Post by Piotr Trzaska
OLED (organic light-emitting diodes) is the display technology based on a thin layer of organic emitters that emit light of a specific color. Key parts of the structure of OLED displays are OLED emitters – the chemical compounds in form of a powder responsible for luminescence – light emission after application of electric current. The quality of the image displayed using OLED technology, the saturation of the color, and the intensity of the light depends for the most part on the emitter parameters, which have evolved with the development of the industry.
OLED technologies have been divided into 4 generations, depending on the characteristics of the emitter, which determine the properties of the diode constructed with its use: 1st generation – fluorescent diodes, 2nd generation phosphorescent diodes, 3rd generation based on the TADF effect, and 4th generation based on hyperfluorescence. Noctiluca develops third- and fourth-generation OLED TADF emitter systems, which will become an alternative to existing technology. Why? Let’s find the answer while analyzing all the generations.
First generation emitters, so called “fluorescent OLEDs”
OLED displays of the first generation are called fluorescent OLEDs and use organic emitters. As the intersystem crossing (ISC) between states of different multiplicity is impossible due to basic laws of physics, only transition S1 -> S0 is allowed and radiative. Upon an electrical excitation, only 25% of formed exciton are of singlet multiplicity and are emissive. The rest 75% are of triplet multiplicity and are not involved in the emission of light. This is why the internal quantum efficiency (IQE) of fluorescent OLEDs is capped to 25% and as a result external quantum efficiency (EQE) to a few percent.
The the most common today second generation emitters
To utilize the rest 75% of excitons, metallorganic complexes were discovered and named as OLED emitters of the second generation. As the presence of rare metals like iridium and platinum causes very strong spin-orbit coupling, in phosphorescent emitters T1 -> S0 transition becomes radiative and ISC process between S1 and T1 also takes place. This approach theoretically allows the utilization of up to a 100%of formed excitons for the emission of light and up to 100% of IQE. The EQE values of the best phosphorescent OLEDs are about 30%.
Even though phosphorescent emitters offer high efficiency and potential for application in OLED technology, the presence of expensive and toxic rare earth metals and precious metals in the structure of complexes causing problems with recycling, limit their application on an industrial scale. Moreover, to date, there has been a notable lack of blue emitter, that represents 70% of display light emission, of second generation.
TADF – third and fourth generation emitters
The problems with 1st and 2nd generation of emitters is what stimulated the intensive research on other efficient emitters but without metals in structure. One of the promising phenomena is thermally activated delayed fluorescence (TADF). TADF emitters thanks to the appropriate design have a very small energy difference between S1 and T1 states (ΔEST). When the exciton lifetime in the triplet state is long enough, the RISC process becomes allowed and is thermally activated. Triplet excitons are converted into singlet excitons – emissive, and emission of delayed fluorescence occurs. The theoretical maximal value of IQE is 100%.
The development of OLED technology has not stopped at the third generation. The requirements for the next generation are high efficiency, high color purity, and considerable lifetime. One of the most promising approaches which meet these conditions is hyperfluorescence – the concept developed by scientists from Kyushu University. Hyperfluorescence is a term known since 2013 when Adachi and his group developed this approach. In a hyperfluorescence, the TADF molecule serves not as an emitter but efficiently transfers the excitation from the host to the fluorescent dopant. During the TADF mechanism, the electrically generated triplet excitons are converted to singlet excitons, then through FRET, the S1 state of fluorescent dopant is filled and light emission occurs. This approach is attractive because of an extremely narrow emission band, better stability, and high color purity. The theoretical limitation of IQE for hyperfluorescence is 100%.
Why do we need the 3rd and 4th generation of emitters?
TADF emitters are expected to drive the growth of the OLED industry in the coming years because they address many problems:
- They are more energy efficient – 5G phones have up to 33% higher power consumption than 4G phones, foldable devices will allow for a bigger display, and slim design means less space for battery – having all this in mind OLED displays with 3rd and 4th emitters could save up to 30% of current energy consumption
- They are better for the environment – OLED displays with 3rd and 4th generation of emitters do not require heavy metals and Rare Earth Elements
- They are cheaper
- They prolong device life and eliminate the burn-in effect of displays
- Additional value: efficient blue emitter, unattainable for 2nd generation
Within the OLED market, the use of second-generation emitters has grown rapidly over the past few years – but not in all colors. Although efficient green and red light emitters have been successfully produced, constructing high-quality blue light emitters seems to be an insurmountable problem – the market is still using blue emitters of the older, 1st generation.
The main technological goal of OLED emitter manufacturers is to create and implement next generation emitters (3rd and 4th generation OLEDs), and to break the technological barrier associated with blue light emitters. To date, however, no one in the world has succeeded in creating an effective 3rd or 4th generation RGB emitter that would be commercially implemented (one use case with yellow monochromatic display has been rolled out). Obtaining 3rd and 4th generation emitters will be a true technological breakthrough.