Experiments of temperature-dependent AS measurement

Chapter 2 Experimental Details and Analytical Methods

2.11 Experiments of temperature-dependent AS measurement

The AS experiments in this thesis were measured in Prof. Jenn-Fang Chen’s laboratory (Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan). The admittances of the devices were measured using HP 4192A LF impedance analyzer. The amplitude of the test signal was 50 mV, which was superimposed on the operating DC bias, and the measurement frequency is in the range from 100 Hz to 10 MHz. All the measurements were carried out in the dark and shielded environment.

In our experiments, the samples were prepared by thermal evaporation on ITO substrates in ULVAC SOLCIET coater. During the film deposition, the chamber pressure was maintained at 10^-6 torr and the evaporation rate was monitored by quartz crystal sensor. After thermal evaporating, the samples were hermetically sealed and immediately housed inside a vacuum cryostat for AS measurements. The temperature-dependent experiments were measured under

vacuum and controlled by LakeShore 330 temperature controller.

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Chapter 3 Development of Di-Chromatic White Organic Light-Emitting Diodes

3.1 Introduction

Solid-state organic devices are at the vanguard of a generation of electronic components that promise to be as easily manufactured as colorful magazines and newspapers. These common printed products are produced using roll-to-roll technologies, where continuous rolls of paper measuring several feet in diameter are fed into machines to be cut, pressed, dyed, and packaged. Similar mass-production techniques for organic electronics may eventually replace traditional semiconductor batch processes, and thereby allow electronics to compete with well-established and inexpensive devices such as the incandescent bulb, and electronic identification tags for low-cost and disposable products.

One device that is on the cusp of widespread use is the OLED. Research over the last twenty years has paved the way for the implementation of efficient blue, green, and red OLEDs in passive and active matrix displays.

Low-information-content OLED displays fabricated by Philips, TDK, Nippon Seiki, Sanyo, and Pioneer have already been commercialized, and various OLED displays like portable smart phones, digital cameras, photo frames, small to medium size TVs have been demonstrated and some of which have already been introduced in the marketplace by Samsung, LG, Sony, AUO, CMEL and Lumiotec.

Interest in the application of WOLED technology for general solid-state lighting applications and FPD backlights is also steadily increasing. Coupled to

the increase in published work in the area of WOLEDs, the power efficiency of WOLEDs has steadily increased over the last ten years and has attained a level requisite WOLED acceptance into lighting market; hence, there is a greater appreciation for potential of energy saving, thin, flexible WOLEDs to replace traditional incandescent white light sources.

3.2 Characteristics of white light

White light has three main characteristics: (1) CIEx,y coordinates; (2) color temperature; and (3) color rendering index (CRI). However, to be meaningful, both the color temperature and CRI should be quoted as the CRI is measured relative to a reference of a given color temoerature.

3.2.1 CIE 1931 chromaticity diagram

The emission color of OLED devices can determined and differentiated by CIE 1931 chromaticity diagram, created by Commission Internationale de l'Eclairage in 1931 [1]. As shown in Figure 3-1, the diagram represents all of the chromaticities visible to the average person. These are shown in color and this region is called the gamut of human vision. The gamut of all visible chromaticities on the CIE plot is the horseshoe-shaped figure shown in color.

The curved edge of the gamut is called the spectral locus and corresponds to monochromatic light, with wavelengths listed in nanometers. The straight edge on the lower part of the gamut is called the line of purples. These colors, although they are on the border of the gamut, have no counterpart in monochromatic light. Less saturated colors appear in the interior of the CIE chart and the white point is at the center and is defined as (0.33, 0.33).

The major disadvantage of this diagram is the color difference is not in average, the green area is much larger than other color on the chart. So it is

difficult to tell the amount of color difference directly from the diagram.

Figure 3-1 CIEx,y chromaticity diagram. All the colors in the visible spectrum lie within or on the boundary of this diagram. The internal arc is the Planckian locus, which is the plot of the coordinates of black body radiation at the temperatures shown, described as color-correlated temperatures.

