Scope of This Work

Many famous companies have been actively developing OLED displays as a future display technology. Active matrix OLED technology opens up the route to high resolution and large color displays, which will be difficult to realize using passive matrix technology.

In this thesis, we propose to enhance the luminance efficiency of conventional time ratio grayscale by increased lighting time and decreased addressing time. Two practical approaches to enhance luminance will be proposed. For decreased addressing time, shift register of sampling adopt double rate serial-in. For increased lighting time, amplitude modulation coding will reduce the number of sub-frame, and increase the lighting time. The combined method is able to enhance the luminance. Besides, the main features of the AMOLED driver include selectable shift register for image mirror, selectable shift register for image handstand, scan signal gating for non-overlap scan, cathode electrical switch for I-R drop issue and voltage regulator which provide a lower voltage from a higher input voltage for different luminance and digital core .

In chapter 2, related technique and characterization for OLED are presented. In chapter 3, we take an AMOLED review for driving design. In chapter 4, design of AMOLED driver is described. Implementation details and the main simulation result are also given. Finally, conclusion and future work are given in chapter 5.

Chapter2

Related Techniques and Characterization for OLED

2.1 LED and OLED

Because they generate their own light, light-emitting diodes (LED) have long been considered the way to a better display. Conventional LED displays have been used successfully in giant screens usually seen in outdoor advertising, but these cannot be easily adapted to the small, high-resolution screens found in notebook computers. However, the OLED is an LED of a totally different kind - based on carbon-based molecules instead of inorganic semiconductors.

LED’s are special diodes that emit light when connected in a circuit. They are frequently used as “pilot” lights in electronic appliances to indicate whether the circuit is closed or not. A clear (or often colored) epoxy case enclosed the heart of an LED, the semi-conductor chip.

The two wires extending below the LED epoxy enclosure, or the bulb indicate how the LED should be connected into a circuit. It is depend on each different material to send out the different wavelength light. Fig. 2-1 shows the typical LED. The product depends on the light wavelength and the factor that eye feel, can be divided into the infrared rays LED and the visible light LED. The material of the usage has decided the wavelength that the LED release light, and the material that suits to make the high brightness LED includes the AlGaAs, AlGaInP and the GaInN etc..

Fig. 2-1 A typical LED

The organic light emitting diode calls Organic Electroluminescence (OEL) also. The principle of luminescence is alike to light emitting diode (LED), make use of the material characteristic equally. When electron and hole come close at emission layer, they capture one another and form a neutral, excited state. This form is called exciton. The exciton then decays and emits a photon. The OLED and LED are similar in many respect, but still have some different in basic structures. The most main discrepancy is the transmission of the charge. The OLED is not a mode that uses the band transmission, but it carry on the transmission of the charge by jump. This also make it the low rate of mobility, the impedance of OLED is higher than the impedance of LED. Besides organic light emitting diode can use various different base (substrates) materials in the low temperature (<150℃.) manufacturing. The defect of the organic light emitting diode lies in its life and operation temperature ranges.

2.2 PLED and OLED

According to the organic thin film material that OLED use, the organic light emitting diode can be divided to small molecule OLED and macromolecule OLED that is polymer-based. In the late 1970s, Eastman Kodak Company scientist Dr. Ching Tang discovered that sending an electrical current through a carbon compound caused these materials to glow. Dr. Tang and Steven Van Slyke continued research in this vein. In 1987, they reported OLED materials that became the foundation for OLED displays produced today.

In 1990, Richard Friend at Cambridge University demonstrated the first polymer-based OLED using conjugated poly pr PPY.

In general, small-molecule devices are fabricated using vacuum deposition [2-1] and PLED are built using spin-coating or ink-jet printing [2-2][2-3]. In addition to in the material of differently, the small molecule OLED and PLED also is different in and component characteristics. Small molecule OLED outpace PLED with respect to efficiencies and lifetimes. Over 40 lm/W for sm-OLED by green phosphorescent and 20 lm/W for green PLED have been reported [2-4].

Over the past decade, great resources have been invested to develop better materials, new device structures, and processing technology for both systems. Today, it is still in technology development cycle, and both technologies hold great promise. In 1997, a glass-base 256x64 pixel display for an automotive radio consol was developed by Pioneer Electronic Corporation.

Recently, Motorola introduced a cellular phone with eye-catching OLED display from Pioneer.

