Probing the thermal stability of OLEDs with neutrons

Organic light-emitting diodes (OLEDs) are subjected to varying conditions of temperature during their lifetime. Using a combination of neutron reflectometry and in situ photoluminescence spectroscopy we show that layers typically found in OLEDs can interdiffuse. The diffusion is temperature dependent and it is found that the iridium(III) complexes can be remarkably mobile, even with relatively bulky alkyl groups attached to the ligand

Influence of dopant concentration and steric bulk on interlayer diffusion in OLEDs

The performance of organic light-emitting diodes (OLEDs) can change when they are subjected to thermal stress after manufacture. The effect of heat on OLED film stacks is studied, in which the emissive layer (EML) comprises either a phosphorescent iridium(III) dopant blended in a host at different concentrations or materials with alkyl substituents to increase the steric bulk of the host and/or dopant. Neutron reflectometry with in situ photoluminescence measurements shows that interdiffusion between the emissive and hole transport layers within the films occurs on thermal annealing. Interdiffusion occurs independent of dopant concentration or steric bulk of the EML components. Importantly, when held at relatively low temperatures, the EML materials are found to only partially diffuse into an adjacent charge transport layer. The movement of materials is found to correlate with the change in luminescence from the hole transport material and an initial enhancement of the emission from the iridium(III) dopant. The results provide an explanation for the burn-in often observed for OLEDs as well as the need to change the driving characteristics over time to ensure that pixels can be held at the requisite brightness

Dependence of organic interlayer diffusion on glass-transition temperature in OLEDs

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T) of the material with the lowest T. Diffusion occurred when the material with the lowest T formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T's. If the supercooled liquid formed at a temperature significantly lower than the T of the higher-T material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes

Diffusion at interfaces in OLEDs containing a doped phosphorescent emissive layer

A common feature of organic light-emitting diodes is their stacked multilayer structure, which is critical for efficient charge injection and transport, and light emission. In this study, it is found that a blended layer of the hole-transport material tris(4-carbazol-9-ylphenyl)amine with 6 wt% fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] readily undergoes interdiffusion with adjacent layers of typical charge transport materials: bathocuproine; 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine; and N,N-bis(naphthalen-1-yl)-N,N-diphenylbenzidine. This process is followed using combined neutron reflectometry and in situ photoluminescence measurements. The temperature at which diffusion occurred is found to correlate with the glass transition temperature of the materials. Importantly, the layer of the material with the lowest T-g is the material that acts as a diffusive host for the adjacent layer, which has a higher T-g. That is, a high T-g material does not necessarily act as a blocking layer for diffusion. Furthermore, the results show that the order of structural change within a film can be predicted on the basis of the thermal properties of the materials. These results confirm the necessity of using materials with high glass transition temperatures throughout the device to minimize performance degradation by layer interdiffusion

Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the Tg of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature

Time-Resolved Neutron Reflectometry and Photovoltaic Device Studies on Sequentially Deposited PCDTBT-Fullerene Layers

We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9 ''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells

An external quantum efficiency of >20% from solution-processed poly(dendrimer) organic light-emitting diodes

Controlling the orientation of the emissive dipole has led to a renaissance of organic light-emitting diode (OLED) research, with external quantum efficiencies (EQEs) of >30% being reported for phosphorescent emitters. These highly efficient OLEDs are generally manufactured using evaporative methods and are comprised of small-molecule heteroleptic phosphorescent iridium(III) complexes blended with a host and additional layers to balance charge injection and transport. Large area OLEDs for lighting and display applications would benefit from low-cost solution processing, provided that high EQEs could be achieved. Here, we show that poly(dendrimer)s consisting of a non-conjugated polymer backbone with iridium(III) complexes forming the cores of first-generation dendrimer side chains can be co-deposited with a host by solution processing to give highly efficient devices. Simple bilayer devices comprising the emissive layer and an electron transport layer gave an EQE of >20% at luminances of up to ≈300 cd/m, showing that polymer engineering can enable alignment of the emissive dipole of solution-processed phosphorescent materials

Repurposing a neurodegenerative disease drug to treat Gram-negative antibiotic-resistant bacterial sepsis

The emergence of polymyxin resistance in carbapenem-resistant and extended-spectrum beta-lactamase (ESBL)-producing bacteria is a critical threat to human health, and alternative treatment strategies are urgently required. We investigated the ability of the hydroxyquinoline analog ionophore PBT2 to restore antibiotic sensitivity in polymyxin-resistant, ESBL-producing, carbapenem-resistant Gram-negative human pathogens. PBT2 resensitized Klebsiella pneumoniae, Escherichia coil, Acinetobacter baumannii, and Pseudomonas aeruginosa to last-resort polymyxin class antibiotics, including the less toxic next-generation polymyxin derivative FADDI-287, in vitro. We were unable to select for mutants resistant to PBT2 + FADDI-287 in polymyxin-resistant E. coli containing a plasmidborne mcr-1 gene or K. pneumoniae carrying a chromosomal mgrB mutation. Using a highly invasive K. pneumoniae strain engineered for polymyxin resistance through mgrB mutation, we successfully demonstrated the efficacy of PBT2 + polymyxin (colistin or FADDI-287) for the treatment of Gram-negative sepsis in immunocompetent mice. In comparison to polymyxin alone, the combination of PBT2 + polymyxin improved survival and reduced bacterial dissemination to the lungs and spleen of infected mice. These data present a treatment modality to break antibiotic resistance in high-priority polymyxin-resistant Gram-negative pathogens