Performance Enhancement of Organic Light-Emitting Diodes with an Inorganically Doped Hole Transport Layer

Organic light-emitting diodes (OLEDs) are generally considered as the next generation display and lighting sources owing to their many attractive properties, including low power consumption, wide viewing angle, vibrant color, high contrast ratios and compatibility with flexible substrates. The research and development of OLEDs has attracted considerable interest and has led to significant progress during the last two decades. The use of OLEDs in small-area displays such as cell phone screens, digital cameras, and wearable devices has become a reality. However, the OLED technology is still far from mature, posing a challenge for their widespread acceptance for applications in large-area displays and solid-state lighting. In particular, the lifetime of OLEDs is too short for many commercial applications, and the degradation mechanisms are still under debate. This work aims to improve the OLED device lifetime by doping of organic hole transport materials with inorganic transition metal oxides (TMOs), and to reduce the cost by simplifying the device layer structure and manufacturing procedure.;First, stress tests under continuous wave and pulsed currents were conducted to gain a better understanding of the key factors governing the degradation process of phosphorescent OLEDs. Through comparative studies of the aging behaviors of OLEDs with different hole transport layers (HTLs) under different stressing conditions, we have found that joule heating plays an important role in device degradation when a large energy level misalignment exists at the indium-tin-oxide (ITO) anode/HTL interface. The heating was effectively suppressed by reducing the interfacial energy barrier, leading to a prolonged lifetime of the OLEDs.;P-type doping of hole transport materials with TMOs was then developed as an effective way to reduce the interfacial energy barrier and the operational voltage of OLED devices. A systematical study was carried out on the effects of doping 4,4\u27-Bis(N-carbazolyl)-1,1\u27-biphenyl (CBP), a wide bandgap organic hole transport material, with WO3 and MoO3. The optimal doping conditions including the doping level and doping thickness have been determined by fabricating and characterizing a series of hole-only devices. Integrating the doped HTL into green phosphorescent OLEDs has resulted in a simplified structure, better optoelectronic characteristics, and improved device reliability.;Finally, selective doping of organic materials with the TMOs was developed and the concept of delta doping was applied to OLEDs for the first time. Selective doping was achieved by simple sequential deposition of the organic host and TMO dopant. Hole-only devices with a HTL comprising alternative 0.5 nm TMO-doped/3-10 nm undoped CBP layers exhibited greatly enhanced hole transport and had a turn-on voltage as low as 1.1 V. Simple fluorescent tris-(8-hydroxyquinoline) aluminum (Alq3)-based green OLEDs with a selectively doped CBP HTL showed a lower voltage and longer lifetime under constant-current stressing compared to similar OLEDs with an undoped HTL. Furthermore. delta doping was realized in more thermally stable organic materials, resulting in a marked conductivity increase along the plane of the doped layers by several orders of magnitude. The delta doping effects were explained by hole accumulation in potential wells formed in nanometer-thick doped regions, as revealed by high-resolution secondary ion mass spectrometry (SIMS) measurements

Modeling of OLED degradation for prediction and compensation of AMOLED aging artifacts

Degradation is still the most challenging issue for OLED, which causes the image-sticking artifact on AMOLED displays and limits their lifetime. To overcome the demerit, OLED degradation is modeled in this thesis, and compensation based on the models is applied for AMOLEDs. A data-counting model is firstly developed to quantitatively evaluate the degradation on OLEDs, with consideration of the accumulation stress during operation. An electro-optical model is further built, based on an equivalent circuit. It can simulate the electro-optical characteristic (I-V, Eff-V) and the degradation behaviors in aging process. Besides, the correlation model is aimed to derive the current efficiency decay with measurable electrical values, delivering more dependable results at strongly aged state. The prediction and compensation are implemented based on developed models. The results show that the models exactly predict the efficiency decay during operation. The image-sticking aging artifact on AMOLED can be suppressed by applying compensation, so that the display lifetime is extended.Durch das Einbrennen von Bildern in AMOLED Displays wird deren Lebensdauer verringert; dieser Qualitätsverlust stellt nach wie vor die größte Herausforderung für die OLED Technologie dar. In dieser Thesis wird die Degradation der OLEDs modelliert und eine Kompensierung anhand der Modelle erreicht. Zunächst wurde ein Data-counting Modell entwickelt, um die Degradation von OLEDs unter Berücksichtigung der akkumulierten Belastung während des Betriebs quantitativ zu bewerten. Des Weiteren wurde ein elektro-optisches Modell entwickelt, das auf einem äquivalenten Schaltungsmodell basiert. Es kann die elektro-optischen Eigenschaft (I-V, Eff-V) und das Degradationsverhalten im Alterungsprozess simulieren. Außer den beiden Modellen wird noch ein Korrelationsmodell entwickelt, das darauf abzielt, die Abnahme der Stromeffizienz aus den messbaren elektrischen Werten abzuleiten. Dieses Modell liefert im stark gealterten Zustand zuverlässigere Ergebnisse. Aufbauend auf die entwickelten Modelle wurden die Vorhersage und die Kompensierung implementiert. Die Ergebnisse zeigen, dass die Modelle den Effizienzverlust während des Betriebes genau vorhersagen. Das Einbrennen des Bildes in das AMOLED-Display kann durch das Anwenden der Kompensierung unterdrückt werden, so dass die Lebensdauer des Displays verlängert wird

