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

Organic Light Emitting Devices

This book describes the state-of-the-art advancement in the field of organic electroluminescence contributed by many researchers with internationally established expertise in the field. It includes original contributions on the synthesis of suitable organic materials, fabrication of organic light emitting devices (OLEDs) and organic white light emitting devices (WOLEDs), characterization of these devices and some designs for optimal performance. All chapters are self-sufficient in presenting their contents. The cost effective chemical technology offers many exciting possibilities for OLEDs and organic solar cells (OSCs) to be futuristic solutions for lighting and power generation. A common flexible substrate can be used to fabricate OLEDs on one side facing a room and OSCs on the other side facing the sun. The device thus fabricated can generate power in the day time and light a room/house at night. The book covers developments on OLEDs, WOLEDs and briefly on OSCs as well

Degradation Mechanisms in Small-Molecule Organic Electronic Devices

Over the last decades organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) have gained considerable attention as efficient, flexible, lightweight, and potentially low-cost technology for lighting and display applications or as a renewable energy source, respectively. However, achieving long-term stability remains challenging. Revealing and understanding aging processes is therefore of great interest. This work presents fundamental investigations to understand and circumvent organic device degradation. In the first part, single materials used in organic devices were investigated. By tailoring an attenuated total reflection infrared (ATR-IR) spectrometer to the specific needs and subsequent measurements, it is shown that the tris(8-hydroxyquinoline)aluminum (Alq3) molecule, a well known fluorescent green emitter, degrades during air exposure by the formation of carbonyl groups. By using a laser desorption/ionization time of flight mass spectrometer (LDI-TOF-MS) it was shown that a,w-bis-(dicyanovinylen)-sexithiophen (DCV6T-Bu4), a well known small-molecule material which is used as part of the active layer, reacts with oxygen during ultraviolet (UV) irradiation. By using climate boxes and a sun simulator the impact of dry and humid air as well as sunlight on C60, a widely-used acceptor molecule in organic solar cells, was investigated. The breaking of the C60 cage to C58 and C56 and the further reaction of these components with oxygen as well as the dimerization of C58 and C56 molecules were found. The degradation products such as C58O increase with air exposure time but they are independent of the humidity level of the ambient air as well as sunlight irradiation. Subsequent annealing leads to a decrease of the C58O concentration. Many efficient n-dopants are prone to degradation in air, due to the low ionization potentials, thereby limiting the processing conditions. It was found that the air exposure of the highly efficient n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4) leads to oxidation reactions of the molecule to [W(hpp)2 + O] and other degradation products. The decay constant of W2(hpp)4 and the matching mean growth time of the [W(hpp)2 + O] degradation as well as a second very quick degradation of the dopant could be determined. The two decay constants can be explained by the assumption that W2(hpp)4 molecules, which are involved in the charge transfer, do degrade slower due to the fact that the charge transfer leads to a downshift of the energy levels of the W2(hpp)4 molecule. Apart from the properties of the organic materials, other effects such as the impact of different purification systems on the material purity as well as the dependence of material purity on the OLED lifetime has been investigated. No correlations between the purification grade and the amount of impurities were found. OLEDs which contain N,N\'-di(naphthalen-1-yl)-N,N\'-diphenyl-benzidine (alpha-NPD) purified in a vertically interlaced stainless steel sublimation systems shows slightly higher external quantum efficiencies compared to tube-based vacuum sublimation systems. The devices which contain alpha-NPD purified by a sublimation system have an extended lifetime. Finally, the impact of residual gases during device fabrication on OLED lifetime and electrical characteristics was investigated. It was found that water vapor introduces an additional series resistance to the OLED, while the other gases do not influence the electric characteristics. The presence of nitrogen or oxygen impacts the lifetime of the OLEDs by the same amount. Nitrogen is non-reactive, this leads to the conclusion that the influence of nitrogen and oxygen on the OLED lifetime is of non-chemical nature, such as changes in the morphology of the organic layers. Water vapor introduces an additional, even faster degradation process within the first hours of OLED operation. As major sources of device degradation, the dimerization of 4,7-diphenyl-1,10-phenanthroline (BPhen) as well as the complexation reaction of alpha-NPD with a bis(1-phenylisoquinoline)iridium(III) (Ir(piq)2) fragment was identified

