Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application

Most recently, organic-inorganic semiconductor light harvester materials, have arisen as a new class of functional element and attracts the research community due to its outstanding optoelectronic properties. Organic-inorganic perovskites are solution process that is easy for the fabrication of devices at low temperature. Additionally, up to date, perovskite quantum dots have emerged as the most efficient light harvester for LEDs and display applications, with high color purity, color tunability, and photoluminescence quantum yield up to 100%. However, the presence of lead in organic-inorganic perovskites and the stability issue of perovskite materials are the significant challenges for the research community. To date, some lead substitute materials have been tried to enhance the film morphology and production of the less toxic light harvester. In this chapter, we focus on the lead substitution on B sight with homovalent cations like Sn2+, Mn2+, Cd2+, and Zn2+ cations. These lead substitutions not only reduce the toxicity of perovskite material while these dopants also enhance the optical and performance of LEDs. We also included the LEDs application of lead substituted perovskite quantum dots (PQDs) that may be useful for the environmental friendly and highly performing perovskite quantum dot LEDs (PQ-LEDs) shortly

Roadmap on Perovskite Light-Emitting Diodes

In recent years, the field of metal-halide perovskite emitters has rapidly emerged as a new community in solid-state lighting. Their exceptional optoelectronic properties have contributed to the rapid rise in external quantum efficiencies (EQEs) in perovskite light-emitting diodes (PeLEDs) from <1% (in 2014) to approaching 30% (in 2023) across a wide range of wavelengths. However, several challenges still hinder their commercialization, including the relatively low EQEs of blue/white devices, limited EQEs in large-area devices, poor device stability, as well as the toxicity of the easily accessible lead components and the solvents used in the synthesis and processing of PeLEDs. This roadmap addresses the current and future challenges in PeLEDs across fundamental and applied research areas, by sharing the community's perspectives. This work will provide the field with practical guidelines to advance PeLED development and facilitate more rapid commercialization.Comment: 103 pages, 29 figures. This is the version of the article before peer review or editing, as submitted by an author to Journal of Physics: Photonics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Roadmap on perovskite light-emitting diodes

In recent years, the field of metal-halide perovskite emitters has rapidly emerged as a new community in solid-state lighting. Their exceptional optoelectronic properties have contributed to the rapid rise in external quantum efficiencies (EQEs) in perovskite light-emitting diodes (PeLEDs) from <1% (in 2014) to over 30% (in 2023) across a wide range of wavelengths. However, several challenges still hinder their commercialization, including the relatively low EQEs of blue/white devices, limited EQEs in large-area devices, poor device stability, as well as the toxicity of the easily accessible lead components and the solvents used in the synthesis and processing of PeLEDs. This roadmap addresses the current and future challenges in PeLEDs across fundamental and applied research areas, by sharing the community’s perspectives. This work will provide the field with practical guidelines to advance PeLED development and facilitate more rapid commercialization

