TFT & OELD – Advancement in Display Technology

TFT Technology:

Thin Film Transistor (TFT full form) monitors are now popular in Computers, TV, Laptops, Mobile phones etc. It gives enhanced quality of images like contrast and address-ability. Unlike the LCD monitors, TFT monitors can be viewed from any angle without image distortion. TFT display is a form of Liquid Crystal Display with thin film transistors for controlling the image formation. Before going into the details of TFT technology, let us see how the LCD works.

The LCD contains Liquid crystals which is a state between liquid and solid. That is the matter can change its form from liquid to solid and viceversa. The liquid crystal flows like a liquid and it can orient to form the solid crystal. In the LCD displays, the liquid crystals used have the property of light modulation. The LCD screen do not emit light directly but it has a number of pixels filled with liquid crystals that pass light. These are arranged in front of a Back light which is the source of light. The pixels are distributed in columns and rows and the pixel behaves like a capacitor. Similar to a capacitor, the pixel has a liquid crystal sandwiched between two conductive layers. The images from The LCD may be Monochrome or colored. Each pixel is connected with a switching transistor.

When compared to the ordinary LCD, TFT monitors give very sharp and crisp text with increased response time. The TFT display has transistors made up of thin films of Amorphous silicon deposited on a glass using the PECVD technology. Inside each pixel, the transistor occupies only a small portion and the remaining space allows the passage of light. Moreover, each transistor can works off at the expense of very little charge so that the image redrawing is very fast and the screen refreshes many times in a second. In a standard TFT Monitor around 1.3 million pixels with 1.3 million thin film transistors are present. These transistors are highly sensitive to voltage fluctuations and mechanical stress and will be damaged easily leading to the formation of Dots of colors . These dots without the image are called as Dead pixels. In the Dead pixels, the transistors are damaged and cannot work properly.

The Monitors using TFT are known as TFT-LCD monitors. The display of TFT monitor has two Glass substrates enclosing a layer of liquid crystal. The Front glass substrate has a color filter. The Back glass filter contains the thin transistors arranged in columns and rows. Behind the Back glass substrate, there is Back light unit that gives light. When the TFT display is charged, the molecules in the liquid crystal layer bend and allow the passage of light. This creates a pixel. The color filter present in the Front glass substrate gives the required color to each pixel.

There are two ITO electrodes in the display to apply voltage. The LCD is placed between these electrodes. When a varying voltage is applied through the electrodes, the liquid crystal molecules align in different patterns. This alignment produces both light and dark areas in the image. This kind of image is called as Grey scale image. In color TFT monitor, the color filter substrate present in the front glass substrate gives color to the pixels. The color or grey pixel formation depends on the voltage applied by the Data driver circuit.

The Thin film transistors play an important role in pixel formation. These are arranged in the Back glass substrate. The pixel formation depends on the On/Off of these switching transistors. The switching controls the movement of electrons into the ITO electrode region. When the millions of Pixels are formed and alighted according to the switching of the transistors, millions of liquid crystal angles are created. These LC angles generates the image in the screen.

Organic Electro Luminescent Display

Organic Electro Luminescent Display (OELD) is the recently evolved solid state semiconductor LED having a thickness of 100-500 nanometers. It is also called as Organic LED or OLED. It finds many applications including the displays in mobile phones, digital camera etc. The advantage of OELD is that it is much thinner than the LCD and consumes less power. OLED is composed of aggregates of Amorphous and Crystalline molecules which are arranged in an irregular pattern. The structure has many thin layers of organic material. When current passes through these thin layers, light will be emitted through the process of Electrophosphorescence. The display can emit colors like Red, Green, Blue, White etc.

Based on the construction, OLED can be classified into

• Transparent OLED- All layers are transparent.
• Top emitting OLED – Its Substrate layer may be either reflective or non reflective.
• White OLED – It emits only White light and makes large lighting systems.
• Foldable OLED – Ideal to make Cell phone display since it is flexible and foldable.
• Active Matrix OLED – The Anode is a transistor layer to control the pixel. All the other layers are similar to the typical OLED.
• Passive OLED – Here the external circuitry determines its pixel formation.

In function, OLED is similar to an LED but it has many active layers. Typically there are two or three organic layers and other layers. The layers are Substrate layer, Anode layer, Organic layer, Conductive layer, Emissive layer and Cathode layer. The substrate layer is a thin transparent glass or plastic layer that supports the OLED structure. Anode later is active and removes electrons. It is also a transparent layer and is made up of Indium Tin Oxide. The organic layer is composed of Organic materials.

Conductive later is an important part and it transports the holes from the Anode layer. It is made up of organic plastic and the polymer used are Light emitting Polymer (LEP), Polymer Light Emitting Diode (PLED) etc. The conductive layer is electroluminescent and uses the derivatives of p-phenylene Vinylene (Poly) and Ployfluorene. The Emissive layer transports electrons from the Anode layer. It is made up of Organic plastic. The Cathode layer is responsible for the injection of Electrons. It may be either transparent or opaque. To make Cathode layer, Aluminum and Calcium are used.

OLED gives excellent display than the LCD and the pictures can be viewed from any angle without distortion. The process of light emission in the OLED involves many steps. When a potential difference is applied between the Anode and Cathode layers, current flows through the Organic layer. During this process, the Cathode layer emits electrons into the Emissive layer. The Anode layer, then releases electrons from the conductive later and the process generates holes. At the junction between the Emissive and the conductive layers, the electrons combine with the holes. This process releases energy in the form of Photons. Color of the Photon depends on the type of material used in the Emissive layer.

Now you have got an idea about the TFT and  OELD advancement in display technology furthermore any queries on this concept or on the electrical and electronic project please leave the comments below.