Methods for modeling degradation of electrical engineering components for lifetime prognosis

Reliability of electrical components is an issue studied to improve the quality of products, and to plan maintenance in case of failure. Reliability is measured by studying the causes of failure and the mean time to failure. One of the methods applied in this field is the study of component aging, because failure often occurs after degradation. The objective of this thesis is to model the degradation of components in electrical engineering, in order to estimate their lifetime. More specifically, this thesis will study large area organic white light sources (OLEDs). These sources offer several advantages in the world of lighting thanks to their thinness, their low energy consumption and their ability to adapt to a wide range of applications. The second components studied are electrical insulators applied to pairs of twisted copper wires, which are commonly used in low voltage electrical machines. First, the degradation and failure mechanisms of the various electrical components, including OLEDs and insulators, are studied. This is done to identify the operational stresses for including them in the aging model. After identifying the main causes of aging, general physical models are studied to quantify the effects of operational stresses. Empirical models are also presented when the physics of degradation is unknown or difficult to model. Next, methods for estimating the parameters of these models are presented, such as multilinear and nonlinear regression, as well as stochastic methods. Other methods based on artificial intelli­gence and online diagnosis are also presented, but they will not be studied in this thesis. These methods are applied to degradation data of organic LEDs and twisted pair insulators. For this purpose, accelerated and multifactor aging test benches are designed based on factorial experimental designs and response surface methods, in order to optimize the cost of the experiments. Then, a measurement protocol is described, in order to optimize the inspection time and to collect periodic data. Finally, estimation methods tackle unconstrained deterministic degradation models based on the measured data. The best empirical model of the degradation trajectory is then chosen based on model selection criteria. In a second step, the parameters of the degradation trajectories are modeled based on operational constraints. The parameters of the aging factors and their interactions are estimated by multilinear regression and according to different learning sets. The significance of the parameters is evaluated by statistical methods if possible. Finally, the lifetime of the experiments in the validation sets is predicted based on the parameters estimated by the different learning sets. The training set with the best lifetime prediction rate is considered the best

