臺灣博碩士論文加值系統

Achieving a full light harvesting to reach up to 100% internal quantum efficiency (IQE) is of outmost important for practical application of OLEDs. This achievement can be done using exciplex formation. An exciplex can be formed between donor and acceptor molecules at the interfacial and bulk layer structure of OLEDs. To achieve highly efficient OLEDs, differences in the mechanism between this two exciplex formations need to be understood. In this work, detailed electrical properties of the interfacial and bulk exciplex emission has been studied using impedance spectroscopy and transient electroluminescence (EL) measurement. The exciplex formation was designed using donor molecule 9,9'-Diphenyl-9H ,9'H -3,3'-bicarbazole (BCzPh) and acceptor molecule 3′,3′″,3′″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1′-biphenyl]-3-carbonitrile)) (CN-T2T). The devices exhibited maximum external quantum efficiency of 7.7 % and 17.1 % and maximum power efficiency of 32.1 lm W-1 and 60 lm W-1 for interfacial and bulk exciplex-formation, respectively. More importantly, interfacial exciplex showed high interfacial charge than a bulk exciplex devices as observed in the C-V plot. The transient EL measurement showed that the decay time for the bulk exciplex was higher due to charge carrier inside the emission layer. The presence of such an emission layer can increase the recombination in the device and reduce the interfacial charge. The faster decay also verifies that charge carrier inside the emission layer of bulk exciplex was not trapped.

Table of Contents

Ming Chi University of Technology Recommendation Letter from the Thesis Advisor i
Ming Chi University of Technology Thesis/Dissertation Oral Defense Committee Certification ii
Abstract iii
Table of Contents iv
List of Figures vi
List of Tables x
Chapter 1 Introduction 1
1.1 Overview 1
1.2 Objectives 4
1.3 Outline 5
Chapter 2 Literature Review 6
2.1 Organic Light Emitting Diodes (OLEDs) 7
2.1.1 Principle and Structure of OLEDS 8
2.1.2 Charge Injection 12
2.1.3 Charge Transportation 16
2.1.4 Charge Recombination 17
2.1.5 Exciplex Formation 18
2.2 Organic Light Emitting Materials 20
2.2.1 Fluorescent Materials 20
2.2.2 Phosphorescent Materials 21
2.2.3 Triplet-Triplet Annihilation (TTA) Materials 22
2.2.4 Thermally Activated Delayed Fluorescence (TADF) Materials 23
2.3 Impedance Spectroscopy 24
2.3.1 Capacitance-Voltage Plot 25
2.3.2 Complex-Z (Cole-cole) Plot 28
2.4 Transient Electroluminescence 30
Chapter 3 Experimental Procedure 34
3.1 OLEDs Fabrication 34
3.1.1 Device Structure 34
3.1.2 Device Fabrication 36
3.2 OLEDs Materials 41
3.3 OLEDs Measurement 44
3.3.1 Optical Measurement PR-655 44
3.3.2 Impedance Spectroscopy Measurement 45
3.3.3 Transient Electroluminescence Measurement 47
Chapter 4 Result and Discussion 49
4.1 Capacitance-Voltage (CV) Plot 52
4.2 Complex Z (Cole-Cole) Plot 57
4.3 Transient Electroluminescence 64
Chapter 5 Conclusion 71
References


