Role of the extra Fe in K2-x Fe4+y Se5 superconductors.

The exact superconducting phase of K2-x Fe4+y Se5 has so far not been conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve this mystery, we have carried out systematic structural studies on a set of well-controlled samples with exact chemical stoichiometry K2-x Fe4+x Se5 (x = 0-0.3) that are heat-treated at different temperatures. Using high-resolution synchrotron radiation X-ray diffraction, our investigations have determined the superconducting transition by focusing on the detailed temperature evolution of the crystalline phases. Our results show that superconductivity appears only in those samples that have been treated at high-enough temperature and then quenched to room temperature. The volume fraction of superconducting transition strongly depends on the annealing temperature used. The most striking result is the observation of a clear contrast in crystalline phase between the nonsuperconducting parent compound K2Fe4Se5 and the superconducting K2-x Fe4+y Se5 samples. The X-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperatures investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at a temperature below the Fe vacancy order temperature. The results strongly suggest that superconductivity in K2-x Fe4+y Se5 critically depends on the occupation of Fe atoms on the originally empty 4d site.

Effects of the charge-transfer reorganization energy on the open-circuit voltage in small-molecular bilayer organic photovoltaic devices: comparison of the influence of deposition rates of the donor.

The theoretical maximum of open-circuit voltage (VOC) of organic photovoltaic (OPV) devices has yet to be determined, and its origin remains debated. Here, we demonstrate that VOC of small-molecule OPV devices can be improved by controlling the deposition rate of a donor without changing the interfacial energy gap at the donor/acceptor interface. The measurement of external quantum efficiency and electroluminescence spectra facilitates the observation of the existence of charge transfer (CT) states. A simplified approach by reusing the reciprocity relationship for obtaining the properties of the CT states is proposed without introducing complex techniques. We compare experimental and fitting results and propose that reorganization energy is the primary factor in determining VOC instead of either the CT energy or electronic coupling term in bilayer OPV devices. Atomic force microscopy images indicate a weak molecular aggregation when a higher deposition rate is used. The results of temperature-dependent measurements suggest the importance of molecular stacking for the CT properties.

Positional Isomeric Cyano-Substituted Bis(2-phenylpyridine)(acetylacetonate)iridium Complexes for Efficient Organic Light-Emitting Diodes with Extended Color Range.

Bis(2-phenylpyridine)(acetylacetonate)iridium, Ir(ppy)2(acac), is a benchmark green emitter for phosphorescent organic light-emitting diodes (PhOLEDs). In this work, we reported three positional isomeric cyano-substituted Ir(ppy)2(acac) complexes, i.e., Ir(3-CN), Ir(4-CN), and Ir(10-CN), with the emission in the yellow to red region (544-625 nm). Through theoretical investigation and single-crystal analysis, it was found that the introduction of cyano substitution at various positions of the ppy ligand allows for tuning the electron distribution and coordination bond length of Ir complexes. Therefore, the charge transfer property of Ir complexes is enhanced such that the energy gap of the cyano-substituted Ir(ppy)2(acac) complexes was reduced. In addition, Ir(3-CN), Ir(4-CN), and Ir(10-CN) exhibited high PLQYs of 83, 54, and 75%, respectively, with the phosphorescence lifetime in the range of 0.79-2.08 µs. Notably, the device utilizing Ir(3-CN) as the emitter exhibited a maximum external quantum efficiency (EQE) of 25.4%, current efficiency of 56.9 cd A-1, power efficiency of 68.7 lm W-1, and brightness of 61,340 cd m-2 at 8 V. The EQE of this device remained 24.3 and 19.9% at luminances of 1,000 and 10,000 cd m-2, corresponding to the efficiency roll-off of 4.3 and 21.7%, respectively. Comparing to the Ir complexes using the ligand with an extended conjugated structure, our results demonstrated a simple molecular design strategy for phosphorescence emitters with reduced molecular weight for efficient PhOLEDs in the yellow to red color region.

Achieving a Highly Stable Perovskite Photodetector with a Long Lifetime Fabricated via an All-Vacuum Deposition Process.