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

Charge-carrier dynamics in organic LEDs

Anyone who decides to buy a new mobile phone today is likely to buy a screen made from organic light-emitting diodes (OLEDs). OLEDs are a relatively new display technology and will probably account for the largest market share in the upcoming years. This is due to their brilliant colors, high achievable display resolution, and comparably simple processing. Since they are not based on the rigid crystal structure of classical semiconductors and can be produced as planar thin-film modules, they also enable the fabrication of large-area lamps on flexible substrates – an attractive scenario for future lighting systems. Despite these promising properties, the breakthrough of OLED lighting technology is still pending and requires further research. The charge-carrier dynamics in an OLED determine its device functionality and, therefore, enable the understanding of fundamental physical concepts and phenomena. From the description of charge-carrier dynamics, this work derives experimental methods and device concepts to optimize the efficiency and stability of OLEDs. OLEDs feature an electric current of charge carriers (electrons and holes) that are intended to recombine under the emission of light. This process is preceded by charge-carrier injection and their transport to the emission layer. These three aspects are discussed together in this work. First, a method is presented that quantifies injection resistances using a simple experiment. It provides a valuable opportunity to better understand and optimize injection layers. Subsequently, the charge carrier transport at high electrical currents, as required for OLEDs as bright lighting elements, will be investigated. Here, electro-thermal effects are presented that form physical limits for the design and function of OLED modules and explain their sudden failure. Finally, the dynamics and recombination of electro-statically bound charge carrier pairs, so-called excitons, are examined. Various options are presented for manipulating exciton dynamics in such a way that the emission behavior of the OLED can be adjusted according to specific requirements.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 249Wer sich heute für ein neues Mobiltelefon entscheidet, kauft damit wahrscheinlich einen Bildschirm aus organischen Leuchtdioden (OLEDs). Durch ihre brillanten Farben, die hohe erreichbare Auflösung und eine vergleichsweise einfache Prozessierung werden OLEDs als relativ neue Bildschirmtechnologie in den nächsten Jahren wohl den größten Marktanteil ausmachen. Da sie nicht auf der starren Kristallstruktur klassischer Halbleiter beruhen und als planare Dünnschichtmodule produziert werden können, ermöglichen sie außerdem die Fertigung großer Flächenstrahler auf flexiblen Substraten – ein sehr attraktives Szenario für zukünftige Beleuchtungssysteme. Trotz dieser vielversprechenden Eigenschaften steht der Durchbruch der OLED-Technologie als Leuchtmittel noch aus und erfordert weitere Forschung. Die Dynamik der Ladungsträger (Elektronen und Löcher) in einer OLED charakterisiert wichtige Teile der Bauteilfunktion und ermöglicht daher das Verständnis grundlegender physikalischer Konzepte und Phänomene. Diese Arbeit leitet anhand dieser Beschreibung experimentelle Methoden und Bauteilkonzepte ab, um die Effizienz und Stabilität von OLEDs zu optimieren. OLEDs zeichnen sich dadurch aus, dass ein elektrischer Strom aus Ladungsträgern (Elektronen und Löchern) möglichst effizient unter Aussendung von Licht rekombiniert. Diesem Prozess geht eine Ladungsträgerinjektion und deren Transport zur Emissionsschicht voraus. Diese drei Aspekte werden in dieser Arbeit zusammenhängend diskutiert. Als erstes wird eine Methode vorgestellt, die Injektionswiderstände anhand eines einfachen Experimentes quantifiziert. Sie bildet eine wertvolle Möglichkeit, Injektionsschichten besser zu verstehen und zu optimieren. Anschließend wird der Ladungsträgertransport bei hohen elektrischen Strömen untersucht, wie sie für OLEDs als helle Beleuchtungselemente nötig sind. Hier werden elektro-thermische Effekte vorgestellt, die physikalische Grenzen für das Design und die Funktion von OLED Modulen bilden und deren plötzliches Versagen erklären. Abschließend wird die Dynamik der stark elektrostatisch gebundenen Ladungsträgerpaare, sogenannter Exzitonen, kurz vor deren Rekombination untersucht. Es werden verschiedene Möglichkeiten vorgestellt sie so zu manipulieren, dass sich das Abstrahlverhalten der OLED anhand bestimmter Anforderungen einstellen lässt.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 24

Eurodisplay 2019

The collection includes abstracts of reports selected by the program by the conference committee

