Semiconductor nanomaterials based photodetectors and organic light-emitting diodes for flexible integrated photonic systems

곽기열 Graduate School, Korea University 2014 국내박사

In recent years, development of several types of flexible photonic devices has received much attention due to their promising applicability as one of basic components for flexible integrated photonic systems. It that regard, it is important to high-performance photonic devices such as photodetectors and organic light-emitting diodes (OLEDs) on flexible substrates. On basis of advance researches, flexible integrated photonic systems are demonstrated on the flexible substrates by combining those photonic devices through simple fabrication procedures. In this thesis, the photodetectors based on semiconductor nanomaterials and the OLEDs with amorphous ZnO-doped In2O3 anodes were fabricated on the flexible substrates and their characteristics were investigated in detail. And then, the flexible integrated photonic systems were developed by means of interconnection of the photonic devices with other photonic and electronic devices. In chapter 1, characteristics of the flexible photodetectors based on semiconductor nanomaterials and the flexible OLEDs were introduced with explanation of their demands for the flexible integrated photonic systems. Chapter 2 described experimental procedures for the fabrication of the photodetectors, the OLEDs, and the integrated photonic systems on the flexible substrates. In Chapter 3, modulation properties of the flexible integrated photonic systems, including investigation of characteristics of individual photonic and electrical devices, were discussed. The promising potential of flexible integrated photonic systems for next generation photonics was described in conclusion.

Highly efficient blue organic light-emitting diodes based on 9,9-diethyl-9H-fluorene with electron-withdrawing moieties

김동영 성균관대학교 일반대학원 2017 국내석사

지금까지 디스플레이는 브라운관(CRT), 액정 디스플레이(LCD), 플라즈마 디스플레이(PDP)를 지나 유기발광다이오드(OLED)와 양자점 디스플레이(QD)에 이르기까지 많은 발전을 이루어왔다. 그 중에서도 유기발광다이오드는 투명하거나 휘어지는 디스플레이로의 적용가능성으로 인해 많은 관심을 갖고 연구되는 디스플레이 물질이며, 미래에는 단순하게 휘어지는 것을 지나 접을 수 있거나 말 수 있는 디스플레이로의 탄생도 기대해볼 수 있는 디스플레이 물질로서 그 가치가 매우 높게 평가되고 있다. Chapter 1 에서는 유기발광다이오드에 대한 기본적인 개념, 구조, 작동 메커니즘, 호스트-도판트 시스템 및 유기발광다이오드에 사용되는 다양한 형광 호스트 물질과 형광 도판트 물질을 특징과 구조를 소개하였다. 기본적으로 유기발광다이오드는 유리나 금속, 플라스틱 재료 위에 여러 층의 유기물을 쌓아 만든 구조이다. 이때, 소자의 양끝에 연결된 양극과 음극을 통해 전기를 흘려주면 각각 정공과 전자가 주입되어 발광층에서 들뜬 상태의 엑시톤을 형성하게 된다. 이 과정에서 형성된 엑시톤이 다시 바닥상태로 내려오면서 그 물질이 갖고 있는 고유의 에너지 갭에 해당하는 에너지를 빛으로 방출하는데, 그 파장에따라서 적색부터 청색까지 다양한 색상의 빛을 띠게 된다. 하지만 발광물질 사이의 파이-파이 상호작용으로 인해 컨쥬게이션 길이가 길어지게되어 물질이 가진 고유의 에너지보다 작은 에너지에 해당하는 빛을 방출할 수 있기 때문에, 호스트-도판트 시스템을 이용하여 색순도와 효율을 개선하는 방법이 많이 사용된다. 따라서, 이에 맞는 적절한 호스트와 도판트의 개발이 필수적이므로 지속적인 연구가 이루어져야 하는 부분이다. Chapter 2에서는 유기발광다이오드의 여러 층 중에서 발광층에 사용될 수 있는 청색 형광 도판트를 새롭게 디자인, 합성하고 그 물질에 관한 연구결과를 소개하였다. 기본 발광 중심의 골격으로는 형광 양자 효율이 높은 것으로 알려진 플루오렌을 사용하였고, 양끝에 전자 받개인 다양한 형태의 N-헤테로고리 구조로 치환하여 전자 이동도를 변화시킴으로써 높은 효율을 지닌 청색 형광 도판트를 합성하고 그 특성을 연구하였다. Section 1 에서는 9,9-디에틸-9H-플루오렌 유도체로서 바이닐기를 도입한 피리딘 계열의 치환체를 도입한 물질이을 사용한 소자의 전기적 발광 특성을 비교하였다. Section 2 에서는 9,9-디에틸-9H-플루오렌을 발광 중심으로 하고 다양한 연결구조를 형성하는 퀴놀린 계열의 여러 치환체를 도입함으로써 변화되는 전기적 발광 특성을 비교하였다. 전체적으로 이 물질들을 발광층의 도판트로 사용한 유기발광다이오드는 높은 효율과 함께 효과적인 청색 발광을 나타냈으며 특히, 기본 발광 중심에 피리딘 유도체로 치환한 물질들 중 하나는 20 mA/cm2에서 7.75 cd/A, 4.14 lm/W, 6.66%의 높은 발광 효율, 전력 효율, 외부 양자 효율을 나타냈으며 8.0 V에서 (0.15, 0.16)의 좋은 순청색의 색좌표를 보여주었다. Section 3에서는 디자인된 물질의 합성 방법, 구조 해석과 더불어 그 물질이 갖고 있는 광학적 특성 측정방법, 유기발광다이오드의 제작방법 및 전기적 발광 특성 측정방법에 대해 정리하였다. 이상으로 본 논문에서는 유기발광다이오드의 발광층에 적용가능한 청색 형광 물질 개발에 대한 연구를 수행하였다. Until now, displays have been hugely developed from cathode ray tube (CRT), liquid crystal display (LCD), plasma display panel (PDP) to organic light-emitting diode (OLED) and quantum dot display (QD). Among them, OLED have been highly evaluated with the possibility of application to folderable and rollable diplay beyond simply flexible diplay based on transparent and flexible properties. In Chapter 1, the basics, structure, operating mechanism, host-dopant system, and various fluorescent host and dopant materials in OLED were introduced. Fundamentally, OLED is multi-layered structure with organic materials on glass, metal, or plastic substrates. When the electicity were drained through cathode and anode of diode, exciton is formed though the combination between injected hole and electron in emitting layer of OLED. Then, the exciton emits various colors of lights according to their intrinsic energy band gaps. However, they could emit red-shifted lights which originated from smaller energy originated from prolonged conjugation lengths due to π-π interactions between emitting materials thus, host-dopant system is used for improvement of color purity and luminescent efficiency. Therefore, continuous researches for proper host and dopants are essencial. In Chapter 2, the newly designed and synthesized blue fluorescent dopants which could be used for emitting layer among the various layes in OLED and the results of studies with them were introduced. The blue fluorescent dopants were designed based on fluorene emitting core well-known for high quantum efficiency with various electron-withdrawing N-heterocyclic structures at each sides of compounds which could change electron mobility and their propereis were investigated. In Section 1, electroluminescent properties of devices using compounds based on 9,9-diethyl-9H-fluorene with various pyridine derivatives which include vinyl groups were compared. In Section 2, electroluminescent properties of compounds based on 9,9-diethyl- 9H-fluorene containing quinoline derivatives with various structures in terms of connections were compared. As a result, all devices using these materials as dopants in emitting layer showed high efficiencies with efficient blue emissions and especially, one of the material which was substituted with pyridine derivative represented 7.75 cd/A, 4.14 lm/W, 6.66% of luminous, power, and external efficiencies at 20 mA/cm2 with efficient blue emission of Commission International de L’Éclairage (CIE) coordinates of (0.15, 0.16). In Section 3, the synthetic methods of designed materials, interpret of their structures, the measuring methods of their optical properties, device fabrication and mesurements of electroluminescent properties were explained and summarized. In this thesis, the researches for development of blue fluorescent materials which could be applied for emitting layer in OLED were performed.