유기 발광 소자에 대한 임피던스 분광 분석 방법에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2019. 2. 홍용택.In this dissertation, the method of analyzing the characteristics of organic light-emitting diodes using impedance spectroscopy was studied. Generally, the organic light-emitting device has a structure in which organic materials necessary for light emission are thinly laminated between a TCO (Transparent Conductive Oxide) and a metal electrode. Due to chemical vulnerability of organic materials as well as the complexity with their hetero-junction, it is necessary to investigate the characteristics of fabricated an organic light-emitting diodes in a non-destructive manner rather than destructive manner. Classically, a method of measuring a current-voltage curve using a current-voltage meter and measuring a luminescence using luminance meter is used to evaluate the characteristics of the OLEDs. However, in order to investigate details at intrinsic interface state or carrier dynamics of OLEDs, it is require measuring the impedance response under operating condition. Impedance spectroscopy (IS) covers all impedance responses in the frequency range from milli hertz to megahertz, while focusing primarily on capacitance in solid-state electronics. This makes it possible to construct a high-resolution equivalent circuit and analyze each measured impedance. Each impedance is measured by applying ac small signal after determining the dc operating voltage. The dc operating voltage and ac small signal should be strategically chosen to describe the behavior of the device. In Chapter 1, an overview of the Impedance Spectroscopy Analysis is introduced and the background of the impedance measurement method is explained. Then the motivation for applying this impedance analysis method to OLEDs is explained and the methods of analyzing the characteristics of OLEDs through impedance spectroscopy are discussed. Chapter 2 reviews previously reported papers about impedance analysis methods of OLEDs and explains the limitations of these papers. In particular, the contributions of the diffusion capacitance that they underestimate are very important to prevent errors when characterizing of OLEDs. To explained diffusion capacitance the Laux & Hess model is applied. This model can explain the impedance response to the residual current and even demonstrate the negative capacitance phenomenon. The analytical results using the Laux & Hess model[1] were verified to describe the characteristics of the OLEDs during operation and an approximate and fitting process for the analytical method of this model is proposed. In Chapter 3, ITO/a-NPD[N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine]/Alq3[Tris-(8-hydroxyquinolinato)aluminum]/LiF/Al type OLEDs were fabricated and investigated using the improved impedance analysis method proposed in Chapter 2. First, according to the structural (thickness) change, the physical analysis was performed quantitatively by changing the impedance response. The measured impedance at high frequencies represents the interface and buck characteristics in the OLEDs, and the impedance at low frequencies explains the dynamics of carriers in the OLEDs. In addition, the impedance changes due to OLEDs degradation are analyzed. Using the proposed Impedance Spectroscopy Analysis method in this paper, the origin of degradation can be accurately and effectively separated from the state change of the interface and the buck. The results strengthen the existing interpretation of the interface trap effect of HTL/EML and showed that it can be traced with the rate of change of the extracted impedance value. Chapter 4 briefly introduces the program tool for analyzing the OLEDs used in this paper, and attached an appendix for derived the formula. And I will discuss the possibility of applying this Impedance Spectroscopy Analysis to mass products industry in the future.