Organic Light-Emitting Diodes | 8 Advantages and Disadvantages

ORGANIC LIGHT-EMITTING DIODE

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What is an OLED? | What does led means?

An OLED is an abbreviation for Organic Light-Emitting Diodes. It is basically a type of light-emitting diode or LED that has an emissive electroluminescent layer that acts as a film of organic compounds and is responsible for emitting light when an electric current is applied. Nowadays, Organic LEDs are extensively used for developing digital displays in several devices such as television, monitors, phones, portable handheld gaming devices, smartwatches, etc. Organic Light-Emitting Diodes are also incorporated in solid-state lighting devices.

organic led or oled
An Organic light-emitting diode or OLED TV. Image source: Steve Liao – https://www.flickr.com/photos/steveliao/2089119570/
The Sony XEL-1 television, the first OLED TV (marketed 2007–2010).
CC BY-SA 2.0

How are OLEDs constructed?

A general Organic Light Emitting Diode comprises a sheet of organic materials deposited on a substrate, which is placed in between the cathode and the anode. The delocalization of pi electrons due to the conjugation over a portion of the whole molecule, resulting in the organic molecules becoming conductive electrically. These materials behave like organic semiconductors as their conductivity typically lies between that of insulators and conductors. In these materials, the role of the valence and conduction bands of inorganic semiconductors is performed by the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO).

Initially, polymer Organic Light-Emitting Diodes were designed to have a single organic layer. However, nowadays multilayer Organic LEDs can be developed having two or more layers for improving the efficacy of the device. Along with the number of layers, the kind of material used for aiding charge injection at electrodes is also important in the final functioning of the device.

The conductive property of the material used decides whether there would be a more gradual electronic flow, or a charge blockage or resistance from traveling to the opposite electrode and being unexploited. The substance is chosen depending on material-properties such as electrical conductivity, optical transparency, and chemical stability. Nowadays, Organic LEDs have a simple bilayer structure that comprises an emissive layer and a conductive layer. Based on the chemical structure of the material, the emitter can either be fluorescent or phosphorescent.

Organic light-emitting diode structure. Image source; AMOLED.png: www.universaldisplay.com derivative work: Pedro Spoladore (talk), AMOLED-enCC BY-SA 3.0

How does an Organic Light-Emitting Diode work?

When the operation begins, a potential difference is applied across the Organic Light-emitting Diode. The anode is kept at a higher potential with respect to the cathode. The material of the anode is based on material-properties such as electrical conductivity, optical transparency, and chemical stability. The lowest unoccupied molecular orbital of the organic layer (at the cathode) receives the injected electrons and the highest occupied molecular orbital (at the anode) withdraws the electrons or in other words, injects electron-hole pairs. In, organic semiconductors the holes are comparatively more mobile than the electrons. Therefore, the recombination of electrons and holes into an exciton occurs closer to the emissive layer.

This results in the decay of an excited state that leads to the emission of radiations having the wavelength ranging in the visible spectrum. The precise wavelength or frequency of the emitted radiation be determined by the bandgap of the material i.e., the difference in energy levels of the HOMO and LUMO. In the case of phosphorescent emitters, the excitons (singlets and triplets) decay radiatively. However, in the case of fluorescent emitters, the triplets do not emit any light. These fluorescent emitters possess a maximum intrinsic efficiency of 25% only. However, the phosphorescent emitters (particularly of short wavelength {blue}) have a lower lifetime compared to fluorescent emitters.

The electron-hole fermions generated have a half-integer spin. Excitons can exist in either singlet or triplet states based on the combination of different spins of electrons and holes. For each singlet exciton, three triplet excitons are formed. The triplet state decay (prevalent in phosphorescent) forbids spin and therefore, increases the transition timespan. Phosphorescent Organic Light-Emitting Diodes facilitate intersystem crossing from both triplet and singlet states by using spin-orbit interactions. This improves internal efficiency. Nowadays, Organic light-emitting diodes are extensively used for developing digital displays in several devices such as television, monitors, phones, portable handheld gaming devices, smartwatches, etc. Organic Light-Emitting Diodes are also incorporated in solid-state lighting devices.

OLED schematic.svg
Organic light-emitting diode schematic – 1. Cathode, 2. Emissive Layer, 3. Emission of radiation, 4 . Conductive Layer, 5. Anode Image source: Organic light-emitting diode Rafał Konieczny, OLED schematicCC BY-SA 3.0

What is the emission spectrum of OLEDs?

The wavelength of radiation emitted depends on the type of material used and the number of layers of the material. The energy of the radiation is equal to the bandgap of the material i.e. the difference in energy levels of the HOMO and LUMO. The final or total emission of an Organic Light Emitting Diode can be tuned virtually to represent any given color including white and black. The color temperature can also be varied by assembling numerous different combinations of layers in a single device. Organic layers are typically transparent in the visible spectral range. Generally, to achieve optimum color combination results, Organic Light Emitting Diodes are equipped with three different color layers, namely – RGB (red, green, and blue).

What are inverted OLEDs?

In the case of inverted Organic Light Emitting Diodes, the anode positioned on the substrate, which is contrary to the conventional Organic LED structure. In an inverted Organic Light Emitting Diode, the cathode is linked to the drain end of an n-channel. This is used in developing devices with AMOLED displays.

Organic light-emitting diodes
AMOLED phones. Organic light-emitting diode Image source: Samsung Galaxy Note 10OLED DisplaysCC BY-SA 4.0

What are Graded Heterojunction OLEDs?

