Dissertations / Theses on the topic 'Heterojunction semiconductor devices'

Hybrid organic-inorganic solar photovoltaic (PV) cells capable of directly converting sunlight to electricity have attracted much attention in recent years. Despite evident technological advancements in the PV industry, the widespread commercialisation of solar cells is still being mired by their low conversion efficiencies and high cost per Watt. Perovskites are an emerging class of semiconductors providing a low-cost alternative to silicon-based photovoltaic cells, which currently dominate the market. This thesis develops a series of studies on “all-solid state perovskite solar cells” fabricated via vapour deposition which is an industrially-accessible technique, to achieve planar heterojunction architectures and efficient PV devices. Chapter 2 presents a general outlook on the operating principles of solar cells, delving deeper into the specific operational mechanism of perovskite solar cells. It also explores the usual methods employed in the fabrication of perovskite thin films. Chapter 3 describes the experimental procedures followed during the fabrication of the individual components constituting the device from the synthesis of the precursors to the construction of the functioning perovskite PV devices. Chapter 4 demonstrates pioneering work involving the dual-source vapour deposition (DSVD) of planar heterojunction perovskite solar cells which generated remarkable power conversion efficiency values surpassing 15%. These significant results pave the way for the mass-production of perovskite PVs. To further expand the range of feasible vapour deposition techniques, a two-layer sequential vapour deposition (SVD) technique is explored in Chapter 5. This chapter focusses on identifying the factors affecting the fundamental properties of the vapour-deposited films. Findings provide an improved understanding of the effects of precursor compositions and annealing conditions on the films. Chapter 5 concludes with a comparison between SVD and DSVD fabricated films, highlighting the benefits of each vapour deposition technique. Furthermore, hysteretic effects are analysed in Chapter 6 for the perovskite PV devices fabricated based on different structural configurations. An interesting discovery involving the temporary functioning of compact layer-free perovskite PV devices suggests the presence of a built-in-field responsible for the hysteresis of the cells. The observations made in this chapter yield a new understanding of the functionality of individual cell layers. Combining the advantages of the optimum vapour deposition technique established in Chapter 4 and Chapter 5, with the enhanced understanding of perovskite PV cell operational mechanism acquired from Chapter 6, an ongoing study on an “all-perovskite” tandem solar cell is introduced in Chapter 7. This demonstration of the “all-perovskite” tandem devices confirms the versatility of perovskites for a broader range of PV applications.

This thesis investigates novel hole extraction interfaces and series interconnects for applications in organic photovoltaics, specifically in single junction solid-state dye-sensitized solar cells (DSSCs) and tandem DSSC/polymer bulk heterojunction solar cells. Improvements in hole extraction and device performance by using materials compatible with scalable deposition methods are presented, including tungsten- and molybdenum-disulphide (WS₂ and MoS₂), and p-type doped spiro-OMeTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene) nanoparticle dispersions. WS₂ and MoS₂ hole extraction layers increase averaged short circuit currents by 20% and 16% respectively, and power conversion efficiencies by 19% and 14% respectively when compared with control devices. Similarly, doped spiro-OMeTAD nano-particle layers improved short circuit current densities by 32% and efficiencies by 9%. Tandem device interconnects using these novel hole extraction formats have been fabricated, but although devices did exhibit rectification, overall performance was poor. Possible reasons for their limited success have been analysed. Dye-sensitized solar mini-modules are also reported. In order to assure the scalability of DSSC technology, these larger area devices were constructed using doctor blade coating to deposit the hole transporter material. As well as achieving a respectable maximum power conversion efficiency of 2.6%, it has also been shown that the extent to which hole transporter infiltrates the mesoporous photoanode of these devices may be tuned by altering substrate temperature during deposition. It was found that an optimal coating temperature of 70 degrees C produced the best efficiency, with a corresponding pore-filling fraction of 41%.

