臺灣博碩士論文加值系統

摘要

高分子發光二極體(PLED)是近年來共軛高分子最具有工業應用發展潛力的項目之一。與小分子發光二極體比較，具有製程簡單，低成本，可大面積化，可做撓曲性面板，輕薄化等優點。但目前遇到操作電壓高，壽命短，發光效率低，電極介面接合較差等問題。
現今顯示器市場廣大，且薄膜電晶體驅動的高分子發光二極體已成為相當重要的發光組件之一，為使薄膜電晶體與高分子發光元件的製程能進一步整合，本論文探討的主題是利用n-型非晶矽氫及摻雜梯度非晶碳化矽氫薄膜材料作為陰極電極與高分子發光層之間的電子注子層，以降低電子注入的能障層而提高電子注入率。另外，我們亦採用很薄的p-型非晶矽氫及摻雜梯度非晶矽碳化矽氫充當 ITO 透明陽極與高分子發光層之間的緩衝層，以緩衝電洞移動率，而此緩衝層也可阻擋陽極中的銦(In)滲透到高分子發光層所引發的劣化現象對元件特性不良的影響。
加入最佳厚度的非晶質電子注入層及很薄緩衝層的PLED結構，在電流密度為 300 mA/cm2 時，本元件的最高亮度可9350 cd/m2，頻譜峰值為579 nm，半高寬為36 nm，發出橘光。

Abstract

In order to improve the electroluminescence (EL) properties of polymer light-emitting diodes (PLEDs) with the increased of electron injection efficiency and balanced hole injection, the thin doped composition-graded (CG) n-a-SiC:H and p-a-SiC:H films were employed as the electron injection layer (EIL) and hole buffer layer in the poly(2-methoxy-5-(2’ethyl-hexoxy)-1,4-phenylene-vinylene (MEH-PPV) polymer PLEDs. Also, surface modification of indium-tin-oxide (ITO) electrode by oxygen-plasma treatment was used in this work. By using the above techniques, the electroluminescence (EL) threshold voltage of a PLED could be reduced and its brightness enhanced. The achieved brightness of the best device was 9350 cd/m2, and its EL threshold voltage was 4.2 V.

Contents

Abstract………………………………………………………………(IV)
Table Captions……………………………………………………… (Ⅴ)
Figure Captions………………………………………………………(Ⅵ)


Chapter 1 INTRODUCTION……………………………… 1


Chapter 2 EXPERIMENTAL PROCEDURES……………… 4

2.1 Thermal Evaporation System… ……………… 4

2.2 Spin Coating System………… ………………… 4

2.3 Preparation of Amorphous Thin-Films………… 5

2.3.1 Deposition system………………………… 5

2.3.2 Deposition of a-SiC:H Film … …………… 5

2.4 Characteristics of Polymer Material…………… 8

2.5 Device Synopsis………………………………… 9

2.6 Device Fabrications…………………………… 14

2.7 Measurement Techniques……………………… 21

2.7.1 Optical bandgap of amorphous films……… 21

2.7.2 Resistivity………………………………… 21

2.7.3 EL intensity and Brightness……………… 22

2.7.4 EL Spectrum……………………………… 22


Chapter 3 RESULTS and DISCUSSION……………………28

3.1 Effect of Oxygen-Plasma Treatment on ITO… 28

3.2 Design Considerations and Characteristics of various PLED's………………………………… 28

3.2.1 Constant bandgap n-a-SiC:H EIL for PLED's………………………………28

3.2.1.2 Effect of thickness of n-a-SiC:H EIL on
PLED (Devices 1, 2, 3, and 6)..................29

3.2.2 Composition-graded (CG) n-a-SiC:H EIL
for PLED (Device 4)…………………… 30

3.2.3 Comparison of Devices 3, 4, and 5……… 30

3.2.4 Comparison of Devices 6 and 7………… 34

3.2.5 PLED with CG n-a-SiC:H EIL and p-a-SiC:H buffer layer (Device 8)……… 34

3.2.6 Comparison of the high performance of PLEDs (Devices 4, 5, and 8………35

3.3 Current-Conduction Mechanism……………… 39

3.3.1 Ideality factor……………………………39

3.3.2 Low Electric Field Region………………… 39

3.3.3 High Electric Field Region………………… 40

3.4 EL Spectrum…………………………………… 44



Chapter 5 CONCLUSION……………………………………… 49



REFERENCES …………………………………………………… 51


Abstract

In order to improve the electroluminescence (EL) properties of polymer light-emitting diodes (PLEDs) with the increased of electron injection efficiency and balanced hole injection, the thin doped composition-graded (CG) n-a-SiC:H and p-a-SiC:H films were employed as the electron injection layer (EIL) and hole buffer layer in the poly(2-methoxy-5-(2’ethyl-hexoxy)-1,4-phenylene-vinylene (MEH-PPV) polymer PLEDs. Also, surface modification of indium-tin-oxide (ITO) electrode by oxygen-plasma treatment was used in this work. By using the above techniques, the electroluminescence (EL) threshold voltage of a PLED could be reduced and its brightness enhanced. The achieved brightness of the best device was 9350 cd/m2, and its EL threshold voltage was 4.2 V.







