The Role of Metallic Dopants in Improving the Thermal Stability of the Electron Transport Layer in Organic Light‐Emitting Diodes

Abstract

tremendous improvement in device lifetime and efficiency. However, ensuring robust stability in applications that require high luminance compared with typical displays (typical display luminance <500 cd m −2 ) remains challenging. Since the thermal conductivity of organic semiconductors is relatively low (typically, <1 W m −1 K −1 ), [7] and no efficient channel to dissipate heat is present in typical OLED stacks, resistive heating can be significant, particularly during high-brightness operation. [8] In addition to intrinsic resistive heating, heating can also occur due to environmental effects during OLED operation (e.g., when OLEDs are used in automotive applications) or due to the need for a certain process temperature during fabrication or encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, highly protective encapsulation layers that are intrinsically conformal and unlikely to be disrupted by local defects; [9] however, state-of-the-art ALD processes require a relatively high temperature during the deposition (typically, >80 °C). [9][10][11] The influence of elevated temperature (due to either resistive or external heating) on device performance has been extensively investigated and is seen as an important factor in OLED degradation. [12][13][14] In general, an increase in temperature can induce morphological changes and may promote crystallization of one or several OLED layers, which in turn can reduce device performance and may lead to catastrophic failure via electric breakdown. [15] Because the thermal durability of organic materials is usually closely correlated to the glass transition temperature (T g ), a straightforward method to enhance device stability is to adapt existing materials for higher T g or to develop new high-T g materials. [16,17] However, great care has to be taken to retain appropriate functionality of the material in the final OLED, e.g., in terms of charge transport or charge blocking performance. Another strategy is to blend an existing low-T g material with a higher T g material. This can result in an intermediate T g and thus in improved morphological stability of the film, even though this effect has been found to be relatively modest. [18] Given that state-of-the-art OLEDs generally comprise 4,7-Diphenyl-1,10-phenanthroline (BPhen) is widely used to create the electron transport layer (ETL) in organic light-emitting diodes (OLEDs) because of its high electron mobility and good compatibility with alkali metal n-dopants. However, the morphology of these ETLs is easily altered by heating due to the relatively low glass transition temperature (T g ) of BPhen and this change often reduces the performance of OLEDs. Here, an enhancement in the thermal stability of OLEDs when doping their BPhenbased ETLs with cesium (Cs) is reported. To investigate the role of the Cs dopant in the BPhen matrix, the crystallization features of Cs-doped BPhen films with different doping concentrations are examined. Next, the electrical and optical ...

A substrateless, flexible, and water-resistant organic light-emitting diode

“…On the other hand, Al 2 O 3 /ZrO 2 nanolaminates provided some barrier function in air, even when used on their own 26 . However, when immersed into a liquid, e.g., cell culture medium, the bare Al 2 O 3 /ZrO 2 nanolaminate failed rapidly, which resulted in the detachment of the entire OLED from the substrate (Supplementary Fig.…”

“…The polymeric films of parylene-C were then formed on the devices in the main vacuum chamber of the parylene coating system which was kept at room temperature and at a base pressure of <25 mTorr. Nanolaminate layers of alternating Al 2 O 3 and ZrO 2 sublayers were deposited following the recipes described in our previous report 26 . In brief, 20 cycles of a 15 ms Trimethylaluminum (TMA) pulse/10 s N 2 purge/15 ms H 2 O pulse/10 s N 2 purge and 28 cycles of a 300 ms tetrakis(dimethylamino)zirconium (TDMAZr) pulse/7 s N 2 purge/30 ms H 2 O pulse/7 s N 2 purge were performed to produce 3 nm thick Al 2 O 3 and ZrO 2 sublayers, respectively.…”

“…Early studies demonstrated TFEs based on inorganic thin films 20,21 , often formed by atomic layer deposition (ALD), which allows the deposition of conformal, densely packed, and hence pin-hole free metal oxide films 22,23 . Nanolaminates formed by alternating ultrathin layers of two different inorganic materials were found to improve encapsulation performance further [24][25][26] . As an extension of this concept and to improve compatibility with flexible substrates, inorganic-polymer multilayer structures have been proposed as TFE barriers 27,28 , and such structures were indeed found to show promising barrier properties [29][30][31][32][33] .…”

Computer aided design of stable and efficient OLEDs

“…14,15,24,57 A further option is improving the thermal stability of materials with high electron mobility, such as BPhen, demonstrated with the use of alkali metal n-dopants. 66 Another alternative is the use of triazine based electron transporters, where substituents can be used to tune the electron mobility and LUMO energy for adjusted injection.…”

Developing Next-Generation Brain Sensing Technologies—A Review

“…By modifying OLED microdisplays that were originally developed for near-to-eye display applications, we demonstrated photostimulation of channelrhodopsin 2 (ChR2) expressing cells from >10 5 individually addressable blue-emitting OLED pixels [107], [108]. A potential weakness of OLEDs is their sensitivity to water and oxygen, and so we have optimized thin-film encapsulation and passivation methods based on atomic layer deposition to enable prolonged operation of OLEDs in a tissue environment [109]. Using the concept of molecular doping, we have also realized OLEDs that achieve brightness levels 100-1000 fold higher than conventional displays and have demonstrated that these allow robust photo-stimulation of individual neurons and small animals genetically transduced to express the latest generation channelrhodospins as well as genetically encoded calcium indicators [110], [111].…”