금속 산화물 및 이차원 나노 물질 기반의 박막 트랜지스터: 성능 최적화 및 양자점 발광 다이오드에 대한 응용

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2021.8. 곽정훈.박막 트랜지스터를 집적한 전자 디스플레이의 개발은 뛰어난 장점으로 인해 큰 관심을 받아왔다. 지난 수십 년 동안 능동 매트릭스 액정 디스플레이 및 유기 발광 다이오드와 같은 평판 디스플레이에 대한 여러 연구가 보고되었습니다. 이러한 우수한 성과에도 불구하고, p형 산화물/나노 반도체의 개발과 백플레인 박막 트랜지스터로 구동되는 양자점 발광 다이오드의 구동에 대한 연구는 제한적입니다. p형 반도체의 전기적 특성은 이동도가 낮고, 오프 전류가 높으며, 소자의 불안정성이 있기 때문입니다. 이러한 의미에서 우리는 앞서 언급한 응용을 위한 p-형 산화물/나노 물질 기반 박막 트랜지스터와 콜로이드 양자점 발광 다이오드를 연구했습니다. 먼저, 우리는 유연한 전자 장치에 잠재적으로 매력적인 나노 와이어 구조로 형성된 대체 전극으로 전기적 특성을 위해 스프레이 코팅된 단일벽 탄소나노 튜브를 소스 및 드레인 전극으로 사용하는 p 형 주석 산화물 (SnO) 박막 트랜지스터를 구현했습니다. 폴리머 에치 스토퍼 층에 SU-8이 있는 SnO 박막 트랜지스터의 소자 구조는 SnO 채널 층의 열화 없이 원하는 영역에서 단일벽 탄소나노튜브의 선택적 에칭을 가능하게 합니다. 또한 단일벽 탄소나노튜브 전극으로 사용하는 SnO 박막 트랜지스터는 적절한 채널폭과 정규화된 전기적 컨택 특성 (~ 1 kΩ cm), 전계 효과 이동성 (~ 0.69 cm2/Vs), 문턱전압이하 스윙 (~ 0.4 V/dec) 및 전류 온-오프 특성 (Ion/Ioff ~ 3.5×103)을 성공적으로 구현하였습니다. 또한 온도에 따른 전기적 컨택 및 채널 특성은 Ni 전극에 필적하는 적절한 접촉 저항과 함께 무시할 수 있는 수준의 가전자띠 테일 스테이트의 3 x 10-3 eV 활성화 에너지로 SnO 채널 전송을 설명합니다. 둘째, 우리는 먼저 상보형 트랜지스터를 구현하여 p (또는 n 형) MoTe2 박막 트랜지스터에 의해 제어되는 양자점 발광 다이오드 구동을 시연합니다. 이 연구에서는 MoTe2 박막 트랜지스터의 유형 변환을 위해 Poly-L-lysine (PLL)에 의한 분자 도핑을 도입하고, 양자점 발광 다이오드 성능 향상을 위해 표면 리간드 변형을 활용합니다. 이와 관련하여 PLL 처리는 전기적 특성의 저하없이 MoTe2 박막 트랜지스터의 뛰어난 유형 변환을 달성하여, 안정적인 p (또는 n 형) 유형 장치를 확보하여 보완 회로의 가용성을 보장합니다. 또한, 옥틸 아민으로 리간드 치환된 양자점은 양자점 발광 다이오드에서 균형 잡힌 전자/정공 주입을 생성하여, 전류 효율 (ηA = 13.9 cd/A)이 개선되고 수명이 더 길어집니다 (L0 = 3000 cd/m2에서 T50 = 66 h). 결과적으로 MoTe2 박막 트랜지스터는 적절한 스위칭 특성을 가진 디스플레이 백플레인 트랜지스터, 광 전류 생성에 대한 내성 및 작동 안정성을 포함하여 양자점 발광 다이오드를 구동하는 능력을 보여줍니다. 이 논문에서 우리는 유망한 응용을 위한 산화물/나노 물질 기반 박막 트랜지스터와 리간드 치환 기술이 적용된 후면 발광 적색 양자점 발광 다이오드에 대해 논의합니다. 백플레인 박막 트랜지스로 p형 SnO와 MoTe2를 사용하고, 새로운 디스플레이 장치로 CdSe 양자점 발광 다이오드를 사용한 우리의 연구 성과는 융합 연구를 위한 구상 분야에서 잠재적으로 사용될 수 있습니다.The development of electronic displays integrated with Thin Film Transistors (TFTs) has been great interests due to their superb merits. For the past decades, several studies have been reported for the flat-panel displays (FPDs) such as active-matrix liquid crystal display (LCD) and organic light-emitting diode (OLED). Despite of these excellent achievements, progress of p-type oxide/nano semiconductors and operation of quantum dot light-emitting diode (QLED) driven by backplane TFTs are limited. This is because the electrical characteristics of p-type semiconductor have low mobility, high off current, and device instability. In this sense, we investigated p-type oxide/nano material-based TFTs and colloidal quantum dot light-emitting diodes (QLEDs) for the afore-mentioned application. First, we implemented p-type tin oxide (SnO) TFTs with spray-coated single-wall carbon nanotube (SWNTs) as source and drain electrodes for their electrical characteristics as alternative electrodes formed of nanowire structures, which are potentially attractive for flexible electronics. The device architecture of SnO TFTs with a polymer etch stop layer (SU-8) enables the selective etching of SWNTs in a desired region without the detrimental effects of SnO channel layers. In addition, SnO TFTs with SWNT electrodes as substitutes successfully demonstrate