Photon Generation and Dissipation in Organic Light-Emitting Diodes

By using phosphorescent and thermally activated delayed fluorescence emitters, the internal quantum efficiency of organic light-emitting diodes (OLEDs) can now reach 100%. However, a major fraction of generated photons is trapped inside the device, because of the intrinsic multi-layer device structure and the mismatch of refractive indices. This thesis comprises different approaches for the efficiency enhancement of planar OLEDs. In particular, outcoupling strategies to extract trapped photons to obtain highly efficient OLEDs are investigated

Luminescent polymer blends in hybrid organic-inorganic polymer light-emitting devices (HyPLEDs)

Electronic MaterialsOrganic/polymer based light-emitting diodes(OLEDs/PLEDs) have enormous technological significance for low-cost solution processing such as spin coating, dip coating and ink-jet printing. A recent promising approach toward air-stable devices without a low work function metal is to use inorganic metal oxides as charge injection and transport layer. In particular, solution processable n-type ZnO layer as electron injection/transport layer was mainly used in hybrid organic-inorganic polymer light-emitting diodes (HyPLEDs) and organic solar cells (OSCs). Based on HyPLEDs, PLED performance was optimized by blending luminescent polymers with various functional molecules or same fluorescent polymers. Blending approaches are generally considered to offer simpler device fabrication, higher device efficiency and performance. Here, we report a luminescent polymer blend system by mixing fluorescent polymer with ionic salt for hybrid organic-inorganic polymer light-emitting electrochemical cells (HyPLECs), and fluorescent red, green, and blue (RGB) polymers for white PLEDs (WPLEDs). Further development was achieved by attaching cholesteric liquid crystal (CLC) reflector in front of the surface of WPLEDs to obtain white circularly polarized (CP) electroluminescence (EL). This thesis is organized as follow. An introduction of semiconducting polymers, characteristics of OLEDs and hybrid PLEDs with diverse transition metal oxide (TMO) layer are mainly described in the chapter 1. In particular, hybrid organic-inorganic polymer light emitting diode (HyPLEDs) are new type of top-emission device in which device can combine with n-type thin-film transistor (TFTs) for application to active matrix structure. The ZnO layer as a cathode material in the inverted configuration takes a role of the electron-injecting layer as they combine properties such as transparency, low resistance, and air-stability. In the chapter 2, we demonstrate enhanced device performance by using a blend of emissive polymer (“Super Yellow”) and mobile ionic liquid molecules (ILMs) in hybrid organic-inorganic polymeric light-emitting electrochemical cells (HyPLECs) with high air stability. The mobile anions and cations redistributed near each electrode/active layer interface make ohmic contacts, thereby enhancing current density and electroluminescence efficiency at relatively low operating voltage. Moreover, a luminescent blend of blue-emitting polymer (“M-blue”), orange-emitting dye (DCM), and ILMs was investigated to achieve white emission in HyPLECs. By using ILMs, we can observe the characteristics of LECs with low operating voltage and air stability of HyPLECs by introducing ZnO layer. Finally, we investigate RGB ternary blend in single active layer for white emission. WPLEDs using polymer blends showed low turn-on voltage, high brightness, efficiency, and color stability. Furthermore, we observed CP-EL by combination of WPLED and cholesteric liquid crystal (CLC) reflector.ope

Small Molecule Organic Optoelectronic Devices

abstract: Organic optoelectronics include a class of devices synthesized from carbon containing ‘small molecule’ thin films without long range order crystalline or polymer structure. Novel properties such as low modulus and flexibility as well as excellent device performance such as photon emission approaching 100% internal quantum efficiency have accelerated research in this area substantially. While optoelectronic organic light emitting devices have already realized commercial application, challenges to obtain extended lifetime for the high energy visible spectrum and the ability to reproduce natural white light with a simple architecture have limited the value of this technology for some display and lighting applications. In this research, novel materials discovered from a systematic analysis of empirical device data are shown to produce high quality white light through combination of monomer and excimer emission from a single molecule: platinum(II) bis(methyl-imidazolyl)toluene chloride (Pt-17). Illumination quality achieved Commission Internationale de L’Éclairage (CIE) chromaticity coordinates (x = 0.31, y = 0.38) and color rendering index (CRI) > 75. Further optimization of a device containing Pt-17 resulted in a maximum forward viewing power efficiency of 37.8 lm/W on a plain glass substrate. In addition, accelerated aging tests suggest high energy blue emission from a halogen-free cyclometalated platinum complex could demonstrate degradation rates comparable to known stable emitters. Finally, a buckling based metrology is applied to characterize the mechanical properties of small molecule organic thin films towards understanding the deposition kinetics responsible for an elastic modulus that is both temperature and thickness dependent. These results could contribute to the viability of organic electronic technology in potentially flexible display and lighting applications. The results also provide insight to organic film growth kinetics responsible for optical, mechanical, and water uptake properties relevant to engineering the next generation of optoelectronic devices.Dissertation/ThesisDoctoral Dissertation Chemical Engineering 201

