OLED Displays

Next-generation Displays Based on Light-emitting Organic Material

Sony publicly unveiled the Organic Light Emitting Diode (hereafter OLED) display at the International CES 2007 in the US this past January and at the Display 2007 Exhibition in Japan this past April. Sony has already announced it will market an OLED TV by the end of this year. This edition of Sony Technology takes an in-depth look at the technology behind OLED displays, which are extremely slim and ultra-light and also boast a high contrast ratio and quick response time.

 What is OLED?

The "organic" in OLED refers to organic material. Carbon is the basis of all organic matter. Examples of carbon-based substances include sugar, wood and the majority of plastics. Inorganic substances include glass, stone and metal. While organic material used to be naturally produced, today it is often artificially manufactured. The "LED" refers to "Light Emitting Diode" and describes the process of converting electric energy into light. Solar batteries absorb light to produce energy but OLED works in reverse: images can be projected onto displays by applying an electric current to light-emitting organic material.

 The two types of OLED

There are two types of organic electroluminescence (OLED): small molecule OLED and polymer OLED. Small molecule OLED uses separate organic molecules and polymer OLED uses chain-linked polymer molecules. Sony uses small molecules, which are better-developed and are characterized by long life.

The polymer type, which uses larger and heavier molecules, is easier to mass produce because the materials can be turned into a sticky ink and applied by inkjet to the substrate. In addition, further development may reduce costs. However, R&D related to the light emitting properties of polymer OLED material has fallen behind. The lifetime of polymer OLED is therefore shorter than the small molecule type because oxygen and water cannot be completely extracted from the solvent which is used to turn the polymer into ink.

On the other hand, vapor deposition extends the lifetime of small molecule OLED. Yet, it is also more difficult to produce large-size displays by vacuum deposition compared to the polymer-related ink jet method. However, Sony has developed a new film deposition method to produce larger displays called laser induced pattern-wise sublimation (LIPS). The organic material layer is composed of an emissive layer, the hole transfer layer that carries positive charges and the electron transfer layer that carries negative charges. With the LIPS method, RGB emitting layers are deposited by laser beams. In turn, the hole and electron layers are deposited by vacuum deposition. Laser deposition does not require a large metal mask and is suited to the manufacture of larger displays.

 OLED Display Structure

Color displays reproduce images in full color via combinations of the three primary colors of light: red, green and blue (RGB). This is how an OLED display works. A layer of organic material, which is several hundred nanometers thick, is sandwiched between a cathode and anode layer (both of which are electroconducive (electrodes)). Applying an electric current causes the organic material to produce RGB colors.

 The structure determines how light travels from light-emitting organic material

Top emission and bottom emission are the two light emission methods for OLED displays. Each is based on a different structure which treats light, generated from the organic material layer, differently. In the bottom emission method, which can be produced via a comparatively simpler manufacturing process, the structure causes light, generated by the organic material, to travel downward toward the TFT backplane glass substrate. However, the presence of opaque pixel driver circuits on the backplane partially block the light---reducing the brightness of the ligh generated by the organic material. By contrast, with top emission (the method used by Sony), the structure causes light to travel to the “top” side of the sealing glass. As light is not blocked by pixel drivers or other such obstructions, this method offers a more efficient means of utilizing light generated by the organic material, resulting in less power consumption to produce an image of superior quality.

  Super Top Emission

Sony has gone further to create a unique “Super Top Emission” method. This method, combining mircrocavity (multiple reflection interference) and color filters, has enhanced color purity and achieved higher color contrast. The mircrocavity structure takes advantage of properties such as wavelength and reflection to achieve this. But because the wavelengths of each RGB color differ, the thickness of the organic material corresponding to each is adjusted nanometer by nanometer to produce the strongest light from each color. However, the cathode electrode, which is a semi-transparent film, does not let through all of the emitted light. Instead, only light that possesses the same wavelength as the distance between electrodes gets repeatedly reflected and thus all other colors are eliminated. In other words, the repeated reflection increases the purity of the required colors. Furthermore, Sony combined color filters with the mircrocavity structure to cut almost all ambient light reflection and achieve higher color contrast.

  The organic material film deposition method

Film deposition, as a method, is crucial in controlling the thickness of the organic material layer at the nano-level. Organic material is vulnerable to oxygen and water. Sony uses a method which forms films by vacuum deposition to glass substrate. Vacuum extraction of oxygen or water not only extends the lifetime of organic material but also reduces power consumption.

So, how does the organic material powder become a film?
When heated, the organic material (powder) evaporates and is deposited onto a glass substrate (vapor phase deposition). A metal mask is partially applied to prevent the molecules of each RGB color from mixing.

 The Merits of OLED: What's behind OLED's super high contrast and quick response time?

OLED has more control over color expression because it only expresses colors when an electric current stimulates the relevant pixels. Because there is no light source, other than the light produced by the organic material itself, when turned “off” the result is complete darkness and therefore contrast is very high. When turned “on,” individual organic elements are stimulated directly by electric current, and therefore response time is incredibly fast. For reference, LCD panels work by applying voltage to shift the direction of liquid crystal molecules – light is let through or blocked, just like window blinds. Even when expressing black, the backlight still emits light – images are displayed by selectively controlling these liquid crystals to block or allow light to pass through.

 The Merits of OLED: Why OLED is so slim and ultra-light

OLED panels are incredibly slim because OLED itself emits light and OLED display panels do not require a backlight. As glass substrates become slimmer, vacuum-deposited OLED displays will also become slimmer.




 The widening world of OLED: Slim, light and flexible displays

On May 24, Sony announced it had developed the Organic Thin Film Transistor (TFT) on a plastic, flexible film. With traditional OLED displays, OLED material is deposited on top of TFT formed on a glass substrate. However, Sony’s newly developed Organic TFT can be deposited on a flexible plastic display. Sony’s Organic TFT, the world’s first full-color display, boasts the world’s highest resolution *1(160x120 pixels, 80 ppi, pixel size: 318 µ m corner) and projects full color moving images even when the display is bent.
The Organic TFT display is expected to be used in the future, amongst others, for flexible flat panel displays, electronic paper and RF-ID tags *2.

*1 As of May 2007, Sony's is the first featuring full color and has highest resolution of all Organic TFT drive OLED display prototypes --- Sony research.
*2 Radio Frequency Identification Tag (radio IC chip): a radio IC chip that identifies and manages information.

-----This article has been translated as it originally appeared in the July 2007 issue of "Sony Family"-----

Copyright 2007 Sony Corporation. All rights reserved.