본 논문은 임피던스 분광법을 이용한 유기 발광 다이오드의 특성 분석 방법에 대한 연구이다. 일반적으로 유기 발광 소자는 TCO (Transparent Conductive Oxide)와 금속 전극 사이에 발광에 필요한 유기 물질이 얇게 적층 된 구조를 갖는다. 유기 물질의 화학적 취약성과 헤테로 접합의 복잡성으로 인해 파괴적인 방식 보다는 비파괴 방식으로 제조된 유기 발광 다이오드의 특성을 조사해야 한다. 고전적으로, 전류-전압계를 사용하여 전류-전압 곡선을 측정하고 휘도 계를 사용하여 발광을 측정하는 방법이 OLEDs의 특성을 평가하는데 사용된다. 그러나 OLEDs의 인터페이스 상태 또는 캐리어 동역학에 대한 세부 사항을 조사하기 위해서는 작동 조건 하에서의 임피던스 응답을 측정해야 한다 일반적인 고체 물리에서의 임피던스 분석 방법은 커패시턴스에 주로 초점을 맞추고 있는 반면에, 임피던스 분광법 (IS)은 miliherz 에서 megaherz의 주파수 범위에서 모든 임피던스 응답을 다루는 것이 특징이다. 이를 통해 분해능이 높은 등가 회로를 구성하고 각각 측정 된 임피던스를 면밀히 분석 할 수 있다. 각 임피던스는 작동 직류 전압을 결정한 후 교류 소신호를 인가하여 측정되는데, 이 작동 직류 전압과 교류 소신호는 소자의 동작을 적절히 설명하기 위해 전략적으로 선택되어야 한다. 제 1 장에서는 임피던스 분광 분석의 개요를 소개하고 임피던스 측정 방법의 배경을 설명한다. 그리고 임피던스 분석 방법을 OLEDs에 적용하려는 동기에 대해서 설명하고 임피던스 분광학을 통해 OLEDs의 특성을 분석하는 다양한 방법들이 논의된다. 제 2 장은 OLEDs의 임피던스 분석 방법에 대해 이전에 보고된 논문을 검토하고 이들 논문의 한계를 설명한다. 특히, 이전에 과소평가된 확산 캐패시턴스의 기여는 OLEDs의 특성을 결정할 때 오류를 방지하는 데 매우 중요하다는 것을 보여줄 것이다. 이러한 확산 용량을 설명하기 위해 Laux & Hess 모델이 적용되었다. 이 모델은 잔류 전류에 대한 임피던스 응답을 매우 잘 설명 할 수 있고 심지어 음의 커패시턴스 현상도 잘 설명해 준다. Laux & Hess 모델을 사용한 분석 결과는 작동 중의 OLEDs의 특성을 잘 묘사 하고 있음을 보였고 이 모델의 분석 방법을 위한 근사 프로세스를 제안하였다. 제 3 장에서는 ITO/a-NPD[N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine]/Alq3[Tris-(8-hydroxyquinolinato)aluminum]/LiF/Al 형 OLED를 제작하여 제 2 장에서 제안된 개선된 임피던스 분석법을 사용하여 조사하였다. 먼저, 구조적(두께) 변화에 따른 임피던스 응답의 변화를 측정하여 물리적, 정량적 해석이 수행되었다. 높은 주파수에서 측정된 임피던스는 OLEDs의 인터페이스 및 버크 특성을 나타내며 저주파수에서의 임피던스는 OLEDs의 캐리어의 동적 이동을 설명한다. 또한 OLEDs의 열화로 인한 임피던스 변화를 분석하였다. 본 논문에서 제안 된 Impedance Spectroscopy Analysis 방법을 사용하면, 열화의 원인으로서 계면과 버크의 상태 변화가 정확하고 효과적으로 분리 될 수 있다. 결과는 HTL/EML의 인터페이스 트랩 효과에 대한 기존 해석을 강화하는 것으로 나왔고 추출된 임피던스 값의 변화율을 추적 할 수 있음을 보여 주었다. 4 장 에서는 이 논문에서 사용된 OLEDs를 분석하기 위한 프로그램 도구를 간략하게 소개하고 수식에 대한 부록을 첨부 했다. 그리고 향후 이 임피던스 분광학 분석을 산업에 적용 시킬 가능성에 대해 논의하고 요약하도록 하겠다.Abstract i Contents v List of Figures viii List of Tables xiii Chapter 1 Introduction 1 1.1 Motivation 1 1.1.1 History of Organic Light Emitting Diodes 6 1.1.2 OLEDs Hetero Structures 8 1.1.3 OLEDs life time 11 1.2 Materials 15 1.2.1 Alq3 16 1.2.2 HAT-CN 17 1.2.3 α-NPB 18 1.2.4 DCM 19 1.3 Equipment & Instrument 20 1.3.1 Thermal Evaporator 20 1.3.2 Impedance Measurement Equipment for OLEDs 23 Chapter 2 Analytic Theory of OLED Impedance Spectroscopy 26 2.1 Problems of Previous Reported Impedance Spectroscopy to extract parameter on OLEDs 28 2.2 Complex Capacitance Concepts for IS 31 2.2.1 ARC 31 2.2.2 ZARC 34 2.2.3 Negative Capacitance 38 2.2.4 Equivalent circuit strategy for OLEDs 43 2.3 Theory 45 2.3.1 Impedance Spectroscopy 45 2.3.2 Small-Signal Model 49 2.3.3 Complex Plane Diagram 51 2.3.4 Equivalent Circuit Modeling 54 2.3.5 Superposition of various Impedance Component 58 2.3.6 Debye relaxation plot( -plot) 60 2.4 How to extract reasonable parameter from impedance spectroscopy 64 Chapter 3 Experiments 71 3.1 Thickness modification OLEDs 72 3.1.1 Analysis of the Interface of the OLEDs 76 3.1.2 Analysis of the Carrier Distribution of OLEDs 78 3.2 Thickness ratio modification 80 3.2.1 Relation between efficiency and M-plot 82 3.3 DCM doping ration Modification 84 3.3.1 Correlation between Current-Voltage-Efficiency and Impedance Response 84 Chapter 4 Discussion 90 4.1 Consideration of Effects of Interface Properties 90 4.2 Consideration of Effects of Bulk properties 92 4.3 Negative Capacitance relation with efficiency analysis 93 4.4 Mobility Measurement Using Impedance analysis 96 Chapter 5 Conclusion 101 Appendix 109 Bibliography 116 Publications 122 초 록 127Docto

