Mixed Host Research Articles

A Macrospirocyclic Carbazole–Fluorene Oligomer as a Solution-Processable Matrix Host Material for Blue Phosphorescent Organic Light-Emitting Diodes with Low Turn-On Voltage and Efficiency Roll-Off

The use of mixed hosts has been demonstrated to be an effective way to improve the efficiency of phosphorescent organic light-emitting diodes (PhOLEDs). In this study, a macrospirocyclic oligomer (Cz-F)4 based on carbazole and fluorene is synthesized as a solution-processable matrix host material for blue PhOLEDs. The oligomer exhibits a high triplet level, good film-forming ability, solution processability, and doping compatibility. As a result, a blue solution-processed PhOLED based on a (Cz-F)4/TAPC [1,1-bis((di-4-tolylamino) phenyl) cyclohexane] mixed host achieves a maximum current efficiency of 18.8 cd/A with a turn-on voltage of 3.3 V. When a ternary mixed-host system composed of (Cz-F)4, TAPC, and OXD-7 [1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene] is employed, the device exhibits a low turn-on voltage of 2.8 V, and the driving voltages are only 3.8–4.6 V at the practical luminance of 100–1000 cd/m2. Even at 5000 and 10 000 cd/m2, the driving voltages are only 5.7 and 6.6 V, resp...

Photoluminescence and electroluminescence of cationic PtAu2 heterotrinuclear complexes with aromatic acetylides.

Cationic PtAu2 heterotrinuclear complexes [PtAu2(dpmp)2(C[triple bond, length as m-dash]CR)2]2+ (dpmp = bis(diphenylphosphinomethyl)phenylphosphine, R = aryl) of aromatic acetylides were prepared. The PtAu2 structures are supported through doubly bridging dpmp and stabilized by a significant Pt-Au interaction. They are highly phosphorescent in fluid CH2Cl2 solution (Φem = 23.5%-78.9%), the solid state (Φem = 15.4%-70.2%), the PMMA film (Φem = 39.9%-71.7%) and the doping film of 61% TCTA : 31% OXD-7 : 8% PtAu2 complex (Φem = 16.9%-67.9%). The phosphorescence arises mainly from 3[π (C[triple bond, length as m-dash]CR) → π* (dpmp)] 3LLCT and 3[π (C[triple bond, length as m-dash]CR) → s/p (PtAu2)] 3LMCT triplet excited states for carbazole-acetylide complexes, whereas other complexes display a 3LLCT character mixed with noticeable PtAu2 centered 3[d → s/p] parentage. Utilizing a mixed host composed of hole-transporting TCTA and electron-transporting OXD-7 doped with 8% PtAu2 species as a light-emitting layer and CuSCN as a hole-transporting layer through an orthogonal solution process, the devices exhibit highly efficient electrophosphorescence with the highest current efficiency (CEmax) of 51.7 cd A-1 and external quantum efficiency (EQEmax) of 14.5%. The efficiency roll-off is small in the practical brightness range of 500-5000 cd m-2. The PtAu2 complexes with carbazole-acetylides display a higher electroluminescence efficiency ascribed to their better hole-transporting character as well as more facile energy transfer from mixed host materials.

Influence of Micro-Cavity Effect and Architecture in the Blue Phosphorescent Organic Light-Emitting Diodes

The performance of the blue phosphorescent organic light-emitting diodes (PHOLEDs) can be affected by the range of the recombination zone (RZ) and the architecture in the emitting layer (EML). In order to change the width and location of RZ, a series of devices with the double emitting layers are fabricated. The range of RZ can be judged by the electroluminescence spectra around 500 nm due to micro-cavity effect of RZ. Then, another series of devices with a single host EML1 (mCP:FIrpic, x nm) and a mixed host EML2 (mCP:TPBi:FIrpic, y nm) are fabricated under the same total thickness of EML (x+y = 30 nm), when x is 15 nm, the maximum luminance and power efficiency can reach 9073 cd/m2 (at 8 V) and 23.4 lm/W (at 103 cd/m2), respectively. Both the driving voltage and efficiency roll-off are also improved.