decent width normalized electrical contact properties (~1 kΩ cm), field effect mobility (~0.69 cm2/Vs), sub-threshold slope (~0.4 V/dec), and current on-off ratio (Ion/Ioff ~ 3.5×103). Furthermore, temperature-dependent electrical contact and channel properties elucidate SnO channel transports with an activation energy of 3×10-3 eV, interpreted as a negligible level of valence-band tail states, together with decent contact resistance comparable to that of Ni electrodes. Second, we firstly demonstrated the QLEDs operation modulated by p (or n-type) MoTe2 TFTs with the realization of complementary type transistor. In this study, molecular doping by Poly-L-lysine (PLL) as an electron dopant is adopted for a type conversion of MoTe2 TFTs, and surface ligand modification is utilized for the improvement of QLED performance. In this regard, the PLL treatment achieves the outstanding type conversion of MoTe2 TFTs without any degradation of electrical properties, leading to securing reliable p (or n-type) devices, thus, availability of complementary circuits. Furthermore, ligand modified QDs capped with octylamine result in balanced electron/hole injection in QLEDs, yielding improved current efficiency (ηA =13.9 cd/A) and longer lifetimes (T50 = 66 h at L0 = 3000 cd/m2). As a result, MoTe2 TFTs demonstrate their capabilities to drive the QLEDs for the envisioned application including display backplane transistor with decent switching properties, immunity for generation of photocurrent, and operation stability. In this thesis, we discuss the oxide/nano material based TFTs and ligand modified bottom emitting red QLEDs for many promising applications. Our research achievements using p-type SnO and MoTe2 as backplane TFTs, and CdSe QLED as novel display device can be used in potentially envision fields for the convergence research.Chapter 1 1 1.1 An Overview of Thin Film Transistors 1 1.2 An Overview of Quantum Dot Light-Emitting Diodes 8 1.3 Outline of Thesis 13 Chapter 2 15 2.1 Materials 15 2.1.1 Synthesis of ZnO Nanoparticles 15 2.1.2 Synthesis of Red Light-Emitting CdSe/Cd1-xZnxS Quantumd Dots 15 2.2 Device Characterization of Thin Film Transistors 17 2.2.1 Characterization for Thin Film Transistor 17 2.2.2 Characterization of Light Response 18 2.3 Device Characterization of Quantum Dot Light-Emitting Diodes 18 2.3.1 Current-voltage-luminance Measurement of QLEDs 18 2.3.2 Efficiency Calculation Methods 21 2.3.3 Other Characterization Methods 22 Chapter 3 24 3.1 Devic Fabrication of SnO TFTs with Spray-Coated Single Walled Carbon Nanotubes as S/D Electrodes 26 3.2 Electrical Performance of SnO TFTs 30 3.3 Contact Resistance of Spray-coated SWNTs as S/D Electrodes 33 3.4 Summary 40 Chapter 4 41 4.1 Description of QLED display driven by MoTe2 TFTs 45 4.2 Ligand Modification of Red CdSe/Cd1-xZnxS Quantum Dots 49 4.3 Type Conversion of MoTe2 TFTs via Electron-Donated Charge Enhancer 54 4.4 Light-Insensitive Behaviors on Photocurrent Generation in MoTe2 TFTs 61 4.5 Operation of QLEDs Driven by Channel-type Controlled MoTe2 TFTs 66 4.6 Summary 70 Chapter 5 71 Bibilography 73 한글 초록 81박

