OLED: Organic Light Emitting Diode
An OLED is a solid-state semiconductor made from a thin film of organic material that emits light when electricity is applied. OLED displays are similar in structure to LEDs (which can be referred to as inorganic LEDs), but unlike LEDs, OLEDs can be more easily used to make emissive displays and area lighting panels.
Organic light-emitting diodes (OLED) have attracted considerable attention over the past decades owing to their promising applications in the field of flat panel displays. Their thin screens and high-color rendition have made a better alternative to cathode-ray tubes (CRT) and liquid crystal displays (LCDs). OLED was first reported in 1960 from single crystals of anthracene and later in 1987 Tang and Van from Kodak achieved a breakthrough in the discovery of OLEDs. They discovered efficient and low voltage OLEDs from p-n heterostructure devices using thin films of organic materials.
OLEDs are a class of light-emitting diodes that work on the principle of electroluminescence. Electroluminescence is the process of emission of light from organic materials when an electric field is applied to them. Electroluminescent materials are of two types, small molecules organic light-emitting diodes (SMOLED) and polymer light-emitting diodes (PLED). The electroluminescent performance of both SMOLED and PLED are similar but the deposition of the organic material is by different mechanisms. In small molecules, organic light-emitting diodes the deposition is by evaporation under vacuum referred to as dry process and in case of PLED they are processed by solution and thereby referred to as a wet process. The construction of SMOLED is much complex and sophisticated than PLED.
Components of OLED
Substrate: It consists of a substrate that is generally made of transparent plastic or glass and forms the base of the OLED.
Anode and cathode: OLED consists of an anode and a cathode. The anode is transparent and is made up of indium tin oxide (ITO) as it has good transparency at the visible range, while the cathode is reflective and is made up of metals with low-work function like Ca, Mg, Ag, Al, etc. Light is produced by the recombination of holes and electrons that are injected into the electrodes. The charge carriers in an anode are holes. Holes are positive charges and can be explained as an absence of electrons. Cathode can be made up of materials that may or may not be transparent. The cathode releases electrons when the current is passed to it.
Organic layer: The organic layer may be small molecules or polymers. The thickness of the organic material is about 100–150 nm.
Conducting and emissive layer: The conducting layer is made up of organic plastic that helps in the movement of holes from the anode. Polyaniline is an example of a conducting polymer used in OLED. The emissive layer is made up of organic plastic molecules that transport electrons from the cathode and is different from the conducting layer. The electrons and holes recombine and light is given in this layer. Polyfluorene is a commonly used polymer for the emissive layer. The tedious part of manufacturing OLEDs is applying the organic layers to the substrate.
Construction of OLED
OLED consists of a thin film of organic material sandwiched between two electrodes. The organic material behaves like an insulator. The materials used in OLED are amorphous or semi-crystalline films. SMOLED uses derivatives of triarylamines for p-type material and derivatives of metal chelates like tris (8-hydroxyquinoline) aluminum (III) Alq3, triazoles, or oxadiazoles for N-type material. Phosphorescent materials have been used in PLED. The efficiency of the emitting layer can be improved by doping it with various organic dyes.
The doping rate is about 1–2 wt% and this solution has been widely used to enhance the color and device lifetime. The main requirements for OLED material are high luminescence, good charge mobility, good thermal and oxidative stability, and excellent colour purity. One of the characteristics of an OLED is the pixel that is an emissive device that can be switched off and be completely black when compared to a liquid crystal where the pixel is a transmissive device which does not allow complete occultation of backlight.
Depositing the organic material onto the substrate to obtain red, green, and blue pixels is a major challenge as it requires accurate poisoning and uniformity in deposition. There are three ways of depositing the organic layer to the substrate:
- Organic vapour phase deposition (OVPD).
- Inkjet printing.
- Vacuum deposition or vacuum thermal evaporation (VTE).
Small molecules are deposited by evaporation through shadow mask and polymers are done by inkjet printing. Conventional techniques like lithography cannot be applied as the materials do not withstand the process and the layers obtained are very thin. The common techniques used for deposition of the organic material onto the substrate are discussed below.
Vacuum Deposition or Vacuum Thermal Evaporation (VTE)
The process of depositing the organic material onto the substrate by vacuum deposition technique can be explained by the following steps:
- The small molecules are placed in crucibles and heated up to 100–500°C
- The shadow masks of thickness 20–100 µm is placed above the crucible. The masks have holes to hold one-third pixels.
- A stack of single colour is deposited and shifted by one pixel and the next colour is deposited.
- The process is repeated until the substrate is completely deposited.
Care is taken while handling large shadow masks as their positioning is to be done with a precision of ±5 µm. As each colour has a different lifetime, the display colour becomes unbalanced. This leads to a further limitation.
Steps involved in depositing the organic material on the substrate by inkjet technique is discussed below.
