Dissertationen zum Thema „Processing of organic semiconducting thin films“

Ce travail de thèse a pour but l’élaboration et la caractérisation de film minces semi-conducteurs à partir de différents matériaux p-conjugués (DAL1, BTBT, C8-BTBT-C8, C10-PBT etd iI(T2)2) pour des applications en optoélectronique. L’objectif général a porté sur l’optimisation de leurs méthodes de fabrication, de manière à améliorer les performances des composants électroniques correspondant (transistors à effet de champs et diodes électroluminescentes organiques)
This PhD work aims at the elaboration and characterization of organic semiconducting thin films based on different π-conjugated materials (DAL1, BTBT, C8-BTBT-C8, C10-PBT and iI(T2)2) for organic optoelectronics applications. The general goal is to optimize their fabrication methods in order to maximize their device performances of the corresponding optoelectronic devices (Organic field effect transistors and Organic light emitting diodes)

Organic semiconductors combine the benefits of organic materials, i.e., low-cost production, mechanical flexibility, lightweight, and robustness, with the fundamental semiconductor properties light absorption, emission, and electrical conductivity. This class of material has several advantages over conventional inorganic semiconductors that have led, for instance, to the commercialization of organic light-emitting diodes which can nowadays be found in the displays of TVs and smartphones. Moreover, organic semiconductors will possibly lead to new electronic applications which rely on the unique mechanical and electrical properties of these materials. In order to push the development and the success of organic semiconductors forward, it is essential to understand the fundamental processes in these materials. This thesis concentrates on understanding how the charge transport in thiophene-based semiconductor layers depends on the layer morphology and how the charge transport properties can be intentionally modified by doping these layers with a strong electron acceptor. By means of optical spectroscopy, the layer morphologies of poly(3-hexylthiophene), P3HT, P3HT-fullerene bulk heterojunction blends, and oligomeric polyquaterthiophene, oligo-PQT-12, are studied as a function of temperature, molecular weight, and processing conditions. The analyses rely on the decomposition of the absorption contributions from the ordered and the disordered parts of the layers. The ordered-phase spectra are analyzed using Spano’s model. It is figured out that the fraction of aggregated chains and the interconnectivity of these domains is fundamental to a high charge carrier mobility. In P3HT layers, such structures can be grown with high-molecular weight, long P3HT chains. Low and medium molecular weight P3HT layers do also contain a significant amount of chain aggregates with high intragrain mobility; however, intergranular connectivity and, therefore, efficient macroscopic charge transport are absent. In P3HT-fullerene blend layers, a highly crystalline morphology that favors the hole transport and the solar cell efficiency can be induced by annealing procedures and the choice of a high-boiling point processing solvent. Based on scanning near-field and polarization optical microscopy, the morphology of oligo-PQT-12 layers is found to be highly crystalline which explains the rather high field-effect mobility in this material as compared to low molecular weight polythiophene fractions. On the other hand, crystalline dislocations and grain boundaries are identified which clearly limit the charge carrier mobility in oligo-PQT-12 layers. The charge transport properties of organic semiconductors can be widely tuned by molecular doping. Indeed, molecular doping is a key to highly efficient organic light-emitting diodes and solar