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Title Page

Abstract

Contents

Chapter 1. Introduction 18

1.1. Metal Halide Perovskites (MHPs) 18

1.1.1. MHPs 18

1.1.2. Characteristics of MHPs 19

1.2. Perovskite Light-Emitting Diodes (PeLEDs) 24

1.2.1. History of PeLEDs 24

1.2.2. Strategy for Enhancing the Efficiency of PeLEDs 26

1.3. Flexible PeLEDs 30

1.3.1. Flexible Electrodes 30

1.3.2. Flexible PeLEDs 32

1.3.3. Mechanical Properties of Multi-layer PeLEDs 36

1.4. Reference 42

Chapter 2. Flexibility of Semitransparent Perovskite Light-Emitting Diodes Investigated Based on Tensile Properties of Perovskite Layer 49

2.1. Research background 49

2.2. Experimental 50

2.3. Results and discussion 53

2.4. Conclusion 62

2.5. Reference 64

Chapter 3. Highly Efficient Flexible Perovskite Light-Emitting Diodes Using Modified PEDOT:PSS Hole Transport Layer and Polymer-Silver Nanowires Composite Electrode 69

3.1. Research background 69

3.2. Experimental 70

3.3. Results and discussion 72

3.4. Conclusion 86

3.5. Reference 87

Chapter 4. Effects of Perovskite Film Morphology on the Efficiency and Flexibility of Perovskite Light-Emitting diodes 93

4.1. Research Background 93

4.2. Experimental 94

4.3. Results and Discussion 96

4.4. Conclusion 105

4.5. Reference 106

Chapter 5. Summary 109

Curriculum Vitae 112

List of Tables

Table 1.1. Linewidth parameters for perovskites and inorganic semiconductors 21

Table 2.1. Summarized electrode performance of various thickness of PDZ. 56

Table 2.2. Summarized electrode performance of PFN/AgNW. 56

Table 2.3. Device performance parameters of perovskite LEDs with and without PFN. 58

Table 2.4. Summarized mechanical properties of constituent materials. 60

Table 3.1. Summarized Device Performance Parameters According to Different Concentrations of... 77

Table 3.2. Summarized Lifetime Parameters of Quasi-2D Perovskite Films on Various HTL Layers. 79

Table 3.3. Summarized device performance using various types of electrodes. 82

Table 3.4. Device Performance Parameters of Flexible PeLEDs Using Various Flexible Electrodes. 83

Table 4.1. Summarized Device Performance Parameters of PeLEDs Using Perovskite-Polymer... 97

Table 4.2. Summarized Lifetime Parameters of Perovskite-Polymer Composite Films Depending on... 101

Table 4.3. Summarized Device Performance Parameters of Flexible PeLEDs Using Perovskite-Polymer... 105

List of Figures

Figure 1.1. Schematic of perovskite crystal structure. 18

Figure 1.2. Calculated tolerance and octahedral factors for various combinations of organic and... 18

Figure 1.3. FWHM emission spectra of organic-based LEDs, QD-based LEDs, and perovskite LEDs 19

Figure 1.4. Emission from CsPbX₃ perovskites (red line) plotted on CIE chromaticity coordinates and... 20

Figure 1.5. FWHM of the steady-state PL spectra as a function of temperature for (a) FAPbI₃, (b) FAPbBr₃... 21

Figure 1.6. (a) Bonding/anti-bonding orbitals in APbX₃ demonstrating the formation of energy bands relative... 22

Figure 1.7. Schematics of (a) defect formation energy of AMX₃ perovskite and (b) electronic band... 23

Figure 1.8. The grain size distribution of MAPbBr₃ nanograin layers of (A) 1:1.05, (B) 1:1, (C) 1.05:1... 24

Figure 1.9. EQE developments depending on the active materials of light-emitting diodes. 25

Figure 1.10. Schematics of general structure of PeLEDs. 26

Figure 1.11. Multi-phase perovskite channel energy across inhomogeneous energy landscape,... 29

Figure 1.12. Chemical structure of PEDOT:PSS polymer chains 30

Figure 1.13. (a) The change in the electrical properties and morphology of PEDOT:PSS with the... 31

Figure 1.14. SEM images of AgNWs produced by the thermal welding process. 32

Figure 1.15. Schematics of the fabrication process of printed PeLEDs (Left). Cross-sectional SEM... 33

Figure 1.16. (a) Schematics of the flexible QD PeLEDs; (b) Light-emitting photographic images of the... 34