2013/2014
La ricerca qui presentata è incentrata sullo sviluppo di tecnologie innovative per la produzione di rivelatori di posizione di fasci fotonici veloci (pBPM) per applicazioni in luce di sincrotrone (SR) e laser a elettroni liberi (FEL). Nel nostro lavoro abbiamo proposto un rilevatore in-situche ha dimostrato velocità di risposta ed omogeneità sia per scopi di diagnostica che di calibrazione. I dispositivi sono basati su pozzi quantici (QW) dimateriali semiconduttori InGaAs / InAlAs,che offrono diversi vantaggi grazie alla loro gap di banda diretta e a bassa energia, e all’alta mobilità elettronica a temperatura ambiente. I QW metamorfici diIn0.75Ga0.25As/In0.75Al0.25As contenenti un gas di elettroni bidimensionali (2DEG) sono staticresciuti tramite epitassia a faci molecolari (MBE). Tali materiali presentano alcune differenze notevoli rispetto al diamante, che è il materiale utilizzato per i rivelatori commerciali allo stato dell’arte. Innanzitutto, i costi di produzione e di fabbricazione sono molto più bassi. Poi, il coefficiente di assorbimento è molto superiore al diamante su una vasta gamma di energie di raggi X, il che li rende ampiamente complementari in possibili applicazioni. Inoltre, utilizzando semiconduttori composti si possono fabbricare dispositivi con diverse combinazioni di materiali per la barriera ed il QW;ciòha permesso di ridurre la gap di energia fino a 0.6 eV. La disponibilità e la ripetibilità di fabbricazione dei dispositivi è migliore rispetto a quelle del diamante. Quattro configurazioni di dispositivi a QW pixelati sono stati testati con diverse fonti di luce, come radiazione di sincrotrone, tubo a raggi X convenzionali e laser ultra veloce nel vicinoUV. In questa tesi, dopo aver introdotto i dispositivi a QW per utilizzo comepBPM, saranno riportati e discussii risultati più importanti ottenuti. Tali risultati indicano che questi rivelatori rispondono con tempi di 100-ps a impulsi laser ultraveloci, cioè un fattore 6 più velocirispetto a rivelatori a semiconduttori commerciali allo stato dell’arte. La precisione raggiunta nella stima della posizione del fascio fotonico è di 800nm, da confrontare con i 150nm di rivelatori a diamante commerciali. Inoltre, i nostri rivelatori di fotoni a QW lavorano a tensioni molto inferiori rispetto aipBPMs esistenti.Infine, test con raggi X da radiazione di sincrotrone mostrano come questi dispositivi presentano elevate efficienze di raccolta di carica, che possono essere imputabili all'effetto di moltiplicazione di carica del gas di elettroni 2D all'interno del pozzo. Tutti questi vantaggi rispetto ai rivelatori esistenti basati sul diamante, rendono i nostri dispositivi potenzialmente molto attrattivi come alternativa a quelli commerciali.
XXVII Ciclo
1984

This dissertation is focused on the research efforts to develop the growth, processing, and modeling technologies for GaN-based Heterojunction Field Effect Transistors (HFETs). The interest in investigating GaN HFETs is motivated by the advantageous material properties of nitride semiconductor such as large band gap, large breakdown voltage, and high saturation velocity, which make it very promising for the high power and microwave applications. Although enormous progress has been made on GaN transistors in the past decades, the technologies for nitride transistors are still not mature, especially concerning the reliability and stability of the device. In order to improve the device performance, we first optimized the growth and fabrication procedures for the conventional AlGaN barrier HFET, on which high carrier mobility and sheet density were achieved. Second, the AlInN barrier HFET was successfully processed, with which we obtained improved I-V characteristics compared with conventional structure. The lattice-matched AlInN barrier is beneficial in the removal of strain, which leads to better carrier transport characteristics. Furthermore, new device structures have been examined, including recess-gate HFET with n+ GaN cap layer and gate-on-insulator HFET, among which the insertion of gate dielectrics helps to leverage both DC and microwave performances. In order to depict the microwave behavior of the HFET, small signal modeling approaches were used to extract the extrinsic and intrinsic parameters of the device. An 18-element equivalent circuit model for GaN HFET has been proposed, from which various extraction methods have been tested. Combining the advantages from the cold-FET measurements and hot-FET optimizations, a hybrid extraction method has been developed, in which the parasitic capacitances were attained from the cold pinch-off measurements while the rest of the parameters from the optimization routine. Small simulation error can be achieved by this method over various bias conditions, demonstrating its capability for the circuit level design applications for GaN HFET. Device physics modeling, on the other hand, can help us to reveal the underlying physics for the device to operate. With the development of quantum drift-diffusion modeling, the self-consistent solution to the Schrödinger-Poisson equations and carrier transport equations were fulfilled. Lots of useful information such as band diagram, potential profile, and carrier distribution can be retrieved. The calculated results were validated with experiments, especially on the AlInN layer structures after considering the influence from the parasitic Ga-rich layer on top of the spacer. Two dimensional cross-section simulation shows that the peak of electrical field locates at the gate edge towards the drain, and of different kinds of structures the device with gate field-plate was found to efficiently reduce the possibility of breakdown failure.

