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Showing 11 to 19 of 19 (0.019 seconds)Spelling suggestions: "subject:"[een] SINGLE CARRIER"" "subject:"[enn] SINGLE CARRIER""
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Block Transmissions On Orthogonal CarriersYazici, Ayhan 01 September 2005 (has links) (PDF)
Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier
Block Transmissions (SCBT) are located at the two opposite edges of block
transmission concept. In this thesis a system which lies between OFDM
and SCBT is proposed. The new system, namely Block Transmissions on
Orthogonal Carriers (BTOC), can be considered as a hybrid form of OFDM
and SCBT. BTOC system is investigated under the redundant filterbank
precoders and equalizers framework. Peak to average power ratio (PAPR) of
BTOC is formulated and compared with the PAPRs of OFDM and SCBT.
Effect of frequency offset for BTOC is investigated and comparison between
OFDM, SCBT, and BTOC is presented. Simulation results of Zero Padded
OFDM (ZP-OFDM), SCBT, and BTOC are included.
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PA efficiency enhancement using digital linearization techniques in uplink cognitive radio systems / Amélioration du rendement de l’amplificateur de puissance en utilisant une technique de linéarisation numérique pour une liaison montante dans un contexte radio intelligente.Ben mabrouk, Mouna 02 December 2015 (has links)
Pour un terminal mobile alimenté sur batterie, le rendement de l’amplificateur de puissance (AP) doit êtreoptimisé. Cette optimisation peut rendre non-linéaire la fonction d’amplification de l’AP. Pour compenser lesdistorsions introduites par le caractère non-linéaire de l’AP, un détecteur numérique fondé sur un modèle deVolterra peut être utilisé. Le comportement de l’AP et le canal étant modélisé par le modèle de Volterra, uneapproche par filtrage de Kalman (FK) permet d’estimer conjointement les noyaux de Volterra et les symbolestransmis. Dans ce travail, nous proposons de traiter cette problématique dans le cadre d’une liaison montantedans un contexte radio intelligente (RI). Dans ce cas, des contraintes supplémentaires doivent être prises encompte. En effet, étant donné que la RI peut changer de bande de fréquence de fonctionnement, les nonlinéaritésde l’AP peuvent varier en fonction du temps. Par conséquent, nous proposons de concevoir une postdistorsionnumérique fondée sur une modélisation par modèles multiples combinant plusieurs estimateurs àbase de FK. Les différents FK permettant de prendre en compte les différentes dynamiques du modèle.Ainsi, les variations temporelles des noyaux de Volterra peuvent être suivies tout en gardant des estimationsprécises lorsque ces noyaux sont statiques. Le cas d’un signal monoporteuse est adressé et validé par desrésultats de simulation. Enfin, la pertinence de l’approche proposée est confirmée par des mesures effectuéessur un AP large bande (300-3000) MHz. / For a battery driven terminal, the power amplifier (PA) efficiency must be optimized. Consequently,non-linearities may appear at the PA output in the transmission chain. To compensatethese distortions, one solution consists in using a digital post-distorter based on aVolterra model of both the PA and the channel and a Kalman filter (KF) based algorithm tojointly estimate the Volterra kernels and the transmitted symbols. Here, we suggest addressingthis issue when dealing with uplink cognitive radio (CR) system. In this case, additionalconstraints must be taken into account. Since the CR terminal may switch from one subbandto another, the PA non-linearities may vary over time. Therefore, we propose to designa digital post-distorter based on an interacting multiple model combining various KF basedestimators using different model parameter dynamics. This makes it possible to track thetime variations of the Volterra kernels while keeping accurate estimates when those parametersare static. Furthermore, the single carrier case is addressed and validated by simulationresults. In addition, the relevance of the proposed approach is confirmed by measurementscarried on a (300-3000) MHz broadband PA.