Great improvement of operation-lifetime for all-solution OLEDs with mixed hosts by blade coating

We investigated some effective device designs and fabrication methods for long operation-lifetime all-solution-processed Phosphorescent OLEDs (PhOLEDs) and fluorescent OLEDs with mixed-hosts system and thin Poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(4-sec-butylphenyl) diphenylamine)] (TFB). The all-solution-processed green PhOLEDs had high current efficiency (30.3 cd/A) and long operation-lifetime. The best half-lifetime of green PhOLEDs with thin HTL, MH-hosts EML and optimized deposition was 310 h at an initial luminance 1000 cd/m2, 250 h at an initial luminance 500 cd/m2 for green PhOLEDs with thin HTL, and MH-hosts EML, and the lifetime of triple layer PhOLEDs device was only 0.5 h for the same materials. The red PhOLEDs exhibited a high current efficiency (10.93 cd/A) and half-lifetime with 157.9 h at an initial luminance 500 cd/m2. For the blue fluorescent OLEDs, the thin polymer TFB, mixed-hosts EML, double EMLs and optimization deposition yield a high current efficiency (5.68 cd/A) and long operation-lifetime with 117.7 h at an initial luminance 500 cd/m2. Single host fluorescent device had half-lifetime of 73.5 h only at an initial luminance 100 cd/m2. Finally, by doping red emitter Rubrene into stable blue device, we achieved soft yellow OLEDs with high efficiency (10.87 cd/A) and 8 fold improvement operation-lifetime (1200 h). We believe that such all-solution-processed OLEDs which showed greatly improved operational lifetimes would be suitable for the indoor supportive lighting with natural colors.

Quantitative Analysis of the Efficiency of OLEDs.

We present a comprehensive model for the quantitative analysis of factors influencing the efficiency of organic light-emitting diodes (OLEDs) as a function of the current density. The model takes into account the contribution made by the charge carrier imbalance, quenching processes, and optical design loss of the device arising from various optical effects including the cavity structure, location and profile of the excitons, effective radiative quantum efficiency, and out-coupling efficiency. Quantitative analysis of the efficiency can be performed with an optical simulation using material parameters and experimental measurements of the exciton profile in the emission layer and the lifetime of the exciton as a function of the current density. This method was applied to three phosphorescent OLEDs based on a single host, mixed host, and exciplex-forming cohost. The three factors (charge carrier imbalance, quenching processes, and optical design loss) were influential in different ways, depending on the device. The proposed model can potentially be used to optimize OLED configurations on the basis of an analysis of the underlying physical processes.

Hole-transporting small molecules as a mixed host for efficient solution processed green phosphorescent organic light emitting diodes

We have demonstrated highly efficient and reduced efficiency roll-off solution processed green phosphorescent organic light emitting diodes (OLEDs) utilizing two hole-transporting small molecular materials of 1,3-bis(carbazol-9-yl)benzene (mCP) and 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as a mixed host. Compared with the corresponding two single host devices, the mixed host one shows a superior overall performance. A low driving voltage of 5.5 V at 1000 cd/m2 with a current efficiency of 39.5 cd/A is achieved in the mCP:TCTA (3:1) mixed host device. Even at the luminance of 10000 cd/m2, the efficiency still reaches 28.6 cd/A. The enhanced charge carrier balance and broadened exciton recombination zone due to the mixed host contribute to the improvement of efficiency and efficiency roll-off.

Efficient white phosphorescent organic light-emitting diodes consisting of orange ultrathin and blue mixed host emission layers

We have successfully demonstrated highly efficient white phosphorescent organic light-emitting diodes (OLEDs) by inserting an ultrathin non-doped orange layer within blue mixed host emission layer. The key feature of the novel device is the employment of blue mixed host and orange ultrathin layers, resulting in an extended recombination region and more balanced charge carrier. The maximum efficiencies of 33.8 lm W−1 and 32.2 cd A−1 are obtained. Moreover, the resulting white device achieves a slight efficiency roll-off and a high luminance at low operating voltage. Our versatile concept suggests a promising simple method to achieve high performance white OLEDs.

A De Novo-Assembly Based Data Analysis Pipeline for Plant Obligate Parasite Metatranscriptomic Studies.