Luminescence in sulfides : a rich history and a bright future

Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles

유기발광소자에 대한 다양한 광추출 구조와 배향 분극 분자의 효과

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 김장주.Since the first electroluminescence (EL) of organic compound was observed in the anthracene single crystal in 1965, organic light-emitting diodes (OLEDs) have undergone numerous developments, and have recently become the mainstream of small-sized displays. In addition, as OLEDs will have a modest substrate dependency, it has the potential to receive even more attention as the next generation display like flexible or transparent displays. The remaining challenges for OLEDs are high efficiency, and operational stability. In general, because organic materials have a wide emission spectrum, OLED can be a good lighting with a high color rendering index (CRI). It can also be used as a display with high color purity by optimizing the cavity length. However, more than half of the photons generated in OLEDs are dissipated due to total internal reflection by high-refractive-index organic layers and the substrate. Therefore, light extraction technology is required to increase the efficiency and reduce the power consumption of OLEDs. In addition, as mentioned before, it is necessary to consider the lifetime of the device. There have been many studies on the short operation lifetime of OLEDs, and several mechanisms have been proposed, but there is no concrete description of the origin of degradation. Nonetheless, it is obvious that the stability of the layers constituting the OLED should be considered for high operational stability. This thesis concerns two research topics: (1) simple and efficient light extraction method for OLED lighting and display, and (2) improving operation lifetime of OLED using spontaneous orientation polarization molecule. In chapter 1, a brief introduction of OLEDs will be provided. In chapter 2, a facile and effective method for fabricating random organic microstructures for efficient light extraction from blue OLEDs is presented. Simple drop casting of a TCTA and B4PyMPM mixed solution followed by UV curing results in films with irregular-shaped microstructures (DACMs), ideal for light extraction without diffraction patterns. An external quantum efficiency (EQE) of 44.3% is realized by attaching DACMs formed on a polymer film to a blue phosphorescent OLED. The efficiency is improved by 35% compared to a planar device without the light extraction layer, greater than the 22% improvement obtained by using microlens arrays. The method is useful for OLED lighting and potentially in displays because of the simple fabrication method that is applicable to a large area on rigid or flexible substrates, the low material cost, the insolubility of the microstructure in alkyl halide solvents such as chloroform, and