Effects of electron transport layer thickness on light extraction in corrugated OLEDs.

This study reported the effects of electron transport layer (ETL) thickness on light extraction in corrugated organic light-emitting diodes (OLEDs) and each layer in OLEDs exhibited a periodical corrugated structure, which was determined by depositing thin films on a glass substrate with a nanoimprinted blazed grating structure. The insight is that light extraction in corrugated OLEDs significantly depends on the ETL thickness. Varying the ETL thickness changed the distribution of carrier recombination and led to exciton formation and optical interference, thereby resulting in different attribution of optical loss modes in OLEDs, which increased or even decreased light extraction and device efficiency. Trapped light extraction from the surface plasmon polariton (SPP) and waveguide (WG) modes was identified by splitting the light into transverse electric and transverse magnetic emissions. Thus, the contributions from the individual SPP and WG modes to the external quantum efficiency (EQE) were distinctly clarified by comparing the experimental results with the theoretical calculations. At the ETL thickness of 115 nm, the corrugated OLED exhibited a significantly enhanced (1.83-fold) EQE compared to the planar one due to the effective extraction of trapped light from the SPP and WG modes. The EQE was enhanced by 0.5%, wherein 0.39% came from the WG mode and 0.11% came from the SPP mode.

Direct detection of virus-like particles using color images of plasmonic nanostructures.

We propose a measurement method for sensitive and label-free detections of virus-like particles (VLPs) using color images of nanoplasmonic sensing chips. The nanoplasmonic chip consists of 5×5 gold nanoslit arrays and the gold surface is modified with specific antibodies for spike protein. The resonant wavelength of the 430-nm-period gold nanoslit arrays underwater environment is about 570 nm which falls between the green and red bands of the color CCD. The captured VLPs by the specific antibodies shift the plasmonic resonance of the gold nanoslits. It results in an increased brightness of green pixels and decreased brightness of red pixels. The image contrast signals of (green - red) / (red + green) show good linearity with the surface particle density. The experimental tests show the image contrast method can detect 100-nm polystyrene particles with a surface density smaller than 2 particles/µm2. We demonstrate the application for direct detection of SARS-CoV-2 VLPs using a simple scanner platform. A detection limit smaller than 1 pg/mL with a detection time less than 30 minutes can be achieved.

Synthesis and Performance in OLEDs of Selenium-Containing Phosphorescent Emitters with Red Emission Color Deeper Than the Corresponding NTSC Standard.

The synthesis of new iridium(III) complexes containing a 2-(benzo[b]selenophen-2-yl)pyridine ligand is reported along with their photophysical, thermal, electrochemical and electroluminescent properties. These complexes are characterized by deep red phosphorescence with photoluminescence quantum yields exceeding 31% in the solid state. Solid layers of the complexes were characterized by ionization potentials of 5.17-5.27 eV and electron affinities of 2.87-2.95 eV. Their thermal and electrochemical stabilities were proved by cyclic voltammetry and thermogravimetric analysis. Deep red selenium-based iridium phosphorescent emitters were used in red electroluminescent devices which were characterized by a deep red color with Commission Internationale de l'Eclairage (CIE 1931) chromaticity coordinates (x, y) of (0.69, 0.31). This color is deeper than that defined by the red color standard (0.67, 0.33) of the National Television System Committee (NTSC) or CIE 1931 of (0.68, 0.32) of the widely known red phosphorescent emitter bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2(acac)). Using newly developed deep red iridium complexes, white hybrid wet-processable light-emitting devices were fabricated, the electroluminescence of which was characterized by a white color with a color rendering index (CRI) reaching 85. White hybrid OLEDs were obtained by mixing blue fluorescence, green thermally activated delayed fluorescence, and red phosphorescence. They showed a maximum brightness exceeding 10000 cd/m2 and a high external quantum efficiency of 6.3% as for solution-processed white devices.

Exciton dynamics in heterojunction thin-film devices based on exciplex-sensitized triplet-triplet annihilation.