Hybrid organic-inorganic metal halide perovskites (HOIP) have become a promising visible light sensing material due to their excellent optoelectronic characteristics. Despite the superiority, overcoming the stability issue for commercialization remains a challenge. Herein, an extremely stable photodetector was demonstrated and fabricated with Cs0.06FA0.94Pb(I0.68Br0.32)3 perovskite by an all-vacuum process. The photodetector achieves a current density up to 1.793 × 10-2 A cm-2 under standard one sun solar illumination while maintaining a current density as low as 8.627 × 10-10 A cm-2 at zero bias voltage. The linear dynamic range (LDR) and transient voltage response were found to be comparable to the silicon-based photodetector (Newport 818-SL). Most importantly, the device maintains 95% of the initial performance after 960 h of incessant exposure under one sun solar illumination. The achievements of these outstanding results contributed to the all-vacuum deposition process delivering a film with high stability and good uniformity, which in turn delays the degradation process. The degradation mechanism is further investigated by impedance spectroscopy to reveal the charge dynamics in the photodetector under different exposure times.

Modulation of the carrier balance of lead-halide perovskite nanocrystals by polyelectrolyte hole transport layers for near-infrared light-emitting diodes.

An alternative material, methylamine (MA)-doped poly[3-(4-carboxymethyl)thiophene-2,5-diyl] (P3CT) as hole transport layer (HTL) was investigated for efficient solution-processed near-infrared perovskite light-emitting diodes (NIR PeLEDs). The best NIR PeLEDs performance was achieved with an optimized composition ratio of the MA-doped P3CT (1:1) due to the balance of the electron and hole carrier in the active layer. The charge-balanced NIR PeLEDs exhibit the highest radiance of 858.37 W sr-1 m-2, a low turn-on voltage of 1.82 V, and an external quantum efficiency of 7.44%. Our findings show that using P3CT as an alternative HTL has the potential to significantly improve PeLED performance, allowing it to play a role in the development of practical applications in high-power NIR LEDs.

A New Dioxasilepine-Aryldiamine Hybrid Electron-Blocking Material for Wide Linear Dynamic Range and Fast Response Organic Photodetector.

A new dioxasilepine and aryldiamine hybrid material DPSi-DBDTA is designed to act as the electron-blocking layer (EBL) for vacuum-processed organic photodetector (OPD). The O-Si-O-linked cyclic structure leads DPSi-DBDTA to have dipolar character, high LUMO, and good thermal and morphology stability suitable for vacuum deposition. An initial trial with C60-based single active layer OPD device manifests the superior capability of DPSi-DBDTA for dark current suppression compared to the typical aryldiamines. Here, the bare and MoO3-doped DPSi-DBDTA is further examined as EBLs for the visible light responsive OPD comprising DTDCPB/C70 bulk heterojunction (BHJ) as the active layer. In sync with the result of C60-based OPD, the low dark current density and high specific detectivity D* (7.085 × 1012 cm Hz1/2 W-1) are achieved. The device with 5% MoO3-doped EBL can exhibit a wide linear dynamic range (LDR) up to 154.166 dB, which is attributed to suppression of both dark current density and carrier recombination. Additionally, the devices also manifest fast time-resolved performance in both frequency and transient response measurements. Especially for the device with 20% MoO3-doped EBL, a wide cutoff frequency response 692.047 kHz and record-high transient response demonstrating ≤0.683 µs for transient photovoltage (TPV) and ≤0.478 µs for transient photocurrent (TPC) have been realized, which is possibly owing to the balance of mobility that mitigates the damage from traps. Such submicrosecond response is comparable with the state-of-the-art perovskite-PDs and Si-PDs.

Carbazole/Benzimidazole-Based Bipolar Molecules as the Hosts for Phosphorescent and Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs.