the controllability of the structure through the solution process. In chapter 3, we show the damageless light extraction structure for top-emittng organic light-emitting diodes (TEOLEDs). TEOLEDs are used in small displays, due to the high aperture ratios, unblurred imaging, and high color purity. However, the strong cavity structures responsible for these advantages result in a high optical loss within metal electrodes. Furthermore, as there is no substrate in the light path, it is rather complicated to form a structure on the emission surface without damage. Here, we present a facile and effective method for light extraction of TEOLEDs. When 1,5-diaminoanthraquinone (DAAQ) is deposited on the Ag electrode, crystallization occurs immediately and nanowire arrays are formed in the out-of-plane direction. The shape and distribution of nanowire arrays can be controlled by deposition thickness and evaporation rate. In addition, morphological changes affect the transmittance of DAAQ-deposited Ag thin films. DAAQ nanowire arrays were applied to a red phosphorescent inverted TEOLED, enhancing the external quantum efficiency (EQE) by 8.6% in a narrow full-width-at-half-maximum (FWHM) device, and by 10.6% in a wide FWHM device. The emission spectra of the out-coupled devices are similar to those of the reference devices, as the DAAQ layer is located outside the OLED stacks sandwiched between two highly reflective metal electrodes. The method is useful for OLED displays because it is simple, vacuum-processable, and does not compromise device lifetime or the emission spectrum. In chapter 4, we suggest the importance of patternability of internal light extraction layer. Integration of internal light extraction layers in OLED displays requires electrical connection between driving circuits in the backplane and an OLED electrode, therefore needs fabrication of a via hole. Generally, internal light extraction layers consist of two materials with different refractive indices; thus, good patternability may be difficult with the matched etch selectivities of these two materials. Both the patternability of the internal light extraction layer and high out-coupling efficiency are important so, it needs a proper etchant which arent lowering the extraction efficiency by demolishing its structure. Here, the patternability of light extraction layers is discussed and demonstrated experimentally. The random scattering layer (RSL) composed of SiOx nano scatterers and a TiO2 planarization layer was used in this study and it was patterned by photolithography and wet etching processes. For matching the etching selectivity of those two different materials, a mixture of buffered oxide etchant (BOE) and phosphoric acid (H3PO4), in a volume ratio of 0.5 of H3PO4 to BOE, shows the best results for forming electrical channels through the out-coupling layer. The