Organic Light-Emitting Diodes (OLEDs) are special types of LEDs that are very useful for electronic displays due to the higher efficiency than your normal run-of-the-mill LEDs. Blue/sky blue phosphorescent OLEDs are a special case of OLEDs. These types of LEDs must not emit any heat to meet the phosphorescent requirement. These types of LEDs have been made, but they haven’t shown high efficiency, long life-times (only 10 hours), or high brightness. These three criteria must be met if these lights were to be used for commercialization.
A new type of blue OLED was introduced by a man who went by the name Forrest, using a new technique during creation called graded-dopant concentration on the emitting layer (EML) (layer of light production). The external quantum efficiency for this type of OLED was reported as 9.5%, an above average number for the efficiency for any LED. The average lifetime for these new OLEDs was 40 hours. This was much better than the original LED.
In 2017, Forrest and his team took it one step further, and an extra molecule was introduced in the creation of the OLED. This molecule would act as an excited-state manager at the EML, preventing an unnecessary use of high energy. A similar external quantum efficiency efficiency was reported for this OLED (9.6%), but the average life-time was improved by leaps and bounds, reaching 140 hours. However, the efficiency of this OLED still led to blue fluorescent LEDs to be favored, as they had an external quantum efficiency of over 10%. The driving voltages for both the OLED and LED blue lights were higher than 8 volts, making both impractical for commercial use in displays.
Turning the clock back 5 years, a more efficient OLED was created, using thermally-activated delayed fluorescence (TADF), introduced by a man named Adachi. These OLEDs clocked out at a 100% internal quantum efficiency, but still a not-so-great external quantum efficiency. The TADF method was quickly adopted to be the best solution for creating blue stable OLEDs. Adachi and his team, however, couldn’t get the TADF OLEDs to cross the 10% mark in terms of external quantum efficiency.
A new blue OLED was developed, using a new fluorescence emitter, and hit a external quantum efficiency of 16.6%, making it great for commercial use. The following results stand to prove the efficiency of these new OLEDs.
One cause of blue OLED instability is the presence of hot triplet excitons (excitons are particles that are said to be in an “excited” state on the molecular level). These excitons are generated by triplet-polar-annihilation processes. Excitons need to be put under control, and that’s why Forrest and his team were able to almost quadruple the life-time of a blue OLED, they added in an extra molecule that manages these types of excited particles. However, with the presence of TADF, shortening the delayed lifetime could actually help with stability. First, these TADF emitters must be created.
Benzonitrile is the most stable material that can be used to construct these TADF emitters, however, blue-Benzonitrile emitters result in very long delayed lifetimes. This, as mentioned earlier, could hurt the device stability.
This delayed lifetime could be decreased with the use of two “donor units”. These units lower the energy release of the triplet-excitons, resulting in a faster reverse intersystem crossing rate. This gives us a shorter delayed lifetime, since the excitons were the primary cause of creating longer delay times to their high release of energy. Along with the resulting shorter delayed life-time, blue OLEDs get a longer device-lifetime and a higher device stability.
In conclusion, the extensive 15 years of work put into developing and innovating perfect blue OLEDs has hit a standstill at several points. However, with the introduction of methods like adding donor units and TADF emitters, we have gotten closer to commercializing blue and sky blue OLEDs. We are now in a stage with a proper device-lifetime and device stability, the two things that were holding the production of perfect blue OLEDs back significantly. With this achievement, we are now on track to commercialize blue and sky blue OLEDs.