3.2.2 Color temperature

The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that light source.

The temperature is conventionally stated in units of absolute temperature, kelvin (K). Color temperature is related to Planck’s law [2] and Wien’s displacement law [3]. Higher color temperatures (5000 K or more) are called cool colors (blueish white color); lower color temperatures (2700-3000 K) are called warm

colors (yellowish white through red color).

However, most light sources, such as incandescent bulb, inorganic light-emitting diodes and even OLEDs are not a real and ideal black-body radiator, thereby, these nearly-Planckian light sources can be judged by the correlated color temperature (CCT), which the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions. As shown in Figure 3-1, CCT can be estimated from the lines project out from the Planckian locus in CIE 1931 chromaticity diagram.

3.2.3 Color rendering index

The color rendering index (CRI) is a numerical measurement of how true colors look when viewed with the light source and can be determined from the output spectrum of the light source. A black-body radiator has a maximum CRI value of 100. Typically the CRI value is higher than 80 for a high quality light source. Moreover, for commercial viability, color temperature and CRI of a light source need to be stable over the source lifetime [4,5].

3.3 Performances of state-of-the-art WOLEDs

Since the first reports of white-emitting devices by the group of Kido [6,7], many approaches to generating white light from organics have been described in the literature. The initial devices exhibited efficiencies of < 1 lm/W, but this value has grown to > 50 lm/W over the last few years [8]. The challenges facing WOLED technology are due, in large part, to the fact that fluorescence or phosphorescence emission from typical organic materials only spans about one third of the visible spectrum. Color tuning molecules to emit in the blue, green, or red portion of the visible spectrum can be accomplished with a variety of molecular structures and their derivatives, however, a single molecule has not

been designed that efficiently emits over the entire visible spectrum, such that high-quality white light is produced. Given the limited spectral bandwidth of single dopants, there are numerous WOLED architectures that combine the emission from multiple dopants. White emission from OLEDs can now be achieved in both small molecule and polymer systems and some review articles which focus on thermally evaporated small molecules and solution-processed polymers have been published [9,10,11].

In recent years, WOLEDs are being considered as practical solid-state light sources and could play a significant role in reducing global energy consumption.

The detailed performances of state-of-the-art WOLEDs of some leading companies in the world are summarized in Table 3-1.

Table 3-1 Performances of state-of-the-art WOLEDs

Company Size

In our lab, the conventional di-chromatic white OLED device is composed of

p-bis(p-N,N-diphenyl-aminostyryl)benzene

(DSA-Ph) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) sky-blue system [15] and 5,6,11,12-tetraphenylnaphthacene, known as rubrene [16] (abbreviation as Rb in this thesis), as the yellow emitter (device architecture is illustrated in Figure 3-2). By making use the bipolar characteristic of Rb molecular [17], doping RB

molecules into the hole transport layer of

N,N’-bis(1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’- diamine (NPB) in

sky-blue device can directly obtain a di-chromatic white emission.

Figure 3-2 Conventional WOLED device architecture and molecular structures of key materials.

After the optimization of layer thickness and doping concentration, the structure of conventional white device is ITO/CFx/NPB (50 nm)/NPB: 1.2% Rb (20 nm)/MADN: 3% DSA-Ph (40 nm)/aluminum tris(8-hydroxyquinoline)

(Alq3, 10 nm)/lithium fluoride (LiF, 1 nm)/Al (200 nm). As shown in Figure 3-3(a), this white device shows a broad EL spectrum in visible region with two distinct peaks, clearly indicating the emissions of DSA-Ph and Rb at 456, 476, 520, and 620 nm, respectively. The device can achieve an EL efficiency of 9 cd/A with CIEx,y coordinates of (0.32, 0.41) at 6.4 V and 20 mA/cm². Moreover, the color stability should be also considered for a white device, because human eyes can sensitively tell the difference when the CIEx,y color change (ΔCIEx,y) is larger than ±(0.04, 0.04). As shown in Figure 3-3(b), the color shift (ΔCIEx,y) of this white device is only of ±(0.02, 0.01) under the wide range of 2 to 300 mA/cm², indicating the emission color of this white device is stable.