Exciton dynamics in a solid-state exciplex sensitized triplet-triplet annihilation (ESTTA) system are studied using transient photoluminescence (TrPL) measurements. The ESTTA system is a trilayer structure with 4,4',4''-tris(N-3-methyphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) acting as the electron donor, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) as a triplet-diffusion-singlet-blocking (TDSB) layer, and 9,10-bis(2'-naphthyl) anthracene (ADN), acting as the electron acceptor and emitter. The thicknesses of the m-MTDATA and ADN layers are 30 nm, while the thickness of the DMPPP layer is varied to characterize its effect on the singlet quenching of the ADN emission. We find that electron transfer via tunneling through the DMPPP layer is the dominant quenching channel, with a characteristic length of ∼5 nm. Doping the high photoluminescence quantum yield molecule 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) into the ADN layer enhanced the overall intensity of the ESTTA signal but did not prevent quenching by exciplex formation. The trilayer configuration (m-MTDATA/DMPPP/ADN) can effectively prevent ADN singlets from being quenched by electron transfer and exciplex formation, and a key property of the DMPPP is its tendency to not undergo electron transfer to the ADN.

Modeling of carrier transport in organic light emitting diode with random dopant effects by two-dimensional simulation.

To model the carrier transport in organic light-emitting diodes (OLEDs) with random dopant effects in the emitting layer, two-dimensional simulation was used. By including the Gaussian shape density of states and field-dependent mobility in the Poisson and drift-diffusion solver, the carrier transport, trapping in the dopant state, and radiative recombination were accurately modeled. To examine the model, the current-voltage characteristics of organic light-emitting devices were compared. The host material in the emitting layer was 2,2-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)biphenyl (BImBP), which was doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) at various concentrations. By including the random doping model, the trend of mobility was altered and the radiative efficiency fitted experimental values well.

High k nanophase zinc oxide on biomimetic silicon nanotip array as supercapacitors.

A 3D trenched-structure metal-insulator-metal (MIM) nanocapacitor array with an ultrahigh equivalent planar capacitance (EPC) of ~300 µF cm(-2) is demonstrated. Zinc oxide (ZnO) and aluminum oxide (Al2O3) bilayer dielectric is deposited on 1 µm high biomimetic silicon nanotip (SiNT) substrate using the atomic layer deposition method. The large EPC is achieved by utilizing the large surface area of the densely packed SiNT (!5 × 10(10) cm(-2)) coated conformally with an ultrahigh dielectric constant of ZnO. The EPC value is 30 times higher than those previously reported in metal-insulator-metal or metal-insulator-semiconductor nanocapacitors using similar porosity dimensions of the support materials.

Organic-based plasmonic emitters for sensing applications.

Plasmonic emissions generated by excitation of an organic layer on a metal grating structure are demonstrated. The emissions correspond to the resonant condition of surface plasmon (SP) modes on the Alq(3)/Au interface, and the grating structure is coupled to the Au/air interface to provide light emissions. Experimental variations in pitch to control plasmonic bandgap obtained highly directional plasmonic emissions with enhanced intensity. This method is readily applicable for detecting refractive index changes by using SP-coupled fluorophores to obtain emissions of varying wavelengths and viewing angles. The calculations showed that the wavelength of the plasmonic emitter changed from 480 to 680 nm at certain viewing angles, while the concentration of contacting glucose increased from 10% to 40%. Accordingly, a device with a pitch size of 500 nm had a sensitivity of Δθe/Δn=37.76° and Δn/Δ=1.681×10(-4) RIU (refractive index unit). Therefore, the proposed approach has potential applications in low-cost, disposable, point-of-care biosensors.

Power Efficiency Enhancement of Organic Light-Emitting Diodes Due to the Favorable Horizontal Orientation of a Naphthyridine-Based Thermally Activated Delayed Fluorescence Luminophore.

Four emitters based on the naphthyridine acceptor moiety and various donor units exhibiting thermally activated delayed fluorescence (TADF) were designed and synthesized. The emitters exhibited excellent TADF properties with a small ΔE ST and a high photoluminescence quantum yield. A green TADF organic light-emitting diode based on 10-(4-(1,8-naphthyridin-2-yl)phenyl)-10H-phenothiazine exhibited a maximum external quantum efficiency of 16.4% with Commission Internationale de L'éclairage coordinates of (0.368, 0.569) as well as a high current and power efficiency of 58.6 cd/A and 57.1 lm/W, respectively. The supreme power efficiency is a record-high value among the reported values of devices with naphthyridine-based emitters. This results from its high photoluminescence quantum yield, efficient TADF, and horizontal molecular orientation. The molecular orientations of the films of the host and the host doped with the naphthyridine emitter were explored by angle-dependent photoluminescence and grazing-incidence small-angle X-ray scattering (GIWAXS). The orientation order parameters (ΘADPL) were found to be 0.37, 0.45, 0.62, and 0.74 for the naphthyridine dopants with dimethylacridan, carbazole, phenoxazine, and phenothiazine donor moieties, respectively. These results were also proven by GIWAXS measurement. The derivative of naphthyridine and phenothiazine was shown to be more flexible to align with the host and to show the favorable horizontal molecular orientation and crystalline domain size, benefiting the outcoupling efficiency and contributing to the device efficiency.