Light extraction from transparent electrode based organic light emitting diodes with high efficiency and high color quality

김정범 서울대학교 대학원 2015 국내박사

Organic light emitting diodes (OLEDs) are moving toward high efficiency and high color quality required to be used in solid state lighting and display industry. This thesis focuses on the investigation of the optical loss channels and introduces a simulation methodology of light extraction in OLEDs. In addition, the theoretical and experimental results of the inverted top emission and transparent OLEDs with light extraction structures are presented to reach high efficiency and high color quality. Chapter 1 explains a basic principle of OLED operation and defines the factors which determine the internal and external efficiencies. Optical analysis method in OLEDs is introduced in terms of the all optical channels such as out-coupled, guided, and lost lights. Especially, the suppression of plasmonic loss will be discussed by using mode analysis. Chapter 2 represents the optical simulation method of the light extraction efficiency of organic light emitting diodes (OLEDs) with a light extraction structures based on the ray optics combined with the classical dipole model. Firstly, the angular distribution of emitted light from top emitting OLEDs (TEOLEDs) into the MLA was calculated by the classical dipole model and was taken as the input for the calculation of light propagation and extraction in the MLA. Secondly, the reflectance at the MLA/air interface of the OLEDs was calculated as functions of wavelength and incident angle of the incident light using the ray tracing Monte Carlo simulation. The enhancement ratio of external quantum efficiencies is calculated as functions of the refractive indices and structure of the MLA. The simulation method combining the classical dipole model and the ray tracing method described the experimental results very well including the external quantum efficiency and angle dependent intensities and spectra. Therefore the model can be utilized to the optimization of MLAs for light extraction. In chapter 3, highly efficient phosphorescent green inverted top emitting OLEDs by using transparent top electrode and horizontally oriented emitter are represented. A highly efficient phosphorescent green inverted top emitting organic light emitting diode with excellent color stability is fabricated by using the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/ indium zinc oxide top electrode and bis(2-phenylpyridine)iridium(III) acetylacetonate as the emitter in an exciplex forming co-host system. The device shows a high external quantum efficiency of 23.4% at 1,000 cd/m2 corresponding to a current efficiency of 110 cd/A, low efficiency roll-off with 21% at 10,000 cd/m2 and low turn on voltage of 2.4 V. Especially, the device showed very small color change with the variation of ∆x=0.02, ∆y=0.02 in the CIE 1931 coordinates as the viewing angle changes from 0 to 60°. The performance of the device is superior to that of the metal/metal cavity structured device. In addition, the phosphorescent green emitter of Ir(ppy)2tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming co-host system is used in the inverted top emitting OLEDs. The device showed a maximum current efficiency of 120.7 cd/A, a maximum external quantum efficiency (EQE) of 27.6% and the power efficiency of 85.9 lm/W at 1000 cd/m2. Moreover the efficiency roll off was small long-lasting to 20,000 cd/m2 with EQE’s and current efficiencies of 26.0% and 113.7 cd/A at 10,000 cd/m2 and 24.5% and 107.6 cd/A at 20,000 cd/m2, respectively. Based on the results, the optical analysis of the maximum achievable and measured EQE was performed using photoluminescence quantum yield (qPL), horizontal orientation ratio (Θ) and electrical loss (Γ). Highly efficient and high color quality white TEOLEDs by using the single layer broadband anti-reflection (AR) coating on the top of the TEOLEDs are represented in chapter 4. The white TEOLEDs with the broadband AR layer shows small variation of spectral change on viewing angles of ∆x = 0.02, ∆y = 0.01 at 0 ~ 60° compared to the reference device of dx = 0.15, dy = 0.05 for W1 (17wt% of B3PYMPM in buffer layer) and ∆x=0.14, ∆y=0.03 for W2 (15wt% of B3PYMPM in buffer layer). Furthermore, the white TEOLEDs with the broadband AR layer show the external quantum efficiencies increase due to the relaxation of the resonance effect of the device structure. As results, the EQEs of 18.8% and 16.9% for W1 and W2 of the single junction white TEOLEDs are realized with the small color variation on viewing angles, resulting in the duv values of -0.0014~0.0009 and 0.0041~0.0058 at 0~60 degrees. Correlated color temperatures of W1 and W2 are ~2200K and ~3000K satisfied with the blackbody radiation white within 5 steps of MacAdam ellipse for the requirement of the solid state lighting application. In chapter 5, a highly enhanced light extractions from an inverted top emission organic light emitting diode with little image blurring and color variation on viewing angles is reported. Direct integration of a high refractive index micro lens array on the top of the transparent indium zinc oxide top electrode of a green phosphorescent OLED showed a significant enhancement of light extraction to get EQE of 44.7% from 27.6%, the power efficiency of 134.7 lm/w from 85.9 lm/W and the current efficiency of 217.2 cd/A from 120.7 cd/A without image blurring. In addition, the device showed excellent color stability on viewing angle with Commission Internationale de l’Eclairage (CIE) coordinate of ∆x = 0.01, ∆y = 0.01 as the viewing angle varied from 0 to 60. In addition, the simulation model for calculation of the light extraction efficiency of indium zinc oxide (IZO) top electrode based top emitting OLEDs (TEOLEDs) with directly formed organic MLAs fabricated using the thermal evaporation of α -NPD on top of the device is also represented. The angular intensity distribution of the light emitted by the TEOLEDs into the MLAs is investigated from the classical dipole model in the cases of 30, 50, and 70 nm thick electron injection layers (EILs), giving the most dramatically change of the angle dependent emission properties. These angle dependent emission properties are used as the light sources set in the calculation of light propagation and extraction to the air by Monte Carlo ray tracing. This method combines coherent and incoherent light characteristics of OLEDs, resulting in the excellent agreement between theoretically calculated and experimentally measured the enhancement of the external quantum efficiencies. In chapter 6, a transparent OLED with an extremely high EQE, achieved by reducing the surface plasmonic and intrinsic absorption loss is introduced, where the transparent indium zinc oxide (IZO) and indium tin oxide (ITO) layers were used as the top and bottom electrodes, respectively. We adopted a special chemical to get the high performance transparent OLED to protect organic layers from the sputtering damage during the deposition of top IZO electrode. To extract the confined light inside the device, a high refractive index micro-cone array was additionally fabricated on the transparent top electrode using a simple evaporation method and a micro-lens array (MLA) sheet was attached on the bottom side of the glass substrate. As a result, the EQE of the device increased from 18.2% to 47.3% by using both microstructures, and this was additionally enhanced to 62.9% by integrating a micro-cone array on one side and a half-sphere lens on the other side. In addition, 75.7% and 77.2% of the total extractable portions of emitted light are calculated from the classical dipole model and combined simulation method.

