RE: Organic LED full report
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Organic Light Emitting Diode is a scalable nano level emerging technology in Flat Panel Displays and as a White Light Source with efficient features. This paper focuses on OLED structure, principle aspects, fabrication methodology and different techniques to replace current white light sources like Incandescent bulbs, Fluorescent tubes, and even display techniques like Liquid Crystal Displays, Plasma technologies. OLEDs can be fabricated using Polymers or by small molecules. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. These are Cheaper, Sharper, Thinner, and Flexible. OLEDs have a potential of being white-light sources that are
• Bright, power-efficient and long lived, by emitting pleasing white light
• Ultra-thin, light weight, rugged, and conformable
• Inexpensive, portable
OLEDs are energy conversion devices (electricity-to-light) based on Electroluminescence. Electro-luminescence is light emission from a solid through which an electric current is passed. OLEDs are more energy-efficient than incandescent lamps. The luminous efficiency of light bulbs is about 13 - 20 lm/W but the latest experimental green emitting OLEDs already have luminous efficiency of 76 lm/W, though at low luminance. The development is on track for OLEDs to effectively compete even with fluorescent lamps, which have the luminous efficiency of 50 - 100 lm/W. One big advantage of OLEDs is the ability to tune the light emission to any desired color, and any shade of color or intensity, including white. Achieving the high Color Rendition Index (CRI) near 100 (the ability to simulate the most pleasing white color, sunlight), is already within the reach of OLEDs. Another advantage of OLEDs is that they are current-driven devices, where brightness can be varied over a very wide dynamic range and they operate uniformly, without flicker.
CRT is still continuing as top technology in displays to produce economically best displays. The first best look of it is its Cost. But the main problems with it are its bulkiness, Difficulties in Extending to Large area displays as per construction. Even though Liquid Crystal Displays have solved one of problem i.e. size, but it is not economical. So in this present scenario the need for a new technology with both these features combined leaded to invention of OLED.OLED which is a thin, flexible, Bright LED with self luminance which can be used as a display device. The main drawback of LCD display is its Less viewing angle and highly temperature depending which moves us towards a new technology. Thus OLED promises for faithful replacement of current technology with added flavors like Less Power Consumption and Self Luminance .Both Active matrix TFT’s and Passive matrix Technologies are used for display and addressing purposes for high speed display of moving pictures and faster response. Already some of the companies released Cell Phones and PDA’s with bright OLED technology for color full displays.
3. Working Principle & Structural Aspects:
Organic Light Emitting Diodes (OLEDs) are thin-film multi-layer devices consisting of a substrate foil, film or plate (rigid or flexible), an electrode layer, layers of active materials, a counter electrode layer, and a protective barrier layer At least one of the electrodes must be transparent to light.
The typical structure of the OLED device. The number of layers may vary, as described.
The OLEDs operate in the following manner: Voltage bias is applied on the electrodes, the voltages are low, from 2.5 to ~ 20 V, but the active layers are so thin (~10Å to 100nm) that the electric fields in the active layers are very high, of the order of 105 – 107 V/cm. These high, near-breakdown electric fields support injection of charges across the electrode / active layers interfaces. Holes are injected from the anode, which is typically transparent, and electrons are injected from the cathode. The injected charges migrate against each other in the opposite directions, and eventually meet and recombine. Recombination energy is released and the molecule or a polymer segment in which the recombination occurs, reaches an exited state. Excitons may migrate from molecule to molecule. Eventually, some molecules or a polymer segments release the energy as photons or heat. It is desirable that all the excess excitation energy is released as photons (light).
The materials that are used to bring the charges to the recombination sites are usually (but not always) poor photon emitters (most of the excitation energy is released as heat). Therefore, suitable dopants are added, which first transfer the energy from
the original excitons, and release the energy more efficiently as photons. In OLEDs, approximately 25% of the excisions are in the singlet states and 75% in the triplet states. Emission of photons from the singlet states (fluorescence), in most cases facilitated by fluorescent dopants, was believed to be the only applicable form of energy release, thus limiting the Internal Quantum Efficiency (IQE) of OLEDs to the maximum of 25%.
Different Light Emitting Polymers for diff colors.
Triplet states in organic materials were considered useless, since the energy of triplets was believed to dissipate non-radiatively, as heat. This low ratio of singlet states to the triplet states and, consequently, low device efficiency, would make the application of OLEDs as sources of light extremely difficult. But by using phosphorescent dopants, the energy from all the triplet states could be harnessed as light (phosphorescence). The energy is transferred from the triplet excitons to the dopant molecules. However, not only excitons in the triplet states are utilized; these dopants, typically containing heavy atoms such as Ir or Pt, facilitate the forbidden "intersystem crossing" from the singlets to the triplet states.
The onset voltage, sometimes as low as 2.4 V is the voltage at which the current begins to flow and enough hole-electron pairs recombine to generate light visible by naked eye. The current and the corresponding light intensity increase with increasing the drive voltage. Two types of materials are needed to bring the charges to the recombination sites: hole transport polymers or small molecules, and electron transport polymers or molecules. The energy mismatch between the electrode and the charge transport layer may require another layer to be sandwiched in between, to facilitate charge injection and thus to reduce the operating voltage.
Energy levels across the simplest organic light emitting diode composed of a single organic layer between two injecting electrodes with a forward bias between anode and cathode. For high efficiency, the top metal electrode (cathode) must have the lowest possible work function.
Some add a "buffer" layer, which may serve the same purpose. Injection of holes is in most cases energetically easier than injection of electrons. This may result in the injection of excess of holes, which could drift to the cathode without meeting electrons. The excessive current would be wasteful and would heat the device. Usually, the electron transport layer acts as a hole blocker, but in some cases a hole-blocking layer is added between the electron and hole transport layers to prevent the escape of holes to the cathode. This has an additional benefit: the excess holes accumulate near the blocking layer and the resulting strong electric field across the cathode-electron transporter interface enhances injection of electrons to the system. This automatically balances the injection rates of both charge carriers, and maximizes recombination. In some cases, exciton blocking layers are added to prevent excitons to reach the electrodes and decay non-radiatively. In other cases, a separate emission layer is sandwiched between the electron transport and hole transport layer.