Alternative transparent electrodes for organic light emitting diodes

Solid state lighting is a new environmentally friendly light source. So far, light emitting diodes (LEDs) and organic LEDs (OLEDs) have been presented as candidates with potentially high efficiency. Recent advances of OLEDs in device architecture, light-out coupling, and materials have ensured high efficiency, exceeding that of incandescent light bulbs. In contrast to conventional point source LEDs, OLEDs distribute light throughout the surface area and are not restricted by their size. Additionally, OLEDs are expected to reach sufficient stability in the near future. The remaining challenge for OLEDs is their cost. New OLED technologies provide cost effective manufacturing methods which could be presented for transparent electrode materials because indium tin oxide (ITO), a widely used material as a transparent electrode for OLEDs, is less than optimal due to its high element price. In this work, alternative transparent electrodes for OLEDs as a replacement of ITO were studied. First, Al doped ZnO (ZnO:Al) which is composed of abundant materials was investigated with DC magnetron sputtering under a wide range of experimental conditions. The optimised ZnO:Al received comparable performance with conventional ITO films, low sheet resistance of 22.8 Ω/sq as well as a high transparency of 93.1 % (average value in the visible range). Various type of p-i-n OLEDs were employed on the structured ZnO:Al using photolithography. Green OLEDs with double emission layers have been archived stable efficiencies even at higher luminance. Also, OLEDs using two fluorescent colour system on ZnO:Al anode showed a purely white emission. It has been found that the OLEDs on ZnO:Al anode has comparable or better device efficiencies and operational lifetime compared to OLEDs on conventional ITO anode. As another alternative electrode, the conductive polymer Baytron®PH510 (PEDOT:PSS) was investigated. Due to a relatively high sheet resistance of PEDOT:PSS, metal grid was designed for large size OLEDs. White OLEDs on PEDOT anode with a size of 5 × 5 cm2 have achieved more than 10 lm/W of power efficiency using a scattering foil. Furthermore, up-scaled devices on 10 × 10 cm2 were also demonstrated. These results showed ZnO:Al and PEDOT are suitable for OLEDs as anode and have high potential as alternative transparent electrode materials

Light-emitting textiles: Device architectures, working principles, and applications

E-textiles represent an emerging technology aiming toward the development of fabric with augmented functionalities, enabling the integration of displays, sensors, and other electronic components into textiles. Healthcare, protective clothing, fashion, and sports are a few examples application areas of e-textiles. Light-emitting textiles can have different applications: Sensing, fashion, visual communication, light therapy, etc. Light emission can be integrated with textiles in different ways: Fabricating light-emitting fibers and planar light-emitting textiles or employing side-emitting polymer optical fibers (POFs) coupled with light-emitting diodes (LEDs). Different kinds of technology have been investigated: Alternating current electroluminescent devices (ACELs), inorganic and organic LEDs, and light-emitting electrochemical cells (LECs). The different device working principles and architectures are discussed in this review, highlighting the most relevant aspects and the possible approaches for their integration with textiles. Regarding POFs, the methodology to obtain side emissions and the critical aspects for their integration into textiles are discussed in this review. The main applications of light-emitting fabrics are illustrated, demonstrating that LEDs, alone or coupled with POFs, represent the most robust technology. On the other hand, OLEDs (Organic LEDs) are very promising for the future of light-emitting fabrics, but some issues still need to be addressed