Exciplex Dynamics and Emitting Dipole Orientation of Organic Light-Emitting Diodes

김권현 서울대학교 대학원 2016 국내박사

Organic light-emitting diodes (OLEDs) are particularly promising organic semiconductor devices for applications in lighting and displays. However, OLEDs exhibit lower power efficiencies than other light sources. Recently reported external quantum efficiencies (EQEs) of 29–30% for phosphorescent OLEDs (PhOLEDs) are close to the theoretical limit for isotropically oriented iridium complexes. The preferred orientation of the transition dipole moments has not been considered for PhOLEDs because of the lack of an apparent driving force for the molecular arrangement, even though horizontally oriented transition dipoles can result in efficiencies of > 30%. The origin of preferred orientation of the emitting dipoles of Ir complexes, and the design of a horizontal orientation for the emitting dipoles of the Ir complexes has not been studied in detail. Furthermore, EQEs in excess of 30% for horizontal emitting dipoles have not previously been reported. Recent investigations of triplet harvesting in pure organic phosphors via reverse intersystem crossing (RISC) have used charge-transfer complexes. Intramolecular charge transfer complexes reduce the energy gap between singlet and triplet states, ΔEST, resulting in more efficient triplet harvesting, as well as an internal quantum efficiency (IQE) of almost 100%, and an EQE of 30%. The excited state intermolecular charge transfer complex (or exciplex) can result in very small values of ΔEST, and highly efficient triplet harvesting via RISC. However, fluorescent OLEDs (FOLEDs) using exciplex emission still exhibit lower efficiencies than FOLEDs based on single molecule thermally activated delayed fluorescent (TADF) emitters; furthermore, there are no clear strategies for improving the efficiency of FOLEDs using exciplex emission. This thesis concerns two research topics: (1) exciplex dynamics for efficient triplet harvesting of FOLEDs, and (2) the emitting dipole orientation of phosphors for PhOLEDs. Our analysis shows that we could achieve unprecedented efficiency for both fluorescent and PhOLEDs. In Chapter 2, the exciplex dynamics that determine the IQE of FOLEDs are analyzed quantitatively using temperature-dependent transient photoluminescence (PL) and electroluminescence (EL) measurements. To date, most researchers have concentrated on reducing ΔEST to obtain high RISC rates, resulting in efficient triplet harvesting; however, this study shows that a high RISC rate does not necessarily lead to efficient triplet harvesting in exciplex emitters if there is a high non-radiative transition rate (knr). Not only efficient RISC, but also low non-radiative losses in both singlet and triplet exciplex emitters is required for efficient triplet harvesting. Triplet harvesting from exciplex emission can be increased by using exciplex emitters with a low knr; furthermore, suppressing knr by cooling the exciplex can result in an IQE of 100%. As a result, FOLEDs using exciplex emission were achieved that exhibited an IQE of 100% and an EQE of 25.2% at 150 K; this compares with an IQE of 48.3% and an EQE of 11.0% at room temperature, and is due to a reduction in the non-radiative transition rate. TADF and exciplex-based FOLEDs have disadvantages compared with conventional FOLEDs in terms of the broad emission spectra and low device stability. In Chapter 3, rather than using exciplex emission, an exciplex system was used as a host for a conventional fluorescent dopant without delayed fluorescence. By exploiting the narrow emission spectra and stability of fluorescent molecules, we use exciplex triplet harvesting via the Förster energy transfer mechanism from the exciplex to the conventional fluorescent dopant. As a result, red FOLED with conventional fluorescent dopant was obtained with an unprecedented EQE of 10.6%. A fraction of radiative excitons greater than 35% was achieved using the exciplex host, which is clearly indicative of triplet harvesting in the system. In Chapters 4–8, we discuss the emitting dipole orientation of the phosphorescent emitter for highly efficient PhOLEDs. Origin of the preferred orientation of phosphorescent emitting dipoles and relationships between the molecular structures of the phosphorescent emitter and the orientation of the emitting dipoles are discussed. OLEDs with unprecedented efficiencies are demonstrated using horizontally emitting dipoles. The orientation of the emitting dipoles of Ir complexes is influenced significantly by the ancillary ligand (Chapter 4) and the main ligand of the Ir complex (Chapter 5). Homoleptic Ir complexes exhibit almost isotropic orientation of the emitting dipoles; by contrast, heteroleptic Ir complexes (HICs) exhibit preferred orientations of emitting dipoles, despite their globular shape. The preferred orientation of the emitting dipole moments of HICs in amorphous host films results from the preferred direction of the triplet transition dipole moments of the HICs and the strong supramolecular arrangement within the co-host environment. This study shows that the C2 axis of HICs preferentially aligns normal to the substrate with some distribution and triplet transition dipole moments in the HICs direct from Ir to N-heterocycles. This results in horizontally emitting dipoles of HICs in the amorphous emitting layer. Furthermore, OLEDs with an EQE of 35.6% (red HICs) and 32.3% (green HICs) are demonstrated, with phosphorescent transition dipole moments oriented in the horizontal (in-plane) direction (Chapters 4 and 5). Based on these observations, HICs with a high fraction of horizontally oriented dipoles (Θ) were designed and synthesized by substituting the main ligand of the HICs. This design strategy includes two strategies: the substituents induce (1) triplet transition dipole moments in HICs located normal to the C2 axis of HICs (Chapter 6), and (2) the doubly degenerated transition dipole moments are parallel to the substrate (Chapter 7). Cyclometalated ligands of HICs were systematically substituted by methyl groups to control the direction of transition dipole moments of HICs. This methyl substitution resulted in different directions of the transition dipole moments in the iridium complexes, giving molecules with larger angles between the C2 axis and the transition dipole moments (Φ) in the molecules, resulting in a larger Θ in the emitting layer. As a consequence, a bis(2-(3,5-dimethylphenyl)-4-methylpyridine) Ir (III) (2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(3′,5′,4-mppy)2tmd) with a high Θ of 80% (which is 6% higher than that of HICs without methyl substituents) was developed, and green OLEDs exhibiting a high EQE of 34.1% were demonstrated using the new emitter (Chapter 6). In Chapter 7, various functional groups were substituted on the 4-position of the pyridine ring of the HICs to align Ir-N bonds of HICs and transition dipole moments parallel to the substrate. These HICs exhibited Θ in the range 80–86.5%, which is 1–7.5% greater than that of the reference HIC (Ir(3′,5′,4-mppy)2tmd), coming from different molecular orientation in the emitting layer. Consequently, we demonstrated unprecedented high EQE of 38% for yellow OLED and 36% for green OLED with new HICs in iridium-based PhOLEDs. An amorphous emission layer can have orientational ordering, as with a liquid crystal; however, the orientation of molecules exhibits some distribution around an average orientation. Thus, it will be difficult to increase Θ due to the amorphous nature of typical emitting layers. In this regard, organic crystals would be the better emitters because of the orientational and positional ordering; however, organic crystals have been rarely used in OLEDs due to low PL quantum yield that results from concentration quenching, and the low device stability that results from the rough surface. In Chapter 8, an unprecedented Θ of 93% and an EQE of 39% were realized for red OLEDs using a crystalline emission layer with perfectly oriented platinum complexes. The relationship between the orientation of the emitting dipole and the crystalline properties of the undoped Pt complex thin film are discussed based on grazing incident wide angle X-ray diffraction analysis and quantum chemical calculations. The orientation of the emitting dipole of the crystalline emission layers was affected not only by the crystallinity of the layer, but also by the molecular arrangement of the crystal, which are both influenced by the symmetry and position of the CF3-substituted pyrazolate units in Pt complexes.

