Chapter 1 Introduction
1.6 Organization of this dissertation
The dissertation is organized as following: The review of OLED technology is presented in Chapter 2. The physical mechanism, fabrication process, full-color technologies and lifetime issues are described in this chapter. In Chapter 3, the basic analysis of passive-matrix addressing scheme is described to derive the limitation of PM-OLED. Following is the fundamental design principle of conventional voltage-driven AM-OLED. To suppress the issues of conventional voltage-driven AM-OLED due to the parasitic resistance effect, AC driving scheme, which can yield
high luminance uniformity and long device lifetime, is shown in Chapter 4. Due to inherent characteristics variations of poly-Si TFT that result in inferior luminance uniformity, using current-driven scheme with full-integrated driver circuitries for enhancing the image uniformity are demonstrated in Chapter 5. In this chapter, different designs of pixel circuit are detailed to improve the long programming time issue induced by current-driven scheme. Moreover, the design for peripheral driver system including current-type DAC, and high-speed current memory are also described. An modified current-driven pixel electrode circuit with current scaling function which can enhance the data programming speed is discussed in Chapter 6.
In Chapter 7, the charge-sensing approach with the modified pixel schematic is presented to easily enhance the functionality testability of AM-OLED. Finally, discussions and summary of this dissertation, and recommendations for the future works are given in Chapter 8.
Reference
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Chapter 2
OLED - Organic Light Emitting Device
2.1 Introduction
During the last fifty years, the semiconductor physics have played an important role in industrial and technical developments and are used in numerous applications, e.g. diodes, transistors and sensors. Compound materials with combinations of the III–
and V-groups of periodic system possess semiconductor properties. The principle of light generated by applying an electrical field to this semiconductor material is known as electroluminescence. A conventional LED constructed of semiconductor materials is based on this luminescence phenomenon and has been mass-produced in the last thirty years. The applications for the LED are found in several areas including optical communication and dot matrix displays because of their ability to emit intensive light.
In spite of the fact that the organic material carbon (C) is positioned in the IV-group as the semiconductors materials, it is considered as an isolator. Nevertheless, Hideki Shirakawa discovered a conducting ability of the organic material in 1977
[1][2][3][4]
for his the Nobel Prize in chemistry in 2000 and started a new era of the semiconductor technology for new applications.
In 1987, the group of Tang and Van Slyke at Eastman-Kodak presented luminescence by an organic material [5]. The group constructed an efficient OLED, which was driven by a rather small voltage. The technique they used to emit light was vacuum deposition of small molecules to form a multi-layer thin-film structure. The layered structure is sandwiched between two electrodes: anode and cathode. The thin layer (<1 µm) of organic material permits a high electrical field at low voltage. Thus,
light from thin film organic materials is usually produced at low voltage. High quality thin molecular layers were an important condition for the development of OLED.
In 1990, a group at the Cavendish Laboratories at Cambridge University presented the first LED using polymers as active material. The active polymer was a conjugated polymer [6]. In comparison to the traditional solid state LED, the new organic approaches were believed to be much more efficient. The conjugated polymers and small molecules are considered as the two classes for OLED. The two classifications of light-emitting organic layers in the OLED devices both possess luminescent and conductive properties. Small molecules have molecular structures of relatively short chain length and consequently low molecular weights. The most commonly used material with the most explored properties among the small molecules is tris-(8-hydroxyquiniline) aluminum (Alq3), as shown in Fig. 2-1(a). The conjugated polymers are composed of a long repeating chain of similar smaller molecules, called monomers. They possess consequently, compared to small molecules, a molecular weight twenty to fifty times higher. A frequently used conjugated polymer is the poly-paraphenylene vinylene (PPV), as shown in Fig.
2-1(b).
(a) (b)
Fig. 2-1. Chemical structures of (a) tris-(8-hydroxyquiniline) aluminum (Alq
3) and (b)
poly-paraphenylene vinylene (PPV).
2.2 Luminescence of organic materials
Organic materials refer to a base of the IV-group carbon and with additional elements such as hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). Since polymers and organic materials are not referred to conducting materials because of the large band gap between conduction and valence bands, a very high electrical field has to be applied. When applying an electrical field, charge carriers including hole and electron are injected from the electrodes into the organic layer respectively and result in geometrical defects on the symmetric organic structure and exhibit a lower band gap according to
Fig. 2-2. The charged carriers move along the structure and the attraction
between the carriers results in a recombination of electron and hole to form an exciton which has a possibility to emit light as photons. The exciton is either in singlet or in triplet-state according to the Pauli’s principle [7]. The exciton will form two new energy bands inside the band gap. The energy level of the exciton is below the conduction band and the released energy is emitted as photons. Upon the relaxation of exciton, heat and photons will be emitted with an energy set according to the gap between the energy bands. The states of the exciton have certain influence of the emission of light and quantum efficiency since the singlet states release its energy as an emission of photons. However the triplet-state is regarded as the heat forming state and can not transfer to light. In some special cases, the triplet-state generates light as well.The backbone of the organic materials is strongly localized bonds between the carbon atoms. The conductivity is enabled through bonds that are orthogonal to the backbone. The length of the conjugation, i.e. the conjugation length, set certain characteristics of the molecule. The conjugation length defines the length where the
electron is free to move within. In general, small molecules tend to have short conjugation length, while longer conjugated molecules, polymers, may have a longer.