Development of hybrid inorganic-organic light-emitting devices with metal oxide charge transport layers

Organic light emitting diodes (OLEDs) are currently being considered as the next generation technology in flat panel displays and solid state lighting applications. Among which, phosphorescent organic light emitting diodes (PhOLEDs) with nearly 100% internal quantum efficiency including other properties such as self emitting, high luminescence efficiency, broad wavelength range, wide viewing angle, high contrast, low power consumption, low weight, and large emitting area are gaining popularity in both academic and industrial research. Although development and commercialization of OLED technology is growing, there are still several key issues that need to be addressed---the external quantum efficiency (EQE) needs to be improved and the biggest technical challenge is to increase the device operational lifetime. Balanced charge injection and transport is vital for improving the device efficiency which demands for selection of better charge injection and transport materials. In addition imbalanced charge injection also degrades the device via joule\u27s heating and charge accumulation thereby limiting the device lifetime. Sensitivity of organic materials to the ambient atmosphere, particularly oxygen and moisture impedes the device performance.;This thesis work attempts to address these issues in the PhOLEDs through selection of proper charge injection and transport material as well as device structure optimization. At first we prepared thin films of thermally evaporated zinc-tin oxide (ZTO) with various ZnO and SnO2 compositions and studied its optical, electrical and morphological properties. After optimization of transparency and conductivity, these ZTO films showed promising materials for alternate transparent conducting oxides and electron transport layer (ETL) functions. Similarly, thin films of thermally evaporated tungsten oxide (WO 3) were prepared and their optical and electrical properties were studied and evaluated as a hole transport layer (HTL) material. We then fabricated and characterized various hybrid light emitting diode (HyLED) structures comprising of---ZTO as an ETL, WO3 as a HTL, and MoO3 as a hole injecting layer (HIL). The device structures were optimized for better performance in terms of efficiency and operational lifetime. Significant enhancement in EQE and operational lifetime were obtained in HyLEDs having WO3 as a HTL than of PhOLEDs with organic HTL. This is because WO3 improved hole injection as well as enabled facile hole transport thereby maintaining the balance of charge injection into the device. Finally, we also prepared inverted HyLEDs using WO3 as HTL and several metals including Ca, Ca/LiF, and Al/LiF as a cathode and their electron injecting capability were studied. Balanced charge injection was observed when a nanometer thick Ca was used as a cathode and WO3 as a HTL. As a result, inverted HyLED with better EQE and operational lifetime were fabricated

A STUDY FOCUSING ON THE EFFECTS OF HTOL STRESS ON THE LUMINESCENCE SPECTRUM OF GAN DIODES TO CHARACTERIZE COMPONENT DEGRADATION

Wide bandgap (WBG) semiconductor technology allows devices to be operated at higher voltages, currents, temperatures, and frequencies than does conventional silicon-based narrow bandgap semiconductors. These characteristics are advantageous to military applications, such as uses in power converters, weapons, and radar systems. Notably, WBG semiconductors have advantages where cooling and space availability for components are concerns, such as unmanned underwater platforms. The ability to monitor the health and performance of these devices passively and remotely would reduce the man-hours required for preventative maintenance; it would also reduce the needs for invasive troubleshooting and needless component replacement. This thesis demonstrates the abilities to measure and analyze the electroluminescence spectrum of WBG devices using a custom-built high-temperature operating life (HTOL) test setup incorporating the ability to sample light spectroscopy.Lieutenant, United States NavyApproved for public release. Distribution is unlimited

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments

Characterization of High Temperature Optocoupler for Power Electronic Systems

High-temperature devices have been rapidly increas due to the implementation of new technologies like silicon carbide, high-temperature ceramic, and others. Functionality under elevated temperatures can reduce signal integrity reducing the reliability of power electronic systems. This study presents an ongoing research effort to develop a high-temperature package for optocouplers to operate at higher temperature compared with commercial devices. Low temperature co-fired ceramic (LTCC) was used as the substrate. Bare die commercial LED and photodetectors were attached to the substrate and tested for functionality. Preliminary results show enhanced performance at elevated temperatures compared to a commercial optocoupler device