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Graduate School of Ajou University Department of Energy Systems 4. Theses(Ph.D)

Development of Solution-Processed High Performance Light-Emitting Devices Based on Colloidal Quantum Dots

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dc.contributor.advisorSoonil Lee-
dc.contributor.authorNGUYEN HUU TUAN-
dc.date.accessioned2018-11-08T08:10:13Z-
dc.date.available2018-11-08T08:10:13Z-
dc.date.issued2014-02-
dc.identifier.other16074-
dc.identifier.urihttps://dspace.ajou.ac.kr/handle/2018.oak/11071-
dc.description학위논문(박사)--아주대학교 일반대학원 :에너지시스템학과,2014. 2-
dc.description.tableofcontentsTable of Contents Abstract i Acknowledgments ii List of Figures vi List of Tables xi Chapter 1: Introduction 1 1.1 Overview 1 1.2 Mechanism and motivation for fabricating QLEDs 3 1.2.1 Mechanisms of charge transfer for QLEDs 3 1.2.2 Colloidal NC QDs 4 1.2.3 Ligand exchange 7 1.2.4 Size dependence 9 1.2.5 Inorganic vs. organic semiconductors 11 1.3 QLED operation 14 1.3.1 Device structure and functions 14 1.3.2 Extracting light from QLEDs 16 References 18 Chapter 2: Experimental Procedure 22 2.1 QLED device fabrication 22 2.1.1 Fabrication equipment 24 2.1.2 Chemicals used to QLED devices 27 2.2 Synthesis of p–type inorganic semiconductor 28 2.3 Synthesis of n–type inorganic semiconductor 28 2.3.1 ZnO sol–gel 28 2.3.2 ZnO nanoparticles 28 2.4 Synthesis of nanocrystalline QDs 29 2.4.1 Synthesis of CdSxSe1–x/ZnS nanocrystals 30 2.4.1.1 CdSxSe1–x core 30 2.4.1.2 CdSxSe1–x/ZnS core/shell 30 2.4.1.3 CdSxSe1–x/ZnS QD ligand exchange 30 2.4.2 Synthesis of CdSe@ZnS gradient green QDs 32 2.5 Measurement methods 33 2.5.1 QLED device measurement 33 2.5.2 PL quantum yield 34 2.5.2.1 Experimental preparation 34 2.5.2.2 Procedure 34 2.5.2.3 Calculation 35 2.5.3 Electron mobility 37 References 39 Chapter 3: All Organic Charge Transport Layers based QLEDs 41 3.1 Introduction 41 3.2 Pure green color emission from QLEDs 43 3.3 Summarry 48 References 49 Chapter 4: Solution–Processed Nickel Oxide layer as the Anode Interfacial Layer based QLED 50 4.1 Introduction 50 4.2 NiO thin film characteristics 52 4.3 QLEDs based on NiO anode interfacial layer 53 4.4 Summarry 64 References 64 Chapter 5: Solution–Processed Metal Oxide Layers as Charge Transport Layers based QLEDs 67 5.1 Introduction 67 5.2 QLEDs based on solution–processed metal oxide charge transport layers 68 5.3 Summarry 76 References 77 Chapter 6: Alloyed CdSxSe1–x core and CdSxSe1–x/ZnS core/shell QD–based LEDs 79 6.1 Introduction 79 6.2 Characteristics of alloyed CdSxSe1–x core and CdSxSe1–x/ZnS core/shell QDs 81 6.2.1 CdSxSe1–x core QDs 81 6.2.2 CdSxSe1–x/ZnS core/shell QDs 83 6.3 Application 86 6.3.1 Alloyed CdSxSe1–x core QDs based LEDs 86 6.3.2 LEDs fabricated with ligand–exchanged alloyed CdSxSe1–x/ZnS core/shell QDs 92 6.4 Summarry 98 References 99 Chapter 7: Ethanedithiol–crosslinked CdSe@ZnS colloidal QDs based LEDs 101 7.1 Introduction 101 7.2 Preparation and characterization of EDT crosslinked CdSe@ZnS QD thin film 102 7.2.1 Thin film deposition and EDT treatment 102 7.2.2 Optical properties of the treated CdSe@ZnS QD thin films 102 7.3 EDT crosslinked CdSe@ZnS QD–based LEDs 105 7.4 Summarry 111 References 112 Chapter 8: Enhancing QLED light extraction with an ordered array of polystyrene beads 113 8.1 Introduction 113 8.2 Formation and characterization of PS beads 115 8.3 Extracting light from QLED with ordered monolayer of polystyrene microbeads 117 8.4 Summarry 126 References 127 Chapter 9: Conclusions and Future Works 129 Appendix 133 |List of Figures Fig. 1.1. Mechanisms of charge transport in QLEDs 4 Fig. 1.2. NC QD (up) and energy–band diagrams for three neighboring QD monolayers (down) 6 Fig. 1.3. Organic and atomic passivation strategies 8 Fig. 1.4. Size–dependent fluorescence emission from quantum dots of various compositions 10 Fig. 1.5. Promotion of electron in ethylene molecule 11 Fig. 1.6. Ethylene molecule with σ and π bonds 12 Fig. 1.7. Energy diagram for