Study of Thermally Activated Delayed Fluorescent Exciplexes and their Practical Applications in OLEDs

Exciplexes are intermolecular charge transfer (CT) complexes in which one electron donating (D) and one electron accepting (A) molecule interact in the excited state. The new bimolecular CT excited state species is the exciplex, a shortening of EXCIted state comPLEX. Until recently exciplexes were avoided in OLEDs structures since they constituted an efficiency loss pathway since they commonly possess low photoluminescence quantum yield (PLQY). Recently, the rise of thermally activated delayed fluorescence (TADF) applications for triplet harvesting in fluorescent OLEDs has resulted in renewed research interest in these bimolecular excited states. The TADF mechanism in fact, allows to upconvert triplet excited states (which are non-emissive in normal fluorescent emitters) into emissive singlet excited states thus boosting the efficiency of the emitter. To be efficient, the TADF mechanism needs to have minimal overlap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). Exciplexes intrinsically possess this characteristic since the CT excited state is formed between two different molecules making exciplexes the perfect candidates as TADF emitters. For this reason, TADF exciplexes are attracting more and more attention although always in the shadow of their more successful intramolecular counterpart (covalently linked D-A fragments in a single molecule) since they could be more easily tailored to maximise their efficiency and modify their properties. The first part of this thesis demonstrates the surprising discovery that exciplex electronic energy and PLQY are not intrinsically fixed by the D/A couple forming the exciplex, and that these characteristics can be tuned and improved through solid state dilution. It is shown that the exciplex electronic energy can be controllably increased by varying average intermolecular distance between the D and A molecule within the exciplex blend by inserting a third inert molecule in the blend forming the film. The change in the exciplex electronic energy and PLQY is rationalised by a general reduction of the coulombic binding energy with D-A separation. In contrast, the PLQY enhancement is not general and determined to be related to the degree of flexibility of the exciplex forming molecules. The second part of this thesis showcases work that broadens the range of potential applications of TADF exciplex OLEDs, demonstrating their suitability as emitters for solution processed devices and how they can be used to confine the recombination zone of a standard phosphorescent OLED - leading to performance and stability improvements

High efficiency top-emitting organic light-emitting diodes: design and fabrication

This thesis focuses mainly on the techniques to achieve high-performance top-emitting OLEDs, regarding device efficiency and lifetime for both non-inverted and inverted structures. It is thus organized as follows: In Chapter 2, the basic physics of organic semiconductor materials are reviewed, including the electronic properties of organic semiconductor materials, molecular excitations and their electronic transitions etc., which are believed to be critical for understanding of the work. Then, the general device physics of OLEDs are reviewed in detail, which includes almost every important electrical and optical process involved in the device. Finally, techniques and methods used to improve the device performance are summarized, which includes electrical doping of charge carrier transport layers. In Chapter 3, all organic materials, experimental techniques, and characterization methods used in this study are briefly described. In the following Chapter 4, techniques that are used for device optimization of non-inverted top-emitting OLEDs are discussed. Also, the mechanism of light outcoupling enhancement by a capping layer is discussed there. In the last part of Chapter 4, the influence of the optical device structure on the intrinsic quantum yield of the emitters is studied. Chapter 5 is focused on inverted top-emitting OLEDs, which are believed to be better applicable with current mainstream n-type amorphous silicon thin film transistor (TFT) technology. In this Chapter, the organic/metal and metal/organic interfaces are investigated in detail and their influence on device performance is discussed. In Chapter 6, the degradation of top-emitting OLEDs is studied, with a focus on the influence of electrode material and electrode thickness on the lifetime of top-emitting devices