Orange and white organic light-emitting diodes using exciplex-forming co-hosts

이성훈 서울대학교 대학원 2014 국내박사

White organic light emitting diodes (WOLEDs) have great potential for applications in displays, lighting, automobiles, imaging, and medicine. In this thesis, various approaches to develop highly efficient WOLEDs using exciplex forming co-hosts are studied in detail. This thesis consists of four parts; (1) orange OLEDs (chapter 2~4), (2) multi-EML WOLEDs (chapter 5), (3) a mechanism of charge generation units (CGUs) and an efficient CGU, and (4) Tandem WOLEDs. All OLEDs use exciplex forming co-hosts to achieve high efficiency, low driving voltage, and low efficiency roll-off, simultaneously. Chapter 1 summarizes the operating principle of OLEDs and what is the exciplex and how to incorporate the exciplex into OLEDs. Various layouts for WOLEDs are introduced and the quality of WOLEDs for lightings is briefly discussed in terms of the efficiency and color quality. Chapter 2~4 introduce orange OLEDs which are the key building block for development of efficient WOLEDs with high color rendering index (CRI). In chapter 2, an exciplex forming co-host is introduced in order to fabricate orange OLEDs with high efficiency, low driving voltage and an extremely low efficiency roll-off, by the co-doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieved a low turn-on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent-green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs showed low efficiency roll-off with an EQE of over 21% at 10,000 cdm-2. The device displayed a very good orange color (CIE of (0.501, 0.478) at 1,000 cdm-2) with very little color shift with increasing luminance. The color shift is supposed to be originated from a light doping concentration of red dopants. The transient electroluminescence (EL) of the OLEDs indicated that both energy transfer (ET) and direct charge trapping took place in the devices. In chapter 3, the recombination mechanism of red emission in orange OLEDs with co-doping of green and red phosphorescent dopants is studied using transient EL analysis in decay region. The dominant recombination type is the ET from green to red dopants and the efficiencies of ET were calculated about 50% in various orange OLEDs using decay rate constants of green emission without red dopant and ET rate constants from green to red dopants. In addition, we showed the energy of transition dipole with the same orientation is more efficiently transferred to that of the same orientation, because the ET is a function of transition dipole orientation. Furthermore, we demonstrated an unprecedentedly efficient orange OLED exhibiting the maximum EQE of 32.2% using green and red dopants having horizontally oriented transition dipole moments. In chapter 4, a high performance orange OLED where red and green phosphorescent dyes are doped in an exciplex forming co-host as separate red and green emitting layers (EMLs). The OLED shows a maximum EQE of 22.8%, a low roll-off of efficiency with an EQE of 19.6% at 10,000 cd/m2, and good orange color with a CIE coordinate of (0.442, 0.529) and no color change from 1,000 to 10,000 cd/m2. In chapter 5, a novel structure for highly efficient WOLEDs approaching the theoretical limit is studied by combining exciplex forming co-hosts, horizontally oriented phosphorescence dyes, and two emission layers for three colour white emission. The WOLED showed a maximum luminous efficacy (LE) of 68 lm W–1 and maximum EQE of 28.8% without an extra light extraction layer. Furthermore, the device exhibited high LE at high luminance with a LE of 58 lm W–1 at 1,000 cd m–2 originating from a high EQE (27.9%), low operating voltage (3.52 V) and low efficiency roll-off. The device emitted high quality warm white light with a CRI of 82. In addition, we achieved a LE of 106 lm W–1 at 1,000 cd m–2 by attaching an index-matched glass half sphere to the glass substrate. In chapter 6, the rate limiting step of charge generation is studed using the CGUs composed of a p-doped hole transporting layer (p-HTL), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) and n-doped electron transporting layer (n-ETL) where 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC) was used as the HTL. Energy level alignment determined by the capacitance-voltage (C-V) measurements and the current density-voltage characteristics of the structure clearly showed that the electron injection at the HATCN/n-ETL junction limits the charge generation in the CGUs rather than charge generation itself at the p-HTL/HATCN junction. Consequently, the CGUs with 30 mol% Rb2CO3 doped BPhen formed with the HATCN layer generates charges very efficiently and the excess voltage required to generate the current density of 10 mA/cm2 was around 0.17 V, which is extremely small compared with the literature values reported up to now. In chapter 7, tandem WOLEDs with EQE approaching theoretical limit are reported by interconnecting high efficiency orange OLEDs with co-doping of green and red phosphorescent dopants having high  andPL in exciplex forming co-host (concept shown in chapter 2, 3) and blue OLED using exciplex forming co-host with an efficient CGU (shown in chapter 6). A tandem WOLED with a high maximum EQE of 54.3% (PE of 63 lm W–1), EQE of 52.6% (PE of 52 lm W–1) at 1,000 cd m–2, low efficiency roll-off, and high color stability was demonstrated. In addition, an EQE of 90.6% at 1,000 cd m–2 by attaching an index-matched glass half sphere on conventional glass substrate was achieved

Development of organic optoelectronic materials for organic light-emitting diodes, organic solar cells, and organic thin-film transistors

권종철 서울대학교 대학원 2014 국내박사

Part 1 describes the studies on the development of blue host materials and dopant materials for organic light-emitting diodes (OLEDs). In the the first section of part 1, we report a new blue host material TCTEB with a conformationally homogenous, sterically bulky structure due to nonbonded interactions between adjacent methylene groups. The maximum quantum efficiency and current efficiency of the OLED using the TCTEB host showed 8.5% and 13.7 cdA-1, respectively, when doped with 8 wt% FIrpic. In the second section of part 1, we report a thermally stable 2,6-di(pyren-1-yl) naphthalene (DPN) and 2,6-di(pyren-1-yl)anthracene (DPA) for use as an emitting layer in OLEDs. An OLED device doped with 5% DPN exhibits National Television Standard Committee (NTSC) pure blue fluorescence emission at 436 nm with a current efficiency of 2.1 cdA–1 at Commission Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.08). The 5% DPA-doped OLED device exhibits blue fluorescence emission at 492 nm with a current efficiency of 6.0 cdA–1 at CIE coordinates of (0.16, 0.27). Part 2 describes the studies on the development of donor materials for organic solar cells (OSCs). In the first and second sections of part 2, we report that triphenylamine based donor materials (M-1, 2) can be successfully utilized to fabricate new OSC devices. As a result, the best device performance optimized at M-1:C60=1:1 and 2:PCBM=1:4 exhibited a power conversion efficiency (PCE) of 1.99% and 0.34%, respectively, under simulated AM 1.5 solar irradiation at 100 mWcm-2. In the third, fourth, fifth, and sixth sections of part 2 are reported vacuum processable and thermally stable new donor materials, 5,5''-di(pyren-1-yl)-2,2':5',2''-terthiophene (3s), 4,7-bis(5-(pyren-1-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BP-3), 2,6-di(pyren-1-yl)di thieno[3,2-b:2',3'-d]thiophene (DDT), and 2,6-di(thiophen-2-yl)dithieno[3,2-b:2',3'-d] thiophene (D1). The maximum PCE values of 3s, BP-3, DDT, and D1 exhibited 1.2%, 3.40%, 3.60%, and 1.85%, respectively, under simulated AM 1.5 solar irradiation at 100 mWcm-2. Part 3 describes the studies on the development of organic dyes for dye-sensitized solar cells (DSCs). We synthesized two organic dyes based on indole for DSCs. They have energy levels suitable for electron transfer from an electrolyte to nanocrystalline TiO2 particles. Under simulated AM 1.5 solar irradiation at 100 mWcm-2, a device using organic dye H-21 exhibited a short circuit current of 13.7 mAcm-2, an open circuit voltage of 0.68 V, a fill factor of 0.70, and a PCE of 4.5%. Part 4 describes the studies on the development of multifunctional materials for organic electronics. In the first section of part 4, we report that naphtho[2,3,a]pyrene (NP) can be commonly used as an effective tri-functional material for active layers of OSCs, OLEDs, and organic thin-film transistor (OTFTs). Consequently, NP turned out to be a promising organic semiconductor material which can be used as an efficient blue or green light emitting material, and an efficient solar cell donor, and an organic semiconductor for OTFTs. In the second section of part 4 is reported an efficient and thermally stable material (4,4′-di(pyren-1-yl)-1,1′-biphenyl, DBP) for use as a deep-blue-emitting layer in OLEDs as well as an active channel layer in OTFTs. An OLED device doped with 3% DBP exhibited pure blue fluorescence emission at 460 nm with a current efficiency of 3.9 cdA–1 at CIE coordinates of (0.15, 0.13). Furthermore, the DBP-based OTFT device exhibited a field-effect mobility of 0.21 cm2 V-1s-1. In the third and fourth section of part 4, we report a multifunctional organic material, 4,4',4''-Tris(4-naphthalen-1-yl-phenyl)amine (1-TNPA) and 4,4',4''-Tris(4-naph thalen-2-yl-phenyl)amine (2-TNPA). In particular, 1-TNPA and 2-TNPA-based devices exhibited efficient, deep blue emission similar to the NTSC standard blue. 1-TNPA and 2-TNPA can be used as hole-transporting materials which are more efficient and thermally stable than NPD. 1-TNPA and 2-TNPA can be also used as donor materials in OSCs and as active materials in OTFTs. Part 5 describes the development of the donor acceptor based phosphorescent iridium metal biotin complexes. In this section, we developed the first phosphorescent sensing system that can overcome intrinsic limitations in the sensitivity of the previous biotin-avidin assays. The neutral sensing system for biotin−avidin assays offers remarkable sensitivity over traditional transition metal based probes, due to the intramolecular energy transfer and increased hydrophobicity associated with the avidin binding site and neutral probe 1, and can be used as a homogenous competitive assay for biotin. New biomolecule probe systems can be produced if biotin is replaced with other recognition elements for various biomolecules.

