Diese Dissertation stellt die erste Anwendung des magneto-optischen Kerr Effektes (MOKE) auf ferromagnetische Metall/Organische Heterostrukturen zur Aufklärung der optischen und chemischen Eigenschaften dar. Die MOKE-Untersuchungen wurden spektroskopisch in einem Energiebereich von 1.7 eV bis 5.5 eV durchgeführt. Heterostrukturen, wie sie hier untersucht werden, sind relevant für Anwendungen in der organischen Spintronik. Die Auswertung der Experimentellen Daten wird unterstützt durch numerische Simulationen eines Schichtmodells und ergänzende Untersuchung der strukturellen und magnetischen Eigenschaften unter Zuhilfenahme von AFM, TEM, SEM, STXM und SQUID-Magnetometrie. In der aktuellen Arbeit wurde Ni als Beispiel einer ferromagnetischen Schicht oberhalb oder unterhalb des organischen Films verwendet. Die organische Schicht besteht jeweils aus den diamagnetischen Molekülen Rubren, Pentacen und Fulleren, welche nur ein vernachlässigbares MOKE-Signal aufweisen. Zum Vergleich wurden das metallfreie Phtalocyanin H2Pc, welche ein nur eine bis zwei Größenordnungen schwächeres MOKE Signal als das genutzte Ni zeigen, betrachtet. Selbst Moleküle, welche kein intrinsisches MOKE-Signal zeigen, können über die optische Interferenz Einfluss auf das MOKE Signal von Ni nehmen. Daher kann die Dicke der organischen Schicht genutzt werden, um den Verlauf des MOKE Spektrum zu kontrollieren. Dies wird für Rubren und C60 gezeigt. Beim Vergleich des MOKE-Spektrums von Rubren/Ni- und Ni/Rubren-Doppelschichten war es möglich zu zeigen, dass die Metallablagerung an der Oberfläche einen Versiegelungseffekt hat, welcher die Oxidation der organischen Unterschicht verlangsamt. AFM und TEM Messungen zeigen, dass Ni die Morphologie der unteren Rubrenschicht annimmt. Die Proben, die mit einer geringen Wachstumsrate von Rubren hergestellt wurden, weisen bei einer nominellen Schichtdicke von 15 nm klar geformte Rubren-Inseln mit großen Abständen zwischen ihnen auf. In diesen Fällen zeigte die magnetische Hysteresemessung von MOKE bei Raumtemperatur eine unterschiedliche Gestalt in Abhängigkeit von der Photonenenergie. Die Hystereseschleifen wurden durch die Präsenz zweier magnetischer Phasen interpretiert. Die MOKE-Spektren dieser beiden Phasen wurden aus dem experimentellen Spektrum separiert. Die Gestalt des gemessenen Spektrums ändert sich mit der Stärke des angelegten Feldes aufgrund der unterschiedlichen Beiträge der zwei Phasen. An den ferromagnetischen Metall/organischen Schichten wurde TEM angewendet, um die Größe der Metallpartikel zu bestimmen, sowie STXM um die Orientierung der organischen Moleküle festzustellen. Die Schichtdicke, das Massenverhältnis sowie die Wechselwirkung zwischen Metall und organischen Material beeinflussen nachweislich das MOKE Signal.

Το ερευνητικό ενδιαφέρον για τους οργανικούς ημιαγωγούς είναι συνεχώς αυξανόμενο τα τελευταία χρόνια, καθώς η αγορά των οργανικών ηλεκτρονικών είναι από τις πιο αναπτυσσόμενες. Για την καλή απόδοση των διατάξεων αυτών σημαντικός είναι ο ρόλος των διεπιφανειών. Οι οικογένειες των n-type οργανικών ημιαγωγών naphthalene bisimides και perylene bisimides έχουν δείξει καλές αποδόσεις σε οργανικά τρανζίστορ. Στην παρούσα εργασία μελετήθηκαν οι διεπιφάνειες νέων οργανικών ημιαγωγών από τις παραπάνω οι οικογένειες οργανικών πάνω σε ανόργανα υποστρώματα με φασματοσκοπίες φωτοηλεκτρονίων. Μελετήθηκε ο σχηματισμός λεπτών υμενίων, πάχους έως τα 10 nm, τριών naphthalene οργανικών ημιαγωγών με διαφορετικό ενεργειακό χάσμα πάνω στον χρυσό και ενός perylene πάνω σε χρυσό και SiO2. Σκοπός ήταν να προσδιοριστεί η επίδραση των διαφορετικών υποκαταστατών του κεντρικού πυρήνα των naphthalene bisimides, στα ενεργειακά χαρακτηριστικά του ημιαγωγού και τα φράγματα έγχυσης των φορέων στην διεπιφάνεια με τον χρυσό. Ο τρόπος ανάπτυξης των όλων των οργανικών ημιαγωγών προσδιορίστηκε ως πολλαπλά στρώματα. Σε μια περίπτωση εντοπίστηκε ότι αλλάζει από οριζόντιο σε κάθετο ο προσανατολισμός των μορίων. Προσδιορίστηκαν όλα τα μεγέθη που χαρακτηρίσουν ενεργειακά την διεπιφάνεια. Συγκεκριμένα, σε όλες τις διεπιφάνειες εμφανίζεται ένα διεπιφανειακό δίπολο λόγω της αναδιάταξης του ηλεκτρονιακού νέφους της επιφάνειας του χρυσού από τα μόρια του οργανικού. Επίσης, οι τιμές των φραγμάτων έγχυσης των ηλεκτρονίων που υπολογίστηκαν είναι αρκετά μικρές που δείχνουν το n-type χαρακτήρα των οργανικών. Οι τιμές του δυναμικού ιονισμού που υπολογίστηκαν ήταν όλες μεγαλύτερες του 5, που είναι προϋπόθεση για τα τρανζίστορ να είναι σταθερά στον αέρα, ενώ σε μια περίπτωση η τιμή ήταν αρκετά μικρή, που δείχνει ότι ο συγκεκριμένος οργανικός ημιαγωγός μπορεί να έχει ambipolar χαρακτηριστικά. Τα αποτελέσματα έδειξαν ότι ο χρυσός μπορεί να χρησιμοποιηθεί αποτελεσματικά ως ηλεκτρόδιο σε τρανζίστορ με n-type οργανικούς ημιαγωγούς. Τέλος, από τα αποτελέσματα τονίστηκε ότι με την υποκατάσταση χημικών ομάδων στον κεντρικό πυρήνα του naphthalene, μια εύκολη διαδικασία, είναι δυνατόν να οδηγηθεί ενεργειακά η διεπιφάνεια προς την επιθυμητή κατεύθυνση. / In the recent years the interest on organic semiconductors is increased as the market of organic electronics is one of most promising. The interfaces between the organic semiconductors with metals or other materials are crucial for the performance of the devices. The study of interfaces by surface sensitive techniques could give useful information for the physics of metal-organic contacts and therefore it is possible the tuning and the improvement of the device performance. The n-type organic semiconductors derivatives of naphthalene bisimides and perylene bisimides, have shown good performance in OFETs. In this work, the interfaces of new synthesized naphthalene bisimides and perylene bisimides molecules with inorganic substrates have been studied by photoelectron spectroscopies. Thin films up to 10 nm thickness of three naphthalene organic semiconductors of different energy gap on Au substrates have been studied. The aim was to investigate the effect of the different substituents of the naphthalene core on the energy characteristics of the organic semiconductors and on the charge injection barriers at the interface. Moreover, the interface of one perylene n-type semiconductor deposited on Au and SiO2 was studied in order to examine the influence of the substrate on the growth mode and the electronic properties. The growth mode of all the organic semiconductors was characterized as simultaneous multilayers. In one case, the orientation of the organic molecules was changed from horizontal to vertical to the surface. In all the interfaces an interface dipole is formed during the early stage of deposition which is attributed to the reorganization of the electron cloud of the Au surface by the organic molecules when they are deposited on Au. The hole and electron injection barriers were also determined. The electron injection barriers were found to be small which indicates the n-type character of these organic molecules. In addition, the results displayed that the Au can be used efficiently as electrode in devices with these organic semiconductors. The ionization potentials of the organic semiconductors were measured and found to be above 5 eV for all and therefore, they are suitable for air-stable transistors. In the case of one organic semiconductor the ionization potential was measured close to the value of five. Thus, this organic semiconductor is suitable for ambipolar transistors. The valance band characteristics near the HOMO, as detected by the UPS spectra, showed that they are affected by the different substituents on the side groups of the imide. These results have shown that changing the substituents of the organic core, which is an easy process; it is possible to tune the energy levels and the electronic characteristics of the interface.

Organic semiconductors show promise to yield a novel class of bendable electronic devices, and much research efforts have focused on the optimization of these films for device performance. It is well known that the structure of organic films has a large influence over the electronic properties. In particular, the carrier mobility is often highly anisotropic, and domain boundaries have a detrimental effect on charge transport. Therefore the domain structure and lattice orientation are of particular interest. However, little is known about the domain structure of organic films, and techniques to study these properties have only begun to emerge in recent years. In this thesis, we apply two experimental techniques, Grazing-Incidence X-ray Diffraction (GIXD) and Lateral Force Microscopy (LFM), toward studying the lattice and domain structure of tetracene films grown on the silicon(001)-monohydride surface. We describe the necessary steps toward optimizing the sensitivity of these techniques to the domain structure. Results show that the crystalline tetracene films form a layered morphology in which the a-b plane lies parallel to the substrate surface. The film lattice structure is similar to bulk tetracene, and the lattice is confined to two orthogonal orientations, forming a partially-commensurate relationship with the substrate surface lattice along the film 'a' axis. LFM images reveal two types of polycrystalline domains. The first type ("major domains") are tens of microns in size, and are classified by their lattice orientation. They are subdivided into the second type ("sub-domains"), which range from 0.1 to 5um in size, and are argued to represent regions of uniform molecular tilt direction. The GIXD data show that the single-crystal domains which comprise these two larger domain types are anisotropic in size, being up to two times longer along the film 'b' axis than along 'a'. The single-crystal domains range from 0.05 to 0.2um in size, depending on lattice orientation and film thickness. The mathematical basis for these single-crystal domain size calculations is presented. The single-crystal domain sizes are thickness-dependent, and are two orders of magnitude smaller than a typical surface island observed in atomic-force microscopy (AFM) topographs. Substrate steps can also significantly influence the film structure by inducing boundaries in the single-crystal domains and sub-domains, but not in the major domains. This detailed knowledge of the domain structure of organic thin-films may assist in our understanding of the factors which affect charge transport in thin films, and may help to direct research efforts in optimizing the film structure for device performance. / Natural Sciences and Engineering Research Council (NSERC), Canadian Foundation for Innovation (CFI), Ontario Innovation Trust (OIT).

The Thesis reports charge transport studies on conducting polymers, polymer carbon nanotube composites and organic semiconductor devices. Conducting and semiconducting polymers consisting of π-conjugated chains have attracted considerable attention as they combine the optoelectronic properties of semiconductors with mechanical properties and processing advantages of plastics. The chemical/electrochemical/photodoping of these semiconducting polymers can tune the Fermi levels and conductivity in a controlled way, and hence the properties of devices can be easily tailored to suit in several applications. Carbon nanotube (CNT) is another another novel promising material for electronic/optoelectronic applications. Lately there has been a great interest in developing composites of polymer and CNTs to utilize the advantages of both CNTs and polymers. The inclusion of CNTs in polymers improves the mechanical, electrical and thermal properties since the aspect ratio (ratio of length to diameter) is very large, as well its density is rather low. The Thesis consists of 6 chapters. First chapter is a brief introduction of general and transport properties of conducting polymers and polymer-carbon nanotube composites. In Chapter 2, the sample preparation and experimental techniques used in this work are discussed. The charge transport in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) is presented in Chapter 3. Chapter 4 focuses on the transport measurements in the polymer-CNT composite samples. Chapter 5 elaborates the ac and dc characterization of organic field-effect transistors (OFETs). And chapter 6 presents the conclusion and future directions of the work that has been presented in the Thesis. Chapter 1: In the scientific and technological revolution of the last few years, the study of high performance materials has been steadily increasing including the study of carbon-based materials. Conducting polymers have special properties that are interesting for this new technology. The charge transport in conjugated polymers is important to optimize the performance of devices. The discovery of CNTs with exceptional thermal, mechanical, optical, electrical and structural properties has facilitated the synthesis of new type of nanocomposites with very interesting properties. Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating CNTs as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. To optimize their electrical properties it is essential to understand the charge transport mechanism in detail. Chapter 2: The multi-wall carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition (CVD) are mixed with a 1:1 mixture of 98% H2SO4 and 70% HNO3 to produce sulfonic acid functionalized multi-wall carbon nanotubes (s-MWNTs). The s-MWNTs are dispersed in a solution of Nafion by ultrasonication and then cast on a glass substrate and slowly dried by moderate heating to obtain the composite films. Polyaniline (PANI)-MWNT composites were obtained by carrying out the chemical synthesis of nanofibrilar PANI in the presence of CNTs. This water dispersible PANIMWNT composite contains well segregated MWNTs partially coated by nanofibrilar PANI. The ac and dc charge transport measurements suggest hopping transport in these materials. OFETs are fabricated with pentacene, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)(PBTTT) and poly(3-hexylthiophene) (P3HT) as active materials. A novel technique is used to characterize the acphotoresponse of these OFETs. Chapter 3: Charge transport studies on PEDOT-PSS have been carried out and found that it correlates with the morphology. The dc conductivity of PEDOT–PSS shows enhanced delocalization of the carriers upon the addition of dimethyl sulfoxide (DMSO) and this is attributed to the extended chain conformation. PEDOT-PSS is known to form a phase-segregated material comprising highly conducting PEDOT grains that are surrounded by a sea of weakly ionic-conducting PSS and a wide variation in the charge transport properties of PEDOT-PSS films is attributed to the degree of phasesegregation of the excess insulating polyanion. The magnetotransport and temperature dependent ac transport parameters across different conducting grades of PEDOT-PSS processed with DMSO were compared. Depending on the subtle alterations in morphology, the transport at low temperatures is shown to vary from the hopping regime (Baytron P) to critical regime of the metal-insulator transition (Baytron PH510) There is a significant positive magnetoresistance (MR) for P–films, but this is considerably less in case of PH510-film. From the low temperature ac conductance it is found that the onset frequency for PH510 is nearly temperature independent, whereas in P type it is strongly temperature dependent, again showing the superior transport in PH510. The presence of ‘shorter network connections’ together with a very weak temperature dependence down to ~ 5 K, suggest that the limitation on transport in PH510 arises from the connectivity within the PEDOT-rich grain rather than transport via the PSS barriers. Chapter 4: DC and AC charge transport properties of Nafion s-MWNT and PANI-MWNT composites are studied. Such a detailed investigation is required to optimize the correlation among morphology and transport properties in these composites towards applications in field-effect transistors, antistatic coating, electromagnetic shielding, etc. The conductivity in Nafion s-MWNT shows a percolative transport with percolation threshold pc = 0.42 whereas such a sharp percolation is absent in PANI-MWNT composite since the conduction via PANI matrix smears out the onset of rapid increase in conductivity. Three-dimensional variable range hopping (VRH) transport is observed in Nafion s-MWNT composites. The positive and negative MR data on 10 wt. % sample are analyzed by taking into account forward interference mechanism (negative MR) and wave-function shrinkage (positive MR), and the carrier scattering is observed to be in the weak limit. The electric-field dependence, measured to high fields, follows the predictions of hopping transport in high electric-field regime. The ac conductivity in 1 wt. % sample follows a power law: ( )  A s , and s decreases with increasing temperature as expected in the correlated barrier hopping (CBH) model. In general, Mott’s VRH transport is observed in PANI-MWNT samples. It is found that the MWNTs are sparingly adhered with PANI coatings, and this facilitates inter-tube hopping at low temperatures. The negative MR of MWNT-PANI composites suggest that the electronic transport at low temperatures is dominated by MWNT network. AC impedance measurements at low temperatures with different MWNT loading show that ac conductivity become temperature independent as the MWNT content increases. The onset frequency for the increase in conductivity is observed to be strongly dependent on the MWNT weight percentage, and the ac conductivity can be scaled onto a master curve given by  ( )  0[1 k( 0 )s ]. Chapter 5: Organic field-effect transistors (OFETs) based on small molecules and polymers have attracted considerable attention due to their unique advantages, such as low cost of fabrication, ease of processing and mechanical flexibility. Impedance characterization of these devices can identify the circuit elements present in addition to the source-drain (SD) channel, and the bottlenecks in charge transport can be identified. The charge carrier trapping at various interfaces and in the semiconductor can be estimated from the dc and ac impedance measurements under illumination. The equivalent circuit parameters for a pentacene OFET are determined from low frequency impedance measurements in the dark as well as under light illumination. The charge accumulation at organic semiconductor–metal interface and dielectric semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under SD electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. Similar charge trapping is observed in an OFET with PBTTT as the active material. This novel method can be used to differentiate the photophysical phenomena occurring in the bulk from that at the metal-semiconductor interface for the polymer. Chapter 6: The conclusions from the various works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up.

Diese Dissertation stellt die erste Anwendung des magneto-optischen Kerr Effektes (MOKE) auf ferromagnetische Metall/Organische Heterostrukturen zur Aufklärung der optischen und chemischen Eigenschaften dar. Die MOKE-Untersuchungen wurden spektroskopisch in einem Energiebereich von 1.7 eV bis 5.5 eV durchgeführt. Heterostrukturen, wie sie hier untersucht werden, sind relevant für Anwendungen in der organischen Spintronik. Die Auswertung der Experimentellen Daten wird unterstützt durch numerische Simulationen eines Schichtmodells und ergänzende Untersuchung der strukturellen und magnetischen Eigenschaften unter Zuhilfenahme von AFM, TEM, SEM, STXM und SQUID-Magnetometrie. In der aktuellen Arbeit wurde Ni als Beispiel einer ferromagnetischen Schicht oberhalb oder unterhalb des organischen Films verwendet. Die organische Schicht besteht jeweils aus den diamagnetischen Molekülen Rubren, Pentacen und Fulleren, welche nur ein vernachlässigbares MOKE-Signal aufweisen. Zum Vergleich wurden das metallfreie Phtalocyanin H2Pc, welche ein nur eine bis zwei Größenordnungen schwächeres MOKE Signal als das genutzte Ni zeigen, betrachtet. Selbst Moleküle, welche kein intrinsisches MOKE-Signal zeigen, können über die optische Interferenz Einfluss auf das MOKE Signal von Ni nehmen. Daher kann die Dicke der organischen Schicht genutzt werden, um den Verlauf des MOKE Spektrum zu kontrollieren. Dies wird für Rubren und C60 gezeigt. Beim Vergleich des MOKE-Spektrums von Rubren/Ni- und Ni/Rubren-Doppelschichten war es möglich zu zeigen, dass die Metallablagerung an der Oberfläche einen Versiegelungseffekt hat, welcher die Oxidation der organischen Unterschicht verlangsamt. AFM und TEM Messungen zeigen, dass Ni die Morphologie der unteren Rubrenschicht annimmt. Die Proben, die mit einer geringen Wachstumsrate von Rubren hergestellt wurden, weisen bei einer nominellen Schichtdicke von 15 nm klar geformte Rubren-Inseln mit großen Abständen zwischen ihnen auf. In diesen Fällen zeigte die magnetische Hysteresemessung von MOKE bei Raumtemperatur eine unterschiedliche Gestalt in Abhängigkeit von der Photonenenergie. Die Hystereseschleifen wurden durch die Präsenz zweier magnetischer Phasen interpretiert. Die MOKE-Spektren dieser beiden Phasen wurden aus dem experimentellen Spektrum separiert. Die Gestalt des gemessenen Spektrums ändert sich mit der Stärke des angelegten Feldes aufgrund der unterschiedlichen Beiträge der zwei Phasen. An den ferromagnetischen Metall/organischen Schichten wurde TEM angewendet, um die Größe der Metallpartikel zu bestimmen, sowie STXM um die Orientierung der organischen Moleküle festzustellen. Die Schichtdicke, das Massenverhältnis sowie die Wechselwirkung zwischen Metall und organischen Material beeinflussen nachweislich das MOKE Signal.

The efficiency of organic light-emitting diodes (OLEDs) typically decreases with increasing current density. This so-called roll-off impedes the market entry of OLEDs in high-brightness applications such as general lighting. One of the most important processes causing roll-off is exciton annihilation, which evolves upon high exciton densities. This mechanism is especially pronounced in phosphorescent molecules due to their long triplet lifetime. In order to reduce the roll-off in phosphorescent OLEDs, this thesis focusses on decreasing the local exciton density by modifying the exciton lifetime, the spatial exciton distribution, and the tendency of emitters to form aggregates. The obtained results lead to a deeper understanding of efficiency roll-off and help sustaining the OLED efficiency at high brightness. The emitter lifetime can be influenced by the optical environment around the emitting dipoles through the Purcell effect. In order to study this effect, the distance between emitter and metal cathode is varied for two different OLED stacks. A strong influence of emitter position and orientation on roll-off is observed and explained by modelling the data with triplet-triplet annihilation theory. Furthermore, design principles for optimal high-brightness performance are established by simulating the roll-off as a function of emitter-cathode distance, emissive dipole orientation, and radiative efficiency. Next, a method is developed that allows extracting the spatial exciton distribution. Therefore, a thin sensing layer that locally quenches excitons is introduced into the emission layer at varying positions. The resulting quenching profile is then fitted using a comprehensive theory based on the diffusion equation, which renders the exciton distribution and diffusion length with nanometer resolution. This method is applied to an emission layer comprising an ambipolar host material. Contrary to expectations which suggest that ambipolar materials exhibit broad exciton formation, a narrow emission zone close to the electron transport layer is found. Additional explorations of structures that might broaden the emission zone point to a narrow emission zone in double emission layers and broader exciton formation in mixed emission layers. Previous investigations revealed a strong correlation between emitter aggregation and molecular dipole moment of the emitter. Within this thesis, the range of studied emitters is significantly extended. It is shown that homoleptic emitters show a stronger tendency to form aggregates than heteroleptic compounds. This is probably not only related to their higher dipole-dipole potential, but also to the molecular structure. Systematic analysis of the deposition parameters shows that aggregate formation depends on the underlying material and increases with increasing substrate temperature and decreasing evaporation rate. The two green emitters Ir(ppy)3 and Ir(ppy)2(acac) are additionally studied by means of X-ray diffraction. Both emitters form crystallite grains and exhibit a preferred orientation. Doping the emitters into an amorphous host, both orientation and crystallite formation retain at the investigated doping concentrations above 20 wt%. This result is a first step toward further understanding of the mechanism of transition dipole orientation.:List of Publications 1 Introduction 2 Principles of Organic Semiconductors 2.1 Molecular Orbitals 2.2 Optical Properties 2.3 Intermolecular Energy Transfer 2.4 Charge Transport 2.5 Organic Light-Emitting Diodes 2.5.1 Structure and Working Principle 2.5.2 Characterization 3 Theory of Efficiency Roll-Off 3.1 Current Status 3.2 Processes Leading to Roll-Off 3.2.1 Triplet-Triplet Annihilation 3.2.2 Triplet-Polaron Interaction 3.2.3 Further Processes Influencing Roll-Off 3.3 Interplay of the Various Processes 3.4 Scope of this Work 4 Experimental Methods 4.1 Sample Preparation 4.2 Measurement 4.2.1 Thin-Film Characterization 4.2.2 OLED Characterization 4.3 Materials 4.3.1 Electrodes, Transport Materials, and Blockers 4.3.2 Materials of the Emission Layer 5 Influence of the Optical Environment 5.1 Introduction 5.2 Influence of Emitter-Cathode Distance 5.3 Emitter Lifetime and Orientation 5.4 Correlation of Roll-Off and Orientation 5.5 Simulation of Roll-Off 5.5.1 Influence of the Electroluminescence Spectrum 5.5.2 Influence of Orientation and Radiative Efficiency 5.6 Conclusion 6 Influence of the Emission Profile 6.1 Preliminary Considerations 6.1.1 Exciton Generation and Diffusion 6.1.2 Width of the Emission Zone 6.1.3 Dependence on the Structure of the Emission Layer 6.2 Measurement of the Emission Profile 6.2.1 Method 6.2.2 Mathematical Description 6.2.3 Experimental Realization and Evaluation 6.3 Ambipolar Matrix Materials 6.3.1 Device Performance 6.3.2 Influence of the Sensing Layer 6.3.3 Emission Profile 6.4 Double- and Mixed Emission Layers 6.4.1 Emission Profile 6.4.2 Influence of the Matrix Ratio 6.5 Summary and Outlook 7 Influence of Molecular Aggregation 7.1 Introduction 7.2 Aggregation of Homoleptic and Heteroleptic Emitters 7.2.1 Photoluminescence Measurements 7.2.2 Time-Resolved Spectroscopy 7.2.3 X-Ray Diffraction 7.2.4 Conclusions on Emitter Orientation 7.2.5 Comparison of the Different Methods—Emitter Aggregation 7.3 Influence of the Matrix Material 7.3.1 Photoluminescence Measurements 7.3.2 Time-Resolved Spectroscopy 7.4 Influence of Processing Parameters 7.4.1 Substrate Heating 7.4.2 Underlying Layer 7.4.3 Evaporation Rate 7.5 Summary and Implications of Aggregation on Efficiency Roll-Off 8 Summary and Outlook 8.1 Summary of Roll-Off Investigations 8.2 Improving the High-Brightness Performance Further 8.3 Concluding Words on Emitter Orientation A Appendix to Theory of Efficiency Roll-Off B Appendix to Emission and Sensing Profiles B.1 Emission Profiles B.2 Emission Profiles Including a Sensing Layer B.3 Sensing Profiles C Appendix to Double- and Mixed Emission Layers C.1 Sample Uniformity C.2 Influence of the Sensor on Current Density C.3 Further D-EML and M-EML structures D Appendix to Molecular Aggregation List of Chemical Compounds List of Abbreviations List of Important Symbols Bibliography Acknowledgement / Die Effizienz organischer Leuchtdioden (OLEDs) nimmt üblicherweise mit ansteigender Stromdichte ab. Dieser so genannte Roll-Off erschwert den Markteintritt von OLEDs in Bereichen, die hohe Helligkeiten erfordern, wie beispielsweise in der Beleuchtung. Einer der wichtigsten Prozesse, die zu Roll-Off führen, ist die Annihilation von Exzitonen. Diese nimmt mit steigender Exzitonendichte zu und ist vor allem in phosphoreszenten OLEDs aufgrund der dort vorhandenen langen Triplettlebensdauer ein großer Verlustfaktor. Im Rahmen dieser Dissertation werden Methoden vorgestellt, die mittels Reduzierung der Exzitonendichte den Roll-Off in phosphoreszenten OLEDs verringern können. Dazu gehören die Veränderung der Exzitonenlebensdauer, die Untersuchung der räumlichen Verteilung der Exzitonen und die Erforschung der Bildung von Emitteraggregaten. Die gewonnenen Ergebnisse führen zu einem besseren Verständnis des Effizienz Roll-Offs und helfen, die Effizienz von OLEDs bei hohen Helligkeiten zu verbessern. Die Emitterlebensdauer kann über den Purcell-Effekt durch Veränderung des die emittierenden Dipole umgebenden elektromagnetischen Felds beeinflusst werden. Dieser Effekt wird genutzt, indem der Abstand zwischen Emitter und Metallelektrode für zwei verschiedene OLED-Aufbauten variiert wird. Der Roll-Off ist stark abhängig von der Position und Orientierung des Emitters und kann durch Modellierung der Daten auf Basis von Triplett-Triplett-Annihilation erklärt werden. Durch Simulation des Roll-Offs in Abhängigkeit des Emitter-Kathode-Abstands, der Orientierung und der strahlenden Effizienz der emittierenden Dipole werden Prinzipien zur optimalen Leistung von OLEDs bei hohen Helligkeiten entwickelt. Als nächstes wird eine Methode eingeführt mittels derer die räumliche Exzitonenverteilung extrahiert werden kann. Dafür wird eine dünne Sensorschicht in die Emissionsschicht eingebracht, die lokal Exzitonen auslöscht. Unter Variation der Position des Sensors wird ein Profil der Auslöschungsintensität bestimmt. Die gemessene Intensität wird mittels einer umfassenden Theorie auf Grundlage der Diffusionsgleichung angepasst, wodurch sich die räumliche Verteilung der Exzitonen und die Diffusionslänge mit einer Auflösung von 1nm ergibt. Die Methode wird auf eine Emissionsschicht angewandt, die das ambipolare Matrixmaterial CBP enthält. Entgegen der Erwartung, dass die Exzitonenbildung in ambipolaren Materialien weiter ausgedehnt ist, ist die gemessene Emissionszone sehr schmal und befindet sich an der Grenze zur Elektronentransportschicht. Um eine Verbreiterung des Emissionsprofils zu ermöglichen, werden weitere Strukturen untersucht. Dabei wird eine schmale Emissionszone in Doppelemissionsschichten beobachtet, wohingegen gemischte Emissionsschichten zu einer Verbreiterung der Exzitonenbildung führen können. Vorangegangene Untersuchungen deckten einen Zusammenhang zwischen der Aggregation von Emittermolekülen und dem Dipolmoment des Emitters auf. In dieser Arbeit werden weitere Emittermoleküle untersucht, wobei eine stärkere Aggregation von homoleptischen Emittern im Vergleich zu heteroleptischen festgestellt wird. Dies ist einerseits im höheren Dipol-Dipol-Potential der homoleptischen Verbindungen und andererseits in der Molekülstruktur begründet. Eine systematische Analyse der Herstellungsparameter zeigt, dass die Aggregatbildung von dem darunter liegenden Material abhängt und mit steigender Substrattemperatur und sinkender Verdampfungsrate zunimmt. Die zwei Grünemitter Ir(ppy)3 und Ir(ppy)2(acac) werden zusätzlich mittels Röntgenspektroskopie untersucht. Beide Emitter bilden kristalline Körner und weisen eine bevorzugte Orientierung auf. Sowohl die Kristallbildung als auch die Orientierung bleiben erhalten, wenn die Emitter mit mehr als 20 Gewichtsprozent in das Matrixmaterial CBP dotiert werden. Dieses Ergebnis ist ein erster Schritt zum besseren Verständnis der in vielen Iridium-Emittern beobachteten Orientierung des Übergangsdipolmoments.:List of Publications 1 Introduction 2 Principles of Organic Semiconductors 2.1 Molecular Orbitals 2.2 Optical Properties 2.3 Intermolecular Energy Transfer 2.4 Charge Transport 2.5 Organic Light-Emitting Diodes 2.5.1 Structure and Working Principle 2.5.2 Characterization 3 Theory of Efficiency Roll-Off 3.1 Current Status 3.2 Processes Leading to Roll-Off 3.2.1 Triplet-Triplet Annihilation 3.2.2 Triplet-Polaron Interaction 3.2.3 Further Processes Influencing Roll-Off 3.3 Interplay of the Various Processes 3.4 Scope of this Work 4 Experimental Methods 4.1 Sample Preparation 4.2 Measurement 4.2.1 Thin-Film Characterization 4.2.2 OLED Characterization 4.3 Materials 4.3.1 Electrodes, Transport Materials, and Blockers 4.3.2 Materials of the Emission Layer 5 Influence of the Optical Environment 5.1 Introduction 5.2 Influence of Emitter-Cathode Distance 5.3 Emitter Lifetime and Orientation 5.4 Correlation of Roll-Off and Orientation 5.5 Simulation of Roll-Off 5.5.1 Influence of the Electroluminescence Spectrum 5.5.2 Influence of Orientation and Radiative Efficiency 5.6 Conclusion 6 Influence of the Emission Profile 6.1 Preliminary Considerations 6.1.1 Exciton Generation and Diffusion 6.1.2 Width of the Emission Zone 6.1.3 Dependence on the Structure of the Emission Layer 6.2 Measurement of the Emission Profile 6.2.1 Method 6.2.2 Mathematical Description 6.2.3 Experimental Realization and Evaluation 6.3 Ambipolar Matrix Materials 6.3.1 Device Performance 6.3.2 Influence of the Sensing Layer 6.3.3 Emission Profile 6.4 Double- and Mixed Emission Layers 6.4.1 Emission Profile 6.4.2 Influence of the Matrix Ratio 6.5 Summary and Outlook 7 Influence of Molecular Aggregation 7.1 Introduction 7.2 Aggregation of Homoleptic and Heteroleptic Emitters 7.2.1 Photoluminescence Measurements 7.2.2 Time-Resolved Spectroscopy 7.2.3 X-Ray Diffraction 7.2.4 Conclusions on Emitter Orientation 7.2.5 Comparison of the Different Methods—Emitter Aggregation 7.3 Influence of the Matrix Material 7.3.1 Photoluminescence Measurements 7.3.2 Time-Resolved Spectroscopy 7.4 Influence of Processing Parameters 7.4.1 Substrate Heating 7.4.2 Underlying Layer 7.4.3 Evaporation Rate 7.5 Summary and Implications of Aggregation on Efficiency Roll-Off 8 Summary and Outlook 8.1 Summary of Roll-Off Investigations 8.2 Improving the High-Brightness Performance Further 8.3 Concluding Words on Emitter Orientation A Appendix to Theory of Efficiency Roll-Off B Appendix to Emission and Sensing Profiles B.1 Emission Profiles B.2 Emission Profiles Including a Sensing Layer B.3 Sensing Profiles C Appendix to Double- and Mixed Emission Layers C.1 Sample Uniformity C.2 Influence of the Sensor on Current Density C.3 Further D-EML and M-EML structures D Appendix to Molecular Aggregation List of Chemical Compounds List of Abbreviations List of Important Symbols Bibliography Acknowledgement

<p>The miniaturization and thinning of wearable, soft robotics and medical devices are soon to require higher performance modeling as the physical flexibility causes direct impacts on the electrical characteristics of the circuit – changing its behavior. As a representative flexible electronic component, the organic field effect transistor (OFET) has attracted much attention in its manufacturing as well as applications. However, as the strain and stress effects are integrated into multiphysics modelers with deeper interactions, the computational complexity and accuracy of OFET modeling is resurfacing as a limiting bottleneck.</p><p>The dissertation was organized into three interrelated studies. In the first study, the Mass-Spring-Damper (MSD) model for an inverted staggered thin film transistor (TFT) was proposed to investigate the TFT’s internal stress/strain fields, and the strain effects on the overall characteristics of the TFT. A comparison study with the finite element analysis (FEA) model shows that the MSD model can reduce memory usage and raises the computational convergence speed for rendering the same results as the FEA. The second study developed the generalized solid-state model by incorporating the density of trap states in the band structure of organic semiconductors (OSCs). The introduction of trap states allows the generalized solid-state model to describe the electrical characteristics of both inorganic TFTs and organic field-effect transistors (OFETs). It is revealed through experimental verification that the generalized solid-state model can accurately characterize the bending induced electrical properties of an OFET in the linear and saturation regimes. The third study aims to model the transient and steady-state dynamics of an arbitrary organic semiconductor device under mechanical strain. In this study, the fractional drift-diffusion (Fr-DD) model and its computational scheme with high accuracy and high convergence rate were proposed. Based on simulation and experimental validation, the transconductance and output characteristics of a bendable OFET were found to be well determined by the Fr-DD model not only in the linear and saturation regimes, but also in the subthreshold regime.</p>

We employ a variant of optical absorption spectroscopy, namely in situ differential reflectance spectroscopy (DRS), for an analysis of the structure-properties relations of thin epitaxial organic films. Clear correlations between the spectra and the differently intense coupling to the respective substrates are found. While rather broad and almost structureless spectra are obtained for a quaterrylene (QT) monolayer on Au(111), the spectral shape resembles that of isolated molecules when QT is grown on graphite. We even achieve an efficient electronic decoupling from the subjacent Au(111) by inserting an atomically thin organic spacer layer consisting of hexa-peri-hexabenzocoronene (HBC) with a noticeably dissimilar electronic behavior. These observations are further consolidated by a systematic variation of the metal substrate (Au, Ag, and Al), ranging from inert to rather reactive. For this purpose, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is chosen to ensure comparability of the molecular film structures on the different metals, and also because its electronic alignment on various metal surfaces has previously been studied with great intensity. We present evidence for ionized PTCDA at several interfaces and propose the charge transfer to be related to the electronic level alignment governed by interface dipole formation on the respective metals. / Zur Analyse der Struktur-Eigenschafts-Beziehungen dünner, epitaktischer Molekülfilme wird in situ differentielle Reflexionsspektroskopie (DRS) als Variante der optischen Absorptionsspektroskopie verwendet. Klare Zusammenhänge zwischen den Spektren und der unterschiedlich starken Kopplung zum jeweiligen Substrat werden gefunden. Während man breite und beinahe unstrukturierte Spektren für eine Quaterrylen (QT) Monolage auf Au(111) erhält, ist die spektrale Form von auf Graphit abgeschiedenem QT ähnlich der isolierter Moleküle. Durch Einfügen einer atomar dünnen organischen Zwischenschicht bestehend aus Hexa-peri-hexabenzocoronen (HBC) mit einem deutlich unterschiedlichen elektronischen Verhalten gelingt sogar eine effiziente elektronische Entkopplung vom darunter liegenden Au(111). Diese Ergebnisse werden durch systematische Variation der Metallsubstrate (Au, Ag und Al), welche von inert bis sehr reaktiv reichen, untermauert. Zu diesem Zweck wird 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA) gewählt, um Vergleichbarkeit der molekularen Filmstrukturen zu gewährleisten, und weil dessen elektronische Anordnung auf verschiedenen Metalloberflächen bereits eingehend untersucht worden ist. Wir weisen ionisiertes PTCDA an einigen dieser Grenzflächen nach und schlagen vor, dass der Ladungsübergang mit der elektronischen Niveauanpassung zusammenhängt, welche mit der Ausbildung von Grenzflächendipolen auf den entsprechenden Metallen einhergeht.

<p>A wide range of optoelectronic applications demand semiconductors optimized for purpose.</p> <p>My research focused on data-driven identification of ABX3 Halide perovskite compositions for optimum photovoltaic absorption in solar cells.</p> <p>I trained machine learning models on previously reported datasets of halide perovskite band gaps based on first principles computations performed at different fidelities.</p> <p>Using these, I identified mixtures of candidate constituents at the A, B or X sites of the perovskite supercell which leveraged how mixed perovskite band gaps deviate from the linear interpolations predicted by Vegard's law of mixing to obtain a selection of stable perovskites with band gaps in the ideal range of 1 to 2 eV for visible light spectrum absorption.</p> <p>These models predict the perovskite band gap using the composition and inherent elemental properties as descriptors.</p> <p>This enables accurate, high fidelity prediction and screening of the much larger chemical space from which the data samples were drawn.</p> <p><br></p> <p>I utilized a recently published density functional theory (DFT) dataset of more than 1300 perovskite band gaps from four different levels of theory, added to an experimental perovskite band gap dataset of \textasciitilde{}100 points, to train random forest regression (RFR), Gaussian process regression (GPR), and Sure Independence Screening and Sparsifying Operator (SISSO) regression models, with data fidelity added as one-hot encoded features.</p> <p>I found that RFR yields the best model with a band gap root mean square error of 0.12 eV on the total dataset and 0.15 eV on the experimental points.</p> <p>SISSO provided compound features and functions for direct prediction of band gap, but errors were larger than from RFR and GPR.</p> <p>Additional insights gained from Pearson correlation and Shapley additive explanation (SHAP) analysis of learned descriptors suggest the RFR models performed best because of (a) their focus on identifying and capturing relevant feature interactions and (b) their flexibility to represent nonlinear relationships between such interactions and the band gap.</p> <p>The best model was deployed for predicting experimental band gap of 37785 hypothetical compounds.</p> <p>Based on this, we identified 1251 stable compounds with band gap predicted to be between 1 and 2 eV at experimental accuracy, successfully narrowing the candidates to about 3% of the screened compositions.</p>

The topic of this work is the structural characterization and theoretical modeling of organic single and heterolayers. The growth of sub-monolayers and monolayers (ML) of the two polycyclic aromatic hydrocarbons quaterrylene (QT) and hexa-peri-hexabenzocoronene (HBC) on Ag(111) and Au(111) was investigated. A transition from a disordered, isotropic phase to an ordered phase with increasing coverage was found. The lattice of the ordered phase turned out to be coverage dependent. The intermolecular potential was modeled including Coulomb and van der Waals interaction by a force-field approach. The postulated repulsive character of the potential could be connected to the non-uniform intramolecular charge distribution and to a screening of the van der Waals forces. Furthermore, the influence of the variable lattice constant on the epitaxial growth of HBC was studied. The second part of this work deals with a ML of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on a ML of HBC. In dependency on the initial lattice constant of HBC, a total of three line-on-line (LOL) and point-on-line coincident phases of PTCDA (with respect to HBC) was found. Following an analysis of the general properties of LOL coincident systems via force-field calculations, a new method to predict the structure of such systems is introduced. / Thema dieser Arbeit ist die strukturelle Charakterisierung von organischen Einfach- und Heterolagen sowie deren theoretische Beschreibung und Modellierung. Es wurden Submonolagen und Monolagen (ML) der polyzyklischen Kohlenwasserstoffe Quaterrylen (QT) und Hexa-peri-hexabenzocoronen (HBC) auf Ag(111) und Au(111) Einkristallen untersucht und ein Übergang von einer ungeordneten, isotropen Phase zu einer geordneten Phase mit steigender Bedeckung beobachtet. Die geordnete Phase wies dabei bedeckungsabhängige Gitterkonstanten auf. Das intermolekulare Potential wurde unter Berücksichtigung von Coulomb und van der Waals Anteilen mittels Kraftfeldmethoden modelliert. Der postulierte repulsive Charakter des Potentials konnte auf die Ladungsverteilung im Molekül und eine Abschwächung des van der Waals Potentials zurückgeführt werden. Weiterhin wurde der Einfluss der variablen HBC Gitterkonstante auf die epitaktische Relation des Gitters zum Metallsubstrat untersucht. Der zweite Teil der Arbeit widmet sich der Untersuchung einer ML 3,4,9,10-Perylenetetracarboxylic dianhydrid (PTCDA) auf einer ML HBC. Dabei wurden, in Abhängigkeit von der HBC Gitterkonstante, insgesamt drei verschiedene Typen von line-on-line bzw. point-on-line Epitaxie nachgewiesen. Im Anschluss an eine Analyse der generellen Eigenschaften solcher epitaktischer Lagen mittels Kraftfeldrechnungen wird eine neue Methode zur Vorhersage der Struktur konkreter Systeme vorgestellt.