A team of researchers at the Semiconductor Energy Laboratory Co., Ltd. (Atsughi, Japan), and headed by Satoshi Seo is developing a novel OLED device architecture. The team reports that it enables high quantum efficiency, is long lasting, has a low drive voltage OLEDs and produces a practical brightness level. The researchers call the new device architecture Exciplex-Triplet Energy Transfer or ExTEF.
Recent results from the team are reported in an article entitled “Organic LEDs with low power consumption and long lifetimes.” This article was posted on the SPIE Newsroom web site on August 24th and can be found here.
The following discussion is paraphrased from the team’s article.
To reduce the power consumption of OLEDs, both the drive current and the voltage need to be reduced. In addition, to minimize the drive current, the photon-to-electron quantum efficiency needs to increase.
To achieve these goals, the team investigated the use of phosphorescence emitted from triplet excitons. The reason for selecting this approach is that the generation percentage of triplet excitons in an OLED is 75%. This is much higher than conventional singlet extinction generation at 25%. Furthermore, phosphorescent emitters can theoretically be used to enable an internal quantum efficiency of 100%. This is because they can convert the singlet excitons to triplet excitons via intersystem crossing. To maximize the quantum efficiency in this method, the phosphorescent emitter must be dispersed within a host material in the emission layer. The introduction of the host material, however, results in an increase to the drive voltage and it often causes degradation of the device.
The ExTET approach addresses these issues. A simplified, graphical explanation of the ExTET OLED light production process is illustrated in the figure below.
To create the emissive layer of the ExTEF, the researcher took a film with an electron-transporting material and a hole-transporting material and doped it with a phosphorescent dopant. Direct recombination between the electrons at the lowest unoccupied molecular orbital level and the hole at the highest occupied molecular orbital level forms a charge-transfer excited complex (exciplex). The phosphorescent emission occurs via energy transfer from the exciplex to the dopant.
The team fabricated green, yellow, orange and red emitting ExTET OLEDs. This was accomplished by the use of various phosphorescent dopants. The team reports achieving extremely high external quantum efficiencies (about 30%) with all these devices. These results indicate that the internal quantum efficiency reached almost 100%, because the theoretical limit of outcoupling efficiency has been reported as 20-30%. The team also reported that the current-voltage characteristics of the ExTET OLEDs overlap perfectly with those of an exciplex-emission OLED that has no dopant. This result demonstrates that the exciplex acts as the recombination site in each ExTET OLED and results in low drive voltages, of about 3V.
The team conducted accelerated life time testing of device luminance. At an initial luminance of 1000 cd/m2, the luminance half-life of the green, yellow, orange and red devices is estimates as 800, 600, 1300, and 100khr, respectively (where khr is 1000 hours). These long lifetimes demonstrate that the exciplex formation does not seriously degrade the OLED, as long as the exciplex acts as a medium for the energy transfer. Furthermore, the measured lifetimes depend on the dopants used. This suggests that the lifetimes of the ExTET devices are determined by the dopants rather than by the exciplex.
The team’s article includes the comment that “Although the realization of a blue PHOLED is widely regarded as implausible, we believe that our ExTET architecture can be used to overcome this limitation.” Consistent with this belief, future research is focused on producing a blue OLED. -Arthur Berman