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Multicarrier communication systems with low sensibility to nonlinear amplificationDeumal Herraiz, Marc 11 July 2008 (has links)
Actualment estem entrant a una nova era de la informació amb gran demanda de sistemes de comunicació sense fils. Nous serveis com dades i video requereixen transmissions fiables d'alta velocitat, fins i tot en escenaris d'alta mobilitat. A més a més, la dificultat d'assignar el limitat espectre radioelèctric juntament amb la necessitat d'incrementar el temps de vida de les bateries dels terminals mòbils, requereix el diseny de transceptors que usin la potència i l'ampla de banda disponibles de manera eficient. Les comunicacions multiportadora basades en OFDM són capaces de satisfer la majoria d'aquests requeriments. Però, entre altres reptes, reduir la sensibilitat a la amplificació no-lineal és un factor clau durant el diseny. En aquesta tesi doctoral s'analitza la sensibilitat dels sistemes multiportadora basats en OFDM a l'amplificació no-lineal i es consideren formes eficients per superar aquest problema. La tesi s'enfoca principalment al problema de reduir les fluctuacions de l'envolupant del senyal transmès. En aquest sentit es presenta també un estudi de les mètriques de l'envolupant del senyal, PAPR i CM. A més a més, basant-nos en l'anàlisis presentat es proposen noves tècniques per sistemes OFDM i MC-SS. Per MC-SS, també es tracta el diseny d'una tècnica de postprocessament en forma de detector multiusuari per canals no-lineals. / Actualmente estamos entrando en una nueva era de la información donde se da una gran demanda de sistemas de comunicación inalámbricos. Nuevos servicios como datos y vídeo requieren transmisiones fiables de alta velocidad, incluso en escenarios de alta movilidad. Además, la dificultad de asignar el limitado espectro radioeléctrico junto con la necesidad de incrementar el tiempo de vida de las baterías de los terminales móviles, requiere el diseño de transceptores que usen eficientemente la potencia y el ancho de banda disponibles. Las comunicaciones multiportadora basadas en OFDM son capaces de satisfacer la mayoría de dichos requerimientos. Sin embargo, entre otros retos, reducir su sensibilidad a la amplificación no-lineal es un factor clave durante el diseño. En esta tesis se analiza la sensibilidad de los sistemas multiportadora basados en OFDM a la amplificación no-lineal y se consideran formas eficientes para superar dicho problema. La tesis se enfoca principalmente al problema de reducir las fluctuaciones de la envolvente. En este sentido también se presenta un estudio de las métricas de la señal, PAPR y CM. Además, basándonos en el análisis presentado se proponen nuevas técnicas para OFDM y MC-SS. Para MC-SS, también se trata el diseño de un detector multiusuario para canales no-lineales. / We are now facing a new information age with high demand of wireless communication systems. New services such as data and video require achieving reliable high-speed transmissions even in high mobility scenarios. Moreover, the difficulty to allocate so many wireless communication systems in the limited frequency band in addition to the demand for long battery life requires designing spectrum and power efficient transceivers. Multicarrier communications based on OFDM are known to fulfill most of the requirements of such systems. However, among other challenges, reducing the sensitivity to nonlinear amplification has become a design key. In this thesis the sensitivity of OFDM-based multicarrier systems to nonlinear amplification is analyzed and efficient ways to overcome this problem are considered. The focus is mainly on the problem of reducing the envelope fluctuations. Therefore, a study of the signal metrics, namely PAPR and CM, is also presented. From the presented analysis, several new techniques for OFDM and MC-SS are proposed. For MC-SS, the design of a post-processing technique in the form of a multiuser detector for nonlinearly distorted MC-SS symbols is also addressed.