OLEDs fabricated on this patterned substrate showed similar current density-voltage (J-V) characteristics to OLEDs on a glass substrate with low leakage levels. The device showed over 50% enhancement of external quantum efficiency (EQE; from 21.7% to 32.7%), similar to the device without via holes. Chapter 5 contains a method for improving the stability of electron transporting layer. After the alignment of permanent dipole moment (PDM) was found in Alq3, it was revealed that spontaneous orientation polarization (SOP) was observed in several electron transporting materials. Although studies on the electrical effects of the polarization have been reported, there are few reports on the effects of polarization on the lifetime of device. Here, we observed the SOP characteristics and the change in the lifetime of device when BAlq was doped in PO-T2T with different volume ratios. As the polarization increased, the operation lifetime also increased and the applied voltage change decreased. In addition, the lifetime enhancement was only observed when there was a polarized layer at the interface with the emtting layer (EML). This shows that hole blocking layer (HBL) can enhance the lifetime when it has a negative surface charge at the interface with EML, and also indicates that the SOP characteristics of the molecule should be considered for improving the lifetime.1965 년, 안트라센 단결정에서 유기물의 첫 전기 발광이 발견된 이후, 유기 발광소자는 많은 발전을 거듭해 최근 소형 디스플레이의 주류를 이뤘다. 또한 유기 발광소자는 적은 기판 의존성을 갖기 때문에 플렉시블 디스플레이나 투명 디스플레이와 같은 차세대 디스플레이로 활용될 수 있는 잠재력을 갖고 있다. 유기 발광소자에서 남은 과제는 높은 효율과 구동 안정성이다. 일반적으로 유기물은 넓은 발광스펙트럼을 보이기 때문에, 유기 발광소자는 연색성이 높은 조명으로 사용될 수 있다. 또한, 광학 공진 구조 최적화를 통해 높은 색순도의 디스플레이로도 활용될 수 있다. 하지만 유기 발광소자가 방출한 광자의 절반 이상은 높은 굴절률의 유기물 및 기판으로 인한 내부 전반사로 소멸된다. 따라서, 유기 발광소자의 효율을 올리고 전력 소모를 감소시키기 위해 광추출 기술은 필요하다. 이에 더불어, 위에서 언급한 바와 같이 소자의 구동시간에 대한 고려도 필요하다. 유기 발광소자의 짧은 구동 수명에 대한 연구는 많이 있고, 몇몇 메커니즘이 발표되었지만, 아직 소자 열화의 전반을 설명할 수 있는 이론은 전무하다. 그럼에도 불구하고, 높은 구동 안정성을 위해선 유기 발광소자를 구성하는 층의 안정성을 고려해야 하는 것은 자명하다. 본 논문은 2 가지 연구주제: (1) 단순하면서도 효율적인 조명용 그리고 디스플레이용 광추출 구조, 그리고 (2) 자발적 배향 분극 분자를 활용한 소자의 수명 향상에 대한 내용을 다루고 있다. 제1 장은 유기 발광소자에 대한 간략한 서론을 담고 있다. 제2 장은 파란색 하부 발광형 유기 발광소자의 외부 광추출 효율 향상을 위한 손쉬우면서도 효율적인 랜덤 유기 마이크로 구조 (DACM)에 대한 내용을 담고 있다. TCTA와 B4PyMPM 혼합 용액을 필름에 드롭 캐스팅 후 UV 경화 시키면 무작위 모양의 마이크로 구조물이 형성되었고, 회절 무늬가 없어 광추출로 적합했다. DACM 필름을 부착한 소자는 44.3%의 외부 광자 효율을 가졌고, 광추출 구조가 없는 소자 평판 소자 대비 35%의 향상을 보였다. 또한, 마이크로 렌즈 어레이 필름에 향상량인 22% 보다 높은 수치를 기록했다. 이 구조는 기판에 상관없이 대면적 공정이 가능하고, 저렴한 제작 공정, 클로로포름과 같은 알킬 할라이드 용매에 강했으며, 구조 컨트롤도 가능하기 때문에 조명용으로 매우 효율적이고, 디스플레이용으로 활용될 잠재력이 있다. 제3 장은 진공 열증착을 통해 손상 없이 형성한 상부 발광형 유기 발광소자용 광추출 구조에 대한 내용을 담고 있다. 상부 발광형 소자는 높은 픽셀 종횡비, 흐려짐 없는 이미징, 높은 색 순도로 인해 소형 디스플레이에 활용되고 있다. 하지만, 이러한 이점을 얻는 강공진 구조는 금속 전극에 의한 높은 광학 손실을 야기한다. 게다가 상부 발광형 소자에는 빛의 경로에 기판이 없는 까닭에 손상 없는 공정이 어렵다. 