Longer conjugation length results in a smaller band gap. It is therefore easier to produce red light with conjugated polymers compared to small molecules, and consequently small molecules can more easily generate blue light.
cathode anode
+
-
-+ +
hole-transporting layer
electron-transporting layer
HTL ETL
HOMO
LUMO
Fig. 2-2. Schematic energy band diagram of OLED and electron-hole recombination in forward bias condition.
2.3 The composition of OLED 2.3.1 The mono-layer device
The simplest OLED, a mono-layer device, is constructed of a thin layer of organic material sandwiched between two electrodes. As shown in Fig. 2-3(a), the function of the anode is to supply positively charged holes and a frequently used material, due to the requirement of transparency, is the Indium Tin Oxide (ITO). The cathode electrode supplies electrons to the organic layers. The charged carriers of electrons and holes are injected into the thin emitting layer material where they form an exciton and generate photons afterwards. The fact of the singlet and triplet states of the excitons has a certain influence and limitation on the device quantum efficiency.
Because of the disordered structure in organic materials, the behaviors of the charged carriers in organic materials are not similar to that in metal. This fact impedes a recombination and decreases its probability [8]. Therefore, there is always one dominant charge carrier that moves through the material without recombining with the opposite charge carrier. Another problem related to mono-layer devices is that the charge carriers tend to remain at one of the electrodes and creating space charges that prevent further hole and electron injections. Furthermore, if charges form an exciton near a metal, quenching mechanism can destroy the exciton and reduce the light generation efficiency.
+ + + + + - - - -
-glass anode cathode
emitting layer
+
glass anode cathode
emitting layer + + + +
-ETL
(a) (b)
Fig. 2-3. Configurations of OLED for (a) fundamental single-layer structure and (b) double-layer heterostructure structure.
2.3.2 The double-layer device
A minimization of the energy barrier between the electrodes and the organic material is another approach to enhance the injection of the carriers. The mono-layer device is then upgraded to a double-layer device. Instead of one organic layer, two layers, an emission and an electron-injection layer, are placed between the electrodes, as shown in Fig. 2-3(b). By choosing the materials according to their properties of mobility, band gap, the charged carriers can be injected and transported in an easier
manner into the emission zone where an emission of light takes place [9].
The band diagram of the materials can be matched better to the specific electrodes through the double-layer construction and result in improved equilibrium of the currents of holes and electrons. The electrical field can consequently be decreased, which increases the efficiency and lifetime of the device. The difference in energy levels between the two organic layers creates a potential barrier at the interface.
The barrier confines the holes and electrons and contributes to an increased recombination probability [10].
2.3.3 The multi-layer device
The functionality of the double-layer device and its components can be further improved with regard to several layers and minimal potential barriers. The greatest advantage of a multi-layer structure, compared to a double-layer, is the possibility to separate transport regions from the emitting region. This multi-layer structure accomplishes a better performance and an increased range of colors of the emitted light. By tuning the voltage over the electrodes, a proper placement of the emitting zone can be achieved, which may improve the efficiency of the device. The structure in Fig. 2-4 is an example of a multi-layer bottom-emission OLED, which has its light emitted through the transparent anode.
The substrate on which the OLED is fabricated consists of a material with a rigid or a flexible structure, e.g. glass or plastics. Compared to flexible substrates, rigid materials are of advantage for their ability to prevent the device from moisture and air.
An outer exposure of moisture and air without any protection significantly degrades the lifetime and performance of the device.
To enhance the injections from the anode, according to Fig. 2-4, a hole injection
layer (HIL) is introduced for control and an enhancement of the injection of the holes into the hole transport layer (HTL). An effective HTL of p-type material can effectively enhance the transportation of holes to the emitting zone. The excitons are expected to be formed in the emitting layer (EL) and eventually emit light. Since equilibrium of mobilities between the carriers is difficult to achieve, a blocking layer can be used to confine the charge carriers to perform a maximum recombination.
glass anode
Emitting Layer (EL)
Hole Transporting Layer (HTL) Hole Injection Layer (HIL) Electron Injection Layer (EIL) Electron Transporting Layer (ETL) cathode
Fig. 2-4. Multi-layer OLED structure with additional functioning HIL, HTL, EIL and ETL.
The electron transport layer (ETL) is of n-type doped material which can enhance the mobility of the charge carriers and the transportation of electrons to the emitting layer. The layer also functions as hole blocking and is suitable to control specific charged carries. The ETL can, similar to the HTL, be used as an emitting layer. Similar to the holes, an organic electron injection layer (EIL) is used to assist the electrons to cross the barrier between the cathode and the ETL. Due to the enhanced transportation of electrons, a low electrical field is adequate for the multi-layer device and leads to a more power efficient OLED. It is furthermore possible to use the same material for the two electrodes, yet at a reduced efficiency.