A series of carbazole/benzimidazole-based molecules, namely, o-CbzBiz, m-CbzBiz, and p-CbzBiz, were readily synthesized in three steps by integrating carbazole with benzimidazole via the ortho-, meta-, and para-positions of phenyl linked to N-phenyl carbazole. These bipolar molecules exhibited a maximum UV absorption band ranging from 310 to 327 nm and a maximum emission band ranging from 380 to 400 nm. Density functional theory calculations showed that the twist angles between the donor and acceptor moieties of these molecules were from 54.9 to 67.1°. Such a twisted structure hampered the π-electron conjugation within the molecule and resulted in high-lying LUMO levels and triplet energies, which make them suitable to be applied as host materials in OLED devices. Our results showed that a maximum external quantum efficiency (EQE) of OLED reached 21.8% when p-CbzBiz was applied as the host of a green phosphorescent emitter, i.e., Ir(ppy)2(acac). In addition, a maximum EQE of OLED reached 16.7% when o-CbzBiz with the host of a green TADF emitter, i.e., 4CzIPN. Moreover, these devices exhibited lower efficiency roll-off than the CBP-hosted device using the same emitters, which demonstrated the bipolar charge carrier property of carbazole/benzimidazole-based molecules.

High-Efficiency Red and Near-Infrared Organic Light-Emitting Diodes Enabled by Pure Organic Fluorescent Emitters and an Exciplex-Forming Cohost.

Three D-A-D-configured molecules DTPBT, DTPNT, and DTPNBT with high quantum yield of orange red (628 nm), red (659 nm), and deep-red/NIR (710 nm) fluorescence, respectively, were developed as emitting dopants in an exciplex-forming cohost (TCTA:3P-T2T) for high-efficiency fluorescence-based organic light-emitting diodes (OLEDs). The obtained physical properties together with theoretical calculations analyzed from these new molecules establish a clear structure-property relationship, in which the feature of central acceptor 2,1,3-benzothiadiazole (BT), naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NT), and 2,1,3-naphthothiadiazole (NBT) plays the crucial role for governing the physical characteristics. The optimized device configured as ITO/HAT-CN/TAPC/TCTA/TCTA:3P-T2T:5% emitter/3P-T2T/LiF/Al gave a record-high efficiency of orange red (591 nm, 15%), red (647 nm, 10%), and deep-red/NIR (689 nm, 9%) electroluminescent devices. The effective harvest of triplet excitons with an exciplex-forming system in conjunction with efficient energy transfer between the exciplex and the dopant is beneficial for such high device efficiencies. More importantly, the stable exciplex-forming cohost and fast radiative decay rate of DTPNT render this particular device exhibiting high device stability as indicated by the low efficiency roll-off under high current densities (EQE (external quantum efficiency) values of 8.1% at 1000 cd m-2 and 6.8% at 10,000 cd m-2). These results reveal the potential of employing an exciplex-forming system as cohost for fluorescent dopants to furnish high-efficiency OLEDs with an emission wavelength extending to the red or even the NIR range.

Exciplex-Forming Cohost for High Efficiency and High Stability Phosphorescent Organic Light-Emitting Diodes.

An exciplex forming cohost system is employed to achieve a highly efficient organic light-emitting diode (OLED) with good electroluminescent lifetime. The exciplex is formed at the interfacial contact of a conventional star-shaped carbazole hole-transporting material, 4,4',4″-tris(N-carbazolyl)-triphenylamine (TCTA), and a triazine electron-transporting material, 2,4,6-tris[3-(1H-pyrazol-1-yl)phenyl]-1,3,5-triazine (3P-T2T). The excellent combination of TCTA and 3P-T2T is applied as the cohost of a common green phosphorescent emitter with almost zero energy loss. When Ir(ppy)2(acac) is dispersed in such exciplex cohost system, OLED device with maximum external quantum efficiency of 29.6%, the ultrahigh power efficiency of 147.3 lm/W, and current efficiency of 107 cd/A were successfully achieved. More importantly, the OLED device showed a low-efficiency roll-off and an operational lifetime (τ80) of ∼1020 min with the initial brightness of 2000 cd/m2, which is 56 times longer than the reference device. The significant difference of device stability was attributed to the degradation of exciplex system for energy transfer process, which was investigated by the photoluminescence aging measurement at room temperature and 100 K, respectively.

Versatile Exciplex-Forming Co-Host for Improving Efficiency and Lifetime of Fluorescent and Phosphorescent Organic Light-Emitting Diodes.