Figure 3-3 (a) EL spectrum at 20 mA/cm² and (b) CIEx,y coordinates vs current density characteristics of conventional WOLED device.

When the device is attached with color filter (provided by Allied Material Technology Corp.), the CIEx,y coordinates of RGB and white colors are (0.64, 0.35), (0.28, 0.60), (0.11, 0.17), and (0.35, 0.38), respectively [see Figures 3-4(a) and 3-4(b)]. For full-color display applications, the color gamut should be considered. Color gamut is the range of color a display can reproduce and is

(a) (b)

commonly expressed as percentage of National Television System Committee (NTSC) specification [(0.67, 0.33), (0.21, 0.71), (0.14, 0.08), and (0.310, 0.316) for RGB and white points, respectively] as shown in Figure 3-4(c), 100% of NTSC refers to the full range of color that can theoretically be displayed.

Usually, CRTs is about 70% of NTSC.

Figure 3-4 (a) The transmittance of RGB color filters and EL spectrum of conventional WOLED device. (b) Spectra of RGB colors after attaching color filters (c) The color gamut of NTSC standard and conventional WOLED device.

After calculation, the color gamut of our conventional WOLED device is 61.3% of NTSC standard, such low NTSC ratio can be attributed to this white emission is only composed of sky-blue and yellow colors, which obviously lacks

(a) (b)

(c)

of the green color at wavelength of 520 nm. In addition, the weak intensity of blue region at wavelength < 450 nm and red region at wavelength > 620 nm, also leading to the poor saturation of blue and red colors.

For portable electronics, power consumption is one important issue should be considered, which means the electronic devices should possess high brightness at low drive voltage. However, the luminance of our conventional WOLED device is only 1200 cd/m² at a drive voltage of 6 V. Therefore, it is worthy to research on how to achieve high power efficiency of WOLED devices.

In this chapter, we introduce the p-doped [tungsten oxide (WO3)-doped NPB] and

n-doped

[cesium carbonate (Cs2CO3)-doped 4,7-diphenyl-1,10-phenanthroline (BPhen)] organic layers into the device architecture of di-chromatic WOLED to achieve high power efficiency. The highly conductive p- and n-doped layers could enhance the charge injection from the contacts and reduce the ohmic losses in these layers [18]. To further reduce the drive voltage in p-i-n OLEDs, the thickness of low conductive layer based on organic materials should be as thin as possible. In addition, careful control of the location of exciton recombination zone (RZ) and the energy-transfer between the host and dopant molecules have been shown to be critical in obtaining a balanced white emission of high efficiency [19,20].

However, it is difficult for WOLEDs with multi-emission layer to achieve a stable white color due to the shift of RZ in thin organic layer, which often leads to undesired CIE_x,y

color change with respect to drive current.

3.5 Development of p-i-n di-chromatic WOLED device

In this section, we demonstrate a dual emission layer (DEML) system for

p-i-n WOLEDs in which the first emission layer is the co-dopant emitting layer

with MADN: 5% NPB: 3% DSA-Ph: 0.2% Rb and the second one is a blue emitting layer of MADN: 5% NPB: 3% DSA-Ph. Three types of p-i-n WOLED devices were fabricated as depicted in Figure 3-5. The total thickness of DEML in these p-i-n devices is only 15 nm. Device I is the p-i-n WOLED without co-deposited NPB in the DEML system while device II is the p-i-n WOLED with the DEML system. The pure NPB layer of device II was replaced with 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane (TAPC) [21] as electron-blocker in device III. In our p-i-n architecture, Cs2CO3 is co-evaporated with BPhen [22], which has a high electron mobility of 2.4 × 10^-4 cm²/Vs, as the

n-doped electron transport layer (n-ETL). BPhen also possesses a deep highest

occupied molecular orbital (HOMO) of 6.4 eV, which can effectively block hole carriers in emission layer. On the other hand, NPB doped with WO3 is used as the p-doped hole transport layer (p-HTL) [23]. In the DEML system, MADN, NPB, DSA-Ph and Rb were used as host material, assistant dopant, blue and yellow fluorescent dopants, respectively.