crystalline semiconductors showing valance and conduction bands 13 Fig. 1.8. Schematic of QLED structure 14 Fig. 1.9. Energy–level diagram for QLEDs 15 Fig. 1.10. Schematics illustrating various processes by which excitons form and decay on QD: (a) charge injection, (b) energy transfer from organic thin films, (c) Auger recombination, and (d) field–induced exciton dissociation 17 Fig. 2.1. Flow chart depicting device fabrication 23 Fig. 2.2. Reactive Ion Etching system 24 Fig. 2.3. Closed system consisting of evaporator, vacuum oven, and spin–coater inside glove box 25 Fig. 2.4. Thermal evaporation system: 1) metal tray (holder) and shadow mask holder, 2) shutter, 3) tubular ceramic heating unit inside crucible with metal pot or tungsten boat, 4) substrate turntable, and 5) concussion quartz thickness gauge 26 Fig. 2.5. EDA– and MPA–ligand–mediated transfer of TOP–capped CdSxSe1–x/ZnS core/shell QDs in chloroform to aqueous phase 31 Fig. 2.6. Schematic of luminance meter used to measure L–I–V characteristics of QLEDs 33 Fig. 2.7. Absorption and emission spectra for CdSe@ZnS gradient QDs at five different concentrations 35 Fig. 2.8. Integrated PL intensity plotted as functions of absorbance at 488 nm for CdSe@ZnS gradient QDs measured at five different concentrations 36 Fig. 2.9. J–V characteristic of electron–only LED device 38 Fig. 3.1. (a) A schematic structure and (b) energy level diagram of the green–color QLEDs 42 Fig. 3.2. Current density and luminance of the green–color QLEDs with respect to the bias voltage. The multilayer structure of the QLEDs is ITO/PEDOT:PSS/Poly–TPD/ QD/TPBi/Alq3/LiF/Al 43 Fig. 3.3. Normalized PL spectra of Alq3, TPBi, and green CdSe/ZnS QDs together with a normalized absorption spectrum of QDs 44 Fig. 3.4. EL spectra of the green QLEDs with and without a TPBi layer. Insert shows the CIE color coordinates of the EL of the QLED with a 20–nm–thick TPBi layer at 12 V 46 Fig. 3.5. Luminous and power efficiencies of the green QLEDs with and without a TPBi layer 46 Fig. 3.6. External quantum efficiency of the green QLEDs with and without a TPBi layer 47 Fig. 4.1. XRD pattern of 40 nm NiO film deposited on glass substrate, annealed at 425oC. The inset shows AFM image of NiO film on ITO substrate 52 Fig. 4.2. Energy level diagram for the green–color QLEDs with NiO anode interfacial layer 53 Fig. 4.3. Current density and luminance of QLEDs with NiO and PEDOT:PSS layers with respect to bias voltage measured at room temperature 54 Fig. 4.4. Current versus bias voltage (I–V) characteristics of NiO–QLED at different temperature 55 Fig. 4.5. Poole–Frenkel conduction plots at low bias voltage region as ln J/(V–Vbi) versus the square root of (V–Vbi) 56 Fig. 4.6. The respective m(T) and b(T) coefficients extracted from the slope and intercept of linear fits in Fig. 4.5 versus the reverse of the temperature 58 Fig. 4.7. The SCLC plots at high bias voltage region as ln J/(V–Vbi) versus the square root of (V–Vbi). The inset shows SCLC in linear scale 60 Fig. 4.8. (a) Luminous and power efficiencies versus luminance and (b) QE versus current density of QLEDs with NiO and PEDOT:PSS layers, measured at room temperature 62 Fig. 4.9. EL spectra of QLEDs with NiO and PEDOT:PSS layers. The insert shows the PL spectra of green CdSe/ZnS core–shell QDs and Alq3 63 Fig. 5.1. Current density characteristics of device 1 (ITO/PEDOT:PSS/Poly–TPD/QDs/ZnO/Al), device 2 (ITO/NiO/Poly–TPD/QDs/ZnO/Al) and device 3 (ITO/NiO/Poly–TPD/QDs/Al) with respect to bias voltage 69 Fig. 5.2. Luminance characteristics of device 1 (ITO/PEDOT:PSS/Poly–TPD/QDs/ZnO/Al), device 2 (ITO/NiO/Poly–TPD/QDs/ZnO/Al) and device 3 (ITO/NiO/Poly–TPD/QDs/Al) with respect to bias voltage 70 Fig. 5.3. Energy level diagram for the QLEDs based on multilayer structure of ITO/NiO(or PEDOT:PSS)/Poly–TPD/QD/ZnO(or w/o ZnO)/Al 71 Fig. 5.4. Luminous efficiency of the three QLEDs with respect to the luminance of the QLEDs based on multilayer structure of ITO/NiO(or PEDOT:PSS)/Poly–TPD/QD/ZnO(or w/o ZnO)/Al 71 