We report a new efficient exciplex-forming system consisting of a biscarbazole donor and a triazine-based acceptor. The new exciplex was characterized with a high photoluminescence quantum yield up to 68% and effective thermally activated delayed fluorescence behavior. The BCzPh:3P-T2T (2:1, 30 nm) blend was examined not only as an emitting layer (device D1) but also a reliable co-host of fluorescent and phosphorescent emitters for giving highly efficient exciplex-based organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 15.5 and 29.7% for devices doped with 1 wt % C545T (device D2) and 8 wt % Ir(ppy)2(acac) (device D4), respectively. More strikingly, a strongly enhanced lifetime ( T75 = 16 927 min.) of the C545T-doped device was obtained. The transient electroluminescence measurement as well as capacitance-voltage and impedance-voltage correlations were utilized to explore the factors governing the high efficiency and stability. The obtained results clearly show that the energy transfer and charge transport is highly efficient; they also show the photoelectric semiconducting characteristics of exciplex-based OLEDs, which are significantly different from those of unipolar host-based reference devices D3 (Alq3: 1 wt % C545T) and D5 (CBP: 8 wt % Ir(ppy)2(acac)). Our works have established a systematic protocol to shed light on the mechanisms behind exciplex-based devices. The combined results also confirm the bright prospect of the exciplex-forming system as the co-host for highly efficient and stable OLEDs.

The Incidence and Recurrence of Getting Lost in Community-Dwelling People with Alzheimer's Disease: A Two and a Half-Year Follow-Up.

Getting lost (GL) is a serious problem for people living with Alzheimer's disease (PwAD), causing psychological distress in both PwAD and caregivers, and increasing the odds of being institutionalized. It is thus important to identify risk factors for the GL events in PwAD. Between April 2009 and March 2012, we invited 185 community-dwelling PwAD and their caregivers to participate in this study. At the baseline, 95 had experienced GL (Group B); the remaining 90 (Group A) had not. We focused on the incidence of GL events and the associated factors by way of demographic data, cognitive function assessed by the Cognitive Ability Screening Instrument (CASI), and spatial navigation abilities as assessed by the Questionnaire of Everyday Navigational Ability (QuENA). After a 2.5-year period, the incidence of GL in Group A was 33.3% and the recurrence of GL in Group B was 40%. Multiple logistic regression analysis revealed that the inattention item on the QuENA and orientation item on the CASI had independent effects on the GL incidence, while the absence of a safety range was associated with the risk of GL recurrence. During the 2.5 years, the PwAD with GL incidence deteriorated more in the mental manipulation item on the CASI than those without. We suggest that before the occurrence of GL, the caregivers of PwAD should refer to the results of cognitive assessment and navigation ability evaluation to enhance the orientation and attention of the PwAD. Once GL occurs, the caregivers must set a safety range to prevent GL recurrence, especially for younger people.

Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit Voltage of Rubrene/Fullerene Organic Photovoltaic Device.

We demonstrated that the open-circuit voltage (VOC) of rubrene/C60 organic photovoltaic (OPV) devices can be substantially improved by changing the rubrene thickness. A shoulder exhibited in a range of 500-550 nm was observed. This result indicated that the singlet excitons of rubrene were increased when the thickness of the rubrene layer was increased. Capacitance-voltage measurements were conducted for estimating the built-in potential of the devices. The calculated VOC was higher than that of the experiment, thus indicating that energetic losses occurred in the devices. We reused the reciprocity and revised Marcus theory for determining the charge-transfer (CT) properties of the devices. The CT properties of the CT states at the rubrene/C60 interface remained similar. The nonradiative energetic losses become smaller when the rubrene layer was increased, thus indicating the bimolecular recombination was increased. The increased recombination thermally activated the electrons in C60 into rubrene for forming the singlet excitons in rubrene. The reduction in reorganization energy indicated that the electroluminescence of rubrene was enhanced, thereby improving VOC. These results proved that the two-step thermal activation of C60 electrons and the improved VOC of rubrene were caused by the increased singlet excitons of rubrene.

Cathodic-controlled and near-infrared organic upconverter for local blood vessels mapping.

Organic materials are used in novel optoelectronic devices because of the ease and high compatibility of their fabrication processes. Here, we demonstrate a low-driving-voltage cathodic-controlled organic upconverter with a mapping application that converts near-infrared images to produce images of visible blood vessels. The proposed upconverter has a multilayer structure consisting of a photosensitive charge-generation layer (CGL) and a phosphorescent organic light-emitting diode (OLED) for producing clear images with a high resolution of 600 dots per inch. In this study, temperature-dependent electrical characterization was performed to analyze the interfacial modification of the cathodic-controlled upconverter. The result shows that the upconverter demonstrated a high conversion efficiency of 3.46% because of reduction in the injection barrier height at the interface between the CGL and the OLED.