cells. Despite this vital role, it is still not understood how mobile charge carriers are induced into the bulk semiconductor upon the doping process. This thesis contains a detailed study of the doping mechanism and the electrical properties of P3HT layers which have been p-doped by the strong molecular acceptor tetrafluorotetracyanoquinodimethane, F4TCNQ. The density of doping-induced mobile holes, their mobility, and the electrical conductivity are characterized in a broad range of acceptor concentrations. A long-standing debate on the nature of the charge transfer between P3HT and F4TCNQ is resolved by showing that almost every F4TCNQ acceptor undergoes a full-electron charge transfer with a P3HT site. However, only 5% of these charge transfer pairs can dissociate and induce a mobile hole into P3HT which contributes electrical conduction. Moreover, it is shown that the left-behind F4TCNQ ions broaden the density-of-states distribution for the doping-induced mobile holes, which is due to the longrange Coulomb attraction in the low-permittivity organic semiconductors.
Organische Halbleiter kombinieren die molekulare Vielfalt und Anpassbarkeit, die mechanische Flexibilität und die preisgünstige Herstellung und Verarbeitung von Kunststoffen mit fundamentalen Halbleitereigenschaften wie Lichtabsorption und -emission und elektrischer Leitfähigkeit. Unlängst finden organische Leuchtdioden Anwendung in den Displays von TV-Geräten und Smartphones. Für die weitere Entwicklung und den Erfolg organischer Halbleiter ist das Verständnis derer physikalischer Grundlagen unabdingbar. Ein für viele Bauteile fundamentaler Prozess ist der Transport von Ladungsträgern in der organischen Schicht. Die Ladungstransporteigenschaften werden maßgeblich durch die Struktur dieser Schicht bestimmt, z.B. durch den Grad der molekularen Ordnung, die molekulare Verbindung von kristallinen Domänen und durch Defekte der molekularen Packung. Mittels optischer Spektroskopie werden in dieser Arbeit die temperatur-, molekulargewichts- und lösemittelabhängigen Struktureigenschaften poly- und oligothiophenbasierter Schichten untersucht. Dabei basiert die Analyse der Absorptionsspektren auf der Zerlegung in die spezifischen Anteile geordneten und ungeordneten Materials. Es wird gezeigt, dass sich hohe Ladungsträgerbeweglichkeiten dann erreichen lassen, wenn der Anteil der geordneten Bereiche und deren molekulare Verbindung in den Schichten möglichst hoch und die energetische Unordnung in diesen Bereichen möglichst klein ist. Der Ladungstransport in organischen Halbleitern kann außerdem gezielt beeinflusst werden, indem die Ladungsträgerdichte und die elektrische Leitfähigkeit durch molekulares Dotieren, d.h. durch das Einbringen von Elektronenakzeptoren oder -donatoren, eingestellt werden. Obwohl der Einsatz dotierter Schichten essentiell für effiziente Leuchtdioden und Solarzellen ist, ist der Mechanismus, der zur Erzeugung freier Ladungsträger im organischen Halbleiter führt, derzeit unverstanden. In dieser Arbeit wird der Ladungstransfer zwischen dem prototypischen Elektronendonator P3HT und dem Akzeptor F4TCNQ untersucht. Es wird gezeigt, dass, entgegen verbreiteter Vorstellungen, fast alle F4TCNQ-Akzeptoren einen ganzzahligen Ladungstransfer mit P3HT eingehen, aber nur 5% dieser Paare dissoziieren und einen beweglichen Ladungsträger erzeugen, der zur elektrischen Leitfähigkeit beiträgt. Weiterhin wird gezeigt, dass die zurückgelassenen F4TCNQ-Akzeptorionen Fallenzustände für die beweglichen Ladungsträger darstellen und so die Ladungsträgerbeweglichkeit in P3HT bei schwacher Dotierung absinkt. Die elektrischen Kenngrößen Ladungsträgerkonzentration, Beweglichkeit und Leitfähigkeit von F4TCNQ-dotierten P3HT-Schichten werden in dieser Arbeit erstmals in weiten Bereichen verschiedener Akzeptorkonzentrationen untersucht.