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters.

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQEmax) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQEmax of 15.4% and CIE coordinates of (0.14, 0.09). The EQEmax of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S1 state to give lower ΔEST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Semi-Transparent, Pixel-Free Upconversion Goggles with Dual Audio-Visual Communication.

The intractable brittleness and opacity of the crystalline semiconductor restrict the prospect of developing low-cost imaging systems. Here, infrared visualization technologies are established with large-area, semi-transparent organic upconversion devices that bring high-resolution invisible images into sight without photolithography. To exploit all photoinduced charge carriers, a monolithic device structure is proposed built on the infrared-selective, single-component charge generation layer of chloroaluminum phthalocyanine (ClAlPc) coupled to two visible light-emitting layers manipulated with unipolar charges. Transient pump-probe spectroscopy reveals that the ClAlPc-based device exhibits an efficient charge dissociation process under forward bias. This process is indicated by the prompt and strong features of electroabsorption screening. Furthermore, by imposing the electric field, the ultrafast excited state dynamic suggests a prolonged charge carrier lifetime from the ClAlPc, which facilitates the charge utilization for upconversion luminance. For the first time, >30% of the infrared photons are utilized without photomultiplication strategies owing to the trivial spectrum overlap between ClAlPc and the emitter. In addition, the device can broadcast the acoustic signal by synchronizing the device frequency with the light source, which enables to operate it in dual audio-visual mode. The work demonstrates the potential of upconversion devices for affordable infrared imaging in wearable electronics.

Transparent organic upconversion devices displaying high-resolution, single-pixel, low-power infrared images perceived by human vision.

Crystalline photodiodes remain the most viable infrared sensing technology of choice, yet the opacity and the limitation in pixel size reduction per se restrict their development for supporting high-resolution in situ infrared images. In this work, we propose an all-organic non-fullerene-based upconversion device that brings invisible infrared signal into human vision via exciplex cohost light-emissive system. The device reaches an infrared-to-visible upconversion efficiency of 12.56% by resolving the 940-nm infrared signal (power density of 103.8 µW cm-2). We tailor a semitransparent (AVT, ~60%), large-area (10.35 cm2), lightweight (22.91 g), single-pixel upconversion panel to visualize the infrared power density down to 0.75 µW cm2, inferring a bias-switching linear dynamic range approaching 80 dB. We also demonstrate the possibility of visualizing low-intensity infrared signals from the Face ID and LiDAR, which should fill the gap in the existing technology based on pixelated complementary metal-oxide semiconductors with optical lenses.

Optical effects of shadow masks on short circuit current of organic photovoltaic devices.

In this paper, we have employed different shadow masks attached on top of organic photovoltaic (OPV) devices to study the optical effects of the former on the short circuit current (J(SC)). To rule out possible lateral electrical conduction and simplify the optical effects inside the device, a small-molecular heterojunction OPV device with a clear donor/acceptor interface was employed with a hole extraction layer exhibiting high resistance intentionally. Careful calibration with a shadow mask was employed. By attaching two layers of opaque masks in combination with a suitable holder design to shield the light from the edges and backside, the value of J(SC) approached that of the dark current, even under 1-sun radiation. With different illumination areas, we found that the photons illuminating the non-active region of the device contributed to 40% of the J(SC) by optical effect within the width of about 1 mm around the active region. When illuminating the non-active area with 12 mm to the active area, a 5.6 times improvement in the J(SC) was observed when the incident angle was 75°. With the introduction of a microstructured film onto the OPV device and an increase in the reflection from the non-active region, a 15% enhancement of the J(SC) compared to the control device was achieved.

An extended π-backbone for highly efficient near-infrared thermally activated delayed fluorescence with enhanced horizontal molecular orientation.