Synthesis and Characterization of Novel Organic Semiconductor Materials Applicable to Organic Light Emitting Diodes and Organic Photovoltaics : Improving Device Performance and Stability

Na Yeon Kwon 고려대학교 대학원 2024 국내박사

Synthesis and Characterization of Novel Conjugated Materials Applicable to Organic Photovoltaics and Organic Light Emitting Diodes: Eco-Friendly Solution Processable Organic Semiconductor

Su Hong Park 고려대학교 대학원 2024 국내박사

유기 반도체는 조정 가능한 광전자 특성 및 화학 구조를 기반으로 하는 용액 공정을 포함하여 무기 반도체에 비해 여러 가지 장점을 가지고 있다. 이러한 장점으로 인해 유기반도체는 유연한 소자로 저비용, 대면적 제조가 가능하며, 유기 태양전지용 광 활성 소재나 유기 발광 다이오드용 발광 재료로 활용될 수 있다. 유기 태양전지는 유기 물질을 사용하여 햇빛을 전기로 변환하는 광전지 기술의 일종이며, 유기 발광 다이오드는 전기가 흐를 때 유기 물질을 발광 물질로 사용하여 빛을 내는 발광 다이오드의 한 종류이다. 유기 태양전지와 유기 발광 다이오드 기술은 유기 물질의 독특한 특성을 활용하여 다양한 응용 분야에 맞게 서로 다른 용도에 사용된다. 이러한 기술들은 재생 가능 에너지와 디스플레이 기술 분야에서 흥미로운 새로운 지평을 열고 있으며 지속적인 연구 및 개발 노력이 그 가능성을 계속 확장시키고 있다. 용액 공정을 통한 유기 태양전지 및 유기 발광 다이오드 소자의 제작에는 일반적으로 방향족 및 할로겐화 용매(예: 톨루엔, 실렌, 메시틸렌, 클로로포름, 클로로벤젠, 1,2-디클로로벤젠)를 사용하는데, 효율적인 전하 수송에 도움이 되는 유리한 형태를 생성하는 능력으로 인해 대부분 위의 용매들을 사용한다. 유감스럽게도, 이러한 용매들은 용매 비용과 인간의 건강 및 환경에 대한 독성으로 인해 대면적 생산과 상업화를 방해한다. 따라서 최첨단 유기 반도체에서는 높은 성능과 더불어 환경 친화적이며 인간과 환경에 해를 끼치지 않는 용매를 사용해야하고 높은 용해도 또한 중요한 우선 순위이다. 본 학위 논문은 4개의 장으로 구성되어 있다. 첫 번째 장에서는 유기 반도체 재료와 유기 태양전지 및 유기 발광 다이오드의 이론적 배경, 특히 광 활성층과 발광층의 재료에 대해 각각 설명한다. 2장에서는 도너 단위체인 BDT-T-Cl과 견고한 구조의 BDD, 그리고 유연한 구조를 가진 2F-BTA, 두 가지 억셉터 단위체를 이용하여 새로운 공액 랜덤 삼원공중합체인 PJ-25, PJ-50 및 PJ-75를 성공적으로 중합하였다. 삼원공중합체에서 2F-BTA의 비율이 증가함에 따라 비할로겐화 용매에 대한 용해도가 크게 향상되었다. 이 세 가지 공중합체 중 PJ-50과 IT-4F 블렌드를 기반으로 한 실내 및 실외 태양전지는 미세한 표면 형태, 우세한 face-on 방향성 및 혼합 박막에서의 효율적인 빛 유도 전하 분리로 인해 높은 광전 변환 효율을 보여주었다. 3장에서는 도너 블록1로 PM6를 선택하고 도너 블록2로 PBDT-TT를 선택하여 새로운 도너블록1-도너블록2 공액형 블록 공중합체인 PM6-b-TT를 합성하였다. PM6-b-TT은 PBDT-TT과 PM6 고분자 각각의 흡수 대역을 결합한 흡수 스펙트럼을 보여주고, 반치폭은 약 25 nm 증가하여 상대적으로 더 넓은 흡수 스펙트럼을 가진다. PM6-b-TT:Y6-BO 혼합 박막은 결정질 내부 형태와 향상된 π-π 고분자 체인 패킹을 갖춘 벌크 이종접합을 생성하였다. 이러한 시너지 효과 덕분에 PM6-b-TT 기반의 슈도-삼원 고분자 태양전지는 높은 광전 변환 효율을 달성하였으며, 특히 PM6:PBDT-TT 기반의 삼원 고분자 태양전지보다 높은 성능을 기록하였다. 또한, 이 소자는 150°C에서 12시간 동안도 뛰어난 안정성을 보여주었다. 4장에서는 다양한 유기 용매에서 뛰어난 용해도를 나타내는 용액 공정용 MR-TADF 발광체인 BCzBN-3tCz를 성공적으로 합성하였다. 이 발광체는 BCzBN에 덴드리틱 분자 구조인 tert-부틸 카바졸(3tCz)이 도입되었다. BCzBN-3tCz는 톨루엔, 자일렌, 클로로폼, 클로로벤젠 및 다이클로로벤젠과 같은 일반적인 방향족 및 할로겐화 용매에서도 뛰어난 용해도를 보여주었다. 흥미로운 점은 비할로겐화 지방성 용매인 에틸 아세테이트에서도 높은 용해도를 보여주었다. 물, 사이클로헥사논, 에틸 아세테이트 등 비할로겐화 지방족 용매를 사용하여 다층 OLED 소자를 제작하고 용액 공정 OLED 소자를 성공적으로 구동하였다. 결과적으로 BCzBN-3tCz를 적용한 친환경 용액공정 OLED는 우수한 외부양자효율과 30nm의 매우 좁은 반치폭을 달성했다. Organic semiconductors (OSCs) offer numerous benefits compared to inorganic semiconductors. These advantages include fine-tuning their optoelectronic properties and capacity for solution processing due to their versatile chemical structures. These features allow OSCs to facilitate cost-effective, large-scale production of flexible devices. They can be used as photoactive materials for organic photovoltaics (OPVs) or emitting materials for organic light emitting diodes (OLEDs). OPVs, also known as organic solar cells, are a type of photovoltaic technology that uses organic materials to convert sunlight into electricity, and OLEDs are a type of light-emitting diode that uses organic materials as the emissive material to produce light when an electric current is applied. The OPV and OLED technologies both harness the unique properties of organic materials for different applications. These technologies represent exciting frontiers in the fields of renewable energy and display technology, with ongoing research and development efforts pushing the boundaries of what's possible. The fabrication of devices based on OPV and OLED typically involves the use of aromatic and/or halogenated solvents such as toluene, xylene, mesitylene, chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (o-DCB) in most solution processes. These solvents are chosen for their ability to create a favorable morphology that facilitates efficient charge transport in OPV and OLED-based devices. However, the widespread use of these solvents poses challenges for large-scale production and commercialization due to their high cost and harmful effects on human health and the environment. Therefore, in addition to high performance, achieving high solubility in environmentally friendly and non-toxic solvents is a crucial priority for cutting-edge OSCs. This dissertation includes four chapters. The first chapter explains OSC materials and the theoretical background of OPVs and OLEDs, especially, about the materials in active layer and emitting layer, respectively. In Chapter 2, new conjugated random terpolymers, PJ-25, PJ-50, and PJ-75 were successfully synthesized from 4,8-bis(4-chlorothiophen-2-yl)benzo[1,2-b:4,5-b`]-dithiophene (BDT-T-Cl) as a donating unit, fluorine-substituted benzotriazole (2F-BTA) as a ductile accepting unit, and 1,3-Bis(5-bromo-2-thienyl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c`]dithiophene-4,8-dione (BDD) as a rigid accepting unit. With an increase in the proportion of 2F-BTA in the terpolymers, their solubility in non-halogenated solvents improved. Among these terpolymers, outdoor PV cells and indoor PV cells based on the PJ-50: 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene) -6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2`,3`-d`]-s-indaceno[1,2-b:5,6-b`]dithiophene (IT-4F) blend exhibited high power conversion efficiency (PCE). This efficiency is attributed to the fine surface morphology, predominant face-on orientation, and efficient photoinduced charge separation in its blend film. In Chapter 3, a novel conjugated block copolymer (CBP) named PM6-b-TT was developed, incorporating two different donor units, D1 (PM6) and D2 (PBDT-TT), using a one-pot polymerization method. PM6-b-TT exhibited an absorption spectrum combining the characteristic absorption bands of PBDT-TT and PM6 polymers. The full width at half-maximum (FWHM) of the absorption spectrum increased by approximately 25 nm, resulting in a relatively broad complementary absorption spectrum. The blend film of PM6-b-TT and Y6-BO achieved bulk-heterojunctions (BHJs) with crystalline internal morphology and improved π–π polymer chain packing. These synergistic effects led to a high PCE in the pseudo-ternary polymer solar cell (PSC) based on PM6-b-TT, surpassing the PCE of the ternary PSC using PM6:PBDT-TT. Furthermore, this device demonstrated outstanding PCE stability, maintaining efficiency at 150 °C for 12 hours. In Chapter 4, the solution-processable MR-TADF emitter BCzBN-3tCz was successfully synthesized, which exhibits excellent solubility. This emitter was created by integrating MR units (BCzBN) into dendritic molecular structures with tert-butyl carbazole (3tCz). The BCzBN-3tCz demonstrates remarkable solubility at room temperature (RT) in common aromatic and halogenated solvents like toluene, xylene, CF, CB, and o-DCB. Interestingly, it exhibits high solubility even in ethyl acetate (EA), a non-halogenated aliphatic solvent. Remarkably, fabricated multi-layered OLED devices employing non-halogenated aliphatic solvents such as water, cyclohexanone (Anone), and EA achieved the successful operation of solution-processed OLED devices. Consequently, eco-friendly solution-processed OLEDs employing the BCzBN-3tCz achieved an impressive external quantum efficiency (EQE) and a remarkably narrow FWHM of 30 nm.