Conjugated polymers have attracted much interest as promising alternatives to inorganic semiconductors, due to their low-temperature, solution-based processability, which may provide for low-cost, large-area electronic device fabrication. However, commercialization of polymer-based electronic devices has been restricted owing to low device performance of solidified thin-films. In order to enhance charge transport of polymer semiconductor thin-films, the self-organization of organic polymer semiconductors into ordered supramolecular assemblies has been achieved by tuning a range of process parameters including film deposition method (spin vs. drop cast), solvent boiling point (low vs. high boiling point), polymer-dielectric interface treatment, and post-deposition processing (solvent vapor or thermal annealing). However, these strategies give rise to limitations for large-scale high-throughput processing due to associated pre- and/or post semiconductor deposition steps. Therefore, in this thesis, we identify alternative processing parameters (i.e., hydrogen bonds between good and poor solvents, UV irradiation to polymer precursor solutions, and combination of sonication and subsequent UV irradiation to polymer precursor solutions) which can contribute to enhancement in charge transport of a model polymer semiconductor, poly(3-hexylthiophene) (P3HT), eliminating the additional pre- and/or post-steps mentioned above. Further, we understand of how the processing parameters effect intra- and intermolecular interactions of the polymer chains, micro- through macroscopic morphologies, and charge transport characteristics of the resultant films.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Kippelen Bernard; Committee Member: Brand Oliver; Committee Member: Bredas Jean-Luc; Committee Member: Dupuis Russell D.; Committee Member: Smith Glenn S.. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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Dissertação (Mestrado em Tecnologia Nuclear)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

碩士
國立臺灣師範大學
物理學系
95
The pentacene and organic-light-emitting-device (OLED, C47H32N2O2) thin films were grown on glass substrate by thermal evaporation technique. The optical constants of these thin films are studied using variable spectroscopic ellipsometry. Room-temperature ellipsometric spectra of the pentacene thin film exhibit a main absorption peak at about 1.87 eV, which can be assigned to the HOMO-LUMO gap of pentacene. Moreover, other high-frequency peaks above 2 eV are likely due to the transitions of an electron excited to higher orbital or localized excitations. On the other hand, the HOMO- LUMO transition energy of OLED thin film moves toward 2.48 eV, indicating its more insulating character. Temperature-dependent ellipsometric spectra of both thin films were measured in the range from 200 K to 450 K. With decreasing temperature, the Davydov splitting of pentacene thin film increases from 0.08 eV at T = 450 K up to 0.122 eV at T = 200 K. This behavior is due to molecular reorientations that cause changes in mutual molecular overlap within the unit cell. A similar change of absorption bands is also observed for OLED thin film.

博士
國立清華大學
化學系
102
Recent years have seen an increasing number of studies on the utilization of fused thiophene-based materials for organic filed effect transistor (OFET) applications due to their strong intermolecular S-S interaction, large ionization potential energy, and better ambient stability. In OFETs, the interface between the electrodes and the organic semiconductor plays a critical role in affecting the device performance. In our work, the 2-phenylbenzo [d,d’]thieno-[3,2-b;4,5-b’] dithiophene (P-BTDT) and its derivative, monofluorine-substituted 2–phenylbenzo [d,d_]thieno [3,2-b;4,5-b_]–dithiophene (m-FP-BTDT), were grown on the Au (111) and modified Au (111) substrate as the organic semiconductor thin films, respectively. The growth mechanisms, the molecular orientations and the electronic structure of the thin films were analyzed via XPS, NEXAFS and UPS. In addition, the above results were combined with the XRD and AFM data to reach a thorough understanding of the performances of various OFETs. The thermal desorption data show that the chemisorbed P-BTDT layer remains thermally stable up to 750 K on Au(111). However, P-BTDT desorption on Au-BT substrate is found to derive exclusively from physisorption state, and chemisorption desorption is entirely suppressed. Based on the change of XPS intensity with the amount of P-BTDT deposited, it is concluded that the growth of P-BTDT film on Au(111) follows the Stranski-Krastanov mechansim, i.e., a completion of one monolayer (2D growth) followed by a 3D crystallite growth. In comparison, for the growth of P-BTDT on Au(111)-benzenethiolate, a pseudo 2D-like growth is observed. From the results of UPS, the change of work function and valence band spectrum with the film-thickness is in accord with this growth behavior. The results obtained by XPS are also in agreement with UPS observations. The molecules of the multilayer grown on Au(111)-BT have their aromatic rings inclined toward the surface by an average 66°, as determined by NEXAFS data. For the thick film grown on Au(111), out-of-plane x-ray diffraction reveals the presence of several crystallite alignment schemes. In contrast, the XRD data of the thicker layer grown on Au(111)-BT show (002) diffraction peaks only, indicating that the crystal orientation of the thin film is (002) preferred. Therefore, we can change the crystal structure and control the growth behavior of the thin film through different modification of substrates, leading to an improved performance of the OFETs. The results show that P-BTDT thin film that is grown on BT-Au can yield a better OFET mobility up to 3.0 × 10-2 cm2/Vs with thickness of 120 nm. In order to shift energy level of LUMO toward lower value, we also prepared m-FP-BTDT organic semiconducting thin films and investigated their properties. XPS intensity analysis shows that on Au(111), m-FP-BTDT film grows according to Stranski–Krastanov (SK) mode. It is interesting to note an abrupt increase of Au 4f signal at around one ML of m-FP-BTDT, which is explained by the production of excess Au adatoms, accompanied by the lifting of the herringbone reconstruction of Au(111). In comparison, the initial growth of m-FP-BTDT on Au-BT proceeds via a pseudo layer-by-layer growth mechanism. NEXAFS data show that m-FP-BTDT molecules on Au-BT adopt a more erected configuration, the angle between the molecule and substrate is 62.5°, resulting in a better cofacial π-stacking. The XRD pattern reveals that the crystal growth orientation of m-FP-BTD thin film is along c axis. Work function for the thick m-FP-BTDT film on Au-BT is determined with UPS as 4.62 eV and the hole injection barrier as 0.94 eV. According to above results the thin film of m-FP-BTDT grown on Au(111)-BT is expected to produce better carrier transport phenomenon.