Many companies are focused on introducing monochrome, multi-color PMOLED display.

Rapid progress is also being made to develop full-color AMOLED for higher-information display, e.g., internet-compatible cell phones, digital cameras and GPS.

2.3 Light Emission Structure

Fig. 2.2 shows the typical structure of an OLED device. OLED is a monolithic, thin film, semi-conductive device that emits light when voltage is applied to it. In its most basic form, an OLED consists of a series of organic thin films that are sandwiched between two thin-film conductors. On a glass substrate, the anode is an Indium Tin Oxide (ITO) layer, the hole-injection, hole-transport emissive and the electron transport layers are organic thin films which are optimized for each color. When voltage is applied, one layer becomes negatively charged relative to another transparent layer. As energy passes from the negatively charged (cathode) layer to the other (anode) layer, it stimulates organic material between the two, which emits light visible through an outermost layer of glass.

A wide variety of colors can be made by employing the specific organic materials. Doping or enhancing organic material helps control the brightness and color of light [2-5]. To maximize the device performance, the cathode is usually a reflective thin film of a low work function metal to maximize the device performance, another electrode which is anode use a high work function material. The overall thickness of the device is less than 2000 Å .

Metal Cathode

2.4 Emission Properties

The structure of a conventional OLED is shown in Fig. 2-2. Electrons and holes are correspondingly injected from the cathode and anode, and migrate through the electron and hole transport layers. Electroluminescence is generated by radiative recombination of these carriers near the interface between the two transport layers.

The relation between the drive current density J and instantaneous luminance in the direction of substrate normal L(0) if an OLED is given by [2-4] [2-6]

J= index or the emission spectrum of the material. For an OLED using Alq3 as the emissive material, Cn=1.1, Cv=427 lm/W, and CE=0.44V^-1. Equation (2-1) gives the drive current density required to achieve a given luminance. A luminance of 100cd/m² is usually considered sufficient for video displays, corresponding to a drive current density of 3.6mA/cm² for η=1%.

2.5 Color Present

The color presents in OLED and can be divided into monochromatic, partial color and full color. Because the OLED has the characteristic of the self-moving optics and high brilliance, the display method of the monochromatic display and partial color still does not lack its usage situation. Current, green technology of organic light emitting diode in small molecule is the most mature, the blue and red optics have also started commercializing.

For high-information content display, the full color is an essential condition on the application of the display. Applications aiming at a full-color OLED display are making steady progress. Red, green and blue are necessary in each full color pixel. Up to present, several schemes exist for generating full color on the display of OLED. Prototypes have been demonstrated or reported by several research organizations, and each of them took a different approach to the fabrication. For example, the first approach employs patterned lateral RGB emitters, as shown in Fig. 2-3 [2-7]. This method has a good possibility of high luminous efficiency although patterning of organic layers is difficult. The second one utilizes blue OLEDs as a light source with fluorescent color arrays as color-changing media (CCM) to obtain RGB colors, as shown in Fig 2-4 [2-8]. This method does not require organic layer

patterning, but its low color change efficiency especially for red color is undesirable. The third approach uses white OLEDs with color filter array, as shown in Fig. 2-5 [2-9] [2-10].

This method has no need to pattern organic materials and is able to adopt the color filter technique used in LCD panel. But the efficiency of emitting light has a significant drop by passing through the color filters. Furthermore, there are other methods such as stacked RGB emitters. A stacked OLED was built at Princeton University with separate red, green and blue sub-pixels in a vertical, coaxial emitting geometry. Single pixels can be tunable to red, green, blue and white, as shown in Fig. 2-6 [2-11].

2.5.1 Patterned Lateral RGB Emitters

The simplest is to place red, green and blue(R, G and B) sub-pixels side by side and address them separately. Red, blue and green materials can be arranged in triads for each pixel through techniques such as precision shadow masking. This method blocks areas of the substrate surface for exact deposition and registration without gaps or overlap in color areas.

Organic light emitting diode technology uses substances that emit red, green, blue or white light. Without any other source of illumination, OLED materials present bright, clear video and images that are easy to see at almost any angle.

To make use of the precise metal mask and CCD images position, red, green and blue by the order deposition constitute a pixel. Because the precise metal mask makes use of the super and thin lamella of metal, mask in the mechanical strength after taking shape will be under the influence of certain restrict, also cause an issue for position accuracy. OLEDs display development incline to the big size, over 10 inches, so will cause very big challenges, including the manufacture for the precise mask in big size and position technique.