우리는 1,5-diaminoanthraquinone (DAAQ) 유기물의 진공 열증착을 통해 상부 발광형 소자의 은 (Ag) 박막 위에 손상 없이 광추출 구조를 형성했다. DAAQ는 은 박막 위에서 증착 즉시 결정화되었으며, 기판에 수직 방향으로 나노와이어를 형성했다. 나노와이어 어래이의 높이, 둘레, 그리고 주기는 증착 속도 및 두께로 조절 가능했다. 이를 상부 발광형 소자에 접목시켰고, 좁은 반치폭의 소자에서는 8.6%, 넓은 반치폭의 소자에서는 10.6%의 향상량을 얻었다. 이 방법은 쉬우며, 소자에 손상을 주지 않아 열화 시키지 않으며, 패터닝 공정이 필요 없고, 진공공정이 가능한 까닭에 매우 유용하다. 제4 장은 내부 광추출 구조의 패터닝 특성의 중요성에 대해 서술한다. 내부 광추출 구조를 디스플레이에 활용하기 위해선 박막 트랜지스터 (TFT) 와 유기 발광소자 전극 사이에 신호를 주고받을 수 있는 통로, 즉 비아홀 (viahole) 이 필요하다. 내부 광추출 구조는 위치가 박막 트랜지스터와 유기 발광소자 사이에 위치한 까닭에, 구조 설계 시 viahole 패터닝을 고려해야 하지만, 지금까지 이런 고려는 적었다. 또한, 일반적인 내부 광추출구조는 고 굴절 물질 내에, 낮은 굴절률의 광 결정이 위치하는데, 이종의 물질은 식각 용액에 대해 서로 다른 에칭 선택비 (etching selectivity)를 갖기 때문에, 내부 광추출구조의 손상 없이 패터닝이 가능한 식각 용액을 찾는 것 또한 필요하다. 우리는 SiOx 스캐터와 TiO2 평탄층을 갖는 내부 광추출 구조의 비아홀 패터닝을 성공적으로 구현한 소자를 보고한다. SiOx와 TiO2에 대해 유사한 에칭 선택비를 갖는 buffered oxide etchant (BOE) 와 인산 (H3PO4) 의 혼합 식각 용액을 활용해 언더컷 (undercut) 문제가 해결된 비아홀 패터닝 공정을 확립했으며, 제작된 유기 발광소자의 낮은 누설 전류 및 손상 없는 광추출 효율을 통해 비아홀 패터닝이 성공적으로 되었음을 실험적으로 보였다. 제5장은 전자 전달층 (Electron transporting layer, ETL) 의 안정성을 향상시키기 위한 방법을 담고 있다. 영구 쌍극자 모멘트 (permanent dipole moment)의 정렬이 비정질의 Alq3 유기물에서도 발현됨을 발견한 이후, 여러 전자전달물질에서 이러한 특성 (배향 분극, orientation polarization)이 나타남이 밝혀졌다. 지금까진 배향 분극이 소자의 전기적 특성에 어떻게 영향을 끼치는지에 대한 연구가 주류를 이뤘다. 하지만 우리는 배향 분극이 유기 발광소자의 수명에 어떤 영향을 주는지 밝혔다. 소자의 에너지 전달 과정에 참여하지 않는 BAlq 분자를 활용해 전자 전달층과 발광층 (EML) 사이에 음의 표면 전하 밀도를 높혔을 때, 소자의 구동 수명이 증가함을 보였다. 또한 물질의 열화 정도를 파악할 수 있는 전압 변화가 낮아짐을 통해 BAlq에 의한 표면전하 변화가 전자전달층의 안정성을 올림을 알 수 있었다. 또한 분극된 층이 발광층과의 계면에 위치할 경우에서만 수명이 증가하는 결과를 통해 정공 차단층 (hole blocking layer, HBL) 이 음의 표면 전하를 발광층과의 계면에 가질 경우, 향상된 수명을 얻을 수 있다는 결론을 얻었으며, 이를 통해 분자의 배향 분극 특성도 소자의 수명향상을 위해 고려해야 함을 보였다.Chapter 1. Introduction 2 1.1 Brief history of Organic Light-Emitting Diodes (OLEDs) 2 1.2 Efficiency of OLEDs 10 1.3 Light extraction methods for OLEDs 16 1.4 Finite difference time domain (FDTD) method 24 1.5 Operational stability of OLEDs 26 1.6 Outline of the thesis 28 Chapter 2. Random Organic Nano-textured Microstructures by Photo-Induced Crosslinking for Light Extraction of Blue OLEDs 31 2.1 Introduction 31 2.2 Experimental section 34 2.3 Results and Discussion 36 2.4 Conclusion 57 Chapter 3. Random Nanowire Arrays Spontaneously Formed via Vacuum Deposition for Enhancing Light Extraction from Inverted Top-Emitting Organic Light-Emitting Diodes 59 3.1 Introduction 59 3.2 Experimental section 62 3.3 Results and Discussion 65 3.4 Conclusion 77 Chapter 4. Via Hole Patterning of Light Extraction Layer for Electrical Connection 78 4.1 Introduction 78 4.2 Experimental section 80 4.3 Results and Discussion 83 4.4 Conclusion 92 Chapter 5. Improving Operation Lifetime of OLEDs using Spontaneous Orientation Polarization 93 5.1 Introduction 93 5.2 Experimental section 97 5.3 Results and Discussion 99 5.4 Conclusion 124 Chapter 6. Summary and Conclusion 125 Bibliography 128 초 록 142 CURRICULUM VITAE 147 List of Publications 149 List of Presentations 151Docto

