Change Memory (PCM), which overcomes some of the shortcomings of the Flash such as durability and scalability. This alternative non-volatile memory technology, which uses resistance contrast in phase-change materials, offers more density relative to DRAM, and can help to increase main memory capacity of future systems while remaining within the cost and power constraints.
Chalcogenide materials can suitably be exploited for manufacturing phase-change memory devices. Charge transport in amorphous chalcogenide-GST used for memory devices is modeled using two contributions: hopping of trapped electrons and motion of band electrons in extended states. Crystalline GST exhibits an almost Ohmic I(V) curve. In contrast amorphous GST shows a high resistance at low biases while, above a threshold voltage, a transition takes place from a highly resistive to a conductive state, characterized by a negative differential-resistance behavior. A clear and complete understanding of the threshold behavior of the amorphous phase is fundamental for exploiting such materials in the fabrication of innovative nonvolatile memories. The type of feedback that produces the snapback phenomenon is described as a filamentation in energy that is controlled by electron–electron interactions between trapped electrons and band electrons. The model thus derived is implemented within a state-of-the-art simulator. An analytical version of the model is also derived and is useful for discussing the snapback behavior and the scaling properties of the device.Lo sviluppo dei sistemi di memoria di futura generazione è guidato principalmente dalla ricerca di una tecnologia in grado di superare quelle attuali in ogni loro specifica di funzionamento, dalla ritenzione di dato alla velocità di accesso, migliorandone la durata e riducendo il dispendio energetico. Il sottosistema delle memorie assorbe una parte significativa delle risorse del macro sistema costituito dal calcolatore, tanto da aver quasi raggiunto il limite tecnologico nel caso delle odierne memorie di tipo DRAM. La soluzione più promettente sembra essere quella delle memorie a cambiamento di fase (PCM), in grado di colmare anche i limiti mostrati dalla tecnologia Flash nell’ambito della durata e scalabilità. I materiali che consentono di realizzare dispostivi a cambiamento di fase pilotato elettricamente appartengono alla famiglia dei calcogenuri. Tra i diversi composti calcogenuri quello attualmente identificato come soluzione più promettente è il Ge2Sb2Te5 (GST). Il trasporto di carica all’interno di dispositivi di memoria realizzati con tali materiali è stato modellato considerando l’azione di due contributi differenti: hopping di cariche intrappolate e moto di elettroni liberi in stati estesi. Il GST mostra un comportamento elettrico pressoché Ohmico in fase cristallina mentre, in fase amorfa, risulta essere poco conduttivo per basse correnti fino al superamento di una tensione di soglia oltre la quale si assiste al passaggio da uno stato altamente resistivo ad uno altamente conduttivo, caratterizzato da un andamento a resistenza differenziale negativa (NDR). Il meccanismo retroattivo che induce il fenomeno di snapback viene descritto come filamentazione in energia controllata dalle interazioni tra elettroni liberi ed elettroni intrappolati. Il modello fisico ricavato è stato implementato all’interno di un simulatore di dispositivi di ultima generazione ed è stato in seguito riprodotto in una versione analitica semplificata in grado, però, di permettere una prima analisi del comportamento elettrico del dispositivo e delle sue proprietà di scaling.
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Engels, Stephan [Verfasser], Christoph [Akademischer Betreuer] Stampfer, and F. [Akademischer Betreuer] Guinea. "Limitations to charge carrier transport in high quality single and bi-layer graphene / Stephan Engels ; Christoph Stampfer, F. Guinea." Aachen : Universitätsbibliothek der RWTH Aachen, 2015. http://d-nb.info/1130590135/34.
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Pahner, Paul Verfasser], Karl [Akademischer Betreuer] [Gutachter] [Leo, and Björn [Gutachter] Lüssem. "Charge Carrier Trap Spectroscopy on Organic Hole Transport Materials / Paul Pahner ; Gutachter: Karl Leo, Björn Lüssem ; Betreuer: Karl Leo." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://d-nb.info/1124777849/34.
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Pahner, Paul [Verfasser], Karl [Akademischer Betreuer] [Gutachter] Leo, and Björn [Gutachter] Lüssem. "Charge Carrier Trap Spectroscopy on Organic Hole Transport Materials / Paul Pahner ; Gutachter: Karl Leo, Björn Lüssem ; Betreuer: Karl Leo." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://d-nb.info/1124777849/34.
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Ito, Hiroyuki. "High quality high current ion beam generation and transport systems including plasma-based space charge neutraliser." Thesis, University of Salford, 2003. http://usir.salford.ac.uk/42995/.
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This thesis discusses the development of an ion beam generation and transport system for the purpose of high current ion implantation and presents a method to achieve high quality beam generation and high transport efficiency. An advanced method of achieving ideal space charge neutralization of an ion beam as well as charge neutralization of implanted targets has also been developed and presented. This study was carried out in five main categories as described below. 1. Beam optics: The optical properties of the ion beam have to be well controlled through various beam transport devices. Beam transfer matrix calculation and Poisson's equation solver were used to determine beam optics. 2. Plasma generation: The ion beam is extracted from a plasma generated in the ion source. Therefore, the quality of ion beams depends largely on the characteristics of the source plasma. Plasma diagnostic tools were used to understand the desired plasma conditions. 3. Space charge neutralisation: Space charge neutralisation is essential to transport high perveance beams because a beam without neutralisation develops a high electric potential in itself that would scatter beam ions away. A plasma based charge neutraliser "Plasma Flood System" was developed to maintain space charge neutrality during implantation. 4. Low energy beam extraction and transport: Beam extraction and transport become difficult at lower energy due to growth of emittance and space charge. Beam divergence had to be minimized by tight optical control. 5. Microwave plasma sources as ultimate ion source: A microwave source was developed to obtain high current, ultra clean ion beams and long source life. The characteristics of microwave plasmas differ significantly from whose generated by other methods such as DC excitation. The source achieved the highest recorded beam currents for various species. All the technology and the methods described above were combined to develop a state-of-the-art high current ion implanter that has achieved world leading beam performance.
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Roland, Paul Joseph. "Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride." University of Toledo / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685.
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Widmer, Johannes. "Charge transport and energy levels in organic semiconductors." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-154918.
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Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devicesOrganische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile
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Wang, Chien-Lung. "Synthesis and Characterization of C60-Porphyrin Derivatives for Enhanced Photovoltaic Performance through Efficient Charge Generation and Transport." University of Akron / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=akron1301353045.
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Cadirci, Musa. "Ultrafast charge dynamics in novel colloidal quantum dots." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/ultrafast-charge-dynamics-in-novel-colloidal-quantum-dots(865aba90-9d60-478d-8f49-ad4785516688).html.
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In this thesis ultrafast exciton dynamics of several colloidal quantum dots have been studied using visible transient absorption spectroscopy. The resultant transient decays and differential transmission spectra were analysed to determine the ultrafast relaxation channels, multiple exciton generation (MEG) efficiency and multi-exciton interactions in the observed materials. All QDs were preliminarily optically characterized using steady state absorption and photoluminescence spectroscopies. In addition, a high repetition infrared femtosecond pump probe experiment was designed and built to detect the picosecond intraband carrier relaxations in quantum dots. Picosecond carrier dynamics of type-II ZnTe/ZnSe and of CuInSe2 and CuInS2 type-I quantum dots were investigated. The common feature of these materials is that they are eco-friendly materials, being alternatives to the toxic Cd- and Pb- based materials. It was found that surface trapping occurred in both cases for electrons in the hot states, and in the minimum of the conduction band for ZnTe/ZnSe core/shell materials. Trion formation was observed in ZnTe/ZnSe core/shell dots at high power and unstirred conditions. The hot and cold electron trapping processes in type-II dots and CuInS2 and CuInSe2 dots shifted, distorted and moderately cancelled the bleach features. In addition, intra-gap hole trapping was observed in CuInS2 and CuInSe2 dots which results in a long decay feature in the recorded transients. MEG competes with Auger cooling, surface mediated relaxation and phonon emission. To enhance the MEG quantum yield, the rival mechanisms were suppressed in well-engineered CdSe/CdTe/CdS and CdTe/CdSe/CdS core/shell/shell and CdTe/CdS core/shell type-II quantum dots. The MEG slope efficiency and threshold for a range of different core size and shell thickness were found to be (142±9)%/Eg and (2.59±0.16)Eg, respectively. The observed threshold was consistent with the literature, whereas, the obtained slope efficiency was about three times higher than the previously reported values. The biexciton interaction energy of the dots stated in the previous paragraph was also studied. To date, time-resolved photoluminescence (TRPL) has been employed to study exciton interactions in type-II quantum dots and large repulsive biexciton interaction energy values between 50-100 meV have been reported. However, unlike the TRPL method, the TA experiment ensures that only two excitons remain in the band edge of the dot. Using this method, large attractive biexciton interaction energies up to ~-60 meV was observed. These results have promising implications regarding enhancing the MEG quantum yield.
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Wehrenfennig, Christian. "Ultrafast spectroscopy of charge separation, transport and recombination processes in functional materials for thin-film photovoltaics." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:1f812413-4a2f-418f-a7fd-d749e88cc2e1.
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Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO2-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH3NH3PbI3 and CH3NH3PbI3-xClx, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH3NH3PbI3 and several microns for CH3NH3PbI3-xClx. These results are shown to hold for both, solution processed and vapour-deposited CH3NH3PbI3-xClx and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH3NH3PbI3-xClx, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.
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Huang, Wei-Jie, and Wei-Jie Huang. "Towards Increased Photovoltaic Energy Generation Efficiency and Reliability: Quantum-Scale Spectral Sensitizers in Thin-Film Hybrid Devices and Microcracking in Monocrystalline Si." Diss., The University of Arizona, 2016. http://hdl.handle.net/10150/623175.
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The present work focuses on two strategies contributing to the development of high efficiency, cost-effective photovoltaic (PV) technology for renewable energy generation: the design of new materials offering enhanced opto-electronic performance and the investigation of material degradation processes and their role in predicting the long-term reliability of PV modules in the field. The first portion of the present work investigates the integration of a novel CdTe-ZnO nanocomposite material as a spectral sensitizer component within a thin-film, hybrid heterojunction (HJ) PV device structure. Quantum-scale semiconductors have the potential to improve PV device performance through enhanced spectral absorption and photocarrier transport. This is realized via appropriate design of the semiconductor nanophase (providing tunable spectral absorption) and its spatial distribution within an electrically active matrix (providing long-range charge transport). Here, CdTe nanocrystals, embedded in an electrically active ZnO matrix, form a nanocomposite (NC) offering control of both spectral absorption and photocarrier transport behavior through the manipulation of nanophase assembly (ensemble effects). A sequential radio- frequency (RF) magnetron sputter deposition technique affords the control of semiconductor nanophase spatial distribution relative to the HJ plane in a hybrid, ZnO-P3HT test structure. Energy conversion performance (current density-voltage (J-V) and external quantum efficiency (EQE) response) was examined as a function of the location of the CdTe nanophase absorber region using both one dimensional solar cell capacitance simulator (SCAPS) and the experimental examination of analogous P3HT-ZnO based hybrid thin films. Enhancement in simulated EQE over a spectral range consistent with the absorption region of the CdTe nanophase (i.e. 400–475 nm) is confirmed in the experimental studies. Moreover, a trend of decreasing quantum efficiency in this spectral range with increasing separation between the CdTe nanophase region and the heterojunction plane is observed. The results are interpreted in terms of carrier scattering/recombination length mitigating the successful transport of carriers across the junction. The second portion of the research addresses the need for robust PV performance in commercial module as a primary contributor to cost-effective operation in both distributed systems and utility scale generation systems. The understanding of physical and chemical mechanisms resulting in the degradation of materials of construction used in PV modules is needed to understand the contribution of these processes to module integrity and performance loss with time under varied application environments. In this context, the second part of present study addresses microcracking in Si–an established degradation process contributing to PV module power loss. The study isolates microcrack propagation in single-crystal Si, and investigates the effect of local environment (temperature, humidity) on microcrack elongation under applied strains. An investigation of microindenter-induced crack evolution with independent variation of both temperature and vapor density was pursued in PV-grade Si wafers. Under static tensile strain conditions, an increase in sub-critical crack elongation with increasing atmospheric water content was observed. To provide further insight into the potential physical and chemical conditions at the microcrack tip, micro-Raman measurements were performed. Preliminary results confirm a spatial variation in the frequency of the primary Si vibrational resonance within the crack-tip region, associated with local stress state, whose magnitude is influenced by environmental conditions during the period of applied static strain. The experimental effort was paired with molecular dynamics (MD) investigations of microcrack evolution in single-crystal Si to furnish additional insight into mechanical contributions to crack elongation. The MD results demonstrate that crack-tip energetics and associated cracking crystal planes and morphology are intimately related to the crack and applied strain orientations with respect to the principal crystallographic axes. The resulting fracture surface energy and the stress-strain response of the Si under these conditions form the basis for preliminary micro-scale peridynamics (PD) simulations of microcrack development under constant applied strain. These efforts were integrated with the experimental results to further inform the mechanisms contributing to this important degradation mode in Si-based photovoltaics.
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Petsiuk, Andrei [Verfasser], Dieter [Akademischer Betreuer] Neher, Thomas [Akademischer Betreuer] Unold, Dieter [Gutachter] Neher, Vladimir [Gutachter] Dyakonov, and Thomas [Gutachter] Unold. "Investigation of charge carrier transport in metal halide perovskites by THz Spectroscopy / Andrei Petsiuk ; Gutachter: Dieter Neher, Vladimir Dyakonov, Thomas Unold ; Dieter Neher, Thomas Unold." Potsdam : Universität Potsdam, 2021. http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-515441.
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Bondi, Luca. "Photocurrent generation at organic heterojunction-electrolyte interface for optoelectronic biosensor implementation." Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2019.
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This thesis project investigates an organic heterojunction made by common and largely available inert dyes which can operate in cellular fluid without being toxic for their environment. The heterojunction/electrolyte interface converts light into an ionic current pulse strong enough to depolarize adjacent cells. Indeed, such a photoelectrode is envisioned to act as a building block in an artificial retina.
In order to achieve this goal, detailed characterization and understanding of the charge generation mechanism is necessary. To this end I realized two experimental configurations that characterize a photoelectrode made of a p/n junction cast over ITO. In the first configuration, the photoelectrode was studied in a three-electrode electrochemical cell under pulsed LED light stimulation. Using different analytical techniques, I characterized the photoelectrodes impedance and photocurrent transients as a function of a well-defined externally applied potential. In the second configuration I extended the experiment to assess the impact of a more realistic floating photoelectrode operation. In this situation the photoelectrode circuit is closed by a second, passive electrode interface that converts the ionic current back into an electronic one. As a consequence, the potential is no longer defined and I demonstrate that the properties of the passive electrode can be optimized to maximize the capacitive currents and minimize the faradic ones.
The following thesis is organized in three main chapters: first an introduction, where I provide an essential background; then methods and materials, where are described the investigated system, the models used for its interpretation and the methods used to take the measurements; then result and discussion, where the measurements made in the non-floating and floating arrangements and their explicative model are presented and discussed, and simulation are performed and compared to the experimental results.