Figure 3-5 Schematic device architecture of p-i-n WOLEDs.

It is known that Rb with LUMO/HOMO of 3.2/5.4 eV can be a carrier trap for electrons, especially at low electric field [24], which will cause the problematic white emission color change with various drive currents in thin emission layer of p-i-n di-chromatic WOLEDs. In order to alleviate the unstable color issue associated with the carrier-trapping property of Rb, we have purposely co-evaporated Rb with low doping concentration of 0.2% and 3%

DSA-Ph in MADN, which would cause the yellow emission generated by the energy-transfer process from blue to yellow emitter.

The energy-transfer process can be demonstrated by the solid-state emission spectra depicted in Figure 3-6, the thin film composed of MADN: 3% DSA-Ph:

0.2% Rb (90 nm) emits intense yellow emission and relatively weak blue-greenish emission. It is evident that the emission of MADN around 430 nm essentially quenched and there is an effective energy-transfer characteristic from DSA-Ph to Rb, which is primarily due to the favourable spectral overlap between the emission peak of DSA-Ph and the absorption peak of Rb at 495 nm [25].

Figure 3-6 Absorption spectrum of Rb and solid PL spectra of DSA-Ph and composite thin film.

(a) (b)

Therefore, we designed and developed a DEML system by inserting a blue emitting layer of MADN: 3% DSA-Ph after the co-evaporated EML into the structure of device I to enhance the blue emission intensity. From Figure 3-7(a), the EL spectrum of device I indeed shows the enhancement of the blue emission intensity with respect to the solid PL of co-evaporated thin film. However, the yellow emission intensity is still much higher than the blue emission, leading to undesirable white emission with CIEx,y

coordinates of (0.37, 0.47). Furthermore,

a significant EL color shift of device I is observed with respect to various drive currents as the CIEx,y

coordinates is shifted from (0.410, 0.496) at 1 mA/cm

² to (0.321, 0.419) at 100 mA/cm² with ΔCIEx,y = ±(0.089, 0.077) as shown in Figure 3-7(b), in which the yellow emission intensity decreases with the increasing current density. We inferred the unstable EL color is due to the RZ shifts towards the blue emitting layer under high current stress.

Figure 3-7 (a) Solid PL spectrum of composite film and EL spectra of device I at 20 mA/cm². (b) CIE_x,y coordinates vs current density characteristics of device I.

Therefore, we co-deposited 5% NPB in the DEML system of device II as the assistant dopant with the purpose of shifting the RZ towards the blue

emitting layer to balance the blue and yellow emission intensity under low current density. Figure 3-8(a) reveals that device II also has a better J-V characteristic than device I which indicates that NPB molecules play an important role in enhancing the hole-transport in the DEML system. In addition, the EL spectrum of device II depicted in Figure 3-8(b) shows a more balanced white emission with CIEx,y of (0.33, 0.43) at 20 mA/cm² and the relative intensity of blue light has been increased as compared with device I. This phenomenon exhibits the co-deposited NPB molecules can indeed shift the RZ to the blue emitting layer and improve white CIEx,y coordinates. Moreover, the EL color shift with respect to varying drive currents has also been improved to ΔCIEx,y = ±(0.05, 0.04) from 1 mA/cm² to 100 mA/cm² as shown in Figure 3-8(c). However, the reduced relative intensity of yellow emission leads to a lower EL efficiency of 7.4 cd/A at 20 mA/cm².