This thesis is a step towards exploiting singlet and triplet excitons in organic semiconductors for multi-functional diodes. It details the device design and fabrication processes for realisation of reversible organic optoelectronic diodes that can sense as well as emit light on demand. It explores new avenues for multi-exciton harvesting and triplet energy transfer in organic semiconductors in conjunction with physical mechanisms of singlet fission and triplet-triplet annihilation. It details optoelectronic characteristics of multi-chromophore, organic cascades that operate as photodetectors, light-emitting diodes and photovoltaic device with spectral response extending from visible to NIR.

The objective of this work is to investigate the performance of SiGe HBTs and scaled CMOS devices in extreme environments. In this work, the inverse mode operation of SiGe HBTs is investigated as a potential solution to the vulnerability of SiGe HBTs to single event effects. The performance limitations of SiGe HBTs operating in inverse mode are investigated through an examination of the effects of scaling on inverse mode performance and optimization schemes for inverse mode performance enhancements are discussed and demonstrated. In addition the performance of scaled MOSFETs, that constitute the digital backbone of any BiCMOS technology, is investigated under radiation exposure and cryogenic temperatures. Extreme environments and their effects on semiconductor devices are introduced in Chapter 1. The immunity of 90nm MOSFETs to total ionizing dose damage under proton radiation is demonstrated. Inverse mode operation of SiGe HBTs is introduced in Chapter 2 as a potential radiation hard solution by design. The effect of scaling on inverse mode performance of SiGe HBTs is investigated and the performance limitations in inverse mode are identified. Optimization schemes for improving inverse mode performance of SiGe HBTs are discussed in Chapter 3. Inverse mode performance enhancement is demonstrated experimentally in optimized device structures manufactured in a commercial third generation SiGe HBT BiCMOS platform. Further, a cascode device structure, the combines the radiation immunity of an inverse mode structure with the performance of a forward mode common emitter device is XIV discussed. Finally, idealized doping profiles for inverse mode performance enhancement is discussed through TCAD simulations. The cryogenic performance of inverse mode SiGe HBTs are discussed in Chapter 4. A novel base current behavior at cryogenic temperature is identified and its effect on the inverse mode performance is discussed. Matching performance of a 90nm bulk CMOS technology at cryogenic temperatures is investigated experimentally and through TCAD simulations in Chapter 5. The effect of various process parameters on the temperature sensitivity of threshold voltage mismatch is discussed. The potential increase of mismatch in subthreshold MOSFETs operating in cryogenic temperatures due to hot carrier effects is also investigated.

Thesis (Ph. D)--Ohio State University, 2002.
Title from first page of PDF file. Document formatted into pages; contains xxx,198 p.: ill. (some col.). Includes abstract and vita. Advisor: Leonard J. Brillson, Dept. of Electrical Engineering. Includes bibliographical references (p. 188-198).

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm2 V−1 s−1 in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 109 (±2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10−9 Acm−2 is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.