Transparent organic upconversion devices for near-infrared sensing.

Transparent organic upconversion devices are shown in a night-vision demonstration of a real object under near-infrared (NIR) illumination in the dark. An extraordinarily high current gain - reflecting the on-off switching effect - greater than 15 000 at a driving voltage of 3 V is demonstrated, indicating the high sensitivity to NIR light and potential of using the proposed upconverter in practical applications. A maximum luminance exceeding 1500 cd m(-2) at 7 V is achieved. Unlike previous studies, where 2D aperture projection is reported, the current study shows 3D images of real objects under NIR illumination in the dark.

Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement.

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 µm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-µm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

Improving performance and lifetime of small-molecule organic photovoltaic devices by using bathocuproine-fullerene cathodic layer.

In this study, we compared the use of neat bathocuproine (BCP) and BCP:C60 mixed buffer layers in chloroboron subphthalocyanine (SubPc)/C60 bilayer organic photovoltaic (OPV) devices and analyzed their influence on device performance. Replacing the conventional BCP with BCP:C60 enabled manipulating the optical field distribution for optimizing the optical properties of the devices. Estimation of the interfacial barrier indicated that the insertion of the BCP:C60 between the C60 and electrode can effectively reduce the barrier for electrons and enhance electron collection at the electrode. Temperature-dependent measurements of the OPV devices performed to calculate the barrier height at the SubPc/C60 interface suggested that band bending was larger when the BCP:C60 buffer layer was used, reflecting increased exciton dissociation efficiency. In addition, the device lifetime was considerably improved when the BCP:C60 buffer layer was used. The device performance was stabilized after the photodegradation of the active layers, thereby increasing the device lifetime compared with the use of the neat BCP buffer layer. Atomic force microscopy images showed that the neat BCP was easily crystallized and could degrade the cathodic interface, whereas the blend of C60 and BCP suppressed the crystallization of BCP. Therefore, the optimal buffer layer improved both the device performance and the device lifetime.

Validation of the route map recall test for getting lost behavior in Alzheimer's disease patients.

Getting lost (GL) behavior is among the early symptoms in Alzheimer's disease (AD). Only a few tests, however, have been developed to screen for this symptom. The aim of this study was to develop an instrument, the Route Map Recall Test (RMRT), for the screening of the GL problem in AD patients. We examined the psychometric properties of the RMRT and its clinical utility to predict the GL risk in 23 AD patients and 43 cognitively healthy older adults. The results showed that the RMRT has a sound reliability (test-retest, r = .752, p < .001; Cronbach's α = 0.887, p < .001). The convergent validity was supported by the high correlations with Trail Making Test A and B. With the optimal criteria (93.5/104), the discriminative validity for the diagnosis of AD showed good sensitivity (86%) and specificity (70%), and sensitivity (100%) and specificity (67%) for GL in AD patients. The findings support the RMRT to be a useful tool for clinical screening of AD patients and their GL risk.

Development of a questionnaire on everyday navigational ability to assess topographical disorientation in Alzheimer's disease.

We developed a Questionnaire on Everyday Navigational Ability (QuENA) to detect topographical disorientation (TD) in patients with Alzheimer's disease (PwAD). In the QuENA, 3 items were designed to assess landmark agnosia, 2 for egocentric disorientation, 3 for heading disorientation, and 2 for inattention. The PwAD and their caregivers rated QuENA according to which TD symptoms would occur. Regarding the construct validity, confirmatory factor analysis showed that the caregiver version of the QuENA fits the proposed TD model well but the patient version does not. Regarding the internal consistency, the Cronbach's α for the caregiver version was 0.91 and that for the patient version was 0.87. A discrepancy existed between the appraisal of navigational abilities by PwAD and by caregivers, and it was correlated with the number of getting lost (GL) events. The caregiver version of QuENA is a feasible, reliable, and valid instrument to assess TD and it also discriminates well between the PwAD with GL and those without.