Table Captions

Table 1. Comparison of Organic and Polymer Materials………………3

Table 2. Deposition conditions and Eopt’s of various amorphous films..10

Table 3. The summaries of EL characteristics for all fabricated devices....
…………………………………………………………………45














Figure Captions

Fig. 2-1 The PECVD system with a stainless steel mesh attached to
upper (cathode) electrode…………………………………… 6

Fig. 2-2 Variation of optical bandgap of i-a-SiC:H with carbon hydride gas fraction (C/(C + Si)) in C2H2-SiH4, C2H4-SiH4 and CH4-SiH4 gas mixtures………………………………………..7

Fig. 2-3 (a) Synthesization, and (b) Spectral analysis of MEH-PPV.....11

Fig. 2-4 (a) The basic structure, and (b) schematic optical bandgap

diagram of a PLED under forward-bias.……………………...12

Fig. 2-5 The schematic optical bandgap diagram of a heterostructure
PLED………………………………………………………..13

Fig. 2-6 (a) The schematic cross-section of n+-a:SiC:H PLED, and

(b) the corresponding optical bandgap diagram under forward -bias……………………………………………………….17

Fig. 2-7 (a) The schematic cross-section of the PLED with Composition

(carbon)-graded n+-a-SiC:H EIL, accompanied with the gas

flow-rates diagram, and (b) the schematic optical bandgap

diagram of the PLED under forward-bias……….…………...18

Fig. 2-8 (a) The schematic cross-section of the PLED with

Composition (carbon)-graded p+-a-SiC:H buffer layer PLED

(Device 7), accompanied with the gas flow-rates diagram, and

(b) the schematic optical bandgap diagram of the PLED under

forward-bias…………………..................................................19

Fig. 2-9 (a) The schematic cross-section of of the PLED with

Composition (carbon)-graded n+-a-SiC:H EIL and p+-a:SiC:H

buffer layer PLEDs, and (b) the corresponding optical bandgap

diagram for the PLED under forward-bias…………………...20

Fig. 2-10 The setup of UV/VIS/NIR spectrometer for measuring optical

bandgap of films……………………………………………...23

Fig. 2-11 Plot of (αhv)1/2 vs. photon energy (hv) for a MEH-PPV polymer film…………………………………….……………………..24

Fig.2-12 The schematic diagram of four point probe measurement setup.

The outer two probes force a current through the sample; the

inner two probes were used to measure the voltage drop…….25

Fig. 2-13 Schematic diagram of EL intensity measurement setup……...26


Fig. 2-14 Setup for measuring EL spectrum of PLED……………….....27

Fig. 3-1 B-V curves of the PLEDs with various constant bandgap n-a-SiC:H EIL thickness….…………………………………..31

Fig. 3-2 The current density and brightness versus applied voltage curves for Device 4…………………………………………...32

Fig. 3-3 Comparison of B-V for Devices 3, 4, and 5. The n-a-SiC:H layers in these devices were all deposited at 70。C substrate temperature…………………………………………………...33

Fig. 3-4 B-V curves for Devices 6 and 7………………………………36

Fig. 3-5 The current density and brightness versus applied voltage curves for Device 8…………………………………………...37

Fig. 3-6 B-V curves for the Devices 4, 5 and 8………………………..38

Fig. 3-7 Plots of semi-logarithmic J-V curves for devices 4 and 8, where
η is the ideality factor………………………………………...41

Fig. 3-8 Plots of log (J) versus log (V) for Device 4…………………..42

Fig. 3-9 Plots of log (J) versus log (V) for Device 8…………………..43

Fig. 3-10 Normalized EL spectra of (a) Device 3, and (b) Device 4……46

Fig. 3-11 Normalized EL spectra of (a) Device 5, and (b) Device 6……47

Fig. 3-12 Normalized EL spectra of (a) Device 7, and (b) Device 8……48

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