OLED

 

OLED

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

 

Fuller viewing angles

 

Lighter weight

 

Greater environmental durability

 

More power efficiency

 

Broader operating temperature ranges

 

Greater cost-effectiveness

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Dept of ECE, NMIT, Bangalore

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OLED

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: co lors: 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 polarizer’s and two glass plates (or between one glass plate and one Thin Film Transistor (TFT) layers). The polarizer’s 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 polarizer’s’ polari zer’s’ 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 th e second polarizer.

er’s and the LC material absorb light. On a Note that the polariz polarizer’s typical monochrome LCD display the polarizer 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 Dept of ECE, NMIT, Bangalore

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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.

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.

Dept of ECE, NMIT, Bangalore

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OLED

ORGANIC LED AND LIQUID CRYSTAL DISPLAY COMPARISON An

organic

LED Liquid crystal Panel

panel Self emission

A luminous form

of  Back light or outside

light Consumption

of  It

Electric power

is

about

light is necessary lowered

to It is abundant when

mW though back light is used

it is a little higher than the reflection type liquid crystal panel Colour

Indication The

form

flourscent A

colour

filter

is

material of RGB is used. arranged in order and or a colour filter is used. 2

2

High brightness

100 cd/m

6 cd/m  

Contrast

100:14

6:1

The thickness of the It is thin with a little When back light is panel

over 1mm

used it is thick with 5mm.

Answer time A

wide

Several us use

of  86 *C ~ -40 *C

Several ns ~ -10 *C

temperature range The corner of the Horizontal 180 *

Horizontal

view

170*

Dept of ECE, NMIT, Bangalore

120*

~

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OLED

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 is shown in Figure

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.

Dept of ECE, NMIT, Bangalore

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OLED

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.

Dept of ECE, NMIT, Bangalore

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OLED

1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation,

4. Conductive La Layer, yer, 5. Anode (+) Dept of ECE, NMIT, Bangalore

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OLED

MODERN TECHNOLOGIES IN OLEDs OLED (Organic Light Emitting Device) technology is focused on a number of key areas, including:

 

Transparent OLED (TOLED)

 

Flexible OLED (FOLED)

 

Passive and Active Matrices

 

Vertically Stacked, High Resolution OLED (SOLED)

 

Organic Lasers

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TOLED

The Transparent OLED (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight. Because TOLEDs are 70% transparent when turned off, they may be integrated into car windshields, architectural windows, and eyewear. Their transparency enables TOLEDs to be used with metal, foils, silicon wafers and other opaque substrates for top-emitting devices.

FOLED

FOLEDs are organic light emitting devices built on flexible substrates. Flat panel displays have traditionally been fabricated on glass substrates because of structural and processing constraints. Flexible materials have significant performance advantages over traditional glass substrates.

Dept of ECE, NMIT, Bangalore

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OLED

PASSIVE AND ACTIVE MATRIX

Dept of ECE, NMIT, Bangalore

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OLED

VERTICALLY STACKED HIGH RESOLUTION OLED (SOLED)

The Stacked OLED (SOLED) uses Universal Display Corporation's award-winning, novel pixel architecture that is based on stac stacking the red,

Dept of ECE, NMIT, Bangalore

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OLED

Dept of ECE, NMIT, Bangalor Bangalore