Current and emerging plant diseases caused by obligate parasitic microbes such as rusts, downy mildews, and powdery mildews threaten worldwide crop production and food safety. These obligate parasites are typically unculturable in the laboratory, posing technical challenges to characterize them at the genetic and genomic level. Here we have developed a data analysis pipeline integrating several bioinformatic software programs. This pipeline facilitates rapid gene discovery and expression analysis of a plant host and its obligate parasite simultaneously by next generation sequencing of mixed host and pathogen RNA (i.e., metatranscriptomics). We applied this pipeline to metatranscriptomic sequencing data of sweet basil (Ocimum basilicum) and its obligate downy mildew parasite Peronospora belbahrii, both lacking a sequenced genome. Even with a single data point, we were able to identify both candidate host defense genes and pathogen virulence genes that are highly expressed during infection. This demonstrates the power of this pipeline for identifying genes important in host–pathogen interactions without prior genomic information for either the plant host or the obligate biotrophic pathogen. The simplicity of this pipeline makes it accessible to researchers with limited computational skills and applicable to metatranscriptomic data analysis in a wide range of plant-obligate-parasite systems.

A bipolar macrospirocyclic oligomer based on triphenylamine and 4,5-diazafluorene as a solution-processable host for blue phosphorescent organic light-emitting diodes

A bipolar macrospirocyclic trimer based on 4,5-diazafluorene and triphenylamine was prepared as a host for solution-processed blue phosphorescent organic light emitting diodes. The oligomer exhibits a high triplet energy of 2.88 eV, excellent thermal and morphological stability, good film-forming ability and solution processability. A solution-processed phosphorescent device based on FIrpic and the trimer achieved a maximum current efficiency of 10.0 cd A−1 with a low current efficiency roll-off at the practical luminance (9.6, 10.0, 9.4 cd A−1 at 102, 207, 1096 cd m−2, respectively). The oligomer additionally showed excellent chemical compatibility with the common host for solution-processing technology. A blue phosphorescent device using mixed hosts of the trimer/4,4′-cyclohexylidenebis(N,N-bis(4-methyl -phenyl)benzenamine) achieved a low turn-on voltage of 3.0 V, a maximum luminance of 28,368 cd m−2, and a maximum current efficiency of 17.7 cd A−1 (11.9 lm W−1), which is almost three times that of a single 4,4′-cyclohexylidenebis(N,N- bis(4-methylphenyl)benzenamine)-hosted device.

24-1: Device Stability Enhancement In TADF OLEDs

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9Hcarbazol- 9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.

The influence of the mixed host emitting layer based on the TCTA and TPBi in blue phosphorescent OLED

The performance of the blue phosphorescent organic light-emitting diodes (PHOLEDs) can be improved by changing the proportion and structure of the host materials in the emitting layer. A series of devices with the mixed host (TCTAx: TPBi1-x) single emitting layer is fabricated and the best performance appears when x is 1/2. Then, the highest luminance and power efficiency reach 7189 (cd/m2) at 10.5V and 41.7lm/W at 2.75V respectively after changing the position of the single host (TCTA or TPBi) emitting layer and the mixed host (TCTA1/2: TPBi1/2) emitting layer in the multiple emitting layers device. The power efficiency is almost 37% improved in device with the optimized mixed host multiple emitting layers than that of device with the mixed host single emitting layer, the former device shows great potential to realize the high efficiency blue PHOLEDs.

A Mixed Host Emitting Interlayer Based on CBP:TPBi in Green Phosphorescent Organic Light-Emitting Diodes

A series of green phosphorescent organic light-emitting diodes based on bipolar-transporting material 4,4'-bis-(carbazol-9-yl) biphenyl (CBP) are prepared. We insert a mixed host emitting interlayer (CBPx: electron-transporting material 1,3,5-tris (N-phenylbenzimidazole-2yl) (TPBi)1–x) in the middle of the emitting layer, and the best performance appears when x is 2/3. The position of this interlayer can also affect the performance of phosphorescent organic light-emitting diodes. When this interlayer is close to the side of the electron transporting layer, the maximum value of luminance, the current efficiency and the power efficiency are 34090 cd/m2 at 12 V, 60.6 cd/A and 56.6 lm/W, respectively.

Link between hopping models and percolation scaling laws for charge transport in mixtures of small molecules

Mixed host compositions that combine charge transport materials with luminescent dyes offer superior control over exciton formation and charge transport in organic light emitting devices (OLEDs). Two approaches are typically used to optimize the fraction of charge transport materials in a mixed host composition: either an empirical percolative model, or a hopping transport model. We show that these two commonly-employed models are linked by an analytic expression which relates the localization length to the percolation threshold and critical exponent. The relation is confirmed both numerically and experimentally through measurements of the relative conductivity of Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) :1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPb) mixtures with different concentrations, where the TCTA plays a role as hole conductor and the BmPyPb as hole insulator. The analytic relation may allow the rational design of mixed layers of small molecules for high-performance OLEDs.

