Optically pumped colloidal-quantum-dot lasing in LED-like devices with an integrated optical cavity.

Realization of electrically pumped lasing with solution processable materials will have a revolutionary impact on many disciplines including photonics, chemical sensing, and medical diagnostics. Due to readily tunable, size-controlled emission wavelengths, colloidal semiconductor quantum dots (QDs) are attractive materials for attaining this goal. Here we use specially engineered QDs to demonstrate devices that operate as both a light emitting diode (LED) and an optically pumped laser. These structures feature a distributed feedback resonator integrated into a bottom LED electrode. By carefully engineering a refractive-index profile across the device, we are able to obtain good confinement of a waveguided mode within the QD medium, which allows for demonstrating low-threshold lasing even with an ultrathin (about three QD monolayers) active layer. These devices also exhibit strong electroluminescence (EL) under electrical pumping. The conducted studies suggest that the demonstrated dual-function (lasing/EL) structures represent a promising device platform for realizing colloidal QD laser diodes

Light-Emitting Devices with Conjugated Polymers

This article introduces a previous study and tremendous progress in basic theoretical modeling, material developments and device engineering for polymer light-emitting devices (PLEDs)

Intrinsically stretchable and transparent thin-film transistors based on printable silver nanowires, carbon nanotubes and an elastomeric dielectric.

Thin-film field-effect transistor is a fundamental component behind various mordern electronics. The development of stretchable electronics poses fundamental challenges in developing new electronic materials for stretchable thin-film transistors that are mechanically compliant and solution processable. Here we report the fabrication of transparent thin-film transistors that behave like an elastomer film. The entire fabrication is carried out by solution-based techniques, and the resulting devices exhibit a mobility of ∼30 cm(2) V(-1) s(-1), on/off ratio of 10(3)-10(4), switching current >100 μA, transconductance >50 μS and relative low operating voltages. The devices can be stretched by up to 50% strain and subjected to 500 cycles of repeated stretching to 20% strain without significant loss in electrical property. The thin-film transistors are also used to drive organic light-emitting diodes. The approach and results represent an important progress toward the development of stretchable active-matrix displays

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

Synthesis and charaterisation of phosphorescent copper (I) complexes for light emitting devices

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Over the last decade, many significant developments have been made to improve the active materials in a new generation of organic light emitting devices (OLEDs). Current OLED technology is focused on organo-transition metal complexes, which emit from the triplet excited state and exhibit bright phosphorescence. Efficient in devices have been reported using these luminescent materials, such as iridium and platinum complexes, however, rare metal abundance concerns, high price and toxicology have inspired the study of alternative phosphorescent materials, such as copper or silver complexes. In this research, novel copper complexes have been synthesized, such as trinuclear and mononuclear copper (I) complexes, using a range of ligands, such as alkynyl, phosphine alkynyl and pyridine ligands. The synthesised complexes have been characterised by with a range of techniques, such as UV/Vis absorption and emission spectroscopy, nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), cyclic voltammetry (CV) and scanning electron microscopy (SEM). Most of the copper complexes have shown very interesting luminescent properties in solution and solid state and some of them were studied for future application in a device.Research Council of U

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

High Photovoltaic Quantum Efficiency in Ultrathin van der Waals Heterostructures

We report experimental measurements for ultrathin (< 15 nm) van der Waals heterostructures exhibiting external quantum efficiencies exceeding 50%, and show that these structures can achieve experimental absorbance > 90%. By coupling electromagnetic simulations and experimental measurements, we show that pn WSe2/MoS2 heterojunctions with vertical carrier collection can have internal photocarrier collection efficiencies exceeding 70%.Comment: ACS Nano, 2017. Manuscript (25 pages, 7 figures) plus supporting information (7 pages, 4 figures