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[en] CHIP SPREAD CDMA TRANSMISSION UNDER NON-IDEAL CONDITION: A COMPARATIVE ANALYSIS / [pt] TRANSMISSÃO CHIP-SPREAD CDMA EM CONDIÇÕES NÃO IDEAIS: UMA ANÁLISE COMPARATIVADARWIN MARCIAL PEREIRA ELVIR 23 March 2018 (has links)
[pt] Este trabalho apresenta uma análise detalhada de uma recente proposta de combinação da transmissão em blocos com portadora única e a técnica CDMA, referida como CS-CDMA (Chip Spread CDMA). Uma característica marcante desta técnica de transmissão é que diferentemente do que ocorre nos sistemas DS-CDMA, a ortogonalidade entre os códigos dos diferentes usuários é mantida mesmo quando a transmissão é feita através de um canal multipercurso seletivo em frequência, suposto invariante no tempo, permitindo assim que os usuários possam ser identicamente desacoplados na recepção. Além desta vantagem, resultados de desempenho indicaram uma significativa superioridade deste sistema sobre o tradicional DS-CDMA. Entretanto comparações existentes, consideraram apenas o up-link, do sistema e adotaram algumas premissas, que incluem, a utilização de códigos ortogonais pelos diversos usuários e a suposição de canais de transmissão invariantes no tempo. O presente trabalho apresenta uma análise detalhada da técnica (CS-CDMA e uma análise comparativa dos sistemas em condições menos favoráveis. Os sistemas operam em ambientes invariante e variante no tempo, com códigos ortogonais e não ortogonais e em dois cenários diferentes, down- link e up-link. Os resultados consideram recepção com equalização no domínio da frequência utilizando equalizadores do tipo ZF (Zero Forcing) e MMSE (Minimum Mean Squared Error). Simulações foram realizadas no intuito de se avaliar o desempenho dos dois sistemas considerados. Curvas de probabilidade de erro foram obtidas e ilustram e comparam tais desempenhos em diferentes situações e cenários de interesse. / [en] This dissertation proposes detailed analysis of a recent combined mechanism for transmission in blocks with a single carrier and CDMA technique known as Chip Spread CDMA (CS-CDMA). An important feature
of this transmission technique is that unlike what happens in Direct Sequence (DS-CDMA) systems, the orthogonality between codes of different users is maintained even when the transmission, considered time-invariant, is made through a selective multipath channel frequency, which ideally allows users to be uncoupled in reception. However, existing comparisons only consider the up-link transmission and adopt certain assumptions, which include orthogonal codes for different users and time invariant channels. This technique have shown a significant superiority as compared with the traditional DS-CDMA. A comparison of (CS-CDMA systems in more realistic conditions are presented in this work. Various environments were tested in the presence of BPSK modulation systems, as well as invariant and time-varying transmission.
Comparison between scenarios down-link and up-link are also presented. The results consider equalized reception in the frequency domain using the ZF (Zero Forcing) equalizers and MMSE (Minimum Mean Squared Error). Simulations were carried out in order to evaluate the performance of the two systems considered. Error probability curves were obtained to illustrate and compare the performances in different situations and scenarios.
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Joint multiuser power allocation and iterative multi-antenna receiver designTervo, V. (Valtteri) 20 January 2015 (has links)
Abstract
This thesis concentrates on joint optimization of transmit power allocation and receive filtering in multiuser, multi-antenna communications. Due to the increasing number of wireless devices, the design of energy-efficient communication links is becoming increasingly important. In cellular mobile communications, reducing the average power consumption in uplink transmission is beneficial for users in order to extend battery life and, hence, energy efficiency in general. However, the power consumption of the high power amplifier (HPA) at the transmitter depends on the peak power of the transmission. This thesis focuses on power allocation problems for single-carrier (SC) frequency division multiple access (FDMA) and orthogonal FDMA (OFDMA) transmission assuming iterative reception.
The goal in the first scheme presented in this thesis is to reduce the average power consumption by designing a power allocation method that takes into account the convergence properties of an iterative receiver in multiuser uplink communications. The proposed scheme can guarantee that the desired quality of service (QoS) is achieved after a sufficient number of iterations.