Les études qui seront présentées dans le cadre de cette thèse portent sur le développement et l’optimisation des techniques pour la modélisation compacte des transistors bipolaires à hétérojonction (TBH). Ce type de modélisation est à la base du développement des bibliothèques de composants qu’utilisent les concepteurs lors de la phase de simulation des circuits intégrés. Le but d’une technologie BiCMOS est de pouvoir combiner deux procédés technologiques différents sur une seule et même puce. En plus de limiter le nombre de composants externes, cela permet également une meilleure gestion de la consommation dans les différents blocs digitaux, analogiques et RF. Les applications dites rapides peuvent ainsi profiter du meilleur des composants bipolaires et des transistors CMOS. Le défi est d’autant plus critique dans le cas des applications analogiques/RF puisqu’il est nécessaire de diminuer la puissance consommée tout en maintenant des fréquences de fonctionnement des transistors très élevées. Disposer de modèles compacts précis des transistors utilisés est donc primordial lors de la conception des circuits utilisés pour les applications analogiques et mixtes. Cette précision implique une étude sur un large domaine de tensions d’utilisation et de températures de fonctionnement. De plus, en allant vers des nœuds technologiques de plus en plus avancés, des nouveaux effets physiques se manifestent et doivent être pris en compte dans les équations du modèle. Les règles d’échelle des technologies plus matures doivent ainsi être réexaminées en se basant sur la physique du dispositif. Cette thèse a pour but d’évaluer la faisabilité d’une offre de modèle compact dédiée à la technologie avancée SiGe TBH de chez ST Microelectronics. Le modèle du transistor bipolaire SiGe TBH est présenté en se basant sur le modèle compact récent HICUMversion L2.3x. Grâce aux lois d’échelle introduites et basées sur le dessin même des dimensions du transistor, une simulation précise du comportement électrique et thermique a pu être démontrée.Ceci a été rendu possible grâce à l’utilisation et à l’amélioration des routines et méthodes d’extraction des paramètres du modèle. C’est particulièrement le cas pour la détermination des éléments parasites extrinsèques (résistances et capacités) ainsi que celle du transistor intrinsèque. Finalement, les différentes étapes d’extraction et les méthodes sont présentées, et ont été vérifiées par l’extraction de bibliothèques SPICE sur le TBH NPN Haute-Vitesse de la technologie BiCMOS avancée du noeud 55nm, avec des fréquences de fonctionnement atteignant 320/370GHz de fT = fmax
The aim of BiCMOS technology is to combine two different process technologies intoa single chip, reducing the number of external components and optimizing power consumptionfor RF, analog and digital parts in one single package. Given the respectivestrengths of HBT and CMOS devices, especially high speed applications benefit fromadvanced BiCMOS processes, that integrate two different technologies.For analog mixed-signal RF and microwave circuitry, the push towards lower powerand higher speed imposes requirements and presents challenges not faced by digitalcircuit designs. Accurate compact device models, predicting device behaviour undera variety of bias as well as ambient temperatures, are crucial for the development oflarge scale circuits and create advanced designs with first-pass success.As technology advances, these models have to cover an increasing number of physicaleffects and model equations have to be continuously re-evaluated and adapted. Likewiseprocess scaling has to be verified and reflected by scaling laws, which are closelyrelated to device physics.This thesis examines the suitability of the model formulation for applicability to production-ready SiGe HBT processes. A derivation of the most recent model formulationimplemented in HICUM version L2.3x, is followed by simulation studies, whichconfirm their agreement with electrical characteristics of high-speed devices. Thefundamental geometry scaling laws, as implemented in the custom-developed modellibrary, are described in detail with a strong link to the specific device architecture.In order to correctly determine the respective model parameters, newly developed andexisting extraction routines have been exercised with recent HBT technology generationsand benchmarked by means of numerical device simulation, where applicable.Especially the extraction of extrinsic elements such as series resistances and parasiticcapacitances were improved along with the substrate network.The extraction steps and methods required to obtain a fully scalable model library wereexercised and presented using measured data from a recent industry-leading 55nmSiGe BiCMOS process, reaching switching speeds in excess of 300GHz. Finally theextracted model card was verified for the respective technology

This work represents several years' research into the field of radiation hardening by design. The unique characteristics of a SiGe HBT, described in Chapter 1, make it ideally suitable for use in extreme environment applications. Chapter 2 describes the total ionizing dose effects experienced by a SiGe HBT, particularly those experienced on an Earth-orbital or lunar-surface mission. In addition, the effects of total dose are evaluated on passive devices. As opposed to the TID-hardness of SiGe transistors, a clear vulnerability to single-event effects does exist. This field is divided into three chapters. First, the very nature of single-event transients present in SiGe HBTs is explored in Chapter 3 using a heavy-ion microbeam with both bulk and SOI platforms [31]. Then, in Chapter 4, a new device-level SEU-hardening technique is presented along with circuit-design techniques necessarily for its implementation. In Chapter 5, the circuit-level radiation-hardening techniques necessarily to mitigate the effects shown in Chapter 3 are developed and tested [32]. Finally, in Chapter 6, the performance of the SiGe HBT in a cryogenic testing environment is characterized to understand how the widely-varying temperatures of outer space may affect device performance. Ultimately, the built-in performance, TID-tolerance, and now-developing SEU-hardness of the SiGe HBT make a compelling case for extreme environment electronics. The low-cost, high-yield, and maturity of Si manufacturing combine with modern bandgap engineering and modern CMOS to produce a high-quality, high-performance BiCMOS platform suitable for space-borne systems.

