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1. INTRODUCTION

Organic light emitting diodes (OLEDs) are optoelectronic devices based on small molecules or polymers that emit light when an electric current flows through them. simple OLED consists of a fluorescent organic layer sandwiched between two metal electrodes.Under application of an electric field, electrons and holes are injected from the two electrodes into the organic layer, where they meet and recombine to produce light. They have been developed for applications in flat panel displays that provide visual imagery that is easy to read, vibrant in colors and less consuming of power.

OLEDs are light weight, durable, power efficient and ideal for portable applications. OLEDs have fewer process steps and also use both fewer and low-cost materials than LCD displays. OLEDs can replace the current technology in many applications due to following performance advantages over LCDs.
• Greater brightness
• Faster response time for full motion video
• Fuller viewing angles
• Lighter weight
• Greater environmental durability
• More power efficiency
• Broader operating temperature ranges
• Greater cost-effectivenes


LIMITATIONS OF LCD- EVOLUTION OF OLED

Most of the limitations of LCD technology come from the fact that LCD is a non-emissive Display device. This means that they do not emit light on their own. Thus, an LCD Operates on the basis of either passing or blocking light that is produced by an external light Source (usually from a backside lighting system or reflecting ambient light). Applying an electric field across an LCD cell controls its transparency or reflectivity. A cell blocking (absorbing) light will thus be seen as black and a cell passing (reflecting) light will be seen as white. For a color displays, there are color filters added in front of each of the cells and a single pixel is represented by three cells, each responsible for the basic colors: red, green and blue.

The basic physical structure of a LCD cell is shown in Figure.The liquid crystal (LC) material is sandwiched between two polarizers and two glass plates (or between one glass plate and one Thin Film Transistor (TFT) layers). The polarizers are integral to the working of the cell. Note that the LC material is inherently a transparent material, but it has a property where its optical axis can be rotated by applying an electric field across the material. When the LC material optical axis is made to align with the two polarizers’ axis, light will pass through the second polarizer. On the other hand, if the optical axis is rotated 90 degrees, light will be polarized by the first polarizer, rotated by the LC material and blocked by the second polarizer.



Note that the polarizers and the LC material absorb light. On a typical monochrome LCD display the polarizers alone absorb 50% of the incident light. On an active matrix display TFT layer, the light throughput may be as low as 5% of the incident light. Such low light output efficiency requires with a LC based displays to have a powerful backside or ambient light illumination to achieve sufficient brightness. This causes LCD’s to be bulky and power hungry.The LC cells are in fact relatively thin and their operation relatively power efficient. It is the backside light that takes up most space as well as power. In fact with the advent of low power microprocessors, the LCD module is the primary cause of short battery life in notebook computers.

Moreover, the optical properties of the LC material and the polarizer also causes what is known as the viewing angle effect. The effect is such that when a user is not directly in front of the display, the image can disappear or sometime seem to invert (dark images become light and light images become dark).

With these disadvantages of a LC based display in mind, there has been a lot of research to find an alternative. In recent years, a large effort has been concentrated on Organic Light Emitting Device (OLED) based displays. OLED-based displays have the potential of being lighter, thinner, brighter and much more power-efficient than LC based displays. Moreover, OLED-based displays do not suffer from the viewing angle effect. Organic Optoelectronics has been an active field of research for nearly two decades. In this time device structures and materials have been optimized, yielding a robust technology. In fact, OLEDs have already been incorporated into several consumer electronic products. However, there are basic properties of organic molecules, especially their instability in air, that hamper the commercialization of the technology for high quality displays.


ORGANIC LED STRUCTURE AND OPERATION

An Organic LED is a light emitting device whose p-n junction is made from an organic compound such as: Alq3 (Aluminum tris (8-hydroxyquinoline)) and diamine (TPD). A typical structure of an OLED cell and the molecular structure of some typical organic materials used are shown in Figure

Fig. 2 Typical structure of an Organic LED and the Molecular Structure of Alq3 & TPd

For an Organic LED, the organic layer corresponding to the p-type material is called the hole-transport layer (HTL) and similarly the layer corresponding to the n-type material is called the electron-transport layer (ETL). In Figure 2, Alq3 is the ETL and TPD is the HTL.

Similar to doped silicon, when ETL and HTL materials are placed to create a junction, the energy bands equilibrates to maintain continuity across the structure. When a potential difference is applied across the structure, a drift current flows through the structure. The injected carries recombination at the junction consists of both thermal and optical recombination, which emits photons.
Figure 3 shows the optical recombination from the energy band perspective. Note that LUMO is a short form for Lowest Unoccupied Molecular Orbital, which corresponds to the conduction band in the energy diagram of doped silicon, and HOMO is a short form for Highest Occupied Molecular Orbital, which corresponds to the valence band in the energy diagram of doped silicon.


Since an OLED emits light through a recombination process, it does not suffer from the viewing angle limitation like an LC based device. Note that for any device to become a viable candidate for use in flat panel displays it has to be able to demonstrate high brightness, good power efficiency, good color saturation and sufficient lifetime. Reasonable lower limits specifications for any candidate device should include the following: brightness of ~ 100cd/m 2 , operating voltage of 5-15V and a continuous lifetime of at least 10,000h.