Innovative organic electroluminescent devices: diarylethenes as light-responsive switches and emissive graphene quantum dots

The awarding in 2016 of the Nobel Prize in Chemistry to Prof. J. Sauvage, J. F. Stoddart and B. L. Feringa, “for the design and synthesis of molecular machines” proves the interest of the scientific community and high-tech industry towards stimuli-responsive multifunctional materials and devices. In this direction, the research activity described in this thesis is focused on light-responsive emissive devices, which can be controlled remotely and reversibly via irradiation with light of specific wavelengths. A range of photochromic diarylethene derivatives were judiciously selected in combination with commercially available organic semiconductors to generate the light-responsivity in our devices. At first, optically switchable green-emitting OLEDs and micro-OLEDs were investigated achieving the maximum ON/OFF ratio of ~20 and ~90 for current density and luminance, respectively. Additionally, through the studying of the performance of light-responsive single carrier devices, it was demonstrated that electrons are more affected than holes by the switching of the photo-active dopant. The device emissive area was further scaled-down working on the characterization of light-emitting transistors (OLETs) having channel length of 2.5 µm. For the first time such devices covered the whole visible spectrum and a maximum ON/OFF ratio exceed 500 was achieved for both drain current and luminance. Finally, another macro-trend of the display community was addressed, the constant search for innovative chromophores as alternative to poorly stable and highly expensive iridium-based phosphorescent materials or highly toxic cadmium-based colloidal quantum dots. In this direction, air stable and potentially non-toxic red-emitting graphene quantum dots (GQDs) were embedded in electroluminescent devices. The LEDs showed high colour purity (FWHM= 30 nm), maximum luminance of 1300 cd/m2 and EQE of 0.67 % which are among the best performance ever reported for LEDs based on red-emitting GQDs

TADF OLEDs: characterization, modelling and detailed degradation analysis

The need of high quality and efficient displays is continuously increasing. The organic light-emitting diode (OLED) technology is certainly one of the most important in this sense, thanks to their high contrast, excellent color purity and wide viewing angle. Despite being already widely used in commercial products, scientific research on OLED materials is still ongoing to improve their efficiency and durability. A new technology which might replace currently used emissive materials in OLED is the so called thermally-activated delayed fluorescence (TADF). With these emitters, the display efficiency can be improved without the need of expensive and pollutant heavy metal atoms. This PhD project is focused on these materials and their use in OLEDs. The study of TADF OLEDs presented in this thesis has been structured in three main parts. The first study allows to clarify a frequent misconception about these emitters: the portion of excitons leading to an emissive event, usually approximated to 100%, can actually be much lower when electrically excited. A method to estimate this value is provided from the analysis of transient and steady state optical measurements. In a state-of-the-art OLED, the emission layer (EML) consists of two or more components. The adjustment of each material component and the optimization of the concentration largely impact the OLED performance. In the second study, OLEDs containing different concentrations of the TADF molecule in the emissive layer are investigated. Several experimental techniques are used and, with the use of software simulations, the effect of emission layer composition on the charge and excitonic processes is analysed. The key aspect which must be improved in order to make TADF a suitable technology in commercial products, is the lifetime. Two studies about this topic have been included in this thesis. To effectively measure the lifetime of emissive devices, one would need to operate them for several thousands of hours. This approach is definitely not applicable on a large scale, when a multitude of different devices need to be tested, since it requires a lot of time and resources. Such characterization is therefore typically done under accelerated stressing conditions, with high currents and/or temperatures. The use of appropriate scaling laws allows to estimate the durability of the device in standard operating conditions from the accelerated ones. In the first study described in this work, several identical TADF OLEDs have been stressed with different current at different temperature, and the complete set of luminance decay is fed into a global fitting algorithm. With this approach the expected lifetime can be estimated in a shorter amount of time, yet with a high accuracy. To improve the device lifetime, a detailed understanding of the processes causing it is necessary. The second study on device lifetime goes more into detail of the degradation processes occurring in a specific TADF OLED stack. The devices are stressed with constant current, and during stressing interruption a series of experimental techniques are used. Electrical device simulations are used to model these OLEDs and qualitatively identify the degradation causes. Specifically, it is found that the generation of trap states causes a variation of the charge injection and accumulation inside the device

Excited State Interactions and Management in Organic Light Emitting Diodes.