Figure 3-8 (a) J-V characteristics of devices I and II. (b) Solid PL spectrum of composite thin film and EL spectra of device I and II at 20 mA/cm². (c) CIEx,y

coordinates vs current density characteristics of device I and II.

To further enhance the efficiency and improve the color stability of these

p-i-n white devices, we turn to refine the exciton confinement in device III, in

which the NPB layer was replaced by TAPC with a high LUMO energy level (2.0 eV) and high hole mobility (10^-4–10^-3 cm²/Vs) [21], which can be an effective electron-blocking as well as hole transport material. Indeed, both blue and yellow intensity of device III have been enhanced as shown in Figure 3-9(a) and its emission achieved a white CIE_x,y of (0.32, 0.43). Furthermore, the EL performance can be boosted to 9.9 cd/A and 8.2 lm/W at 20 mA/cm². It is also observed that the white emissive color becomes more stable with respect to

(a) (b)

(c)

drive current density as the EL color shift is only of ΔCIEx,y = ±(0.013, 0.009) from 1 mA/cm² to 100 mA/cm² as shown in Figure 3-9(b).

Figure 3-9 (a) EL spectra at 20 mA/cm². (b) CIEx,y coordinates vs current density characteristics of devices I, II, and III.

Detailed EL performances of these devices are summarized in Table 3-2.

The voltage, current efficiency, power efficiency, external quantum efficiency (E.

Q. E.), and color coordinates were measured at 20 mA/cm². All p-i-n white devices show a much lower drive voltage and a dramatic gain in power efficiency as compared with those of conventional white device. Device III can achieve 10 cd/A and 9.3 lm/W at 1000 cd/m²which are considerably better than those of device II with 7.3 cd/A and 6.8 lm/W. It is noteworthy that both L-V and

J-V curves of device III are steeper than those of conventional white device as

depicted in Figure 3-10. The threshold voltage of device III is around 2.9 V.

When driven at 6 V, device III can reach 10000 cd/m² which is nearly 8 times brighter than the conventional device. These results prove that the efficient exciton confinement is one of the most important factors in controlling the RZ shift under various drive currents and it is also indispensable for the

(a) (b)

development of a highly efficient and color stable p-i-n WOLED.

Table 3-2 EL performances of p-i-n white OLED devices at 20 mA/cm².

Figure 3-10 L-J-V characteristics of conventional white device and device III.

3.6 Development of tandem p-i-n di-chromatic WOLED device

For applications of WOLEDs, it is important that WOLEDs possess high brightness and EL efficiency at a lower current density and stable spectral characteristics in a wide range of injection current. In the past years, many monochrome tandem OLEDs have been reported to be useful for providing high luminance [26,27], the luminance at a fixed current density increases linearly with the number of stacked and independent OLED elements. This can lead to a

Device Voltage (V)

Current Eff.

(cd/A)

Power Eff.

(lm/W)

E. Q. E.

(%) CIE_x,y

Conventional 6.4 9.0 4.4 3.5 (0.32, 0.41)

I 4.4 9.1 7.1 3.0 (0.37, 0.47)

II 3.6 7.4 6.4 2.7 (0.33, 0.43)

III 3.8 9.9 8.2 3.6 (0.32, 0.43)

significant improvement in lifetime by reducing the degradation that accompanies the high drive currents required to achieve similarly high brightness in a single-unit device.

In this section, we consolidate both the structural features of p-i-n technique and that of the tandem device concept into one WOLED device to modify the high drive voltage issue of reported tandem WOLEDs [28,29]. The major challenge in tandem OLEDs in general is to prepare the effective connecting layer between emitting units so that the current can smoothly flow through

在文檔中以蒽為主體之非晶態材料雙極性載子傳輸特性與摻雜電性研究及其在有機電激發光元件之應用 (頁 63-0)