In the case of Graded Heterojunction Organic Light Emitting Diodes, there is a gradual reduction in the fraction of electron holes to electron transporting chemicals. This is done to achieve almost 200% more the quantum efficiency than conventional Organic Light Emitting Diode structure.

What are Stacked OLEDs?

In the case of Stacked Organic Light Emitting Diodes, the pixel architecture used, arranges the red, green, and blue sub pixels vertically on top of each other rather than horizontal next to one another arrangement. This leads to a great increase in color depth, gamut, and a considerable reduction in pixel gap. The other display methods generally use the next to one another arrangement decreasing the potential resolution.

What are the technical characteristics of a conventional OLED?

Characteristics of Organic Light-Emitting Diode

The technical characteristics of a conventional Organic Light Emitting Diode is shown below:

Energy Efficiency180 lm/Wt
Сurrent Efficiency40 cd/A
Internal Quantum Efficiency (Exiton/Photon)100%
External Quantum Efficiency (Illuminated photon/Formed photon)40%
Opearting Voltage5 – 8 V
Inclusion Voltage3 – 9 V
Angle of View180°
Brightness1000 cd/m2
Contrast100:1
Life Time6 – 11 years
Temperature Range-40…+50°C

What are the advantages of OLEDs?

The advantages of OLED

  1. Organic Light-Emitting Diodes are biodegradable substances.
  2. Organic Light-Emitting Diodes are comparatively lighter, thinner, and more elastic than the crystalline layers in liquid crystal displays or Light Emitting Diodes.
  3. Organic Light-Emitting Diodes are very flexible and therefore, they can be easily folded and rolled up as required in roll-up displays inserted in certain fabrics these days. The reason behind this is that the substrate used in Organic LED is polymer rather than the glass used for an LED or an LCD.
  4. Organic Light-Emitting Diodes are comparatively brighter than normal Light Emitting Diodes. The artificial contrast ratio of Organic LEDs is higher. This is due to the fact that the organic layers of Organic LEDs are much narrower than the analogous inorganic crystal layers of an LED. Moreover, the conductive and emissive layers of Organic LEDs do not use glass (which absorbs some portion of light) and can have a multi-layered design.
  5. Unlike an LCD, an Organic Light Emitting Diode setup has no requirement of a backlight. This helps in reducing the energy or power consumption by an Organic LED device. LCDs need illumination to help in producing a visible image which necessitates more energy, whereas OLEDs are capable of generating their own light.
  6. The production process of an Organic Light Emitting Diode is easier and it can be processed into large thin sheets. Comparatively, it is a lot more difficult to produce such a large number of layers of liquid crystal.
  7. Organic Light-Emitting Diodes provide a wider viewing angle compared to LCDs. This is because an Organic LED pixel emits light directly. The pixel colors of an Organic LED are not shifted along with the change in the angle of observation from normal to a right angle.
  8. An Organic Light Emitting Diode has more rapid response time compared to an LCD.

Organic LED pixel

Organic LED pixel emits light directly and pixel colors of an Organic LED are not shifted along with the change in the angle of observation from normal to a right angle. Organic Light Emitting Diodes provide a wider viewing angle compared to LCDs.

World largest Oled array. 2850 LG OLED light .jpg
World’s largest Organic light-emitting diode array. Image source: organic LED Sebastien JurkowskiWorld largest Oled array. 2850 LG OLED light CC BY-SA 4.0

What are the disadvantages of an OLED?

The disadvantages of using an Organic Light Emitting Diode are

  1. The lifespan of an Organic Light Emitting Diode is lower than LCD. The green and red Organic LED films have longer lifespans of about 46,000 to 230,000 hours; however, the blue Organic LEDs have much shorter lifetimes up to 13-14,000 hours approximately.
  2. The substances used for producing blue light in an OLED degrades more rapidly than the substances producing other colors which cause a reduction in the overall luminescence of the Organic LED.
  3. Organic Light Emitting Diodes should not come in contact with water because it leads to instant degradation.
  4. Organic Light Emitting Diode needs about three times more power for displaying an image having a white background. The extensive use of white backgrounds can lead to a reduced battery life in mobile phones and other devices.
  5. Organic Light Emitting Diodes are expensive. They cost around 10 to 20 times more than the similar performing LED.
  6. There is a lack of wide range of commercially available Organic Light Emitting Diode products.
  7. Organic Light Emitting Diodes have high capacitance that limits the device modulation bandwidth to about 100 kHz range.
  8. Organic Light Emitting Diodes have low light efficiency.

What are the differences between an OLED and an LED?

The differences between an OLED and an LED are:

Organic Light Emitting Diode or OLEDLight Emitting Diode or LED
In the case of Organic light-emitting diodes, the emissive electroluminescent layer is made up of organic compounds.In the case of LEDs, the emissive electroluminescent layer is made up of inorganic substances.
In Organic LED television, each pixel works individually.LEDs cannot be used as a pixel properly in television due to the size.
They have a lower light efficiency.They have higher light efficiency.
They can be thin, and small due to their flexibility.They are comparatively less flexible.
They do not use a backlight as they can produce their own light.They cannot produce their own light and therefore, use a backlight.
They are expensive.They have comparatively lower manufacturing costs.
Organic LEDs do not require any kind of glass support.LEDs require glass support.
They provide a wider viewing angle.They have a comparatively lower angular range.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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