Reducing the peak-to-average power ratio (PAPR) in any transmission system is beneficial because it allows the use of inexpensive, energy-efficient power amplifiers. The goal in the second scheme presented in this thesis is to control the PAPR of the transmitted signal. Hence, in addition to the QoS constraint, the instantaneous PAPR constraint is derived for SC-FDMA and OFDMA transmission. Moreover, a statistical approach is considered in which the power variance of the transmitted waveform is controlled. The QoS and PAPR constraints are considered jointly and, therefore, the proposed power allocation strategy jointly takes into account the channel quality and the PAPR characteristics of the power amplifier. However, the PAPR constraint can be adopted to any SC-FDMA or OFDMA framework and it is not restricted to the scheme presented in this thesis. The objective of the optimization problems considered throughout the thesis is to minimize the sum power. The majority of the derived constraints are non-convex and therefore, two alternative successive convex approximations (SCAs) are derived for all the non-convex constraints considered.
The numerical results show that the proposed power allocation strategies can significantly reduce the average transmission power of users while allowing flexible PAPR control. Hence, the proposed methods can be used to extend battery life for users and especially improve the QoS at the cell edges. / Tiivistelmä
Väitöskirjassa tutkitaan lähettimessä tapahtuvan tehoallokoinnin sekä vastaanottimessa tapahtuvan signaalin suodatuksen yhteisoptimointia monikäyttöön suunnatussa langattomassa moniantennikommunikaatiossa. Langattomien laitteiden lukumäärän kasvaessa energiatehokkuuden merkitys tiedonsiirtolinkkien suunnittelussa korostuu. Soluihin perustuvassa langattomassa tietoliikenteessä keskimääräisen tehonkulutuksen pienentäminen ylälinkkilähetyksessä (käyttäjältä tukiasemaan) on tärkeää käyttäjän kannalta, sillä se pidentää laitteen akun kestoa. Lähettimen tehovahvistimen (high power amplifier (HPA)) tehonkulutus on kuitenkin verrannollinen lähetyksen huipputehoon. Väitöskirjassa luodaaan uusia menetelmiä sekä vertaillaan tehoallokointia yhden kantoaallon taajuustason monikäyttöön (single carrier frequency division multiple access (SC-FDMA)) ja ortogonaalisen taajuustason monikäyttöön (orthogonal FDMA (OFDMA)) perustuvissa lähetysteknologioissa.
Työn ensimmäisessä osiossa tavoitteena on keskimääräisen tehonkulutuksen pienentäminen monen käyttäjän ylälinkkikommunikaatiossa suunnittelemalla tehoallokointimenetelmä, joka ottaa huomioon iteratiivisen vastaanottimen konvergenssiominaisuudet. Työssä ehdotettu menetelmä takaa vastaanotetun informaation halutun laadun (quality of service (QoS)) riittävän monen vastaanottimessa tehdyn iteraation jälkeen.
Huipputehon ja keskitehon suhteen (peak to average power ratio (PAPR)) pienentäminen missä tahansa lähetyksessä on hyödyllistä, sillä sen ansiosta voidaan käyttää energiatehokkaampia ja halvempia tehovahvistimia. Työn jälkimmäisessä osiossa tavoitteena on kontrolloida lähetetyn signaalin huipputehon ja keskitehon suhdetta. Työn ensimmäisessä osiossa esitetyn QoS-rajoitteen lisäksi tehoallokointia rajoitetaan symbolisekvenssikohtaisella PAPR-rajoitteella SCFDMA- ja OFDMA-lähetyksessä. Lisäksi esitetään tilastollinen menetelmä, jossa rajoitetaan lähetetyn signaalin tehon varianssia. Kun käytetään yhtäaikaisesti QoS- ja PAPR-rajoitteita, voidaan tiedonsiirtokanavaan suunnitella optimaalinen tehoallokointi ottaen huomioon tehovahvistimen epälineaarisuudet. Työssä esitetty PAPR-rajoite on kuitenkin geneerinen, ja se voidaan sovittaa mihin tahansa SCFDMA- tai OFDMA- optimointikehykseen. Työssä esitettävien optimointiongelmien tavoitteena on käyttäjien summatehon minimointi. Suurin osa työssä esiintyvistä ongelmista on ei-konvekseja, joten siinä esitetään kaksi vaihtoehtoista peräkkäinen konveksi approksimaatio (successive convex approximation (SCA)) -menetelmää kaikille ei-konvekseille rajoitteille.