Highly Efficient Fluorescent Organic Light-Emitting Diodes using Delayed Fluorescence

선진원 서울대학교 대학원 2017 국내박사

When organic light emitting diodes (OLEDs) appeared commercially aming to replace conventional display devices in early 2000s, it was not easy to guess OLEDs would become a dominant lighting source in display and lighting products industry. With ability of displaying colors in higher quality and wider viewing angle than those of liquid crystal display (LCD), and its adaptability to other technology from being able to be flexible and transparent, now OLEDs have become the ultimate display means. However, there are unsolved issues that need to draw our attention. Competition rose between phosphorescent and fluorescent OLEDs especialy to take better position for the commercial blue OLED. Blue phosphorescent OLEDs exhibit highly efficient blue light, however the stability issue still need to be cleared and further, phosphorescent OLEDs are considered to be toxic for containing heavy metal complex. Therefore, blue fluorescent OLEDs are being used commercially in spite of their low EL efficiency from limitation of exciton production portion of singlet to ~25%. Theoretically, if singlet-triplet splitting () is small enough, then reverse intersystem crossing (RISC) can take place harvesting additional triplets in fluorescent OLEDs. In order to have small, it is necessary to have spatial separation of the highly occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). Recently, by taking advantage of small, efficient intra- and intermolecular charge transfer (CT) materials have been reported, which are referred to thermally activated delayed fluorescence (TADF) material and excited charge transfer complex (exciplex) system, respectively. Especially, OLEDs adopting exciplex system exhibit high EL efficiency from efficient energy transfer (ET) of host to dopant material and charge balance in emission layer (EML). Further investigating into the method to boost EL efficiency in fluorescent OLEDs, there are sensitizing and heavy atom effect (HAE). Implementing phosphorescent or TADF material in EML working as assisted dopants in organized structure of cascading enegy levels of singlet and triplet among the constituents, improved EL efficiencies in fluorescent OLEDs were reported. Also, in CT type host material, HAE can be induced to improve EL efficiency of fluorescent OLED by enhancing mixing of singlet and triplet state taking advantage of both the sensitizing and HAE. In this dissertation, the work is focused on the investigation of full potential of fluorescent OLEDs, especially blue fluorescent OLEDs, attempting to answer the series of questions: 1) Is it possible to turn all the triplets into light in fluorescent OLED? 2) Can there be efficient host system for blue fluorescent OLED? 3) Is it possible to achieve high EL efficiency and color purity in a same blue fluorescent OLED, simultaneously? 4) Is it possible to achieve high EL efficiency in conventional blue fluorescent OLEDs? Starting from the first question of the possibility of achieving high EL efficiency in fluorescent OLED equivalent to that of phosphorescent OLED. In Chapter 2, 100% internal quantum efficiency (IQE) is achieved in a green fluorescent OLED exhibiting 30% external quantum efficiency (EQE) comparable to that of phosphorescent OLED. The OLED comprises an exciplex-forming cohost system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming cohosts contributed efficient ET and charge balance in the system. Large distribution of exciton in exciplex cohost lowered exciton density in EML resulting in the smallest efficiency roll-off among the other reported OLEDs using the same fluorescent emitter. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device. Motivated by the result of the highly efficient green fluorescent OLED using exciplex cohost system in Chapter 2, the efficient mixed cohost system for blue fluorescent OLED was investigated in Chapter 3. Selecting host material for blue emitter is particulary difficult for the requirement of high triplet energy, therefore the majority of reported efficient blue fluorescent OLEDs have been dependent on the same single host material of high triplet energy, which arouse the second question for efficient host system for blue fluorescent OLEDs. The efficient mixed cohost suggested in this work boosted electorluminescence (EL) efficiency of the blue fluorescent OLED by fully utilizing the ability of blue TADF emitter. The EQE was increased from previously reported ~13% to 21.8% from perfect charge balance through bipolar character of the mixed cohost. The achieved EQE was the highest among the blue fluorescent OLEDs of the same emitting material in the single host. The achieved EQE was one of the highest EQEs in blue fluorescent OLEDs and identical to the theoretically achievable maximum EL efficiency using the emitter under consideration of none electrical loss. Through the works in chapter 2 and 3, the fluorescent OLED was proven to be able to achieve the equivalent EL efficiency to that of phosphorescent OLED achieving 100% IQE, and highly efficient blue fluorescent OLED based on the mixed cohost was reported, therefore the motivation of further investigating into the potential of blue fluorescent OLEDs has risen. Despite of numerous reports on efficient blue fluorescent OLEDs, not many have achieved color purity and high EL efficiency at the same time. In Chapter 4, the mixed cohost was further utilized to realize deep blue emission and high EL efficiency at the same time using blue TADF emitter based on azasiline unit. For electroluminescence with delayed fluorescence, the azasiline unit has been introduced for the first time as a donor in a TADF material. The TADF material of 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPDDA) shows strong intramolecular CT character with large spatial separation with the acceptor of triazine leading to narrow splitting of singlet and triplet excited states