Fig. 5.5. Power efficiency of the three QLEDs with respect to the luminance of the QLEDs based on multilayer structure of ITO/NiO(or PEDOT:PSS)/Poly–TPD/QD/ZnO(or w/o ZnO)/Al 72 Fig. 5.6. QE of the three QLEDs with respect to the luminance of the QLEDs based on multilayer structure of ITO/NiO(or PEDOT:PSS)/Poly–TPD/QD/ZnO(or w/o ZnO)/Al 73 Fig. 5.7. The normalized EL spectra of the three QLEDs measured at 10 V. The insert show photograph of working pixel of device 2 at 10 V 74 Fig. 5.8. PL spectrum of green CdSe/ZnS core–shell QDs 75 Fig. 6.1. Absorbance of alloyed CdS0.2Se0.8 core QDs prepared for different reaction times 81 Fig. 6.2. PL spectra for alloyed CdS0.2Se0.8 core QDs prepared for different reaction times 82 Fig. 6.3. TEM image and corresponding EDS spectrum for alloyed CdS0.2Se0.8 QDs grown for 1 h. Average QD size is 8.0 nm 82 Fig. 6.4. Absorbance and PL emission measured at 488–nm excitation for CdSxSe1–x–core QDs 84 Fig. 6.5. Absorbance and PL emission spectra measured at 488–nm excitation for TOP–capped CdSxSe1–x/ZnS core/shell QDs 84 Fig. 6.6. Absorbance and PL emission spectra measured at 488–nm excitation for MPA–capped CdSxSe1–x/ZnS core/shell QDs 85 Fig. 6.7. PL spectra for MPA–capped CdSxSe1–x/ZnS core/shell, QD–TOP, and core QDs. Inset shows light emitted from QD–MPA in DI water under UV irradiation 86 Fig. 6.8. Energy–level diagram for alloyed CdSxSe1–x core QLEDs 87 Fig. 6.9. Current density and luminance plotted as functions of bias voltage for QLEDs fabricated with different–sized alloyed CdSxSe1–x QDs. Multilayer structure of QLEDs consisted of ITO/PEDOT:PSS/poly–TPD/QD/Alq3/LiF/Al 88 Fig. 6.10. Luminous efficiency plotted as functions of luminance for QLEDs fabricated with different–sized alloyed CdSxSe1–x QDs 89 Fig. 6.11. Normalized EL spectra for QLEDs fabricated with different–sized alloyed CdSxSe1–x QDs 89 Fig. 6.12. PL spectra for QLEDs fabricated with different–sized alloyed CdSxSe1–x QDs. Inset shows PL spectrum for Alq3 91 Fig. 6.13. QE of pure color emitted from QDs. Inset shows EL spectra for QLEDs after evolution of color independence 91 Fig. 6.14. Current density plotted as functions of bias voltage for devices fabricated with different thicknesses of QD EMLs 93 Fig. 6.15. Luminance plotted as functions of bias voltage for devices fabricated with different thicknesses of QD EMLs 93 Fig. 6.16. Luminous efficiency plotted as functions of luminance for QLEDs fabricated with different thicknesses of QD of QD EMLs 94 Fig. 6.17. Power efficiency plotted as functions of luminance for QLEDs fabricated with different QD EML thicknesses 95 Fig. 6.18. Normalized EL spectra measured at 10 V for QLEDs. Inset shows emission image measured at 10 V for working pixel of device fabricated with 10–nm–thick QD layer 95 Fig. 6.19. QE plotted as functions of current density for QLEDs fabricated with different QD EML thicknesses 96 Fig. 6.20. Energy–level diagram for QLEDs fabricated with ITO/ZnO/QD/TPD/MoO3/Al multilayer structure 97 Fig. 7.1. PL spectra for EDT–, BDT–, and formic–acid–treated and untreated QD films deposited onto ITO/ZnO substrates 103 Fig. 7.2. PL spectra for EDT– and BDT–treated and untreated QD films deposited onto glass/ZnO substrates 104 Fig. 7.3. PL spectra for EDT– and BDT–treated and untreated QD films deposited onto glass–only substrates 105 Fig. 7.4. Energy–level diagram for QLEDs fabricated with multilayered structure 106 Fig. 7.5. Current density plotted as functions of bias voltage for QLEDs treated with various EDT concentrations 107 Fig.7.6. Luminance plotted as functions of bias voltage for QLEDs treated with various EDT concentrations 107 Fig. 7.7. Luminous efficiency of QLEDs treated with various EDT concentrations 108 Fig. 7.8. Power efficiency of QLEDs treated with various EDT concentrations 108 Fig. 7.9. Normalized EL spectra for green QLEDs treated with various EDT concentrations 109 Fig. 7.10. PL spectrum for green–gradient CdSe@ZnS QDs 110 Fig. 7.11. QE of the green QLEDs treated with various EDT