Robust deep learning for computational imaging through random optics

Light scattering is a pervasive phenomenon that poses outstanding challenges in both coherent and incoherent imaging systems. The output of a coherent light scattered from a complex medium exhibits a seemingly random speckle pattern that scrambles the useful information of the object. To date, there is no simple solution for inverting such complex scattering. Advancing the solution of inverse scattering problems could provide important insights into applications across many areas, such as deep tissue imaging, non-line-of-sight imaging, and imaging in degraded environment. On the other hand, in incoherent systems, the randomness of scattering medium could be exploited to build lightweight, compact, and low-cost lensless imaging systems that are applicable in miniaturized biomedical and scientific imaging. The imaging capabilities of such computational imaging systems, however, are largely limited by the ill-posed or ill-conditioned inverse problems, which typically causes imaging artifacts and degradation of the image resolution. Therefore, mitigating this issue by developing modern algorithms is essential for pushing the limits of such lensless computational imaging systems. In this thesis, I focus on the problem of imaging through random optics and present two novel deep-learning (DL) based methodologies to overcome the challenges in coherent and incoherent systems: 1) no simple solution for inverse scattering problem and lack of robustness to scattering variations; and 2) ill-posed problem for diffuser-based lensless imaging. In the first part, I demonstrate the novel use of a deep neural network (DNN) to solve the inverse scattering problem in a coherent imaging system. I propose a `one-to-all' deep learning technique that encapsulates a wide range of statistical variations for the model to be resilient to speckle decorrelations. I show for the first time, to the best of my knowledge, that the trained CNN is able to generalize and make high-quality object prediction through an entirely different set of diffusers of the same macroscopic parameter. I then push the limit of robustness against a broader class of perturbations including scatterer change, displacements, and system defocus up to 10X depth of field. In the second part, I consider the utility of the random light scattering to build a diffuser-based computational lensless imaging system and present a generally applicable novel DL framework to achieve fast and noise-robust color image reconstruction. I developed a diffuser-based computational funduscope that reconstructs important clinical features of a model eye. Experimentally, I demonstrated fundus image reconstruction over a large field of view (FOV) and robustness to refractive error using a constant point-spread-function. Next, I present a physics simulator-trained, adaptive DL framework to achieve fast and noise-robust color imaging. The physics simulator incorporates optical system modeling, the simulation of mixed Poisson-Gaussian noise, and color filter array induced artifacts in color sensors. The learning framework includes an adaptive multi-channel L2-regularized inversion module and a channel-attention enhancement network module. Both simulation and experiments show consistently better reconstruction accuracy and robustness to various noise levels under different light conditions compared with traditional L2-regularized reconstructions. Overall, this thesis investigated two major classes of problems in imaging through random optics. In the first part of the thesis, my work explored a novel DL-based approach for solving the inverse scattering problem and paves the way to a scalable and robust deep learning approach to imaging through scattering media. In the second part of the thesis, my work developed a broadly applicable adaptive learning-based framework for ill-conditioned image reconstruction and a physics-based simulation model for computational color imaging

Study of Drop-on-Demand Inkjet Printing Technology with Application to Organic Light-Emitting Diodes

Ph.DDOCTOR OF PHILOSOPH

Encoding high dynamic range and wide color gamut imagery

In dieser Dissertation wird ein szenischer Bewegtbilddatensatz mit erweitertem Dynamikumfang (High Dynamic Range, HDR) und großem Farbumfang (Wide Color Gamut, WCG) eingeführt und es werden Modelle zur Kodierung von HDR und WCG Bildern vorgestellt. Die objektive und visuelle Evaluation neuer HDR und WCG Bildverarbeitungsalgorithmen, Kompressionsverfahren und Bildwiedergabegeräte erfordert einen Referenzdatensatz hoher Qualität. Daher wird ein neuer HDR- und WCG-Video-Datensatz mit einem Dynamikumfang von bis zu 18 fotografischen Blenden eingeführt. Er enthält inszenierte und dokumentarische Szenen. Die einzelnen Szenen sind konzipiert um eine Herausforderung für Tone Mapping Operatoren, Gamut Mapping Algorithmen, Kompressionscodecs und HDR und WCG Bildanzeigegeräte darzustellen. Die Szenen sind mit professionellem Licht, Maske und Filmausstattung aufgenommen. Um einen cinematischen Bildeindruck zu erhalten, werden digitale Filmkameras mit ‘Super-35 mm’ Sensorgröße verwendet. Der zusätzliche Informationsgehalt von HDR- und WCG-Videosignalen erfordert im Vergleich zu Signalen mit herkömmlichem Dynamikumfang eine neue und effizientere Signalkodierung. Ein Farbraum für HDR und WCG Video sollte nicht nur effizient quantisieren, sondern wegen der unterschiedlichen Monitoreigenschaften auf der Empfängerseite auch für die Dynamik- und Farbumfangsanpassung geeignet sein. Bisher wurden Methoden für die Quantisierung von HDR Luminanzsignalen vorgeschlagen. Es fehlt jedoch noch ein entsprechendes Modell für Farbdifferenzsignale. Es werden daher zwei neue Farbräume eingeführt, die sich sowohl für die effiziente Kodierung von HDR und WCG Signalen als auch für die Dynamik- und Farbumfangsanpassung eignen. Diese Farbräume werden mit existierenden HDR und WCG Farbsignalkodierungen des aktuellen Stands der Technik verglichen. Die vorgestellten Kodierungsschemata erlauben es, HDR- und WCG-Video mittels drei Farbkanälen mit 12 Bits tonaler Auflösung zu quantisieren, ohne dass Quantisierungsartefakte sichtbar werden. Während die Speicherung und Übertragung von HDR und WCG Video mit 12-Bit Farbtiefe pro Kanal angestrebt wird, unterstützen aktuell verbreitete Dateiformate, Videoschnittstellen und Kompressionscodecs oft nur niedrigere Bittiefen. Um diese existierende Infrastruktur für die HDR Videoübertragung und -speicherung nutzen zu können, wird ein neues bildinhaltsabhängiges Quantisierungsschema eingeführt. Diese Quantisierungsmethode nutzt Bildeigenschaften wie Rauschen und Textur um die benötigte tonale Auflösung für die visuell verlustlose Quantisierung zu schätzen. Die vorgestellte Methode erlaubt es HDR Video mit einer Bittiefe von 10 Bits ohne sichtbare Unterschiede zum Original zu quantisieren und kommt mit weniger Rechenkraft im Vergleich zu aktuellen HDR Bilddifferenzmetriken aus