Red OLED Green OLED Blue OLED

RED GREEN BLUE

Fig. 2-3 One pixel by lateral RGB Emitter

2.5.2 Color Change Media

Single blue-emitting OLED may serve as a pump of R and G fluorescent color-changing media (CCM), which efficiently absorb blue light and reemit the energy as green or red light.

This method does not require organic layer patterning. Its advantage is not to need the precise alignment to the technique, only needing the deposition for blue glow OLED component. A full-color organic display with 10 in. diagonal size was reported. This display was based on blue OLEDs with color-changing media. A potential problem to overcome in such displays is that “color bleeding” as light waveguided in the substrate can result in unintentional pumping of photoluminescence in adjacent pixels [2-6][2-12].

RED GREEN BLUE

Red CCM Green CCM Transparent Spacer

Blue OLED

Fig. 2-4 One pixel by color change media

2.5.3 Color Filter

The way is to use a single white-emitting organic LED and filter. The light pass through R, G, and B media or dielectric. Full-color effects also can be produced using a white emitting layer. A color filter array made through photolithographic techniques is inserted between the anode and substrate layers to produce red, blue and green effects. The combination of a white electroluminescent material and a color filter does not require precise alignment as rigorous as pixelized OLED displays. Nevertheless, the principal drawback of this approach is that much of the white OLED output must be removed by the filter to obtain the required primary colors.

For example, up to 90% of the optical power from the white OLED is filtered in order to obtain a sufficiently saturated red pixel, with the result that the OLED must be driven up to ten times brighter than the required R-G-B pixel brightness.

RED GREEN BLUE Red Passband Filter Green Passband Filter Blue Passband Filter

White Light OLED

Fig. 2-5 One pixel by color filter

2.5.4 Color Stacked OLED

Current research on OLEDs is focusing on the integration of OLEDs into full-color, flat panel displays. The recent success in surface-emitting OLEDs has led to a new kind of integrated full-color pixel: the stacked OLED. In this technique, the layers that emit different colors are stacked on top of each other along with the required electrode to independently address each layer. The advantages of this approach in processing are that the patterning step and process control requirements are now essentially the same as for a monochrome display.

Assuming that the patterning and addressing issue could be satisfactorily resolved, stacked OLEDs will triple resolution offered by the conventionally patterned RGB sub-pixels[2-12][2-13].

R-G-B

Color-Stacked OLED Color-Stacked OLED Color-Stacked OLED

R-G-B R-G-B

2.6 Electric Illumination Characterization

Two drive methods are used in active-matrix OLED display: voltage driving and current driving. Voltage driving works by applying image signals to pixels as voltage data, which are converted to current data by pixel drive transistors. Converted current data flow to pixel OLED devices, which then emit light. Current driving works by applying image signals to pixels as current data, which are retained by pixel capacitors. Retained current data flows from pixel drive transistors to the OLED devices, which then emit light.

The voltage driving method applies voltage data to the source signal lines of the OLED display. Applied voltage data are converted to current data by pixel drive transistors and applied to OLED devices. Because current data converted by drive transistors have a nonlinear relationship to voltage data, the OLED device luminance has a nonlinear relationship to voltage data, as shown in Fig. 2-7. Current driving applies current data to the source signal line of the OLED display. Current data and OLED device luminance have a linear relationship, as shown in Fig. 2-8. Because OLED devices of RGB differ from on the current-luminance characteristics and the voltage-luminance characteristics, a adjustment circuit for brightness is necessary.

Fig. 2-7 A typical brightness and voltage data relation for different color OLED

Fig. 2-8 A typical brightness and current data relation for different color OLED

2.7 Matrix Addressing

A display is an array of independently controllable pixels, the number of which depends on its dimension and resolution required by a particular application. Very large pixel counts are encountered in high-information content displays. For example, an NTSC standard TV screen requires 1.5x10⁵ pixels. The addressing of a large number of pixels in an array is an important issue in the display technology. Among the five addressing schemes used in electronic displays[2-14], direct addressing and matrix addressing are suitable for OLED-based systems.

