An organic light-emitting diode (OLED) is required to exhibit long-time operation without degradation as an inorganic LED. Sufficiently long operation time has been demonstrated for green-and red-emitting OLEDs. However, a blue device that is important for full-color display and lighting exhibits a much shorter operational lifetime than the other color devices. The short lifetime is mainly attributed to the molecular dissociation and the defects and radical species formation through various unimolecular and bimolecular processes, including direct photolysis, exciton-exciton interaction, and exciton-polaron interaction, and so on. Different novel techniques of chemistry and physics have been employed for blue devices to suppress the degradation process induced by high-energy excitons. A deep understanding of the degradation mechanism is still needed for all three kinds of blue OLEDs employing fluorescence, phosphorescence, and thermally active delayed fluorescence. In this brief review, we introduce and discuss several degradation mechanisms for these three kinds of OLEDs.

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BackgroundButyric acid is an important chemical currently produced from petrochemical feedstocks. Its production from renewable, low-cost biomass in fermentation has attracted large attention in recent years. In this study, the feasibility of corn husk, an abundant agricultural residue, for butyric acid production by using Clostridium tyrobutyricum immobilized in a fibrous bed bioreactor (FBB) was evaluated.ResultsHydrolysis of corn husk (10% solid loading) with 0.4 M H2SO4 at 110 °C for 6 h resulted in a hydrolysate containing ~ 50 g/L total reducing sugars (glucose:xylose = 1.3:1.0). The hydrolysate was used for butyric acid fermentation by C. tyrobutyricum in a FBB, which gave 42.6 and 53.0% higher butyric acid production from glucose and xylose, respectively, compared to free-cell fermentations. Fermentation with glucose and xylose mixture (1:1) produced 50.37 ± 0.04 g L−1 butyric acid with a yield of 0.38 ± 0.02 g g−1 and productivity of 0.34 ± 0.03 g L−1 h−1. Batch fermentation with corn husk hydrolysate produced 21.80 g L−1 butyric acid with a yield of 0.39 g g−1, comparable to those from glucose. Repeated-batch fermentations consistently produced 20.75 ± 0.65 g L−1 butyric acid with an average yield of 0.39 ± 0.02 g g−1 in three consecutive batches. An extractive fermentation process can be used to produce, separate, and concentrate butyric acid to > 30% (w/v) sodium butyrate at an economically attractive cost for application as an animal feed supplement.ConclusionA high concentration of total reducing sugars at ~ 50% (w/w) yield was obtained from corn husk after acid hydrolysis. Stable butyric acid production from corn husk hydrolysate was achieved in repeated-batch fermentation with C. tyrobutyricum immobilized in a FBB, demonstrating that corn husk can be used as an economical substrate for butyric acid production.

A lack of an efficient and stable blue device is a critical factor restricting the development of organic light-emitting diode (OLED) technology that is currently expected to be overcome by employing thermally activated delayed fluorescence (TADF). Here, we investigate the TADF and electroluminescence (EL) performance of six carbazole/triphenyltriazine derivatives in different hosts. A good linearity between lg(LT50/k F 2 ) and the EL emission wavelength is found, where LT50 is the half-life of the devices and k F is the fluorescence rate of the emitters, suggesting the dominance of the singlet exciton energy and lifetime in device stability. An indolylcarbazole/triphenyltriazine derivative (ICz-TRZ) with the capability to suppress solid-state solvation exhibits blue-shifted emission and an increased k F (1.5 × 10 8 s −1 ) in comparison to the control emitters in doped films. ICz-TRZ-based devices achieve a maximum external quantum efficiency (EQE) of 18% and an EQE of 5.5% at a very high luminance of 7 × 10 4 cd/m 2 . Ignoring the poor electrochemical stability of ICz-TRZ, the device offers an LT50 approaching 100 h under an initial luminance of 1000 cd/m 2 and CIE coordinates of (0.14, 0.19). The findings in this work suggest that computer-aided design of high k F TADF emitters can be an approach to realize efficient and stable blue OLEDs.