for the efficient RISC. A blue OLED based on DTPDDA not only displayed deep blue emission in the Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.197) but also exhibited EQE of 22.3% which is the highest value ever reported for a blue fluorescent OLED. Theoretical prediction based on transient photoluminescence (PL) and optical simulation result agrees well with the achieved EQE indicating the successful conversion of triplet excitons to singlet in the blue fluorescent OLED by using DTPDDA. In Chapter 5, further utilizing azasiline unit for blue TADF emitter, donor-connector-acceptor (D-C-A) and donor-acceptor-donor (D-A-D) type blue TADF emitters were implemented into the mixed cohost in order to investigate the full potential of blue fluorescent OLEDs based on azasilin unit. Utilizing the azasiline unit, 5-(4’-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1’-biphenyl]-4-yl)-10,10- diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPPDDA), the TADF blue emitter of D-C-A type resulted in deep blue emission with CIE coordinate of (0.151, 0.087), close to the blue standard of the National Television System Committee (NTSC) of (0.140, 0.080) with 4.7% EQE. In D–A–D type materials of bis(4-(10,10-diphenyldibenzo[b,e]-[1,4]azasilin-5(10H)-yl)phenyl)methanone (BDAPM) and 5,5’-(sulfonylbis(4,1-phenylene))bis(10,10-diphenyl-5,10-dihydro dibenzo[b,e][1,4]azasiline) (SPDDA), carbonyl and sulfone units were used as the acceptors where the azasiline moeity was used as the donor unit, respectively. The sulfonyl unit contributed to a large twist of the molecular structure while the carbonyl unit led to a small twist of the molecular structure. As a result, the blue fluorescent OLEDs containing BDAPM and SPDDA demonstrated 11.4% and 2.3% EQEs with CIE y-values of 0.310 and 0.107, respectively. However, emitting from TADF material often shows broad EL spectrum due to strong CT state of the material, and the stability of the TADF based OLEDs still need to be improved. As an alternative, conventional fluorophore can be used as an emitter, where assisted dopants either phosphorescent or TADF material is strategically implemented to enhance spin mixing of singlet and triplet excited states. Recently, the high EL efficiencies were reported from blue fluorescent OLEDs based on conventional blue fluorescent dye, taking advantage of TADF material as a sensitizer. With smallof TADF material, RISC can occur effectively harvesting additional triplet excitons. Also, adopting heavy metal compound of platinum or iridium enhances spin mixing of singlet and triplet from HAE, consequently EL efficiency of OLED can be improved. Even more, increased spin reversal of triplet to singlet was observed by taking advantage of CT state of host on the induction of HAE from iridium complex, eventually enhancing the singlet to triplet ratio (ηs/T). In Chapter 6, almost all the triplets were harvested in the conventional blue fluorescence OLED by promoting both sensitizing and HAE through co-doping TADF material and phosphorescent material into EML. Comparison of the theoretical and experimental data indicates that ηs/T was increased from 0.23 to 0.94 with increased EQE from 5.6% to 12.3% in blue fluorescent OLED when the both TADF material and phosphorescent material were doped together as assisted dopants, indicating the nearly all the triplets harvested in the conventional blue fluorescent OLED. 유기발광소자는 현존하는 디스플레이 및 조명을 구현하는 방법중 독보적인 우위를 점하고 있다. 기존 업계에서 내세우던, 단순한 색순도 및 시야각의 장점을 벗어나, 기판의 선택에 따라 유연하게 휘어지고 투명하게 제작될 수 있어, 유기발광소자의 적용범위는 계속 확장되고 있는 실정이다. 이에 따라, 그 어느때 보다도 유기발광소자의 효율 및 수명을 증진시키는 연구가 물질 및 소자 측면에서 활발하게 진행되고 있다. 이러한 관점에서, 3원색중 청색 유기발광소자 연구가 가장 활발한데, 그 이유로 인광물질과 형광물질에 기반한 청색 유기발광소자의 장점 및 단점에 기인한 경쟁이 있다. 인광물질에 기반한 청색 유기발광소자는 고효율의 외부양자효율을 구현할 수 있지만 단수명으로 인해 상용화의 한계에 도달해 있어, 낮은 외부양자효율을 보이는 형광물질 기반의 청색 유기발광소자가 상용화 되어 사용되고 있다. 하지만, 이론적으로 형광유기발광소자는 25%의 여기자만이 발광에 사용되어, 인광유기발광소자의 외부양자 효율에 접근할 수 없다. 이러한 형광유기발광소자의 낮은 외부양자효율을 높이기 위한 방법으로, 최근 전하이동 복합체의 역항간 교차를 이용하여 삼중항 여기자를 수확한 연구가 활발히 보고 되었다. 분자내 전하이동 복합체인 열활성화지연형광 (TADF) 발광체는 삼중항-일중항 에너지 차이를 줄여 삼중항 여기자를 발광 가능한 단일항 여기자로 수확할 수 있다. 들뜬상태 분자간 전하이동 복합체인 엑시플렉스도 삼중항-일중항 에너지 차이가 작아서 역항간 교차를 이용하여 효율적인 삼중항 수확을 할 수 있다. 본 논문에서는 이러한 역항간교차 방법을 활용해 형광유기발광소자의 외부양자효율을 증진시키는 방법을 유기발광 소자적 측면에서 연구하였다. 제 2장에서는, 형광유기발광소자의 외부양자효율이 인광유기발광소자에 근접할 수 있는지를 확인하였다. 엑시플렉스는 전술한 바와 같이 삼중항-일중항 차이가 적어 역항간 교차를 이용할 수 있는 장점이외에도, 호스트 물질로 사용시 발광층으로의 주입장벽이 없어져 전하-정공 균형을 맞출 수 있으며, 발광층내 여기자가 넓게 분포하여, 여기자 농도에 의한 급격한 효율감소를 방지할 수 있는 등의 장점이 있다. 엑시플렉스에서 도판트로의 에너지 전이를 고려하여, 발광체로는 기존 높은 절대발광효율에도 낮은 외부양자효율이 보고된 바 있는 녹색 열활성화지연형광 물질을 사용하였다. 제작된 녹색형광유기발광소자는 30%의 외부양자효율을 나타내어, 인광유기발광소자의 외부양자효율과 동일한 외부양자효율이 가능함을 증명하였다. 하지만, 엑시플렉스는 이종의 물질 각각의 최고 점유 분자궤도 (HOMO)와 최저 비점유 분자궤도 (LUMO)에 의해 형성되어, 높은 삼중항에너지가 요구되는 청색 도판트에 적합한 엑시플렉스 호스트의 발견이 용이하지 않다. 이에 따라, 소자에 적용시 엑시플렉스가 갖고 있는 양극성에서 나오는 전하균형의 장점을 사용할 수 있도록, 제 3장에서는 혼합 공동호스트를 사용한 고효율 청색형광유기발광소자를 연구하였다. 고효율의 높은 삼중항에너지의 호스트물질의 부재로, 현재까지 청색형광유기발광소자 연구는 단일호스트인 특정물질에 의존해 왔다. 본 연구에서는 높은 절대발광효율에도 불구하고 낮은 외부양자효율을 보이는 청색 열활성화지연형광 물질을 사용하여, 기존 보고된 ~13%의 외부양자효율 대비 향상된 21.8%의 외부양자효율을 발표하였다. 소자내에서 전기적 손실이 없을때를 가정한 이론적 한계수치와 동일한 외부양자효율이다. 소자적인 측면에서, 기존보고된 청색형광유기발광소자 대비 구동전압은 낮고, 20%가 넘는 외부양자효율을 보여 청색형광유기발광소자용으로 적용된 혼합 공동호스트가 효율적임을 증명하였다. 하지만, 기존 보고된 바에 의하면, 색순도와 소자성능 두가지를 모두 구현한 청색형광유기발광소자는 전무한 실정이었다. 제 4장과 5장에서는, 혼합 공동호스트에 아자질린과 트리아진 물질을 기반으로하는 청색열활성화지연형광 발광체를 적용하여, 발광체의 원활한 역항간 교차와 혼합 공동호스트의 장점을 결합하여, 22.3%의 외부양자효율과 y-색좌표기준 0.2이하의 색 순도 높은 청색형광유기발광소자를 구현할 수 있었다. 이러한 결과를 통해 아자질린 물질이 갖는 열활성화지연형광물질에서의 공여체로서의 장점을 발견할 수 있었다. 아자질린 공여체를 활용한 다른 세가지 열활성화지연형광물질을 공동혼합호스트에 적용하여, 각각의 물질들이 갖는 구조와 수용체의 종류에 따른 현상을 소자 및 광물리적 측면에서 분석하였다. 하지만, 청색 열활성화지연형광 물질을 발광체로 사용시, 물질의 전하이동 특성에 따라 소자의 발광 스펙트럼이 넓어져 색수도가 떨어질 수 있으며, 장수명의 청색 열활성화지연형광 물질이 적어 실질적인 산업체 적용에 한계가 있을 수 있다. 이를 극복하기 위해, 일반 청색형광물질을 발광체로 사용하고, 이를 센시타이징 또는 중원자 효과(heavy atom effect)를 활용하여 외부양자효율을 향상시킬 수 있다. 제 6장에서는, 센시타이징과 중원자효과를 복합적으로 활용하여, 일반 청색형광물질 기반의 청색형광유기발광소자의 외부양자효율을 2.5배 이상 향상시킨 연구 결과이다. 본 연구에서 제안된 방법을 통해, 발광에 사용된 여기자가 기존 ~25%에서, ~94%로 증가하여, 일반청색형광유기발광소자에서 비발광 삼중항 여기자를 거의 모두 수확할 수 있었다.