Organic light emitting diodes (OLEDs) have been leading the research and development in organic semiconductors, and representing a primary driving force in information display as well as solid-state lighting innovations. In organic semiconductors, excitons are responsible for optical transitions, and are thus central to the operation of all organic optoelectronic devices. This dissertation aims at understanding the fundamental physics of exciton interactions and their effects on the performance of OLEDs. We show that managing exciton interactions based on exciton physics results in significantly improved device characteristics. Organic light emitting diodes based on singlet and triplet exciton emission are called fluorescent OLEDs and phosphorescent OLEDs (PHOLEDs), respectively. The first part of this dissertation studies exciton interactions in fluorescent OLEDs. We begin by identifying singlet-triplet annihilation as a loss mechanism in fluorescent efficiency, and thus propose a triplet management strategy to de-excite the detrimental non-emissive triplet. This strategy leads to more than 100% improvement in fluorescent OLED efficiencies, and also a more than 100-fold increase in lasing duration in organic semiconductor lasers (OSLs), thus allowing for the first observation of the continuous-wave threshold in OSLs. Further, since triplet-triplet annihilation (TTA) contributes to fluorescent emission, we analyze the trade-off between STA and TTA, and propose optimal fluorescent material properties needed for high fluorescent efficiency. The second part of this work focuses on exciton interactions in PHOLEDs. Triplet-triplet annihilation is studied through transient photoluminescence, and Dexter-type triplet diffusion is identified as the dominant mechanism leading to TTA. Thus, minimizing the Stokes shift between the molecular emission and absorption is introduced as a route leading to high efficiency PHOLEDs at high luminance. Indeed, exciton interactions are important for not only OLED efficiency but also operational lifetime. Based on the understanding that triplet-polaron annihilation (TPA) is a fundamental intrinsic degradation mechanism in blue PHOLEDs, we designed a novel OLED whose phosphorescent emitter concentration is varied linearly with position. This doping profile results in a low and uniform exciton density and thus a higher efficiency and suppressed TPA, leading to a significantly extended operational lifetime over conventional blue PHOLEDs.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107267/1/yfz_1.pd

A Study on Charge Selective Transport for Highly Efficient Polymer Based Optoelectronic Devices

Department of Materials Science EngineeringPolymer based optoelectronic devices including polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) have been recently focused for display, energy source and flexible electronic applications because of their advantages such as low cost, light weight, easy solution process fabrication and mechanical flexibility. Moreover, so much effort has been made to maximize their device performance through optimization of device configuration and charge selective transport. In particular, balanced charge transport via charge selective interfacial engineering or surface modification is promising for optimized device performance. According to the device configuration, interfacial engineering can improve the minority carrier transport with well-matched energy level, passivate the charge trap sites and enhance the materials compatibility. It can also block abundant majority carrier and reduce the exciton quenching, leading to improving the recombination rate of balanced charges in PLEDs while disrupting bimolecular recombination in PSCs. Here, I present variety interfacial engineering strategies employing modified charge transport layer such as graphene oxide (GO) as a hole transport layer (HTL) in conventional PLEDs and surface modified zinc oxide (ZnO) as an electron transport layer (ETL) using ionic liquid molecules (ILMs), conjugated polyelectrolyte (CPE) and amine-based polar solvents in inverted polymer light-emitting diodes (iPLEDs) and polymer solar cells (iPSCs). A GO layer with a wide band gap blocks transport of electrons from an emissive layer to an indium tin oxide (ITO) anode while reduces the exciton quenching between the GO layer and the emissive layer. As a result, the GO layer maximizes hole-electron recombination within the emissive layer leading to improvement of device performance in PLEDs. In addition, surface modified ZnO layers with various interfacial layers such as ILMs, CPE and amine-based polar solvents remarkably enhance the devices performance by introducing spontaneously oriented interfacial dipoles between the ZnO layer and active layer in iPLEDs and iPSCs. This charge selective interfacial engineering is a promising way for organic optoelectronic devices such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic laser diodes (OLDs).ope