2.3.4 Modifications
A bottom-emission OLED has a transparent substrate such like the glass. A silicon backplane can be used as the substrate as well and with a different combination of the electrodes and the organic layers to form a top-emission device can be accomplished. In comparison with conventional bottom-emission OLED, the top-emission OLED presents more advantageous for some micro-displays appllications[11][12]. When using a transparent cathode in the bottom-emission OLED, the device has become a transparent OLED (TOLED). The transparency of the device then is nearly as transparent as the chosen substrate. For advanced multi-stacked OLED devices, e.g. stacked OLED (SOLED), the transparent device is required.
It is also possible to apply the OLED on a flexible material to make a bendable or foldable display, enabling a completely new era of display types, e.g. displays directly deposited on the windshield or the helmet visor. Although more research has to be done in order to make the efficient non-glass encapsulation against humidity and oxidation, flexible OLED also brings a more rugged structure due to no need for the glass substrate and their bendable nature. Since standard OLED uses a cathode of a metallic material, approximately 75% of the incident light is reflected back and causes a poor contrast when ambient light passes through the display [13]. The use of a TOLED in combination with an absorber or an optical interference structure improves the contrast, e.g. Luxell’s black layer results in a 180° phase shift to cancel out the ambient radiation [14].
2.4 Fabrication of OLED devices
The difference between the small molecules and conjugated polymers is mainly
the fabrication and patterning process. All processes need a pure deposition of the organic material on a cleaned substrate. As moisture and UV-light have certain effects on the degradation mechanism of the device during manufacturing [ 15 ][ 16 ]
, the processes for either small molecules or conjugated polymers should be elaborated for long device lifetime.
2.4.1 Patterning small molecules
Small molecule layers are deposited through a vacuum evaporation process, as shown in Fig. 2-5. The surfaces roughness of the organic layers deposited by evaporation process sequentially are important for the lifetime of the device. A disadvantage of vacuum evaporation process is the different for large-size substrate.
turbo pump
cold trap
rough pump
holder
shutter
source boat
vacuum chamber substrate
power supply
Fig. 2-5. Schematic diagram of vacuum evaporation system for OLED fabrication.
2.4.2 Patterning polymers
Conjugated polymers can be applied to a surface through either dip-coating or spin-coating. Dip-coating is a method where the substrate is slowly dipped into the polymer solution, which results in a cover of polymers at both sides. The spin-coating
method is performed by dripping the polymer solution onto a rotated plate where the polymer solution is spread out to form a uniform thin film, as revealed in Fig. 2-6(a).
The thickness of the layers is dependent on the composition of the polymer and the concentration of the polymer solution. However, spin-coating results in non-smooth and non-parallel surfaces, which would cause degradation of devices because of the different distances between the electrodes.
(a) (b)
Fig. 2-6. Fabrication approaches for solvent based PLED. (a) spin-coating process and (b) ink-jet printing process.
An advantage of the spin-coating method compared to the vacuum deposition is the simplicity. However, it is also a less material efficient utilized process, and it should be taken into account in fabricating large size display, the spin-coating method can result in much higher costs. A drawback of spin-coating is the requirement of the removal of water and oxygen before the metal deposition [ 17 ]. However, the conventional spin-coating process is difficult to integrate the multiple color materials on the pixels individually with high resolution to realize a full-color display.
Ink jet printing is another promising candidate for the patterning of pixels to achieve the full-color display with polymers. This allows direct pattering of polymer
layers with high resolution and large-size substrate to yield full-color displays without the use of expensive vacuum deposition systems [18][19][20]
, as shown in
Fig. 2-6(b).
The ink-jet printing technology requires optimization of the solvents used to dissolve the polymers to formulate the inks. Selection of a suitable solvent is important since it affects the film morphology, which can influence the device performance in terms of device efficiency and stability. In addition, the surface energy of ITO electrode and the material of bank which defines the pixel region are as well the key parameters that control the spreading of polymer solvent and the resulted film uniformity.
2.5 Full-color approaches in manufacturing
In a similar manner for small molecules and polymers, fabrication technologies to achieve higher color purity have been proceed. The synthesizing and doping of organic materials have modified their chain structure and color characteristics. For emission of several colors, a host material can be doped with dye and new luminescent properties are then accomplished [21]. In order to achieve the full-color displays, several approaches will be introduced in following sections.
2.5.1 Patterning RGB
The patterning RGB is a straightforward approach in much the same way as CRT to achieve colors without an additional color-filter. As shown in Fig. 2-7(a), each pixel is divided into three parts called subpixels. By the combination of various gray levels of three primary colored subpixels, an arbitrary color can be generated. Since each OLED must have a different organic thin film as its light emitting layer, a drawback of this approach is in high-resolution because the arrangement of the subpixels becomes three times tighter to achieve a similar resolution compared to other approaches. In
other words, for an achievement of high-resolution, the patterning process is
other words, for an achievement of high-resolution, the patterning process is