List of Figures

Figure 1.1 Flexible OLEDs Display. 1
Figure 2.1 Configuration of OLEDs by Tang and Van Slyke. 7
Figure 2.2 Electron spin configuration in the ground and excited states. 8
Figure 2.3 Emission mechanism of fluorescent light (singlet) (a); phosphorescence light (triplet) (b) by Jablonski diagram. 8
Figure 2.4 Single layer structure of OLEDs. 10
Figure 2.5 Double layer structure of OLEDs. 10
Figure 2.6 Multi layers structure of OLEDs. 11
Figure 2.7 Field-assisted thermal activation of an electron over the coulombic barrier. 13
Figure 2.8 Tunneling injection through the barrier. 15
Figure 2.9 Energy-level alignment vs vacuum-level shift for electron injection. 16
Figure 2.10 Langevin recombination mechanism. 18
Figure 2.11 The trap-assisted recombination process, electron capture (a); electron emission (b); hole capture (c); hole emission (d). 18
Figure 2.12 The exciplex formation emission mechanism. 19
Figure 2.13 Interfacial exciplex formation emission. 19
Figure 2.14 Bulk exciplex formation emission. 20
Figure 2.15 The possible emission of singlet and triplet exciton using fluorescent emitters. 21
Figure 2.16 The possible emission of singlet and triplet exciton using phosphorescent emitters. 22
Figure 2.17 The possible emission of singlet and triplet exciton using TTA emitters. 23
Figure 2.18 The possible emission of singlet and triplet exciton using TADF emitters. 24
Figure 2.19 Capacitance – voltage under different area. 26
Figure 2.20 The OLEDs device at condition V0 < Vt. 27
Figure 2.21 The OLEDs device at condition V = Vt. 27
Figure 2.22 The OLEDs device at condition Vt < V0 < Vbi. 27
Figure 2.23 The OLEDs device at condition V0 = Vbi. 28
Figure 2.24 The OLEDs device at condition V0 > Vbi. 28
Figure 2.25 Complex-Z (cole-cole) plot. 29
Figure 2.26 The Cole-cole plot for capacitor (a); resistor in series with capacitor (b); resistor in parallel with capacitor (c); resistor in series with parallel RC circuit (d). 29
Figure 2.27 Equivalent circuit of OLEDs using double layer model. 30
Figure 2.28 Initial time delay under pulse bias of 10µ. 31
Figure 3.1 Interface exciplex device structure. 34
Figure 3.2 Bulk exciplex device structure. 35
Figure 3.3 Photolithography process. 36
Figure 3.4 Thermal Evaporation System. 37
Figure 3.5 OLEDs fabrication process. 38
Figure 3.6 Anode mask (left); Holder (right). 38
Figure 3.7 Organic chamber (left); Metal chamber (right). 39
Figure 3.8 Normal cathode mask (28 x 18) (left); Cathode mask (29 x 19) (right). 40
Figure 3.9 Final packaging OLEDs. 40
Figure 3.10 The clean cover boat covered in aluminum foil. 41
Figure 3.11 Energy level diagram of the material used in fabricated OLEDs. 41
Figure 3.12 Molecular structure of HAT-CN. 42
Figure 3.13 Molecular structure of TAPC. 42
Figure 3.14 Molecular structure of BCzPh. 43
Figure 3.15 Molecular structure of CN-T2T. 43
Figure 3.16 PR-655 Spectrascan. 44
Figure 3.17 Keithley 2400 source meter. 44
Figure 3.18 The connection for optical measurement in PR-655. 45
Figure 3.19 XM Solartron Analytical (Material Lab). 46
Figure 3.20 The connection for impedance measurement. 46
Figure 3.21 The impedance measurement setup. 47
Figure 3.22 TimeHarp 260. 47
Figure 3.23 The connection for transient electroluminescence measurement. 48
Figure 3.24 The transient electroluminescence measurement setup 48
Figure 4.1 EQE characteristic of proposed devices. 50
Figure 4.2 Luminance-voltage-current density characteristic (J-V-L) of proposed devices. 50
Figure 4.3 Current efficiency – luminance – power efficiency characteristics of proposed devices. 51
Figure 4.4 EL spectrum of proposed devices. 52
Figure 4.5 C-V plot in the interfacial (reff) devices. 53
Figure 4.6 C-V plot in the bulk (reff) devices. 53
Figure 4.7 C-V plot in the interfacial (split) devices. 54
Figure 4.8 C-V plot in the bulk (split) devices. 55
Figure 4.9 C-V plot in the all proposed devices. 56
Figure 4.10 Equivalent circuit 58
Figure 4.11 Complex-Z (cole-cole) plot in the interfacial (reff) devices. 58
Figure 4.12 Complex-Z (cole-cole) plot in the bulk (reff) devices. 60
Figure 4.13 Equivalent circuit for bulk (reff) devices. 61
Figure 4.14 Complex-Z (cole-cole) plot in the interfacial (split) devices. 61
Figure 4.15 Complex-Z (cole-cole) plot in the bulk (split) devices. 62
Figure 4.16 Complex-Z (cole-cole) plot in the all proposed devices. 63
Figure 4.17 Normalized intensity transient EL of interfacial (reff) devices depending on, reverse bias after turn-off (a); frequency (b); at the decay part, depending on the reverse bias (c). 65
Figure 4.18 Normalized intensity transient EL of bulk (reff) devices depending on, reverse bias after turn-off (a); frequency (b); at the decay part, depending on the reverse bias (c). 66
Figure 4.19 Normalized intensity transient EL of interfacial (split) devices depending on, reverse bias after turn-off (a); frequency (b); at the decay part, depending on the reverse bias (c). 67
Figure 4.20 Normalized intensity transient EL of bulk (split) devices depending on, reverse bias after turn-off (a); frequency (b); at the decay part, depending on the reverse bias (c). 68


List of Tables

Table 3.1 Device configuration of testing devices. 36
Table 4.1 The device performances of testing devices. 49
Table 4.2 C-V plot parameter. 55
Table 4.3 Parameter of equivalent circuit interfacial (reff) device 59
Table 4.4 Parameter of equivalent circuit bulk (reff) device. 60
Table 4.5 Parameter of equivalent circuit interfacial (split) device. 61
Table 4.6 Parameter of equivalent circuit bulk (reff) device. 62
Table 4.7 Decay time of all purposed devices. 69

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