Chapter 1 Introduction
1.4 OLED combined with poly-Si TFT
Since OLED is a current driven device, higher current supplied to OLED results
in higher brightness. In the case of a-Si TFT, a device with larger W/L and higher operation voltage are essential to provide enough current supply to the OLED.
Nonetheless, larger W/L TFT design results in less OLED emission, which means smaller aperture ratio in the bottom emission structure AM-OLED. Because a pixel consists of TFT area and OLED emitting area, the larger the occupation area of TFT, the smaller the OLED emitting area. As the resolution gets higher, pixel size becomes smaller but the area of TFT remains the same. Hence, the OLED emitting area becomes smaller, prohibiting the a-Si TFT to be used in small sizes with very high-resolution AM-OLED applications. Therefore, the motivation for the integration of OLED and poly-Si TFT technology is desired to give a power-efficient operation and increase the reliability of the panel. This is due to the high drive currents and the long term stability of the poly-Si TFT compared to the a-Si TFT, while a sufficient current supply from TFT is essential for panel operation [11][12][13]
.
Nonetheless, even poly-Si technology offers the potential of great driving capability and driver circuitry integration, the AM-OLED design still has several technical challenges:
Poly-Si TFT Vth uniformity - Poly-Si TFT threshold voltage uniformity is primarily a function of the laser annealing process stability. The uniformity variance should be of less than 5% across the panel to allow for adequate margins in designing the proper gate and data voltages. When the uniformity exceeds the design margins, the potential for not fully charging the pixel exists. That will result in uneven brightness for a give gray level. Compensating for non-uniformity by increasing voltage inversely increases the power consumption.
Lifetime - It was already known that a high drain voltage and a relatively high gate voltage (hot-carrier conditions) in poly-Si TFT can decrease the maximum
transconductance and causes the variation of threshold voltage [14][15][16][17]
. The degradation of transconductance and the threshold voltage variation associating with bias and temperature stress are of great concern to integrate TFTs on the substrate.
Besides, the degradation alters the characteristics of poly-Si TFTs in pixel electrode circuitry so that the output current for driving OLED deviates with given voltage signals as well. In other words, the variation of display luminance accompanied with device degradation results in image quality degradation.
1.5 Motivation and objective of this dissertation
New generation mobile communication and personal information systems, such as mobile phone, hand-held personal computer (HPC), digital camera, and game-boy player have progressed rapidly. Displays with the features of light weight, low power, bright, wide viewing-angle, and full color are essential for above applications. OLED combined with poly-Si TFT technology to form an AM-OLED is an unique feature those requirements. However, the AM-OLED still has several issues needed to be improved.
This dissertation presents a detailed study of the design of pixel electrode and integrated driver circuitries to improve the image quality and functionality testability for AM-OLED based on poly-Si TFT technology. The theoretical analysis of the performance of pixel electrode circuit and integrated driver circuitries will base on the principle of poly-Si TFT. Additionally, the 9-masks fabrication process for poly-Si TFT was utilized to realize the design.
Since the luminance of OLED is directly proportional to the driving current, each pixel in active-matrix TFT array consumes a certain amount of current supplied by the power electrode. The power electrode in active-matrix array is usually fabricated by
metal materials such as molybdenum (Mo), aluminum (Al) and copper (Cu). The current signal passing through these metal wires can generate the voltage drop due to the intrinsic resistance and affect the luminance uniformity. Increasing the wire width of the electrode can reduce the parasitic resistance, however, the aperture ratio will be decreased. It is expected that the voltage drop caused by the parasitic resistance will become the critical drawback in display applications of large size and high resolution.
In this dissertation, we propose a simple AC voltage driving scheme with an adequate design of conventional two transistors (2-T) pixel circuit for AM-OLED displays. By means of the charge feed-through mechanism of storage capacitor, the proposed AC driving scheme can counteract the voltage drop caused by the parasitic resistance.
Without employing the complicated driving system, the AC driving scheme can effectively improve the brightness uniformity.
Ensuring the brightness uniformity of each pixel is essential for rendering of high quality images of poly-Si TFT AM-OLED. The variations of device characteristics caused by device aging or manufacturing process are still an issue in poly-Si TFT technology. More specifically, the variations of threshold voltage can affect the drain current and lead to change of the pixel luminance. Due to the variation of device characteristics, the current driving technology with self-compensation function is the leading scheme for achieving the uniform image quality for AM-OLED. In this research, we proposed a fully-integrated current-type driving system including the pixel electrode circuitry, current generator, current-type DAC, and current-type memory. With this integrated driver, only small digital voltage signals are needed for the display interface so that the conventional driver ICs for active-matrix display still can be used. Besides, the precise current signal for various gray levels can be correctly manipulated in this driver system to compensate the characteristic variations
of poly-Si TFT.
In active-matrix TFT array processes for both AM-LCD and AM-OLED applications, TFT array inspection and yield management are important to ensure the reliability of display applications. In-line testing of TFT array in a manufacturing process is beneficial for yield improvement because the faulty TFT array can be repaired or scrapped before encapsulation, and external driver assembly. Likewise, utilizing TFT array testing for failure analysis can detect the location of the faults and identify the categories of faults in TFT array. The conventional AM-OLED pixel circuit does not provide for fully functional testing with electrical charge sensing scheme as the AM-LCD pixel does, unless an additional component can be added-in.
Here, we propose the modified pixel circuit for not only the voltage-driven but also the current-driven pixel circuitries to measure the characteristics of TFT and detect defects. The convenient charge sensing scheme can be used to effectively determine and analyze the circuit breaks, shorts, and leaky transistors or capacitances. The proposed TFT array testing scheme is demonstrated to be a good tool for managing the yield of the array process of AMOLED.
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