This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within the inorganic semiconductors. Inorganic semiconductor nanowires and their blends with semiconducting polymers have been investigated using state-of-the-art ultrafast optical techniques to provide information on the sub-picosecond to nanosecond photoexcitation dynamics in these systems. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising nanowires in hybrid organic photovoltaic devices, revealing the performances to date. The experimental methods used during the thesis are detailed in Chapter 3. Chapter 4 describes the crucial roles of surface passivation on the ultrafast dynamics of exciton formation in gallium arsenide (GaAs) nanowires. By passivating the surface states of nanowires, exciton formation via the bimolecular conversion of electron-hole plasma can observed over few hundred picoseconds, in-contrast to the fast carrier trapping in 10 ps observed in the uncoated nanowires. Chapter 5 presents a novel method to passivate the surface-states of GaAs nanowires using semiconducting polymer. The carrier lifetime in the nanowires can be strongly enhanced when the ionization potential of the overcoated semiconducting polymer is smaller than the work function of the nanowires and the surface native oxide layers of nanowires are removed. Finally, Chapter 6 shows that the carrier cooling in the type-II wurtzite-zincblend InP nanowires is reduced by order-of magnitude during the spatial charge-transfer across the type-II heterojunction. The works decribed in this thesis reveals the crucial role of surface-states and bulk defects on the carrier dynamics of semiconductor nanowires. In-addition, a novel approach to passivate the surface defect states of nanowires using semiconducting polymers was developed.

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 devices
Organische 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

Organic solar cells have great potential for cost-effective and large area electricity production, but their applicability is limited by the relatively low efficiency. In this dissertation I report investigations of novel materials and the underlying principles of organic solar cells, carried out at the University of St Andrews between 2011 and 2015. Key results of this investigation: • The charge carrier mobility of organic semiconductors in the active layer of polymer solar cells has a rather small influence on the power conversion efficiency. Cooling solar cells of the polymer:fullerene blend PTB7:PC₇₁BM from room temperature to 77 K decreased the hole mobility by a factor of thousand but the device efficiency only halved. • Subphthalocyanine molecules, which are commonly used as electron donor materials in vacuum-deposited active layers of organic solar cells, can, by a slight structural modification, also be used as efficient electron acceptor materials in solution-deposited active layers. Additionally these acceptors offer, compared to standard fullerene acceptors,advantages of a stronger light absorption at the peak of the solar spectrum. • A low band-gap polymer donor material requires a careful selection of the acceptor material in order to achieve efficient charge separation and a maximum open circuit voltage. • Metal structures in nanometer-size can efficiently enhance the electric field and light absorption in organic semiconductors by plasmonic resonance. The fluorescence of a P3HT polymer film above silver nanowires, separated by PEDOT:PSS, increased by factor of two. This could be clearly assigned to an enhanced absorption as the radiative transition of P3HT was identical beside the nanowires. • The use of a processing additive in the casting solution for the active layer of organic solar cells of PTB7:PC₇₁BM strongly influences the morphology, which leads not only to an optimum of charge separation but also to optimal charge collection.

Des heterostructures de semiconducteurs ii-vi en cdte-cdmnte sont elaborees par la technique d'epitaxie par jets moleculaires (ejm). Elles sont ensuite caracterisees et etudiees en vue d'applications. Les conditions de croissance des couches de cdmnte sont optimisees selon les deux directions cristallographiques <001> et <111>. Le desaccord de maille entre le puits cdte et les barrieres cdmnte limite les structures coherentes realisables. La composition de l'alliage cdmnte, ainsi que les epaisseurs des couches sont maitrisees. Des structures de bonne qualite structurale et optique ont ete obtenues, ce qui a permis d'etudier des structures laser et des structures piezoelectriques. Des laser visibles compacts pompes electroniquement ont ete realises. Le dispositif s'appelle laser a semiconducteur a micropointe (lsm). La zone active est constituee d'une heterostructure a confinement separe et a gradient d'indice lineaire (l. Grinsch) epitaxiee selon la direction <001>. La longueur d'onde d'emission laser atteinte avec cdmnte est dans le rouge a temperature ambiante. La croissance de puits quantiques selon l'axe polaire <111> est tres interessante: l'existence d'un grand champ piezoelectrique (de l'ordre de 100000 v/cm) a ete mis en evidence experimentalement. Ce champ modifie la structure electronique et les proprietes optiques de l'heterostructure. Lors de l'ecrantage du champ par des porteurs photocrees, la raie se decale vers le bleu, ce qui est prometteur pour les applications en optique non lineaire ou en electro-optique. Pour expliquer les resultats experimentaux, l'hypothese d'une non-linearite du coefficient piezoelectrique a ete emis