- The inkjet printing technique involves dispensing polymer materials that are soluble in solvent.
- The solution is dispensed by inkjet nozzles on the substrate.
- The drops measuring a few picolitres are injected accurately by the inkjet head.
- Polyamide banks are built around the pixel area forming a well.
- These banks are water repellent and the pixel is hydrophilic to prevent sticking on the banks.
- The pixels are filled properly and once the deposition is complete the droplets are dried, the solvent is evaporated and the film is formed.
Working of OLEDs
The voltage is applied to the OLED from the battery. When a voltage is applied between the electrodes, charges are produced. Holes from anode and electrons from cathode are injected into the organic material. During the recombination process electrons recombine with the holes at the boundary of the emissive layer. Charges move inside the material and form excitons. The recombination zone depends on the charge mobility of the organic material and the strength of the electric field applied. Hence luminance is proportional to current density.
After diffusion, exciton recombines and photon is emitted. The colour of the photon depends on the energy difference between the highest occupied molecular orbit and lowest unoccupied molecular orbit. In simple words, it depends on the organic materials used. The wavelength can be controlled by the extent of conjugation in the molecule or polymer.
The OLEDs work in two different modes of operation:
- Active matrix displays (AMOLED): AMOLEDs are packed with layers of cathodes, organic molecules, and anodes. The anode has a thin film transistor (TFT) arrangement that forms a matrix. The transistor circuit controls each pixel which turns it ON to get the image. AMOLEDs consume less power than PMOLEDs as transistor circuitry requires less power than external circuitry and hence they are efficient for large displays. A large number of transistors are involved in controlling a large number of pixels in high resolution or large displays. The restore time is faster for AMOLEDs which makes them suitable for video displays. These types of displays are used in computer screens, electronic billboards, and large TV screens.
- Passive matrix displays (PMOLED): PMOLEDs consist of strips of anode and cathode. The anode and cathode strips are arranged perpendicular to each other. The intersections of the cathode and anode make up the pixels where light is emitted. The pixels are controlled by an external circuit that decide the ON/OFF state of the required pixel. PMOLEDs are easy to make, but they consume more power than other types of OLED but definitely less than an LCD display. The power consumption is due to the power needed for the external circuitry. PMOLEDs are most efficient for texts and icons and are best suited for small screens like cell phone panels and MP3 players.
Types of OLEDs
- Transparent OLEDs: Transparent OLEDs have only transparent materials for the making of all their components (substrate, cathode, and anode). A transparent OLED allows the emitted light to pass in both the directions when it is turned ON. They work with both active-matrix and passive-matrix modes of displays.
- Top-emitting OLED: These types of OLEDs have their substrate made up of an opaque or reflective material. They emit light only in one direction and work best in the active-matrix display mode.
- Foldable OLED: Foldable OLEDs have substrates made of very flexible metallic foils or plastics. They are lightweight and long-lasting. Since the substrate is a flexible material they reduce breakage and hence repair.
- White OLEDs: White OLEDs emit white light that is brighter, more uniform, and more energy efficient. These are more efficient than fluorescent lights. They show excellent true-colour qualities. The white OLED have started to replace fluorescent lights that are currently used in homes and buildings as they consume less energy and cost lesser.
Lifetime and Device Stability of OLED
Device stability is an important issue for any emissive technology such as OLED and particularly when it comes to the ageing of the three primary colours. The lifetime can be simply defined as the mean time to half brightness. The display applications generally have a lifetime over 20,000 hours. Degradation occurs both under operating conditions and while it is used as storage device. The process of degradation is by dark spot degradation, catastrophic failure, and intrinsic degradation.
The key advantages of OLED for flat panel display applications are their
- Self-emitting property
- High luminous efficiency
- Colour rendition capability
- Wide viewing angle
- Lightweight, transparency, and flexibility
- High contrast
- Low-power consumption
- Potentially large area color display
- OLED has a color imbalance due to the lifetime of organic materials. The red and green OLED films have longer lifetimes (46,000–230,000 hours) while the blue films have much shorter lifetimes (up to around 14,000 hours). This leads to the non-uniformity of the films as each has a different lifetime thereby creating a color imbalance and poor image quality.
- Processes involved in the manufacturing of OLED are expensive and time-consuming.
- They possess poor water resistivity and hence can be easily spoiled when water spills on its surface.
Applications of OLED
OLEDs are used in small-screen devices such as cell phones, PDAs, and digital cameras. Rapid progress in OLED research and development has led to a flexible display screen known as flexible organic light-emitting diodes (FOLED). Instead of glass surfaces as display screens, the FOLEDs are made on flexible substrates (transparent plastic to opaque metal foils). OLEDs have a faster response time than LCDs and hence video images could be much more realistic and constantly updated.