A Physics of Failure Based Qualification Process for Flexible Display Interconnect Materials

The next paradigm shift in display technology involves making them flexible, bringing with it many challenges with respect to product reliability. To compound the problem, industry is continuously introducing novel materials and experimenting with device geometries to improve flexibility and optical performance. Hence, a method to rapidly qualify these new designs for high reliability applications is imperative. This dissertation involves the development of a qualification process for gate line interconnects used in flexible displays. The process starts with the observed failure mode of permanent horizontal lines in the displays, followed by the identification of the underlying failure mechanism. Finite element analyses are developed to determine the relationship between the physical flexing and the mechanical stress imposed on the traces. The design of an accelerated life test is performed based on the known agent of failure being cyclic bending that induces a tensile strain. A versatile dedicated test system is designed and integrated in order to rapidly capture changes in resistance of multiple traces during test. Dedicated test structures are also designed and fabricated to facilitate in-situ electrical measurements and direct observations. Since the test structures were consumed during the integration of the test system, random failure times are used in the process of determining a life-stress model. Different models are compared with respect to their applicability to the underlying failure mechanism as well as parameter estimation techniques. This methodology may be applied towards the rapid qualification of other novel materials, process conditions, and device geometries prior to their widespread use in future display systems

Bio- und Umweltsensorik basierend auf organischer Optoelektronik

The integration of organic light emitting diodes (OLEDs) and organic photodetectors (OPDs) promises compact and low-cost hybrid integrated sensors for optical detection. The thermal evaporation-based device fabrication technique allows for all optical sensing elements being permanently aligned with a high degree of miniaturization, creating more portable, energy-efficient and multiplexing-capable devices; these may be easily combined with microfluidic units resulting in a minimal sample and reagent volume demand of the sensor. This dissertation deals in particular with the system design, development, characterization and deployment of a monolithic integrated sensor unit with 8 OLED and 8 OPD pixel pairs for different applications. The following work provides an extensive study of the system efficiency via ray tracing simulations, investigating crucial boundary conditions for efficient analyte detection. The proposed sensing unit contains OLED and OPD devices with an individual pixel size of 0.5mm × 0.5mm fabricated on a 12.5mm × 12.5mm glass substrate. The developed sensor system was successfully characterized and applied in a biosensing application by detecting fluorescence labelled single-stranded DNA (ssDNA) after forming the Förster resonance energy transfer (FRET) upon the hybridization of two ssDNA strands. This optoelectronic sensor has the potential to enable compact and low-cost fluorescence point-of-care (POC) devices for decentralised multiplex biomedical testing. Additionally, this sensing platform was deployed in environmental and agricultural applications to detect nutrients such as nitrite and nitrate. In this colorimetric application the popular Griess reaction was utilized to form the nitrite concentration dependent amount of azo dye, which absorbs light around 540nm