Near-infrared thermally activated delayed fluorescence (NIR-TADF) materials with emission over 700 nm have been insufficiently investigated mainly due to the limited choice of strong donor/acceptor units for molecular construction and the limited electronic coupling between the donors and acceptors. Herein, a novel D-A1-A2-A3 configuration was developed for the design of a NIR-TADF material (TPA-CN-N4-2PY), in which three types of sub-acceptor units (CN: cyano; N4: dipyrido[3,2-a:2',3'-c]phenazine; PY: pyridine) were incorporated into a molecular skeleton to reinforce the electron-accepting strength. The attachment of two pyridine units on TPA-CN-N4 produced TPA-CN-N4-2PY with an extended π-backbone, which shifted the electroluminescence (EL) emission into the NIR region and enhanced the horizontal ratio of emitting dipole orientation (Θ//) simultaneously. TPA-CN-N4-2PY-based OLEDs demonstrated a record-high external quantum efficiency (EQE) of 21.9% with an emission peak at 712 nm and Θ// = 85% at the doping ratio of 9.0 wt%. On the contrary, the parent compound TPA-CN-N4-based OLEDs at the same doping ratio achieved an EQE of 23.4% at 678 nm with Θ// = 75%. This multiple sub-acceptors approach could enrich the design strategy of the NIR-TADF materials, and the large conjugated system could improve the Θ// for achieving efficient emitters.

High photoelectric conversion efficiency of metal phthalocyanine/fullerene heterojunction photovoltaic device.

This paper introduces the fundamental physical characteristics of organic photovoltaic (OPV) devices. Photoelectric conversion efficiency is crucial to the evaluation of quality in OPV devices, and enhancing efficiency has been spurring on researchers to seek alternatives to this problem. In this paper, we focus on organic photovoltaic (OPV) devices and review several approaches to enhance the energy conversion efficiency of small molecular heterojunction OPV devices based on an optimal metal-phthalocyanine/fullerene (C(60)) planar heterojunction thin film structure. For the sake of discussion, these mechanisms have been divided into electrical and optical sections: (1) Electrical: Modification on electrodes or active regions to benefit carrier injection, charge transport and exciton dissociation; (2) Optical: Optional architectures or infilling to promote photon confinement and enhance absorption.

Nanostructured Molybdenum Trioxide Layer on the Silver Anode of a Top-Incident Organic Photovoltaic Device.

A nanostructured molybdenum trioxide (MoO3) layer was successfully fabricated utilizing various deposition rates, employed as an anodic buffer layer to separate the active layer from a silver anode and modifying the anodic surface to facilitate hole transportation for top-incident organic photovoltaic (TIOPV) devices. The deposition rate and thickness of the MoO3 layer were crucial parameters for determining the surface morphology and work function, and the internal optical field distribution, respectively. These factors affected the performance of the devices in terms of their open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). The baseline TIOPV device without a buffer layer had a power conversion efficiency (PCE) of only 0.47%. By contrast, with a smooth 20-nm MoO3 buffer layer fabricated using a deposition rate of 1 Å/s (which prevented problems caused by the Ag anode), another fabricated TIOPV device had substantially higher VOC, JSC and FF values, which improved the PCE by a factor of 6.2 to 2.92%. When an additional 5-nm nanostructured MoO3 layer was deposited at a deposition rate of 0.5 Å/s, the most efficient TIOPV device had an even greater PCE, a factor of 7.5 times higher at 3.53%.

Lifetime elongation of quantum-dot light-emitting diodes by inhibiting the degradation of hole transport layer.

Developing a colloidal quantum-dot light-emitting device (QDLED) with high efficiency and good reliability is necessarily preliminary for the next-generation high-quality display application. Most QDLED reports are focused on efficiency improvement, but the device operational lifetime issue is less addressed and also the relevant degradation mechanisms. This study achieved a 1.72 times elongation in the operational lifetime and a 9 times improvement in the efficiency of QDLED by inserting a hole-transporting/electron-blocking poly(9-vinylcarbazole) (PVK) layer, which prevented operational degradation on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-secbutylphenyl))-diphenylamine)] (TFB) hole-transporting layer and also confined the electron in the QD-emitting layer. Although the TFB/PVK HTL structure is a well-known pair to enhance the device performance, its detailed mechanisms were rarely mentioned, especially for relative operational lifetime issues. Herein, a new insight behind operational lifetime elongation of QDLED is disclosed through various fundamental experiments including steady-state photoluminescence, transient electroluminescence and single-carrier only devices. Evidently, other than QD degradation, this study found that the other crucial factor that decreased the device lifetime was TFB-HTL degradation using steady-state photoluminescence and transient electroluminescence analyses. The PVK electron-only device exhibited a stable voltage value when it was driven by fixed current, which also affirmed that PVK has excellent electron-stability characteristics.