Doctor of Philosophy

dissertationIn Part 1, we demonstrate the fabrication of organic light-emitting devices (OLEDs) with precisely patterned pixels by the spin-casting of Alq3 and rubrene thin films with dimensions as small as 10　μm. The solution-based patterning technique produces pixels via the segregation of organic molecules into microfabricated channels or wells. Segregation is controlled by a combination of weak adsorbing characteristics of aliphatic terminated self-assembled monolayers (SAMs) and by centrifugal force, which directs the organic solution into the channel or well. This novel patterning technique may resolve the limitations of pixel resolution in the method of thermal evaporation using shadow masks, and is applicable to the fabrication of large area displays. Furthermore, the patterning technique has the potential to produce pixel sizes down to the limitation of photolithography and micromachining techniques, thereby enabling the fabrication of high-resolution microdisplays. The patterned OLEDs, based upon a confined structure with low refractive index of SiO2, exhibited higher current density than an unpatterned OLED, which results in higher electroluminescence intensity and eventually more efficient device operation at low applied voltages. We discuss the patterning method and device fabrication, and characterize the morphological, optical, and electrical properties of the organic pixels. In part 2, we demonstrate a new growth technique for organic single crystals based on solvent vapor assisted recrystallization. We show that, by controlling the polarity of the solvent vapor and the exposure time in a closed system, we obtain rubrene in orthorhombic to monoclinic crystal structures. This novel technique for growing single crystals can induce phase shifting and alteration of crystal structure and lattice parameters. The organic molecules showed structural change from orthorhombic to monoclinic, which also provided additional optical transition of hypsochromic shift from that of the orthorhombic form. An intermediate form of the crystal exhibits an optical transition to the lowest vibrational energy level that is otherwise disallowed in the single-crystal orthorhombic form. The monoclinic form exhibits entirely new optical transitions and showed a possible structural rearrangement for increasing charge carrier mobility, making it promising for organic devices. These phenomena can be explained and proved by the chemical structure and molecular packing of the monoclinic form, transformed from orthorhombic crystalline structure

Microbial Quality and Pathogen Decontamination Strategies for Locally-Grown, Fresh Produce from West Virginia and Kentucky

This study aimed to evaluate the microbiological quality/safety of fresh produce from farmers\u27 markets (FM) and assess the post-harvest washing practice with antimicrobials to inactivate Salmonella and Listeria monocytogenes on fresh produce. In study I, 212 produce samples were tested for the presence of Salmonella and Listeria spp. using modified FDA-BAM methods. Aerobic plate counts (APCs), total coliforms (TCCs), and yeast/molds were analyzed on petri-films. Among the 212 samples, the APCs, TCCs, and yeast/molds were 3.72-5.63, 3.67-5.47, and 3.07-4.13 log CFU/g, respectively, with spinach containing the highest (P\u3c0.05) populations. Among all tested samples, Salmonella enterica spp. enterica was detected on 18.6% of spinach, 10.9% of tomatoes, 18.5% of peppers, and 56.3% of cantaloupes, which is much higher than previous reported. Only 3.78% of the samples were confirmed for Listeria spp., and 50% of them were identified as L. monocytogenes, based on multiplex PCR results. Due to the high percentage of pathogens detected on the farmers\u27 marker produce, an evaluation of post-harvest produce washing with various antimicrobials was conducted in study II. Specifically, spinach, tomatoes, green peppers and cucumbers were inoculated with S. Typhimurium and Tennessee or L. monocytogenes and washed in tap water, vinegar water (10%), lactic acid (5%), a lactic and citric acid blend (2.5%), and sodium hypochlorite (200 ppm) for 30 sec or unwashed. Vinegar water (10%) showed better (P\u3c0.05) reduction of S. Typhimurium and Tennessee on tomatoes and cucumbers, and L. monocytogenes on tomatoes and peppers than tap water. The three antimicrobials reached an additional reduction level of 0.9 to 2.7 (S. Typhimurium and Tennessee) and 0.2 to 1.4 log CFU/g ( L. monocytogenes) compared to tap water. Lactic acid caused the greatest (P\u3c0.05) reduction of S. Typhimurium and Tennessee on spinach and green peppers, and sodium hypochlorite showed the great (P\u3c0.05) reduction of L. monocytogenes on cucumbers. The results supplied important information for FM vendors to develop post-harvest protocols to control foodborne pathogens

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