Numeeriset tulokset osoittavat, että esitetyt tehoallokointimenetelmät pienentävät merkittävästi keskimääräistä tehonkulutusta mahdollistaen lisäksi adaptiivisen PAPR-kontrolloinnin. Väitöskirjassa esitettyjen menetelmien avulla voidaan pidentää mobiilikäyttäjien akun kestoa sekä erityisesti parantaa solun reunakäyttäjien palvelun laatua.
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[pt] BRINCANDO COM BLOCOS: VARIAÇÕES DA TRANSMISSÃO EM BLOCOS E SUAS COMBINAÇÕES COM A TÉCNICA DE MÚLTIPLO ACESSO CDMA / [en] PLAYING WHIT BLOCKS: VARIATIONS OF BLOCKS TRANSMISSION AND THEIR COMBINATIONS WITH THE TECHNIQUE OF MULTIPLE ACCESS CDMA17 November 2021 (has links)
[pt] Além de apresentar uma visão unificada das técnicas de transmissão digital
em blocos e suas combinações com a técnica de múltiplo acesso CDMA
(Code Division Multiple Access), esta dissertação examina uma nova possível
técnica de transmissão para sistemas com múltiplo acesso, aqui chamada
CS-CDMA (Chip-Spread Code Division Multiple Access), combinada com
as técnicas de transmissão em blocos, em portadora única (SC-Single Carrier
), gerando o esquema SC CS-CDMA, e em múltiplas portadoras OFDM
(Orthogonal Frequency Division Multiplexing), gerando o esquema MC CSCDMA.
Comparações de desempenho com os sistemas de transmissão por
blocos CDMA de portadora única conhecido como SC CDMA (Single Carrier
Code Division Multiple Access) e de múltiplas portadoras conhecido como
MC CDMA (Multi Carrier Code Division Multiple Access) são apresentadas.
Uma característica marcante da técnica de transmissão enfocada é que,
diferentemente do que ocorre nos sistemas CDMA, a ortogonalidade entre os
códigos dos diferentes usuários é mantida mesmo quando a transmissão é feita
atravéss de um canal multipercurso seletivo na frequência, permitindo assim
que os usuários possam ser idealmente desacoplados na recepção. Além desta
vantagem, e muito devido a ela, os resultados de desempenho, obtidos via
simulação, indicam uma significativa superioridade do sistema proposto CSCDMA,
quando os sinais são transmitidos em portadora única, sobre o tradicional
CDMA. Os resultados consideram recepção com equalização no domínio
da frequéncia utilizando equalizadores do tipo ZF (Zero Forcing) e MMSE
(Minimum Mean Squared Error). / [en] In addition to presenting a unified view of digital blocks transmission
technologies and their combinations with the technique of multiple access
CDMA (Code Division Multiple Access), this dissertation addresses a new
possible transmission technique for multiple access sytem, called here CS
-CDMA (Chip-Spread Code Division Multiple Access), combined with blocks
transmissions techniques, in single carrier (SC- Single Carrier ), generating
scheme SC CS-CDMA, and in multi carrier OFDM (Ortogonal Frequency
Division Multiplexing), generating escheme MC CS-CDMA. Performance
comparisons with the traditional SC CDMA (Single Carrier Code Division
Multiple Access) and MC CDMA (Multi Carrier Code Division Multiple
Access) block transmission systemare presented. A key feature of the proposed
transmission technique is that, differently from what occurs with CDMA, the
original code orthogonality between users is preserved despite propagation
through multipath frequency selective channels, thus ideally allowing the
decoupling of the signals at the receiver side. Beyond this advantage, and
mostly due to it, the performance results indicate a clear superiority of the
proposed system CS-CDMA, over the traditional CDMA, when signals are
transmitted in single carrier. The simulation results consider frequency domain
ZF (Zero Forcing) and MMSE (Minimum Mean Squared Error).