Today there is a great deal of interest in the development of gas sensors for applications like air pollution monitoring, indoor environment control, detection of harmful gases in mines etc. Based on different sensing principles, a large variety of sensors such as semiconductor gas sensors, thermoelectric gas sensors, optical sensors and thermal conductivity sensors have been developed. The present thesis reports a detailed account of a novel method followed for the design and development of a thermoelectric gas sensor for sensing of Volatile Organic Compounds. Thermoelectric effect is one of the highly reliable and important working principles that is widely being put into practical applications. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor that has been developed. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor. The deposition parameters for sputtering of tin oxide film have been optimized to obtain a high seebeck coefficient. A test set-up to characterize the deposited films for their thermoelectric property has been designed and developed. A novel method of increasing the seebeck coefficient of tin oxide films has been successfully implemented. Thin films of chromium, copper and silver were used for this purpose. Deposition of the semiconducting oxide on strips of metal films has led to a noticeable increase in the seebeck coefficient of the oxide film without significantly affecting its thermal conductivity. The next part of our work involved development of a gas sensor using this thermoelectric film. These sensors were further tested for their response to volatile organic compounds. The sensor showed significant sensitivity to the test gases at relatively low temperatures. In addition to this, the developed sensor is also selective to acetone gas.

Today there is a great deal of interest in the development of gas sensors for applications like air pollution monitoring, indoor environment control, detection of harmful gases in mines etc. Based on different sensing principles, a large variety of sensors such as semiconductor gas sensors, thermoelectric gas sensors, optical sensors and thermal conductivity sensors have been developed. The present thesis reports a detailed account of a novel method followed for the design and development of a thermoelectric gas sensor for sensing of Volatile Organic Compounds. Thermoelectric effect is one of the highly reliable and important working principles that is widely being put into practical applications. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor that has been developed. The thermoelectric property of semiconducting tin oxide film has been utilized in the sensor. The deposition parameters for sputtering of tin oxide film have been optimized to obtain a high seebeck coefficient. A test set-up to characterize the deposited films for their thermoelectric property has been designed and developed. A novel method of increasing the seebeck coefficient of tin oxide films has been successfully implemented. Thin films of chromium, copper and silver were used for this purpose. Deposition of the semiconducting oxide on strips of metal films has led to a noticeable increase in the seebeck coefficient of the oxide film without significantly affecting its thermal conductivity. The next part of our work involved development of a gas sensor using this thermoelectric film. These sensors were further tested for their response to volatile organic compounds. The sensor showed significant sensitivity to the test gases at relatively low temperatures. In addition to this, the developed sensor is also selective to acetone gas.