The direct addressing scheme, where each pixel is connected to an individual driver, can only be used for discrete indicators and simple alphanumeric displays with few characters. In this case, complex character patterns be realized using shadow masks. By comparison, inorganic LED alphanumeric displays are expensive to fabricate because of the many individual diodes required to make up a single character, each with its own contacts and leads. In a matrix addressed display, pixel are organized in rows (scan line) and columns (data line), and each pixel is electrically connected between one row lead, and one column lead. The addressing schemes where active electronic components are added to pixels are called active matrix (AM) addressing; while those without extra active components in the pixels are termed passive matrix (PM) addressing.

2.7.1 Passive matrix

2.7.1.1 Pixel Structure

As shown in Fig 2-9, passive matrix (PM) OLED displays stack layers in a linear pattern, much like a grid, with “columns” of organic and cathode materials superimposed on “rows” of anode material. Each intersection or pixel contains all three substances. External circuitry controls electrical current passing through the anode “rows” and cathode “columns,”

stimulating the organic layer within each pixel. As pixels turn on and off in sequence, pictures form on the screen.

Fig. 2-9 Structure of a simple matrix-type display

2.7.1.2 Pixel Addressing

To address a matrix-type array of OLEDs is to perform a passive multiplexing. This mode of operation is well adapted since the electroluminescence phenomenon is fast (intrinsic response time in the 10^-9 s range) and the current voltage characteristics of OLEDs are non-linear (diode-like). To achieve gray scales, the column drivers must be current sources since the pixel luminance is proportional to the drive current. There are two types of driving method [2-15] to implement gray scale . PAM (pulse amplitude modulation) and PWM (pulse width modulation) driving methods are well known. In increasing of the full color grayscale level and panel resolution, It is hard to exact current control. Therefore, the developed data driver IC is made by PWM method which has constant current source. The method of the

driving PM OLED is different from that of the LCD (liquid crystal display).

In this addressing mode, the matrix-type display is scanned line by line over the frame time. An electronic switch is employed to address each of the rows sequentially. When a line is selected, each pixel in the line is fed with a constant current pulse whole time duration is adapted to the desired level of brightness (pulse width modulation technique). Fig.2-10 schematically shows this principle of operation.

Fig. 2-10 Multiplexing principle for a matrix-type array of organic LEDs

2.7.1.3 Pixel Concern

To achieve an average display luminance of Ld , the pixel must be driven to an instantaneous luminance of :[2-4] [2-6]

L(0)=

d r

L M

a

d ……… (2-2)

where M is number of rows of a display, d is the duty cycle of a pulsed current addressing the pixels and r_a is the aperture ratio of the display. The aperture ratio

r =Wa rWc/[(Wr+Dr)(Wc+Dc)] , where Wr, Wc are the row-line width and column-line width respectively, and Dr, Dc are the gap width between the row lines and between the column lines in a PMOLED display respectively.

The instantaneous luminance requirement may limit the number of rows in a display. For a mean luminance 100 cd/m², an instantaneous luminance up to 10⁵cd/m² limits the display row number to 500 for d = 50%, r = 100%. For a VGA display, this instantaneous luminance _a translated to a peak current about 1A on the row lines. These huge currents cause large voltage drops in the row lines and push the OLED operation to higher voltage, and then lowers the power efficiency. Table 2-1 [2-16] shows the power dissipation increases dramatically in PMOLED displays with increasing size and resolution. This simulation shows that when increasing the display area by a factor of 4(2 times larger diagonal), the total power dissipation increases by a factor of 10. It seems a bigger passive matrix display becomes impractical, and it will be necessary to move to an active matrix technology. Therefore, PM OLED displays’ function and configuration are well-suited for text and icon displays in

The instantaneous luminance requirement may limit the number of rows in a display. For a mean luminance 100 cd/m², an instantaneous luminance up to 10⁵cd/m² limits the display row number to 500 for d = 50%, r = 100%. For a VGA display, this instantaneous luminance _a translated to a peak current about 1A on the row lines. These huge currents cause large voltage drops in the row lines and push the OLED operation to higher voltage, and then lowers the power efficiency. Table 2-1 [2-16] shows the power dissipation increases dramatically in PMOLED displays with increasing size and resolution. This simulation shows that when increasing the display area by a factor of 4(2 times larger diagonal), the total power dissipation increases by a factor of 10. It seems a bigger passive matrix display becomes impractical, and it will be necessary to move to an active matrix technology. Therefore, PM OLED displays’ function and configuration are well-suited for text and icon displays in