Figure 1.17. (a) Changes in current density under cyclic bending of the Gr-PeLEDs and ITO-based... 34

Figure 1.18. (a) Light-emitting images of the stretchable PeLEDs with the EInGa top electrode; (b and... 35

Figure 1.19. (a) Optical transmittance spectrum of a flexible AgNW substrate (inset: photographic... 36

Figure 1.20. The effects of repeated bending deformation of a multi-layer device. 37

Figure 1.21. Surface contours of the Young's modulus of MAMI₃ (M＝Sn, Pb) perovskite compounds. 37

Figure 1.22. Schematic of single-crystal elastic modulus measurement. (1) Excitation is achieved using... 38

Figure 1.23. (a) Typical load-displacement curves of various facets of MAPbX₃ (X＝I, Br and Cl); (b)... 39

Figure 1.24. Multi-layer device with n layers. 40

Figure 1.25. Schematic diagram of the stress states for a specimen under bending. The extrados and... 40

Figure 2.1. Schematic of process for fabricating samples for hole-nanoindentation measurements. 52

Figure 2.2. SEM image of hole-nanoindentation sample before and after testing. 52

Figure 2.3. SEM image of MAPbBr₃ sample used for tensile test attached to push-to-pull device. 53

Figure 2.4. Schematic of process for fabricating flexible and semitransparent perovskite LEDs. 53

Figure 2.5. Schematic diagram, energy band diagram, and photographs of flexible and semitransparent... 54

Figure 2.6. XRD and SEM analysis. (a) XRD pattern and (b) SEM image of MAPbBr3 film on PDZ. 55

Figure 2.7. Optical characteristics of (a) PDZ anode and (b) AgNWs cathode. 55

Figure 2.8. Optical characteristics of perovskite LEDs. (a) Photographs and (b) transmittance spectrum... 56

Figure 2.9. Cyclic bending test. Changes in sheet resistances of (a) PDZ and (b) PFN/AgNW layers... 57

Figure 2.10. Device performance parameters of perovskite LEDs with and without PFN... 57

Figure 2.11. Cyclic bending test of the perovskite LEDs. (a) Side-view images of emissive the... 58

Figure 2.12. (a) Change in luminance of PeLEDs after cyclic bending deformation with rb of 1.0 mm...[이미지참조] 59

Figure 2.13. Hole-nanoindentation tests performed on PDZ, MAPbBr3, and SPW-111 layers... 60

Figure 2.14. In situ microtensile test of perovskite MAPbBr3. (a) Typical stress-strain curve for the... 61

Figure 3.1. (a) Schematic diagram of fabricated flexible PeLEDs (NOA 63 substrate/composite... 73

Figure 3.2. Cross-sectional TEM image of composite electrode. 74

Figure 3.3. (a) The absorption spectra of 3D FAPbBr3 and quasi-2D perovskites. (b) XRD patterns of... 74

Figure 3.4. AFM images of (a) PEDOT:PSS HTL and (b) Zonyl-treated PEDOT:PSS HTL (1 wt.% of... 75

Figure 3.5. Device performance parameters of PeLEDs according to different concentrations of Zonyl... 76

Figure 3.6. PL spectra of quasi-2D perovskite films on PEDOT:PSS films with various concentrations... 78

Figure 3.7. Contact angles of water droplet onto PEDOT:PSS films with Zonyl of (a) 0 wt.%, (b) 0.5... 79

Figure 3.8. UPS data of PEDOT:PSS films with Zonyl of (a) 0 wt.%, (b) 0.5 wt.%, (c) 1.0 wt.%, and... 80

Figure 3.9. Chemical structures of PEDOT:PSS and Zonyl FS-300. 80

Figure 3.10. (a) Schematics of the manufacturing process of the composite electrode. AFM images and... 81

Figure 3.11. Transmittance spectra of H2SO4-treated PEDOT:PSS electrode, AgNWs electrode,... 82

Figure 3.12. (a) Change of sheet resistance under cyclic bending of the composite electrode and... 84

Figure 3.13. SEM images of composite electrode (a) before bending and (b) after 100 cycles bending... 85

Figure 4.1. (a) Device structure and (b) energy level diagram of PeLEDs (ITO / MPS-TEA / perovskite-... 97

Figure 4.2. (a) Absorption spectra and (b) XRD patterns of perovskite film without PMMA and... 98

Figure 4.3. (a) SEM images and (b) schematic illustrations of morphological change of perovskite-... 98