OLEDs with brightness of up to 140,000 cd/m 2 , power efficiencies of up to
40 lm/W , and low operating voltages from 3-10V have been reported. Saturated-color OLEDs have been demonstrated, spanning almost the entire visible spectrum. Moreover, the thickness of an OLED structure, which typically is less than a micrometer, allows for mechanical flexibility, leading to the development of bendable displays indicating the potential development of rolled or foldable displays. Furthermore, the recent development of vapor phase deposition techniques for the OLED manufacturing process may well result in low-cost large-scale production of OLED based flat panel displays as opposed to LC based displays that require extra processes such as layer alignment and tilt angle adjustment.

OLED lifetime exceeding 50,000 h [7] has been reported. Note however, this lifetime number applies to any singular OLED structure. The number does not capture the fact that each. OLED pixel’s electrical characteristics in a display consisting of array of pixels may vary differently than the characteristic of its neighboring pixel. Although all the pixel in the array may have upto 6yrs lifetime display consisting of pixels with differing characteristics will lose its brightness and pixel to pixel accuracy if no adjustments are made to compensate for this variation. OLED-based displays are not so popular among consumer mobile computing device as LC based displays. There are challenges in OLED based flat panel display design which are not found in LC based design. OLED pixel in an array may not have uniform electrical characteristic since OLED are organic devices whose electrical properties are easily effected by the environment and its pattern of usage. In OLED power efficiency degrades with time and use. All pixel have different identical pattern of usage. This causes each pixel to have different colors.

I-V characteristic variation
The I-V characteristics of OLED is also varying with time. Several factors contribute to the I-V characteristic variation. The first and foremost is temperature. As shown in Figure , the I-V characteristic depends quite strongly on the operating temperature. The I-V characteristic variation pose a challenge to the control of OLED based displays as the I-V operating points have to be shifted depending on the operating temperature. Besides temperature, the I-V characteristic also depends strongly on the type of anode/cathode used in the device as well as the thickness of the organic active Electro Luminescence (EL) layer. In particular Figure shows the I-V characteristic variation with the thickness of the organic layer.





Direct Optical Feedback


The electrical feedback signal, which will represent the light output intensity level, is then used to control the driving signal so that the output optical power consistently represents the input reference signal. Figure shows the block diagram for this idea. The idea has the potential to succeed since the sensor can be designed to have a much more reliable and consistent characteristic compared to the OLED.

The goal of the thesis is to create a working 5x5 pixels OLED display, which maintains uniform grayscale reliability despite the varying characteristics of the individual pixels. The final demonstration system includes the 5x5 pixels OLED based displays together with the addressing, the feedback and the driving circuitry implemented using discrete components.

A Feedback Loop Shared by a Column of Pixels
There are several considerations to be made for the feedback loop implementation. Since the demonstration system is geared to building a model for the later integrated implementation, there are many more aspects to be considered. Ideally the discrete demonstration implementation should: use the simplest circuits possible, use as small number of devices as possible, be low power so that the power efficiency potential of OLED-based displays can be achieved and scalable to a much larger number of pixels.

The simplest implementation of the feedback loop of the display system will be to have a loop for every single pixel. However, this is expensive in term of the number of components, which translates to space and complexity if the design is used in an integrated version. Moreover, a continuous running feedback loop around every pixel will also tend to be expensive in terms of Power since the feedback circuitry is also consuming power.

On the other hand, a display design based on a single feedback loop per pixel can be expanded easily to large number of pixels, as every pixel and its control loop is then simply an exact copy of another. Moreover, in the integrated implementation, the light sensor as shown in Figure will be implemented using a simple silicon p-n junction. The close spatial proximity of the sensor to the feedback loop will make the sensing more accurate. As a result each pixel will have less error and more consistent output.

Another possibility is to have a small number of feedback loops, each reusable by a group of pixels using some addressing mechanism. This alternative has the potential of being lower in Power consumption and in the number of devices.



However, with this scheme there are extra requirements on the feedback loop since each of the pixels only has access to the feedback loop for a limited of time within each cycle. In other word, the feedback loop must have a faster step response (larger bandwidth). Furthermore, the Pixel design also has to include a relatively accurate sample and hold circuit so that it can reliably store a driving signal set by the shared feedback loop and maintain it through out a full cycle of refresh time. The basic schematic for this shared feedback loop is shown in Figure.


In this thesis project, a single feedback loop shared by a single column of pixels is chosen as the method to drive the display because a single feedback loop per pixel turns out to be prohibitively expensive in terms of real estate and pixel complexity. Moreover, the driving circuitry in the feedback loop can use the conventional display driver circuitry since a loop per-column topology means that the display is refreshed in a row by row fashion similar to the active matrix topology in the commercially available LC based display. This also means that the same buffering and data format used in any active matrix display can be used to drive the proposed OLED based display. In the demonstration system, a single feedback loop for each column of 5 Pixel is built, together with the sample and hold as well as the addressing circuitry.