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Performance Analysis Of Multiuser/Cooperative OFDM Systems With Carrier Frequency And Timing OffsetsRaghunath, K 12 1900 (has links)
Multiuser and cooperative orthogonal frequency division multiplexing(OFDM) systems are being actively researched and adopted in wireless standards, owing to their advantages of robustness to multipath fading, modularity, and ability to achieve high data rates. In OFDM based systems, perfect frequency and timing synchronization is essential to maintain orthogonality among the subcarriers at the receiver. In multiuser OFDM on the uplink, timing offsets (TOs) and/or carrier frequency offsets (CFOs) of different users, caused due to path delay differences between different users, Doppler and/or poor oscillator alignment, can destroy orthogonality among subcarriers at the receiver. This results in multiuser interference (MUI)and consequent performance degradation. In this thesis, we are concerned with the analysis and mitigation of the effect of large CFOs and TOs in multiuser OFDM systems, including uplink orthogonal frequency division multiple access (OFDMA),uplink single-carrier frequency division multiple access(SC-FDMA), and cooperative OFDM.
Uplink OFDMA: In the first part of this thesis, we analytically quantify the effect of large CFOs and TOs on the signal-to-interference plus noise ratio(SINR) and uncoded bit error rate(BER) performance of uplink OFDMA on Rayleigh and Rician fading channels, and show analytical results to closely match with simulation results. Such an SINR/BER analysis for uplink OFDMA in the presence of both large CFOs as well as TOs has not been reported before. We also propose interference cancelling(IC) receivers to mitigate the performance degradation caused due to large CFOs and TOs of different users.
SC-FDMA versus OFDMA: An issue with uplink OFDMA is its high peak-to-average power ratio(PAPR).Uplink SC-FDMA is proposed in the standards as a good low-PAPR alternative to uplink OFDMA; e.g., SC-FDMA has been adopted in the uplink of 3GPP LTE. A comparative investigation of uplink SC-FDMA and OFDMA from a sensitivity to large CFOs and TOs view point has not been reported in the literature. Consequently, in the second part of the thesis, we carry out a comparative study of the sensitivity of SC-FDMA and OFDMA schemes to large CFOs and TOs of different users on the uplink. Our results show that while SC-FDMA achieves better performance due to its inherent frequency diversity advantage compared to OFDMA in the case of perfect synchronization, its performance can get worse than that of OFDMA in the presence of large CFOs and TOs. We further show that use of low-complexity multistage IC techniques, with the knowledge of CFOs and TOs of different users at the receiver, can restore the performance advantage of SC-FDMA over OFDMA.
Cooperative OFDM: Cooperative OFDM is becoming popular because of its ability to provide spatial diversity in systems where each node has only one antenna. In most studies on cooperative communications, perfect time synchronization among cooperating nodes is assumed. This implies that the transmissions from different cooperating nodes reach the destination receiver in orthogonal time slots. In practice, however, due to imperfect time synchronization, orthogonality among different nodes’ signals at the destination receiver can be lost, causing inter-symbol interference(ISI).In the third part of the thesis, we investigate cooperative OFDM communications using amplify-and-forward(AF) protocol at the relay, in the presence of imperfect timing synchronization. We derive analytical expressions for the ISI as function of timing offset for cooperative OFDM with AF protocol, and propose an IC receiver to mitigate the effects of timing offset induced ISI.
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Charge transport and energy levels in organic semiconductors / Ladungstransport und Energieniveaus in organischen HalbleiternWidmer, Johannes 25 November 2014 (has links) (PDF)
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.
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Charge transport and energy levels in organic semiconductorsWidmer, Johannes 02 October 2014 (has links)
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
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