Perovskite Materials, Devices and Integration

Perovskites have attracted great attention in the fields of energy storage, pollutant degradation as well as optoelectronic devices due to their excellent properties. This kind of material can be divided into two categories; inorganic perovskite represented by perovskite oxide and organic-inorganic hybrid perovskite, which have described the recent advancement separately in terms of catalysis and photoelectron applications. This book systematically illustrates the crystal structures, physic-chemical properties, fabrication process, and perovskite-related devices. In a word, perovskite has broad application prospects. However, the current challenges cannot be ignored, such as toxicity and stability

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Department of Chemical EngineeringHere, the synthetic methods of luminous quantum dots (QDs) and their application for the quantum dot light emitting device (QLED) were introduced. This dissertation composed for the two parts, the metal nitride QDs synthesis, characterization and their application for QLEDs with the perovskite QDs (PeQDs) synthesis, surface treatment, and their application for the PeQLEDs. Normally, the Cd and Pb chalcogenide semiconductors utilized as the QDs materials. However, the Cd and Pb were regulated from many countries. For the replacement of the Cd and Pb based QDs, the heavy metal free materials demanded for the next generation QDs. From this necessity, the metal nitride chosen as the next-generation eco-friendly QDs materials. Also, the metal nitride QDs utilized for the III-V QDs research as a model system. The combination of the metal and nitrogen precursors focused for the finding the novel synthetic routs of the metal nitride QDs. The metal nitride had prominent stability with optoelectronic properties. However, the synthetic methods of the metal nitride not yet optimized from the low reactivity of the nitrogen sources. Normally, the NH3 gas utilized for the metal nitride material synthesis, but this gas phase precursor had hardness for the exact quantization of the ligand quantity. Also, the complex synthetic pathway with low optoelectronic quality of the conventional colloidal metal nitride QDs hindered general usage of the metal nitride nanomaterials for optoelectronic applications, especially QLEDs. The conventional QDs, which composed for the metal chalcogenide or pnictide, had liquid or solid phase anion precursors. For correct of these problems for the synthesis of the metal nitride, the solid state and/or liquid state nitrogen sources utilized for replacing the gas phase NH3 source. For the band gap control of the metal nitride QDs, the quantum confinement effect, host-guest energy transfer, and the metal alloy ratio control were utilized. From these approaches, the red to blue emitting metal nitride colloidal QDs realized via simple wet-chemical methods. Also, the metal nitride QLED was firstly realized from above luminous colloidal metal nitride QDs. Secondly, the CsPbX3 PeQDs synthesized and their surface treatment methods developed for the optimization of the photoemission properties. The PeQDs had weak binding strength between the surface binding ligand and the PeQDs. From weak binding strength, the surface binding ligand easily detached from the surface of the PeQDs. Striping of the surface binding ligands induced surface defect sites, and these surface defects caused the non-radiative recombination. For the correct of this issue, the ligand assisted post treatment (LAPT) and the ligand assisted solubility adjustment (LASA) methods developed for the preventing of ligand diffusing out tendency. Firstly, long chain ligand added for colloidal PeQDs solution for reducing diffusion rate of the surface binding ligand. This long chain ligand adding pathway called LAPT. For removing excess ligands with reducing the internal resistance of the PeQDs film, aromatic short chain ligands utilized for surface treatment of the PeQDs under solution and/or film state. The short chain ligand passivation served slower diffusion rate and shorter particle to particle distance than pristine ligand condition. From these above properties, the aromatic short chain ligand treatment realized for optimization of the PeQLEDs performance via reducing surface defect with internal resistance of the photoactive layer. This short chain ligand based surface treatment pathway called LASA. From these approach, the optoelectronic properties of the PeQDs and PeQLEDs improved via simple surface treatment for the PeQDs. For the deep study of the colloidal QDs synthesis and application, the metal nitride QDs and the PeQDs utilized as model system. From this interdisciplinary research of the synthesis and the device application of the QDs, this dissertation could find and the correct of the various issues of the QDs as described in this dissertation.ope

Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials

Light-emitting diodes (LEDs) are considered to be the most promising energy-saving technology for future lighting and display. Two-dimensional (2D) materials, a class of materials comprised of monolayer or few layers of atoms (or unit cells), have attracted much attention in recent years, due to their unique physical and chemical properties. Here, we summarize the recent advances on the applications of 2D materials for improving the performance of LEDs, including organic light emitting diodes (OLEDs), quantum dot light emitting diodes (QLEDs) and perovskite light emitting diodes (PeLEDs), using organic films, quantum dots and perovskite films as emission layers (EMLs), respectively. Two dimensional materials, including graphene and its derivatives and transition metal dichalcogenides (TMDs), can be employed as interlayers and dopant in composite functional layers for high-efficiency LEDs, suggesting the extensive application in LEDs. The functions of 2D materials used in LEDs include the improved work function, effective electron blocking, suppressed exciton quenching and reduced surface roughness. The potential application of 2D materials in PeLEDs is also presented and analyzed