Quantum-Dot and Organic Hybrid Light-Emitting Diodes Using a Common Layer Structure

이수현 서울대학교 대학원 2023 국내박사

Organic light-emitting diodes (OLEDs) have gained significant attention for their excellent device performance and integration capabilities in flat panel displays. The conventional method of achieving full-color displays involves individually patterning each primary color emitting-layer (EML) using fine metal masks (FMMs) during thermal evaporation. However, this approach has several issues such as the shadow effect, particle contamination, and misalignments, which hamper the large-scale and mass production. Meanwhile, colloidal quantum-dots (QDs) and QD light-emitting diodes (QLEDs) have emerged as promising candidates for next-generation display devices due to their exceptional optoelectronic properties including high photoluminescence quantum yield, narrow emission bandwidth, and good photostability. Nevertheless, the complex solution process to pattern red, green, and blue QDs limits the performance of QLEDs and restricts the resolution of full-color displays. Therefore, one of the urgent issues to utilize the OLEDs and QLEDs in practical is developing simple and low-cost patterning techniques for the realization of the full-color and high-resolution displays. This thesis introduces novel concepts of common layer architectures that integrate QDs into hybrid light-emitting devices to reduce the difficulties in patterning process and manufacturing cost of full-color displays. While a few studies have explored the common layer structure previously, their application has been limited to organic small molecules. Herein, the utilization of the common layer structure is extended into two types of QD-integrated structures for the first time. In the first approach, QDs are commonly deposited across the entire active area using a solution process, eliminating the need for an FMM step for the red sub-pixel in the full-color hybrid device structure. Despite the presence of underlying QDs, the green and blue-emitting OLEDs exhibit emission solely from their respective dyes which is attributed to the confinement of excitons within the EML through effective hole blocking facilitated by the deep highest occupied molecular orbital level of the buffer layer. Consequently, a full-color QD–organic hybrid light-emitting device, employing the QD common layer architecture on a single substrate, successfully maintains the color purity of each sub-pixel. As the second common layer structure, a QD–organic hybrid light-emitting diode is introduced, which incorporates an organic blue common layer (BCL) deposited using a common mask over the entire sub-pixels. The optimized device structure enables red and green-emitting QLEDs to maintain their Commission Internationale de l'Eclairage color coordinates even with the presence of the BCL. Additionally, the adoption of the BCL significantly enhances the external quantum efficiency of the green and red QLEDs by 38.4% and 11.7% respectively, due to Förster resonance energy transfer from the BCL to beneath QDs. By using the BCL structure, a full-color QD–organic hybrid device on a single substrate is demonstrated. Therefore, it is believed that these novel common layer strategies suggested in this thesis, facilitating QLED–OLED hybrid device structure, is practically applicable for easier fabrication of solution-processed, high-resolution, and full-color displays with reduced process steps. 유기발광다이오드는 우수한 장치 성능과 평면 패널 디스플레이 활용 가능성으로 큰 주목을 받고 있다. 풀컬러 디스플레이를 달성하는 기존의 방법은 진공증착 과정동안 파인 메탈 마스크(FMM)를 사용하여 각 기본 색상 발광층을 개별적으로 패터닝하는 것이다. 그러나 이러한 접근 방식은 shadow effect, 입자 오염 및 FMM 정렬 불량과 같은 여러 가지 문제로 인해 대면적 및 대량 생산에 어려움이 있다. 한편, 콜로이드 양자점과 양자점발광다이오드는 높은 발광 양자 수율, 좁은 방출 대역폭, 우수한 광안정성과 같은 탁월한 광전자 특성으로 인해 차세대 디스플레이 장치의 유망한 후보로 부상했다. 하지만 적색, 녹색 및 청색 양자점 패터닝에 이용되는 복잡한 용액 과정은 양자점발광다이오드의 성능을 제한하고 풀컬러 디스플레이의 해상도를 높이기 어렵게 했다. 따라서 유기발광다이오드와 양자점발광다이오드를 실용화하기 위한 시급한 과제 중 하나는 풀컬러 및 고해상도 디스플레이 구현을 위한 간단하고 생산 비용이 낮은 패터닝 기술의 개발이다. 이 논문은 양자점을 복합 발광 장치에 사용하여 풀컬러 디스플레이의 패터닝 과정 및 제조 비용을 줄일 수 있는 새로운 공통층 구조를 소개한다. 공통층 구조에 대한 연구는 기존에도 있었으나 그 활용은 유기물에 제한되었다. 이 논문에서는 지금까지의 연구 중 최초로 공통층 구조의 활용을 두 가지 유형의 양자점 활용 구조로 확장하였다. 첫 번째로 제안하는 방식에서는 용액 공정을 이용해 양자점을 전체 발광 영역에 성막함으로써, 풀컬러 복합 발광 장치 구조에서의 빨간색 픽셀에 대한 FMM 이용 과정을 생략할 수 있도록 했다. 이 구조에서 녹색 및 청색 유기발광다이오드는 아래에 위치한 양자점에서는 발광하지 않고 각각의 발광층에서만 빛을 발생하였으며, 이는 정공제어층의 큰 최고 점유 분자 궤도함수값에 의한 효과적인 정공 차단을 통해 발광층 내에 엑시톤을 국한시킴으로써 가능했다. 이에 따라 단일 기판에 양자점 공통층 구조를 활용한 풀컬러 양자점-유기물 복합 발광 장치는 각 하위 픽셀의 색 순도를 성공적으로 유지하였다. 두 번째 공통층 구조로는 전체 하위 픽셀에 마스크 없이 진공 증착된 유기 청색 공통층을 사용한 양자점-유기 복합 발광 다이오드를 소개한다. 최적화된 소자 구조에서 적색 및 녹색 양자점발광다이오드는 청색 공통층이 있는 경우에도 각각의 색좌표를 유지하였다. 또한 청색 공통층에서 양자점으로의 Förster 공진 에너지 전달로 인해 적색 및 녹색 양자점발광다이오드의 외부 양자 효율이 각각 38.4% 및 11.7% 증가하였다. 이러한 청색 공통층 구조를 사용하여 단일 기판에 풀컬러 양자점-유기 복합 발광 장치를 시연하였다. 따라서 본 논문에서 제시하는 공통층 구조를 활용한 양자점-유기물 복합 발광 다이오드는 발광층 패터닝 공정을 한 단계 줄임으로써 용액 공정을 기반으로 한 고해상도 및 풀컬러 디스플레이의 보다 쉬운 제조에 기여할 수 있을 것이다.