Effect of mixed hole transporting host on the mobility, Gaussian density of states and efficiencies of a heterojunction phosphorescent organic light emitting diode

We present an in-depth study of the hole transport in poly(vinylcarbazole) PVK films blended with small molecule tris(4-carbazoyl-9-ylphenyl)amine (TcTa). Doping TcTa in PVK introduces shallow hole traps when the doping concentration is lower than 20 wt%. It becomes percolative at higher concentrations. The energetic disorder σ of the blended system reduces from ~72 meV at 0 wt% TcTa to ~41 meV at 50 wt% TcTa. A correlation between σ and the film morphologies suggests that the blending of TcTa molecules in the film does not only change the film homogeneity and roughness but also the energetic disorder. In addition to the mobility study, we fabricated a red phosphorescent organic light emitting diode with the same blending system. By doping merely 5 wt% of TcTa into PVK as mixed hole-transporting hosts, the efficiency of the deep red heterojunction phosphorescent organic light emitting diode increased from 2 cd A−1 to 4 cd A−1, suggesting that TcTa molecules assist in hole injection.

Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers.

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.

Co-occurrence of host plants associated with plant quality determines performance patterns of the specialist butterfly, Battus polydamas archidamas (Lepidoptera: Papilionidae: Troidini)

In herbivorous insects, differences in the degree of specialization to host plants emerge when the distribution of an herbivore differs from that of its host plants, which results in a mosaic of populations differing in performance on the different host plants. Using a specialized butterfly, Battus polydamas archidamas Boisduval, 1936, which feeds exclusively on the genus Aristolochia, we test whether host plant co-occurrence and associated differences in host quality modify local adaptation in terms of larval preference and performance. We compared individuals from a monospecific host stand of Aristolochia chilensis with those from a mixed host stand of A. chilensis and A. bridgesii. Individuals were reared in a reciprocal transfer experiment in which source population and the host species fed to larvae were fully crossed in a two-by-two factorial experiment in order to quantify their preference, performance (development time, size and growth rate) and survival. Individuals from both populations preferred the species they ate during their larval development over the other host, which indicates host plant-induced preference with non-adaptive implications. Larvae from mixed and monospecific stands grew faster and survived better when reared on A. bridgesii than A. chilensis. Larvae from a monospecific host stand grew slower and fewer individuals survived under the same local conditions, which is contrary to expectations. Therefore, rearing the butterfly on A. bridgesii consistently resulted in better performance, which indicates that the monospecific population is less well adapted to its host than the mixed population. Variation in the occurrence of the two host plants in the two populations can result in divergent selection due to the variation in plant quality, which in this case could result in opposing adaptive processes.

The effect of the quantum well structure in green phosphorescent OLED with a mixed host emission interlayer

In order to improve the performance of green phosphorescent organic light-emitting diodes, we investigate multiple quantum well (MQW) structure ( ${{\rm CBP/CBP}}_{{{\rm 2/3}}}$ : ${{\rm TPBi}}_{{{\rm 1/3}}}/{{\rm CBP}}$ ) devices with mixed host emission interlayer ( ${{\rm CBP}}_{{{\rm 2/3}}}$ : ${{\rm TPBi}}_{{{\rm 1/3}}}$ ). The four devices with the same thickness of the emission layer (EML) are fabricated, in which the number of quantum well is zero, one, two, and three, respectively. Remarkably, the device with a double MQW structure achieves the maximum luminance, current efficiency and power efficiency values corresponding to ${{\rm 21930\,cd/m}}^{2}$ at 10 V, ${{\rm 48.6\,cd/A}}$ , and ${{\rm 46.4\,lm/W}}$ , respectively. The results also show that the double MQW structure can reduce turn-on voltage from 3.5 to 3 V. We attribute these improvements to the MQW structure which can extend the exciton distribution and avoid space charge accumulation in the EML.