The field of infrared (IR) detector technology has shown vast improvements in terms of speed and performance over the years. Specifically the dynamic range (DR) and sensitivity of detectors showed significant improvements. The most commonly used technique of implementing these IR detectors is the use of charge-coupled devices (CCD). Recent developments show that the newly investigated bipolar complementary metal-oxide semiconductor (BiCMOS) devices in the field of detector technology are capable of producing similar quality detectors at a fraction of the cost. Prototyping is usually performed on low-cost silicon wafers. The band gap energy of silicon is 1.17 eV, which is too large for an electron to be released when radiation is received in the IR band. This means that silicon is not a viable material for detection in the IR band. Germanium exhibits a band gap energy of 0.66 eV, which makes it a better material for IR detection. This research is aimed at improving DR and sensitivity in IR detectors. CCD technology has shown that it exhibits good DR and sensitivity in the IR band. CMOS technology exhibits a reduction in prototyping cost which, together with electronic design automation software, makes this an avenue for IR detector prototyping. The focus of this research is firstly on understanding the theory behind the functionality and performance of IR detectors. Secondly, associated with this, is determining whether the performance of IR detectors can be improved by using silicon germanium (SiGe) BiCMOS technology instead of the CCD technology most commonly used. The Simulation Program with Integrated Circuit Emphasis (SPICE) was used to realise the IR detector in software. Four detectors were designed and prototyped using the 0.35 µm SiGe BiCMOS technology from ams AG as part of the experimental verification of the formulated hypothesis. Two different pixel structures were used in the four detectors, which is the silicon-only p-i-n diodes commonly found in literature and diode-connected SiGe heterojunction bipolar transistors (HBTs). These two categories can be subdivided into two more categories, which are the single-pixel-single-amplifier detectors and the multiple-pixel-single-amplifier detector. These were needed to assess the noise performance of different topologies. Noise influences both the DR and sensitivity of the detector. The results show a unique shift of the detecting band typically seen for silicon detectors to the IR band, accomplished by using the doping feature of HBTs using germanium. The shift in detecting band is from a peak of 250 nm to 665 nm. The detector still accumulates radiation in the visible band, but a significant portion of the near-IR band is also detected. This can be attributed to the reduced band gap energy that silicon with doped germanium exhibits. This, however, is not the optimum structure for IR detection. Future work that can be done based on this work is that the pixel structure can be optimised to move the detecting band even more into the IR region, and not just partially.
Dissertation (MEng)--University of Pretoria, 2013.
Electrical, Electronic and Computer Engineering
unrestricted

abstract: Silicon photonic technology continues to dominate the solar industry driven by steady improvement in device and module efficiencies. Currently, the world record conversion efficiency (~26.6%) for single junction silicon solar cell technologies is held by silicon heterojunction (SHJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). These solar cells utilize the concept of carrier selective contacts to improve device efficiencies. A carrier selective contact is designed to optimize the collection of majority carriers while blocking the collection of minority carriers. In the case of SHJ cells, a thin intrinsic a-Si:H layer provides crucial passivation between doped a-Si:H and the c-Si absorber that is required to create a high efficiency cell. There has been much debate regarding the role of the intrinsic a-Si:H passivation layer on the transport of photogenerated carriers, and its role in optimizing device performance. In this work, a multiscale model is presented which utilizes different simulation methodologies to study interfacial transport across the intrinsic a-Si:H/c-Si heterointerface and through the a-Si:H passivation layer. In particular, an ensemble Monte Carlo simulator was developed to study high field behavior of photogenerated carriers at the intrinsic a-Si:H/c-Si heterointerface, a kinetic Monte Carlo program was used to study transport of photogenerated carriers across the intrinsic a-Si:H passivation layer, and a drift-diffusion model was developed to model the behavior in the quasi-neutral regions of the solar cell. This work reports de-coupled and self-consistent simulations to fully understand the role and effect of transport across the a-Si:H passivation layer in silicon heterojunction solar cells, and relates this to overall solar cell device performance.
Dissertation/Thesis
Doctoral Dissertation Electrical Engineering 2019

The performance of III-V heterojunction bipolar transistors (HBTs) has improved significantly over the past two decades. Today’s state of the art Indium Phosphide (InP) HBTs have a maximum frequency of oscillation greater than 800 GHz and have been used to realize an amplifier operating above 600 GHz . In comparison to silicon (Si) based devices, III-V HBTs have superior transport properties that enables a higher gain, higher speed, and noise performance, and much higher Johnson figure- of-merit . From this perspective, the InP HBT is one of the most promising candidates for high performance mixed signal electronic systems.