Deep Blue Fluorescent Material with an Extremely High Ratio of Horizontal Orientation to Enhance Light Outcoupling Efficiency (44%) and External Quantum Efficiency in Doped and Non-Doped Organic Light-Emitting Diodes.

A novel bis-4Ph-substituted 9,10-dipehnylanthracene deep blue [1931 CIE (0.15, 0.08)] fluorescent compound, AnB4Ph, has been synthesized and characterized for organic light-emitting diode (OLED) applications. Our experimental study of AnB4Ph excludes the possibility of triplet-triplet annihilation, hybridized local and charge transfer, or thermally activated delayed fluorescent characteristics of the material. Since the solid-state photoluminescence quantum yield of AnB4Ph was determined to be 48%, assuming a 100% for the charge recombination efficiency, the light outcoupling efficiency (ηout) of an AnB4Ph non-doped OLED achieving an external quantum efficiency (EQE) of 5.3% is at least 44%, which is more than twofold higher than 20% for conventional OLEDs. Both grazing incidence wide-angle X-ray scattering (GIWAXS) and angle-dependent photoluminescence (ADPL) measurements reveal AnB4Ph having a high value of order parameter (SGIWAXS) of 0.61 for a ππ stacking along the normal direction and an orientation order parameter (SADPL) for a horizontal emitting dipole moment of -0.50 or Θ (horizontal-dipole ratios) of 100%, respectively. Otherwise, a refractive index (n) measurement provides a n = 1.80 for AnB4Ph thin films. Based on ηout = 1.2 × n-2, the calculated ηout is 37%, which is also in accordance with the results of GIWAXS and ADPL. We have also fabricated the classical fluorescent DPAVBi-doped AnB4Ph OLEDs, which display a true blue [1931 CIE (0.15 and 0.16)] electroluminescence with a high efficiency (EQE = 6.9%), surpassing the conventional ∼5% EQE. Based on an ηout of 42% for DPAVBi-doped AnB4Ph OLEDs, our studies suggest that the extremely horizontally aligned AnB4Ph host material exerts the same horizontal alignment on the DPAVBi dopant molecules.

Emitter apodization dependent angular luminance enhancement of microlens-array film attached organic light-emitting devices.

Taking organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of a microlens-array-film (MAF) attached OLED (organic light-emitting device) can be further evaluated by ray-tracing approach. First, we assumed artificial emitters and revealed that not every OLED with MAF has luminance enhancement. Then, the OLEDs of different Alq(3) thickness were fabricated and their angular luminance measurement validated simulation results. Mode analyses for different layers were performed to estimate the enhancement potential of the MAF attached devices. In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF attached.

Partitioning pixel of organic light-emitting devices with center-hollowed microlens-array films for efficiency enhancement.

A pixel partition scheme assisted with patterned or center-hollowed microlens-array films (MAFs) was proposed to improve the optical characteristics and electrical properties of organic light-emitting diodes (OLEDs). In our optical simulation results, a pixel of 1 × 1 mm(2) with a center-hollowed MAF has a 42% luminance enhancement; however, after dividing the large pixel into ten by ten smaller pixels of 100 × 100 µm(2), the partitioned units with a corresponding center-hollowed MAF can have a 104% luminance enhancement under the same total active area and the same optical power of organic emitters. Furthermore, a significant 127% luminance enhancement by the introduction of a high-refractive-index substrate can be obtained.

Morphological control of the electrochemically deposited poly(4-vinyltriphenylamines) (PVTPAs).

The morphology of the thin films obtained from electrochemical deposition of poly(4-vinyltriphenylamine)s (PVTPAs) on ITO surface varies as their molecular weight increases. The SEM studies revealed that while uniform coating could be obtained from low molecular weight PVTPAs (approximately 2700), uneven and rough coating with numerous cracks on the surface would be formed from high molecular weight PVTPAs. These defects could be reduced by using our surface modification strategy, in which the ITO surface was first primed with a thin layer of electrochemical coating of poly(methylenetriphenylamine) (PMTPA). This coating would turn the highly polar ITO surface into a nonpolar conducting surface that benefits for the PVTPA deposition. Our electroluminescence test suggested that this type of PMTPA/PVTPA composite film is good for hole injection and transportation.