Figure 4.4. Schematic illustrations of perovskite crystal growth with PMMA. 99

Figure 4.5. (a) PL spectra and (b) time-resolved PL signals of PL images of perovskite-polymer... 101

Figure 4.6. Top-view of SEM images of perovskite-polymer composite layer before and after 100 cyclic... 102

Figure 4.7. Change in luminance of flexible PeLEDs using perovskite and perovskite-polymer... 103

Figure 4.8. Changes in sheet resistance of (a) AgNWs and PEDOT:PSS composite flexible electrode... 104

Figure 4.9. (a) J-V, (b) L-V, (c) EQE-V, and (e) the EL spectra of flexible PeLEDs using AgNWs and... 104

초록보기

Metal halide perovskites (MHPs) have been intensely investigated as emissive active materials of light-emitting diodes (LEDs) due to their high color purity with a narrow full-width at half maximum (FWHM), high photoluminescence quantum yields (PLQYs), a wide range of tunable emission and low-cost solution processing. Recently, several strategies, including defect passivation of perovskite, reduction in perovskite grain size, and introduction of quasi-2D perovskite, have been employed to enhance the efficiency of perovskite LEDs (PeLEDs). In addition, with the increasing interest in wearable electronic devices, flexible PeLEDs are being actively researched.

Considering the flexibility of PeLEDs, perovskite may show a lower elastic and ductility compared to conventional organic materials due its high crystallinity. Until now, the study for flexibility of PeLEDs has been studied mainly regarding the cyclic bending tests, and the flexibility of the perovskite layer has not been investigated systemically. In addition, most studies on the intrinsic properties of the perovskite have been focused on the single crystal perovskite. However, polycrystalline perovskite, which shows different mechanical property from the single crystalline perovskite, is used in most PeLEDs. Thus, the mechanical property of polycrystalline perovskite used in PeLEDs should be investigated by the tensile test. Moreover, the critical bending radius (rc), which shows a limitation of mechanical flexibility of PeLEDs, can be calculated from the elastic limit of the weakest layer among constituent materials of PeLEDs and the distance from the neutral plain.

One of the methods for enhancing the mechanical flexibility of perovskite layer is to fabricate a composite layer of perovskite and polymer. The polymer matrix can serve as elastic connector and enhance the flexibility. Moreover, optical and electrical properties of the PeLEDs with perovskite-polymer composite layer can be improved by fabricating uniform film with high surface coverage, and passivated small grain boundary with less defects.

In chapter 2, we demonstrate flexible and semitransparent perovskite LEDs using MAPbBr₃ perovskite emissive layer with flexible and semi-transparent poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) anode and Ag nanowire cathode. Moreover, intrinsic mechanical property of each layer including polycrystalline MAPbBr₃ layer, charge transport layers, and flexible electrodes was investigated using cyclic bending test and in situ hole-nanoindentation test and it was confirmed that mechanical failure occurs in the MAPbBr₃ perovskite layer during cyclic bending. Tensile properties of the MAPbBr₃ layer explain the critical bending radius (rc) of the PeLEDs on the order of 1 mm.

In chapter 3, we enhance device efficiency of PeLEDs by introducing an insulating fluorosurfactant, Zonyl FS-300 (Zonyl), into the PEDOT:PSS HTL, reducing the exciton quenching and energy barrier at the PEDOT:PSS/quasi-2D perovskite interface. Moreover, quasi-2D perovskite as emissive layer instead of 3D perovskite was used to obtain enhanced optical property. As a result, a flexible PeLEDs with a polymer-silver nanowire composite electrode is demonstrated, which show a maximum external quantum efficiency (EQEmax) of 3.98 %, and this is maintained even after 1,000 cycles of bending with a 2.5 mm bending radius.

In chapter 4, we report a simple method to enhance the mechanical flexibility of perovskite layer as well as the device efficiency of PeLEDs by fabricating composite layer of perovskite and poly(methyl methacrylate) (PMMA). Perovskite-polymer composite layer remarkably enhances the device efficiency through passivation of defect sites, and optimum morphology control. Highly bright and efficient PeLEDs with perovskite-polymer composite layer are realized showing a maximum luminance (Lmax) of 11,000 cd m-2 and an EQEmax of 16.08 %. In addition, the addition of polymer to perovskite improves the morphology of the perovskite film and increases the mechanical flexibility of the perovskite layer. Finally, device performance of flexible PeLEDs with perovskite-polymer composite layer is maintained even after 1,000 cycles of bending with a 1 mm bending radius.

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