The radiation effects in III-V heterojunction devices are investigated in this thesis. Two types of heterojunction devices studied are InGaP/GaAs single heterojunction bipolar transistors (SHBTs) and GaN-based heterojunction light emitting diodes (LEDs). InGaP/GaAS HBTs are investigated for high energy (67 and 105 MeV) proton irradiation effects while GaN heterojunction LEDs are studied for neutron irradiation effects. A compact model and the parameter extraction procedures for HBTs are developed, and hence the I[subscript C]--V[subscript CE] characteristics of pre- and post-irradiation HBTs can be simulated by employing the developed model. HBTs are electrically characterized before and after proton irradiation. Overall, the studied HBT devices are quite robust against high energy proton irradiation. The most pronounced radiation effect shown in SHBTs is gain degradation. Displacement damage in the bulk of base-emitter space-charge region, leading to excess base current, is the responsible mechanism for the proton-induced gain degradation. The performance degradation depends on the operating current and is generally less at higher currents. Compared to the MBE grown devices, the MOVPE grown HBTs show superior characteristics both in initial performance and in proton irradiation hardness. The 67 MeV protons cause more damage than 105 MeV protons due to their higher value of NIEL (non-ionizing energy loss). The HBT I-V characteristics of pre- and post-irradiated samples can be simulated successfully by employing the developed model. GaN heterojunction LEDs are electrically and optically characterized before and after neutron irradiation. Neutron irradiation causes changes in both the I-V characteristic and the light output. Atomic displacement is responsible for both electrical and optical degradation. Both electrical and optical properties degrade steadily with neutron fluence producing severe degradation after the highest fluence neutron irradiation. The light output degrades by more than 99% after 1.6x10����� n/cm�� neutron irradiation, and the radiation damage depends on the operating current and is generally less at higher currents.
Graduation date: 2003

碩士
國立中央大學
電機工程學系
103
In this thesis, we develop vacuum level system, E_0 method, for numerical 2-D heterojunction device simulation. In the past, our program use intrinsic Fermi level, E_fi, to calculate carrier concentration and potential. However, the E_fi modeling has problem in heterojunction. The E_0 method has been verified by a pN heterojunction. And discuss problems of simulation happened on abrupt. Finally, we use the E_0 method to compare HBT with BJT to discuss the electrical characteristics.

Present day data processing technology requires very high speed signal processing and data conversion rates. Traditionally, these circuits have been implemented in silicon MOS technology, whose high speed performance is limited, due to inherent material properties. Though relatively immature compared to silicon technology, GaAs integrated circuit technology appears to be a potential vehicle for realizing high-speed circuits because of its high electron mobility and low parasitic capacitance. One major drawback of GaAs technology has been the lack of complementary technology in contrast to silicon where CMOS technology has greatly facilitated the development of analog ICs. This thesis investigates the suitability of complementary GaAs Heterojunction FET integrated circuit technology for the realization of high sample-rate switched-capacitor circuits. In order to yield an accurate device model for the design work, model parameters of both n and p GaAs Heterojunction FET devices are extracted from measurement results. Based on the extraction results, a set of analog building blocks are presented. These circuits include a high bandwidth operational amplifier and a fast settling switch which are essential for high sample-rate circuits. A second order switched-capacitor low pass filter sampling at a clock rate of 100MHz is designed using the above building blocks. The designs studied predict better high frequency performance for C-HFETs compared to Si CMOS technology.
Graduation date: 1994

