Mar 20, 2017: Over the past few years, there has been an increasing rate of adoption of organic-LED (OLED) in smartphones, tablets, TVs and other devices. OLED technology not only offers a wide color gamut, high contrast and flexibility for the displays but also is well-known for its thinness and lightweight. However, OLED faces two underlying challenges such as short lifespan and high power consumption, and solutions need to be found at the earliest.
In order to overcome such shortcomings, Japanese researchers have come up with a new device architecture known as exciplex–triplet energy transfer (ExTET) to achieve extremely high internal quantum efficiencies, low drive voltages, and long lifetime for OLEDs.
The concept of the research
Firstly, to reduce the power consumption of OLEDs, it is vital to reduce the drive current and voltage.And to reduce the drive current, it is essential to boost the quantum efficiency (photons/electrons). In order to understand such changes, the utilization of phosphorescence released from triplet excitons is a useful approach as the generation percentages of singlet and triplet excitons in an OLED are 25% and 75%, respectively, according to the researchers.
In addition, phosphorescent emitters such as iridium complexes can be used to facilitate an internal quantum efficiency of 100%. This is due to the fact that the emitters can convert singlet excitons to triplet excitons through intersystem crossing. To make the most of the quantum efficiency in this process, the phosphorescent emitter is required to be dispersed within a host material in the emission layer (EmL).
Elementary process of ExTET
The researchers put in a mixture film, which comprised of a hole-transporting material (HTM) and an electron-transporting material (ETM) in the EmL of the ExTET device.Injected electrons and holes are then transported through ETM and HTM.
The next step is the ExTET OLED design where direct recombination between hole at the highest occupied molecular orbital (HOMO) level of the HTM and electron at the lowest unoccupied molecular orbital (LUMO) level of the ETM takes place. In the concluding part of the method, phosphorescent emission takes place thorough energy transfer via exciplex to the dopant.
PHOLED Vs ExTET OLED
The researchers compared the energy state of their ExTET OLED with a phosphorescent OLED (PHOLED). In a PHOLED, the smallest triplet-excited state (T1) level of the host has to correspond to the dopant so as to avoid luminescence quenching. As a result, the drive voltage gets boosted, and a big S1 energy gives rise to a huge HOMO–LUMO gap (ΔEHL). As against this, the ΔEST of an exciplex is verysmall.Hence, the S1 and T1 energies of the exciplex can be directly harvested to the dopant by regulating their energy levels in the ExTET OLED.
30% quantum efficiencies
Further, the researchers fabricated the ExTET OLEDby doping the particular exciplex with a variety of phosphorescent dopants, thus paving way for extremely high external quantum efficiencies of 30% in ExTET OLED devices. The results revealed that the internal quantum efficiency is nearly 100% since the theoretical limit of out coupling efficiency reached about 20–30%.
This exciplex–triplet energy transfer mechanism for OLEDs seems to be a positive approach towards the improvement of OLED efficiency and lifespan. However, the researchers want to dig deep into the matter and, are looking forward to make a blue OLED with their ExTET approach.