concentrations 111 Fig. 8.1. SEM images of (a) close–packed array of 280–nm PS spheres and (b) nonclose–packed array of randomly sized irregular PS beads 116 Fig. 8.2. Schematic representation of inverted QLED containing PS array 117 Fig. 8.3. (a) Current density and (b) luminance plotted as functions of bias voltage and (c) luminance plotted as functions of current density for four devices 118 Fig. 8.4. Luminous efficiencies of four QLEDs plotted as functions of luminance for inverted–structure QLEDs 119 Fig. 8.5. Power efficiency plotted as functions of luminance for four inverted–structure QLEDs 120 Fig. 8.6. QE of green QLEDs. Inset shows emission images of working pixels of devices 1 and 3 measured at 4.5 V. Light outcoupling from device 3 shows stronger scattering from PS array than that from device 1, which was fabricated without PS–scattering medium 121 Fig. 8.7. EL spectra measured with and without shadow masks for devices 1 and 3 122 Fig. 8.8. Normalized EL spectra measured at 4.5 V for four QLEDs. Insets show CIE color coordinates with respect to luminance for four green QLED devices (upper inset). Color coordinates of devices are coincident at same luminance. PL spectrum for green CdSe@ZnS QDs (lower inset) 123 Fig. 8.9. EL spectrum for QLED device fabricated without any scattering medium integrated with transmission spectrum for nonclose–packed PS array. Transmittance peaks of both spectra almost coincide 124 Fig. 8.10. Angular dependence of normalized EL intensity of light emitted from QLEDs, which strongly follows Lambertian–fitting curve 125 |List of Tables Table 2.1. QLED–fabrication materials and corresponding properties 27 Table 6.1. Dependences of emission peak, FWHM, and QY of as–synthesized alloyed core QDs on reaction time 83-
dc.language.isoeng-
dc.publisherThe Graduate School, Ajou University-
dc.rights아주대학교 논문은 저작권에 의해 보호받습니다.-
dc.titleDevelopment of Solution-Processed High Performance Light-Emitting Devices Based on Colloidal Quantum Dots-
dc.typeThesis-
dc.contributor.affiliation아주대학교 일반대학원-
dc.contributor.department일반대학원 에너지시스템학과-
dc.date.awarded2014. 2-
dc.description.degreeDoctoral-
dc.identifier.localId609806-
dc.identifier.urlhttp://dcoll.ajou.ac.kr:9080/dcollection/jsp/common/DcLoOrgPer.jsp?sItemId=000000016074-
dc.description.alternativeAbstractWe developed methods of combining semiconductor quantum dots (QDs) and organic and inorganic metal oxides in solution–processed light–emitting devices (LEDs). Several distinct QLED structures fabricated with many types of QDs were studied. In addition to controlling the concentrations of holes and electrons within a multilayered device, which determines the luminance and efficiency of QLEDs, using metal oxide layers is advantageous for fabricating QLEDs showing long–term stability because metal oxides are stable in air and prevent the water vapor and oxygen in the ambient air from permeating into the QD–emission layer. Moreover, the wet–chemistry processing required for manufacturing QLEDs makes metal oxide layers attractive for manufacturing QLEDs at low cost and/or on a large scale. As processing methods of QDs depend on the properties of surface groups consisting of the organic–capping ligand surrounding the QDs, these surface groups must also be treated to form close–packed, highly conducting QD layers in order to improve device efficiency. Several QD and structure types enable us to fabricate QLEDs showing various colors and efficiencies for various technological applications. In addition, we developed various methods of fabricating patterned structures that can be integrated into QLEDs to improve light outcoupling efficiency. We determined the physical operation of the QLEDs by performing electrical and optical measurements and morphological analysis, which further provides fundamental physical understanding of the interactions among inorganic, organic, and QD semiconductors and of design and selection of material for QLEDs.-
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