碩士
國立中央大學
電機工程學系
101
Since low-power consumption and high performance are required in the integrated circuits, Sb-based materials are considered to be high potential candidates in high speed electronic device applications, due to the InAs and InGaSb showing highest carrier mobility properties among III-V compound semiconductors. Among a number of high-k dielectric materials, Al2O3 become one of the candidates for high dielectric constant materials used for most electronics. The major reasons are that Al2O3 shows a large dielectric constant, a large bandgap, a high breakdown field, a good thermal stability, and amorphous type of crystal structure. Therefore, in this thesis, the high dielectric constant material of Al2O3 had been used as gate dielectric to develope the Sb-based metal-insulator-semiconductor heterojunction field effect transistors (MIS-HFETs) with in-depth analysis and discussion. 　　We first analyzed physical and optical properties of the Al2O3 thin film deposited by atomic layer deposition. The physical properties analysis included film thickness, surface roughness and the optical properties by ellipsometer. The various surface treatments of InAs substrate using different chemical solutions were studied in order to investigate the properties of the InAs MOS capacitors. The electrical property characteristics included C-V, I-V and J-E measurements were characterized to obtain the optimum conditions of Al2O3 film for MIS-HFET application. Finally, we fabricated MIS-HFETs using the Al2O3 deposited by ALD as gate dielectric on the conventional InAs/AlSb and InGaSb/AlSb HFET epitaxy materials. The fabricated InAs/AlSb n-channel MIS-HFET with a gate length of 2 μm, demonstrated the maximum drain current (IDSS) of 371 mA/mm, a transconductance (Gm) of 604 mS/mm, and a subthreshold slope is 137 mV/dec, and a peak current gain cut-off frequency (fT) of 10.6 GHz. The InGaSb/AlSb p-channel MIS-HFET with 1μm gate length showed, the maximum drain current of 31 mA/mm, transconductance of 43 mS/mm, and the subthreshold slope of 153 mV/dec.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis describes investigations concerning the physics and applications of band edge discontinuities in semiconductor heterojunctions. An historical overview and an assessment of the current experimental and theoretical understanding of band offsets are presented in Chapter 1. The physics and potential device applications of tunneling phenomena in semiconductor heterostructures are also introduced. Part I of the thesis describes measurements of semiconductor heterojunction valence-band offsets by x-ray photoelectron spectroscopy (XPS). In Chapter 2, the basic physical principles of XPS are discussed, with emphasis on capabilities of the technique that are especially relevant in studies of semiconductor interfaces. The experimental procedures and data analysis techniques required to measure band offsets by XPS are also described. A measurement of the GaAs/AlAs (100) valence-band offset is presented in Chapter 3. Our results are discussed in the context of the recent controversy surrounding the value of the GaAs/AlAs band offset. The relationship between band offset commutativity and interfacial quality as elucidated by our experiments and by previously reported results is also explained. A measurement of the valence band offset for the lattice-mismatched Si/Ge (001) heterojunction system is described in Chapter 4. A method developed to extend the applicability of the XPS technique to the determination of strain-dependent band offsets is presented and applied to the Si/Ge (001) material system. Our results demonstrate the profound influence of strain on band offset values in lattice-mismatched heterojunctions. An interpolation scheme for determining band offset values for [...] alloy heterojunctions is described, and is used to demonstrate the consistency of our results with Si/[...] modulation doping experiments that have been reported. Studies of band offsets and interfacial reactions in III-V/II-VI heterojunction systems are reported in Chapter 5. Specifically, band offsets and interface chemistry have been examined in the A1Sb/GaSb/ZnTe material system; the influence of interfacial chemistry on band offset values is demonstrated, and implications of our work for the viability of proposed heterojunction approaches for fabricating visible light emitters are discussed. The measurement of the CdSe/ZnTe (100) valence band offset is described in Chapter 6. Our results are discussed in the context of possible heterojunction approaches for injection of electrons into p-ZnTe for fabricating visible light emitters, and implications of our results for the common anion rule are addressed. Part II of the thesis describes theoretical and experimental studies of tunneling in semiconductor heterostructures. Various theoretical models we have used to simulate the electrical behavior of a wide variety of heterostructure devices are described in Chapter 7. These techniques are applied to the calculation of current-voltage characteristics for double-barrier tunnel structures proposed in the GaAs/ZnSe and InAs/ZnTe material systems. Theoretical and experimental studies of interband transport in the InAs/GaSb/AlSb material system are described in Chapter 8. Band alignments in the InAs/GaSb/AlSb system and material properties of InAs and GaSb are discussed in the context of both fundamental physics and device applications. Attention is then focused on a theoretical and experimental study of transport in the InAs/GaSb/InAs device structure that helped to clarify the basic nature of interband transport and the coupling between conduction-band and valence-band states in different layers of a heterostructure. In Chapter 9, a theoretical study of hole tunneling times in GaAs/AlAs double-barrier heterostructures is presented. The experimental observation of anomalously short hole tunneling times that motivated this work is reviewed, and a phenomenological model explaining this experimental observation as a consequence of valence-band-mixing is developed.

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 devices.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers
Organische 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.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers