Ce manuscrit décrit la synthèse et la caractérisation de nouvelles molécules incluant un cœur organophosphoré, le phosphole. Certaines de ces molécules ont été utilisées pour la fabrication de dispositifs OLEDs ou de cellules photovoltaïques organiques. Le premier chapitre fait un état de l'art de la chimie du phosphole dans le domaine des matériaux organiques entre 2010 et 2013. Le second chapitre décrit la synthèse et l'étude physico-chimique de molécules qui permettent de moduler l'angle de torsion dans les systèmes π conjugués pour faire varier les propriétés optiques et rédox. Une de ces molécules a permis la fabrication d'une diode blanche organique. Le troisième chapitre de ce manuscrit présente une structure tridimensionnelle intéressante, le 1,1-biphosphole. En plus de posséder une structure tridimensionnelle, ces structures présentent un mode de conjugaison original, la conjugaison σ-π, qui permet de réduire l'écart HO-BV de nos systèmes. Une de ces molécules a permis la fabrication de la première cellule photovoltaïque organique avec un dérivé du phosphole inséré dans la couche active. Dans une deuxième partie, ce chapitre traite également de la réactivité originale du 1,1'-biphosphole qui permet de fonctionnaliser l'atome de phosphore par une simple substitution nucléophile, permettant d'insérer une grande variété de substituants pour moduler les propriétés des molécules. Pour finir, ce manuscrit présente un quatrième chapitre qui implique le phosphole comme unité coordinante afin de réaliser des nouveaux complexes qui permettent de réaliser une ortho-métallation par activation C-H. De nouveaux complexes ortho-métallés d'Ir(III) et de Rh(III) ont été synthétisés et caractérisés. / This thesis describes the synthesis and the characterization of new molecules including an organophosphorous unit, the phosphole ring. Some molecules have been used to build devices like organic light emitting diodes or organic photovoltaic cells.The first chapter describes the state of the art of the phosphole chemistry in organic materials between 2010 and 2013. The second chapter describes molecules having a tuneable twist angle allowing a fine control of the properties of the molecules like the HOMO-LUMO gap. One of those molecules has been used to build a white organic light emitting diode. The third chapter of this thesis presents an interesting three dimensional structure, the 1,1'-biphosphole. Beside this three dimensional structure, the molecules possess an original conjugation mode, the σ-π conjugation which allows a decrease of the HOMO-LUMO gap. One molecule from this chapter was used as absorber in organic photovoltaic cell. In the second part of this chapter, the 1,1'-biphosphole structure shows an interesting reactivity toward nucleophilic attack in order to functionalize the phosphorus center. This reactivity has been used to make new molecules and offer the opportunity to attach a wide range of substituents to the phosphorus atom in order to tune the properties of the molecules. The fourth chapter deals with the coordination chemistry of the phosphole in order to realize new ortho-metalated complexes. New Ir(III) and Rh(III) complexes have been synthesized and characterized.

White light emitting diodes (LED) have drawn increasing attention due to their low energy consumption, high efficiency and potential to become primary lighting source by replacing conventional light sources. White light emission is usually generated either by coating yellow phosphor on a blue-LED or blending red, green and blue phosphor in an appropriate ratio. Maintaining appropriate proportions of individual components in the blend is difficult and the major demerit of such system is the overall self-absorption, which changes the solution concentration. This results in uncontrolled changes in the whiteness of the emitted light. Zinc Oxide (ZnO), a wide bandgap semiconductor with a large exciton binding energy at room temperature has been recognized as a promising material for ultraviolet LEDs and laser diodes. Tuning of structural, optical and electrical properties of ZnO thin films by different dopants (Lithium, Indium and Gallium) is dealt in this thesis. The achievement of white light emission from a semiconducting material without using phosphors offers an inexpensive fabrication technology, good luminescence, low turn-on voltage and high efficiency. The present work is organized chapter wise, which has 8 chapters including the summary and future work. Chapter 1: Gives a brief discussion on the overview of ZnO as an optoelectronic material, crystal structure of semiconductor ZnO, the effect of doping, optical properties and its possible applications in optoelectronic devices. Chapter 2: Deals with various deposition techniques used in the present study, includes pulsed laser deposition and thermal evaporation. The experimental set up details and the deposition procedures are described in detail. A brief note on the structural characterization equipments, namely X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the optical characterization techniques namely Raman spectroscopy, transmission spectroscopy and photoluminescence (PL) spectroscopy is presented. The electrical properties of the films were studied by current- voltage, capacitance - voltage and Hall Effect measurements and the experimental details are discussed. Chapter 3: High quality ZnO/Si heterojunctions fabricated by growing ZnO thin films on p-type Si (100) substrate by pulsed laser deposition without using buffer layers are discussed in this chapter. The crystallinity of the heterojunction was analyzed by high resolution X-ray diffraction and atomic force microscopy. The optical quality of the film was analyzed by room temperature (RT) photoluminescence measurements. The high intense band to band emission confirmed the high quality of the ZnO thin films on Si. The electrical properties of the junction were studied by temperature dependent resistivity, current- voltage measurements and RT capacitance-voltage (C-V) analysis. ZnO thin film showed the lowest resistivity of 6.4x10-3 Ω.cm, mobility of 7 cm2/V.sec and charge carrier concentration of 1.58x1019cm-3 at RT. The charge carrier concentration and the barrier height (BH) were calculated to be 9.7x1019cm-3 and 0.6 eV respectively from the C-V plot. The BH and ideality factor, calculated by using the thermionic emission (TE) model were found to be highly temperature dependent. We observed a much lower value in Richardson constant, 5.19x10-7 A/cm2K2 than the theoretical value (32 A/cm2K2) for ZnO. This analysis revealed the existence of a Gaussian distribution (GD) with a standard deviation of σ2=0.035 V. By implementing GD to the TE, the values of BH and Richardson constant were obtained as 1.3 eV and 39.97 A/cm2K2 respectively from the modified Richardson plot. The obtained Richardson constant value is close to the theoretical value for n-ZnO. These high quality heterojunctions can be used for solar cell applications. Chapter 4: This chapter describes the structural and optical properties of Li doped ZnO thin films and the properties of ZnO/Li doped ZnO multilayered thin film structures. Thin films of ZnO, Li doped ZnO (ZLO) and multilayer of ZnO and ZLO (ZnO/ZLO) were grown on silicon and Corning glass substrates by pulsed laser deposition technique. Single phase formation and the crystalline qualities of the films were analyzed by X-ray diffraction and Li composition in the film was investigated to be 15 Wt % by X-ray photoelectron spectroscopy. Raman spectrum reveals the hexagonal wurtzite structure of ZnO, ZLO and ZnO/ZLO multilayer, confirms the single phase formation. Films grown on Corning glass show more than 80 % transmittance in the visible region and the optical band gaps were calculated to be 3.245, 3.26 and 3.22 eV for ZnO, ZLO and ZnO/ZLO respectively. An efficient blue emission was observed in all films that were grown on silicon (100) substrate by photoluminescence (PL). PL measurements at different temperatures reveal that the PL emission intensity of ZnO/ZLO multilayer was weakly dependent on temperature as compared to the single layers of ZnO and ZLO and the wavelength of emission was independent of temperature. Our results indicate that ZnO/ZLO multilayer can be used for the fabrication of blue light emitting diodes. Chapter 5: This chapter is divided in to two parts. The fabrication and characterization of In doped ZnO thin films grown on Corning glass substrate is discussed in the first section. Zinc Oxide (ZnO) and indium doped ZnO (IZO) thin films with different indium compositions were grown by pulsed laser deposition technique. The effect of indium concentration on the structural, morphological, optical and electrical properties of the film was studied. The films were oriented along the c-direction with wurtzite structure and are highly transparent with an average transmittance of more than 80 % in the visible wavelength region. The energy band gap was found to be decreasing with increasing indium concentration. High transparency makes the films useful as optical windows while the high band gap values support the idea that the film could be a good candidate for optoelectronic devices. The value of resistivity observed to be decreasing initially with doping concentration and subsequently increasing. The XPS and Raman spectrum confirm the presence of indium in indium doped ZnO thin films. The photoluminescence spectrum showed a tunable red light emission with different In concentrations. Undoped and In doped ZnO (IZO) thin films were grown on Pt coated silicon substrates (Pt/Si) to fabricate Pt/ZnO:Inx Schottky contacts (SC) is discussed in the second section. The SCs were investigated by conventional two probe current-voltage (I-V) measurement and by the I-V spectroscopy of conductive atomic force microscopy (C-AFM). X-ray diffraction technique was used to examine the thin film quality. Changes in various parameters like Schottky barrier height (SBH) and ideality factor (IF) as a function of temperature were presented. The estimated BH was found to be increasing and the IF was found to be decreasing with increase in temperature. The variation of SBH and IF with temperature has been explained by considering the lateral inhomogeneities in nanometer scale lengths at metal–semiconductor (MS) interface. The inhomogeneities of SBH in nanometer scale length were confirmed by C-AFM. The SBH and IF estimated from I-V spectroscopy of C-AFM showed large deviation from the conventional two probe I-V measurements. IZO thin films showed a decrease in SBH, lower turn on voltage and an enhancement in forward current with increase in In concentration. Chapter 6: In this chapter the properties of Ga doped ZnO thin films with different Ga concentrations along with undoped ZnO as a reference is discussed. Undoped and Ga doped ZnO thin films with different Ga concentrations were grown on Corning glass substrates by PLD. The structural, optical and electrical properties of Ga doped ZnO thin films are discussed. The XRD, XPS and Raman spectrum reveal the phase formation and successful doping of Ga on ZnO. All the films show good transmittance in the visible region and the photoluminescence of Ga doped ZnO showed a stable emission in the blue- green region. The resistivity of Ga doped ZnO thin films was found to be first decreasing and then increasing with increase in Ga concentrations. Chapter 7: The effect of co-doping to ZnO on the structural, optical and electrical properties was described in this chapter. Ga and In co-doped ZnO (GIZO) thin films together with ZnO, In doped ZnO (IZO), Ga doped ZnO (GZO), IZO/GZO multilayer for comparison, were grown on Corning glass and boron doped Si substrates by PLD. GIZO showed better structural, optical and electrical properties compared with other thin films. The Photoluminescence spectra of GIZO showed a strong white light emission and the current-voltage characteristics showed relatively lower turn on voltage and larger forward current. The CIE co-ordinates for GIZO were observed to be (0.31, 0.33) with a CCT of 6650 K, indicating a cool white light and established a possibility of white light emitting diodes. Finally the chapter 8 presents the summary derived out of the work and a few suggestions on future work.

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

Despite enormous efforts and investments, the efficiency of InGaN-based green and yellow-green light emitters remains relatively low, and that limits progress in developing full color display, laser diodes, and bright light sources for general lighting. The low efficiency of light emitting devices in the green-to-yellow spectral range, also known as the “Green Gap”, is considered a global concern in the LED industry. The polar c-plane orientation of GaN, which is the mainstay in the LED industry, suffers from polarization-induced separation of electrons and hole wavefunctions (also known as the “quantum confined Stark effect”) and low indium incorporation efficiency that are the two main factors that contribute to the Green Gap phenomenon. One possible approach that holds promise for a new generation of green and yellow light emitting devices with higher efficiency is the deployment of nonpolar and semi-polar crystallographic orientations of GaN to eliminate or mitigate polarization fields. In theory, the use of other GaN planes for light emitters could also enhance the efficiency of indium incorporation compared to c-plane. In this thesis, I present a systematic exploration of the suitable GaN orientation for future lighting technologies. First, in order to lay the groundwork for further studies, it is important to discuss the analysis of processes limiting LED efficiency and some novel designs of active regions to overcome these limitations. Afterwards, the choice of nonpolar orientations as an alternative is discussed. For nonpolar orientation, the (1-100)-oriented (m-plane) structures on patterned Si (112) and freestanding m-GaN are studied. The semi-polar orientations having substantially reduced polarization field are found to be more promising for light-emitting diodes (LEDs) owing to high indium incorporation efficiency predicted by theoretical studies. Thus, the semi-polar orientations are given close attention as alternatives for future LED technology. One of the obstacles impeding the development of this technology is the lack of suitable substrates for high quality materials having semi-polar and nonpolar orientations. Even though the growth of free-standing GaN substrates (homoepitaxy) could produce material of reasonable quality, the native nonpolar and semi-polar substrates are very expensive and small in size. On the other hand, GaN growth of semi-polar and nonpolar orientations on inexpensive, large-size foreign substrates (heteroepitaxy), including silicon (Si) and sapphire (Al2O3), usually leads to high density of extended defects (dislocations and stacking faults). Therefore, it is imperative to explore approaches that allow the reduction of defect density in the semi-polar GaN layers grown on foreign substrates. In the presented work, I develop a cost-effective preparation technique of high performance light emitting structures (GaN-on-Si, and GaN-on-Sapphire technologies). Based on theoretical calculations predicting the maximum indium incorporation efficiency at θ ~ 62º (θ being the tilt angle of the orientation with respect to c-plane), I investigate (11-22) and (1-101) semi-polar orientations featured by θ = 58º and θ = 62º, respectively, as promising candidates for green emitters. The (11-22)-oriented GaN layers are grown on planar m-plane sapphire, while the semi-polar (1-101) GaN are grown on patterned Si (001). The in-situ epitaxial lateral overgrowth techniques using SiNx nanoporous interlayers are utilized to improve the crystal quality of the layers. The data indicates the improvement of photoluminescence intensity by a factor of 5, as well as the improvement carrier lifetime by up to 85% by employing the in-situ ELO technique. The electronic and optoelectronic properties of these nonpolar and semi-polar planes include excitonic recombination dynamics, optical anisotropy, exciton localization, indium incorporation efficiency, defect-related optical activities, and some challenges associated with these new technologies are discussed. A polarized emission from GaN quantum wells (with a degree of polarization close to 58%) with low non-radiative components is demonstrated for semi-polar (1-101) structure grown on patterned Si (001). We also demonstrated that indium incorporation efficiency is around 20% higher for the semi-polar (11-22) InGaN quantum wells compared to its c-plane counterpart. The spatially resolved cathodoluminescence spectroscopy demonstrates the uniform distribution of indium in the growth plane. The uniformity of indium is also supported by the relatively low exciton localization energy of Eloc = 7meV at 15 K for these semi-polar (11-22) InGaN quantum wells compared to several other literature reports on c-plane. The excitons are observed to undergo radiative recombination in the quantum wells in basal-plane stacking faults at room temperature. The wurtzite/zincblende electronic band-alignment of BSFs is proven to be of type II using the time-resolved differential transmission (TRDT) method. The knowledge of band alignment and degree of carrier localization in BSFs are extremely important for evaluating their effects on device properties. Future research for better understanding and potential developments of the semi-polar LEDs is pointed out at the end.

Organische Farbstoffe mit einem konjugierten pi-Elektronen System zeigen überwiegend ein halbleitendes Verhalten. Daher sind sie potentielle Materialien für elektronische und optoelektronische Anwendungen. Erste Anwendungen in Flachbildschirmen sind bereits in (noch) geringen Mengen auf dem Markt. Die kontrollierte Dotierung anorganischer Halbleiter bereitete die Basis für den Durchbruch der bekannten Halbleitertechnologie. Die Kontrolle des Leitungstypes und der Lage des Fermi-Niveaus erlaubte es, stabile pn-Übergänge herzustellen. LEDs können daher mit Betriebsspannungen nahe dem thermodynamischen Limit betrieben werden (ca. 2.5V für eine Emission im grünen Spektralbereich). Im Gegensatz dazu bestehen organische Leuchtdioden (OLEDs) typischerweise aus einer Folge intrinsischer Schichten. Diese weisen eine ineffiziente Injektion aus Kontakten und eine relative geringe Leitfähigkeit auf, welche mit hohen ohmschen Verlusten verbunden ist. Andererseits besitzen organische Materialien einige technologische Vorteile, wie geringe Herstellungskosten, große Vielfalt der chemischen Verbindungen und die Möglichkeit sie auf flexible große Substrate aufzubringen. Sie unterscheiden sich ebenso in einigen fundamentalen physikalischen Parametern wie Brechungsindex, Dielektrizitätskonstante, Absorptionskoeffizient und Stokes-Verschiebung der Emissionswellenlänge gegenüber der Absorption. Das Konzept der Dotierung wurde für organische Halbleiter bisher kaum untersucht und angewandt. Unser Ziel ist die Erniedrigung der Betriebsspannung herkömmlicher OLEDs durch den Einsatz der gezielten Dotierung der Transportschichten mit organischen Molekülen. Um die verbesserte Injektion aus der Anode in die dotierte Löchertransportschicht zu verstehen, wurden UPS/XPS Messungen durchgeführt (ultraviolette und Röntgen-Photoelektronenspektroskopie). Messungen wurden an mit F4-TCNQ dotiertem Zink-Phthalocyanin auf ITO und Gold-Kontakten durchgeführt. Die Schlussfolgerungen aus den Experimenten ist, das (i) die Fermi-Energie sich durch Dotierung dem Transportniveau (also dem HOMO im Falle der vorliegenden p-Dotierung) annähert, (ii) die Diffusionspannung an der Grenzfläche durch Dotierung entsprechend verändert wird, und (iii) die Verarmungszone am Kontakt zum ITO sehr dünn wird. Der Kontakt aus organischem Material und leitfähigem Substrat verhält sich also ganz analog zum Fall der Dotierung anorganischer Halbleiter. Es entsteht ein stark dotierter Schottky-Kontakt dessen schmale Verarmungszone leicht durchtunnelt werden kann (quasi-ohmscher Kontakt). Die Leistungseffizienz von OLEDs mit dotierten Transportschichten konnte sukzessive erhöht werden, vom einfachen 2-Schicht Design mit dotiertem Phthalocyanine als Löchertransportschicht, über einen 3-Schicht-Aufbau mit einer Elektronen-Blockschicht bis zu OLEDs mit dotierten 'wide-gap' Löchertransport-Materialien, mit und ohne zusätzlicher Schicht zur Verbesserung der Elektroneninjektion. Sehr effiziente OLEDs mit immer noch niedriger Betriebsspannung wurden durch die Dotierung der Emissionsschicht mit Molekülen erhöhter Photolumineszenzquantenausbeute (Laser-Farbstoffe) erreicht. Eine optimierte LED-Struktur weist eine Betriebsspannung von 3.2-3.2V für eine Lichtemission von 100cd/m2 auf. Diese Resultate entsprechen den zur Zeit niedrigsten Betriebsspannungen für OLEDs mit ausschließlich im Vakuum aufgedampften Schichten. Die Stromeffizienz liegt bei ca. 10cd/A, was einer Leistungseffizienz bei 100cd/m2 von 10lm/W entspricht. Diese hohe Leistungseffizienz war nur möglich durch die Verwendung einer Blockschicht zwischen der dotierten Transportschicht und der Lichtemissions-Schicht. Im Rahmen der Arbeit konnte gezeigt werden, dass die Dotierung die Betriebsspannungen von OLEDs senken kann und damit die Leistungseffizienz erhöht wird. Zusammen mit einer sehr dünnen Blockschicht konnte einen niedrige Betriebsspannung bei gleichzeitig hoher Effizienz erreicht werden (Blockschicht-Konzept). / Organic dyes with a conjugated pi-electron system usually exhibit semiconducting behavior. Hence, they are potential materials for electronic and optoelectronic devices. Nowadays, some applications are already commercial on small scales. Controlled doping of inorganic semiconductors was the key step for today's inorganic semiconductor technology. The control of the conduction type and Fermi-level is crucial for the realization of stable pn-junctions. This allows for optimized light emitting diode (LED) structures with operating voltages close to the optical limit (around 2.5V for a green emitting LED). Despite that, organic light emitting diodes (OLEDs) generally consist of a series of intrinsic layers based on organic molecules. These intrinsic organic charge transport layers suffer from non-ideal injection and noticeable ohmic losses. However, organic materials feature some technological advantages for device applications like low cost, an almost unlimited variety of materials, and possible preparation on large and flexible substrates. They also differ in some basic physical parameters, like the index of refraction in the visible wavelength region, the absorption coefficient and the Stokes-shift of the emission wavelength. Doping of organic semiconductors has only been scarcely addressed. Our aim is the lowering of the operating voltages of OLEDs by the use of doped organic charge transport layers. The present work is focused mainly on the p-type doping of weakly donor-type molecules with strong acceptor molecules by co-evaporation of the two types of molecules in a vacuum system. In order to understand the improved hole injection from a contact material into a p-type doped organic layer, ultraviolet photoelectron spectroscopy combined with X-ray photoelectron spectroscopy (UPS/XPS) was carried out. The experimental results of the UPS/XPS measurements on F4-TCNQ doped zinc-phthalocyanine (ZnPc) and their interpretation is given. Measurements were done on the typical transparent anode material for OLEDs, indium-tin-oxide (ITO) and on gold. The conclusion from these experiments is that (i) the Fermi-energy comes closer to the transport energy (the HOMO for p-type doping), (ii) the built-in potential is changed accordingly, and (iii) the depletion layer becomes very thin because of the high space charge density in the doped layer. The junction between a doped organic layer and the conductive substrate behaves rather similar to a heavily doped Schottky junction, known from inorganic semicondcutor physics. This behavior favors charge injection from the contact into the organic semiconductor due to tunneling through a very small Schottky barrier (quasi-ohmic contact). The performance of OLEDs with doped charge transport layers improves successively from a simple two-layer design with doped phthalocyanine as hole transport layer over a three-layer design with an electron blocking layer until OLEDs with doped amorphous wide gap materials, with and without additional electron injection enhancement and electron blocking layers. Based on the experience with the first OLEDs featuring doped hole transport layers, an ideal device concept which is based on realistic material parameters is proposed (blocking layer concept). Very high efficient OLEDs with still low operating voltage have been prepared by using an additional emitter dopant molecule with very high photoluminescence quantum yield in the recombination zone of a conventional OLED. An OLED with an operating voltage of 3.2-3.2V for a brightness of 100cd/m2 could be demonstrated. These results represent the lowest ever reported operating voltage for LEDs consisting of exclusively vacuum sublimed molecular layers. The current efficiency for this device is above 10cd/A, hence, the power efficiency at 100cd/m2 is about 10lm/W. This high power efficiency could be achieved by the use of a blocking layer between the transport and the emission layer.

Organische Farbstoffe mit einem konjugierten pi-Elektronen System zeigen überwiegend ein halbleitendes Verhalten. Daher sind sie potentielle Materialien für elektronische und optoelektronische Anwendungen. Erste Anwendungen in Flachbildschirmen sind bereits in (noch) geringen Mengen auf dem Markt. Die kontrollierte Dotierung anorganischer Halbleiter bereitete die Basis für den Durchbruch der bekannten Halbleitertechnologie. Die Kontrolle des Leitungstypes und der Lage des Fermi-Niveaus erlaubte es, stabile pn-Übergänge herzustellen. LEDs können daher mit Betriebsspannungen nahe dem thermodynamischen Limit betrieben werden (ca. 2.5V für eine Emission im grünen Spektralbereich). Im Gegensatz dazu bestehen organische Leuchtdioden (OLEDs) typischerweise aus einer Folge intrinsischer Schichten. Diese weisen eine ineffiziente Injektion aus Kontakten und eine relative geringe Leitfähigkeit auf, welche mit hohen ohmschen Verlusten verbunden ist. Andererseits besitzen organische Materialien einige technologische Vorteile, wie geringe Herstellungskosten, große Vielfalt der chemischen Verbindungen und die Möglichkeit sie auf flexible große Substrate aufzubringen. Sie unterscheiden sich ebenso in einigen fundamentalen physikalischen Parametern wie Brechungsindex, Dielektrizitätskonstante, Absorptionskoeffizient und Stokes-Verschiebung der Emissionswellenlänge gegenüber der Absorption. Das Konzept der Dotierung wurde für organische Halbleiter bisher kaum untersucht und angewandt. Unser Ziel ist die Erniedrigung der Betriebsspannung herkömmlicher OLEDs durch den Einsatz der gezielten Dotierung der Transportschichten mit organischen Molekülen. Um die verbesserte Injektion aus der Anode in die dotierte Löchertransportschicht zu verstehen, wurden UPS/XPS Messungen durchgeführt (ultraviolette und Röntgen-Photoelektronenspektroskopie). Messungen wurden an mit F4-TCNQ dotiertem Zink-Phthalocyanin auf ITO und Gold-Kontakten durchgeführt. Die Schlussfolgerungen aus den Experimenten ist, das (i) die Fermi-Energie sich durch Dotierung dem Transportniveau (also dem HOMO im Falle der vorliegenden p-Dotierung) annähert, (ii) die Diffusionspannung an der Grenzfläche durch Dotierung entsprechend verändert wird, und (iii) die Verarmungszone am Kontakt zum ITO sehr dünn wird. Der Kontakt aus organischem Material und leitfähigem Substrat verhält sich also ganz analog zum Fall der Dotierung anorganischer Halbleiter. Es entsteht ein stark dotierter Schottky-Kontakt dessen schmale Verarmungszone leicht durchtunnelt werden kann (quasi-ohmscher Kontakt). Die Leistungseffizienz von OLEDs mit dotierten Transportschichten konnte sukzessive erhöht werden, vom einfachen 2-Schicht Design mit dotiertem Phthalocyanine als Löchertransportschicht, über einen 3-Schicht-Aufbau mit einer Elektronen-Blockschicht bis zu OLEDs mit dotierten 'wide-gap' Löchertransport-Materialien, mit und ohne zusätzlicher Schicht zur Verbesserung der Elektroneninjektion. Sehr effiziente OLEDs mit immer noch niedriger Betriebsspannung wurden durch die Dotierung der Emissionsschicht mit Molekülen erhöhter Photolumineszenzquantenausbeute (Laser-Farbstoffe) erreicht. Eine optimierte LED-Struktur weist eine Betriebsspannung von 3.2-3.2V für eine Lichtemission von 100cd/m2 auf. Diese Resultate entsprechen den zur Zeit niedrigsten Betriebsspannungen für OLEDs mit ausschließlich im Vakuum aufgedampften Schichten. Die Stromeffizienz liegt bei ca. 10cd/A, was einer Leistungseffizienz bei 100cd/m2 von 10lm/W entspricht. Diese hohe Leistungseffizienz war nur möglich durch die Verwendung einer Blockschicht zwischen der dotierten Transportschicht und der Lichtemissions-Schicht. Im Rahmen der Arbeit konnte gezeigt werden, dass die Dotierung die Betriebsspannungen von OLEDs senken kann und damit die Leistungseffizienz erhöht wird. Zusammen mit einer sehr dünnen Blockschicht konnte einen niedrige Betriebsspannung bei gleichzeitig hoher Effizienz erreicht werden (Blockschicht-Konzept). / Organic dyes with a conjugated pi-electron system usually exhibit semiconducting behavior. Hence, they are potential materials for electronic and optoelectronic devices. Nowadays, some applications are already commercial on small scales. Controlled doping of inorganic semiconductors was the key step for today's inorganic semiconductor technology. The control of the conduction type and Fermi-level is crucial for the realization of stable pn-junctions. This allows for optimized light emitting diode (LED) structures with operating voltages close to the optical limit (around 2.5V for a green emitting LED). Despite that, organic light emitting diodes (OLEDs) generally consist of a series of intrinsic layers based on organic molecules. These intrinsic organic charge transport layers suffer from non-ideal injection and noticeable ohmic losses. However, organic materials feature some technological advantages for device applications like low cost, an almost unlimited variety of materials, and possible preparation on large and flexible substrates. They also differ in some basic physical parameters, like the index of refraction in the visible wavelength region, the absorption coefficient and the Stokes-shift of the emission wavelength. Doping of organic semiconductors has only been scarcely addressed. Our aim is the lowering of the operating voltages of OLEDs by the use of doped organic charge transport layers. The present work is focused mainly on the p-type doping of weakly donor-type molecules with strong acceptor molecules by co-evaporation of the two types of molecules in a vacuum system. In order to understand the improved hole injection from a contact material into a p-type doped organic layer, ultraviolet photoelectron spectroscopy combined with X-ray photoelectron spectroscopy (UPS/XPS) was carried out. The experimental results of the UPS/XPS measurements on F4-TCNQ doped zinc-phthalocyanine (ZnPc) and their interpretation is given. Measurements were done on the typical transparent anode material for OLEDs, indium-tin-oxide (ITO) and on gold. The conclusion from these experiments is that (i) the Fermi-energy comes closer to the transport energy (the HOMO for p-type doping), (ii) the built-in potential is changed accordingly, and (iii) the depletion layer becomes very thin because of the high space charge density in the doped layer. The junction between a doped organic layer and the conductive substrate behaves rather similar to a heavily doped Schottky junction, known from inorganic semicondcutor physics. This behavior favors charge injection from the contact into the organic semiconductor due to tunneling through a very small Schottky barrier (quasi-ohmic contact). The performance of OLEDs with doped charge transport layers improves successively from a simple two-layer design with doped phthalocyanine as hole transport layer over a three-layer design with an electron blocking layer until OLEDs with doped amorphous wide gap materials, with and without additional electron injection enhancement and electron blocking layers. Based on the experience with the first OLEDs featuring doped hole transport layers, an ideal device concept which is based on realistic material parameters is proposed (blocking layer concept). Very high efficient OLEDs with still low operating voltage have been prepared by using an additional emitter dopant molecule with very high photoluminescence quantum yield in the recombination zone of a conventional OLED. An OLED with an operating voltage of 3.2-3.2V for a brightness of 100cd/m2 could be demonstrated. These results represent the lowest ever reported operating voltage for LEDs consisting of exclusively vacuum sublimed molecular layers. The current efficiency for this device is above 10cd/A, hence, the power efficiency at 100cd/m2 is about 10lm/W. This high power efficiency could be achieved by the use of a blocking layer between the transport and the emission layer.

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