Photoluminescence of europium(III)-doped (YxSc1−x)2O3 nanoparticles: Linear relationship between structural and emission properties

Europium (III)-doped (YxSc1−x)2O3 nanocrystalline powders of different compositions (x=0, 0.25, 0.5, 0.75 and 1) are synthesized using a polymer complex solution method. Comparative structural and optical characterization is carried out with powder X-ray diffraction and photoluminescence excitation, emission and emission decay measurements. Detailed information about unit cell parameter and energy levels of europium (III) ions are presented for all powder compositions. The second rank crystal field parameters (B02) and (B22) and crystal field strength (Nv) are calculated from the energies of Stark components of the 7F1 manifold of the europium (III) emissions. It has been shown that the crystal field parameter changes with the powder composition and shows a linear relationship with the average ionic radii of cations in the solid solutions. A similar dependence is also found for the magnitude of splitting of 7F1 manifold vs. unit cell parameter values. Moreover, this linear relationship is demonstrated for the whole family of europium (III)-doped mixed rare-earth sesquioxide hosts (RR’O3; R, R′=rare earth ions) by taking into consideration results published in the literature so far. A simple, semi-empirical equation which relates structural properties (unit cell parameters, ionic radii) of the sesquioxide host material and the emission properties of europium (III) ions (energy splitting of the 7F1 manifold, crystal field parameters and crystal field strength) are presented.

Characterizing spectral–temporal patterns of defoliator and bark beetle disturbances using Landsat time series

Defoliators and bark beetles are natural disturbance agents in many forest ecosystems around the world. Mapping the spatial and temporal patterns of insect disturbance dynamics can help in understanding their impacts on forest ecosystem resilience and functioning, and in developing adaptive management strategies. In recent years, much progress has been made in landscape-level analyses of insect-induced disturbances using remotely sensed data. However, many studies have focused on single insect agents or aggregated different insect agents into a single group. In this study, we characterized the temporal-spectral patterns associated with bark beetle and defoliator disturbances using Landsat time series between 1990 and 2013, with the objective to test if the two insect disturbances can be separated with Landsat data. We analyzed a recent outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins) and western spruce budworm (Choristoneura freemani Razowski) in British Columbia, Canada. To characterize the disturbance and recovery trends associated with insect disturbances we used the LandTrendr segmentation algorithm. We fitted LandTrendr spectral trajectories to annual normalized burn ratio (NBR) and Tasseled Cap (TC) time series, from which we then extracted a set of disturbance metrics. With these disturbance metrics, two random forest models were trained to a) distinguish insect disturbances from harvest and fire disturbances; and to b) attribute the insect disturbances to the most likely agent, i.e. mountain pine beetle or western spruce budworm. Insect disturbances were successfully mapped with an overall accuracy of 76.8%, and agents were successfully attributed with overall accuracies ranging from 75.3% to 88.0%, depending on whether only pure host-stands or mixed stands with both insect hosts were considered. In the case of mixed host stands, nearly 45% of the western spruce budworm disturbances were falsely attributed to mountain pine beetle. Spectral metrics describing disturbance magnitude were more important for distinguishing the two insect agents than the disturbance duration. Spectral changes associated with western spruce budworm disturbances had generally lower magnitudes than mountain pine beetle disturbances. Moreover, disturbances by western spruce budworm were more strongly associated with changes in TC greenness, whereas disturbances by mountain pine beetle were more strongly associated with changes in TC brightness and wetness. The results reflect the ephemeral nature of defoliators versus the tree mortality impacts of bark beetles in our study area. This study demonstrates the potential of Landsat time series for mapping bark beetle and defoliator disturbances at the agent level and highlights the need for distinguishing between the two insect agents to adequately capture their impacts on ecosystem processes.

Improved Performance of Red Phosphorescent Organic Light Emitting Diodes Using Partial Mixed Host System.

Phosphorescent organic light-emitting diodes (PHOLEDs) have shown a heterojunction system that is a bulk heterojunctioned emitting layer (EML). In our work, we have demonstrated an improvement in efficiency with a partial mixed host system in an EML. A mixed host system, which is mixed with a host material and an electron transporting material, was used on the red PHOLEDs. In general, the recombination zone is located at the electron transporting layer (ETL) side in typical PHOLEDs due to excessive accumulation at the interface. However, this study proposed the removal of interfaces through the advantage of a mixed host system and a mixed layer between EML and ETL sides to increase the electron mobility of the experiments. Detailed optical and electrical analysis of these phenomena, were demonstrated.