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The single relaxation time (SRT) and the revised matrix (RM) lattice Boltzmann models are compared for simulations of three dimensional forced isotropic turbulence with resolutions of 1283 and 2563, respectively. Guo et al.[1] forcing technique with the same forcing parameters and conditions is used to investigate the ability and stability of the two models for simulations of forced turbulence. Some new aspects and results have been confirmed such as the superiority of the MRT model to simulate forced turbulence in comparison with the SRT model. To overcome the stability shortage problem of the lattice Boltzmann models, it is proposed to apply the Courant-Friedichs-Lewy condition (CFL) [2] by multiplying the velocity input with the CFL coefficient which is less than unity and by dividing the output velocity data by the same CFL coeffi-cent. The initial velocity field is chosen as u(x, 0) = 0 and the force is injected at low wave-numbers with a fixed forcing amplitude of 10−4 for all cases. The single relaxation time is set to τ = 0.503 in all SRT simulations. Results show that the obtained turbulent velocity fields yield universal characteristics as proven in previous theoretical, experimental and numerical studies. The Taylor Reynolds number for the simulations are found as : Rλ(SRT) = 62 and Rλ(M RT) = 65 for the resolution of 1283 and Rλ(SRT) = 107 and Rλ(M RT) = 82 for the case of 2563, respectively. To test the weak incompressibility for the SRT model in comparison to the MRT model, the probability distribution function (PDF) of the density ρ is depicted and it is found that ρ is almost unity at all timesteps for the MRT case, while a clear disturbance about unity is observed for the SRT case. Time variation’s statistics such as the Taylor Reynolds number Rλ, the Taylor microscale λ, the Kolmogrov microscale η and energy spectra are calculated and depicted for all cases. One of the disadvantages of the MRT case is the simulations time which is found about twice the SRT simulations time, where about 80 hours are needed for simulating 12000 timesteps in the 2563 case in comparison to 42 hours for the SRT model using the same hardware resources.

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.

Single-crystalline organic semiconductors exhibiting band transport have opened new possibilities for the utilization of efficient charge carrier conduction in organic electronic devices. The epitaxial growth of molecular materials is a promising route for the realization of well-crystallized organic semiconductor p–n junctions for optoelectronic applications enhanced by the improved charge carrier mobility. In this study, the formation of a high-quality crystalline interface upon “quasi-homoepitaxial” growth of bis(trifluoromethyl)dimethylrubrene (fmRub) on the single-crystal surface of rubrene was revealed by using out-of-plane and grazing-incidence X-ray diffraction techniques. Ultraviolet photoelectron spectroscopy results indicated abrupt electronic energy levels and the occurrence of band bending across this quasi-homoepitaxial interface. This study verifies that the minimization of the lattice mismatch enhances the crystalline qualities at the heterojunctions even for van der Waals molecular condensates, potentially opening an untested route for the realization of high-mobility organic semiconductor optoelectronics.

A two dimensional partial differential equation scheme that uses a second-order hyperbolic diffusion equation for image denoising is developed to a three dimensional model for filtering isotropic turbulence. The mathematical derivation of the model is introduced and a consistent finite difference numerical approximation scheme is proposed. The model is tested against the velocity fields of isotropic turbulence that are simulated by solving the lattice Boltzmann(LB) and the Navier–Stokes(NS) equations, respectively. The D3Q15 LB model and the Fourier spectral method are used to solve the LB equation and the Navier–Stokes vorticity equation at grid points of 2563, respectively. Also the filtering method with the choice of the same filtering parameters is applied against a synthetic turbulent field with the same number of points. The high rotation vortex identification method Q is used to visualize the total, coherent and incoherent fields in all cases of the study. Despite the different nature of the LB and NS direct numerical simulations and the synthetic turbulence, all input parameters for the hyperbolic filtering model are chosen the same in all cases. Comparisons between the LB and NS filtered energy spectra, coherent vortices, incoherent regions, skewness, flatness and the fourth order moments are also considered. The same features are calculated for the non-filtered and filtered synthetic turbulent fields. It is shown that the coherent part preserves all statistical features of turbulence. However, the isotropy is the only feature that has been preserved for the incoherent part and other statistical features are divergent in all cases of the study.

In this study, attempts to suppress numerical viscosity in incompressible smoothed particle hydrodynamics (SPH) computations are reported. Two-dimensional computations are performed for inviscid and viscous flows to evaluate the effects of numerical viscosity suppression. The first approach is to reduce numerical viscosity at the wall by considering only the wall-normal components of the forces between fluid particles and wall particles. The second approach is to reduce numerical viscosity within the flow field by employing elliptic kernel functions whose major axes are aligned with the local mean flow direction. It is found that special treatment of the wall radically reduces the numerical wall friction. Using an elliptic kernel function is found to work reasonably well in reducing numerical viscosity.

Numerical simulations of the liquid flow scattering from rotary atomizers are performed using an incompressible smoothed particle hydrodynamics (SPH) method. The influence of grooves at the edges of the atomizers on the formation of ligaments and droplets is investigated changing the numbers and shapes of the grooves. As a result, it is found that small droplets are likely to be generated when the number of grooves is large and the depth of grooves is deep. It is also found that the grooves work more effectively in bell-cup atomizers than in disk type atomizers. Keywords: Smoothed particle hydrodynamics (SPH), Rotary atomizer, Grooves, Droplet

Energy of photons, i.e. the wavelength of light, can be upgraded through interactions with materials—a process called photon upconversion (UC). Although UC in organic solids is important for various applications, such as in photovoltaics and bioimaging, conventional UC systems, based on intersystem crossing (ISC), suffer from low efficiency. Herein, we report a novel UC mechanism at heterojunctions of organic semiconductors in bilayer structures. The UC occurs by triplet formation during the charge separation and recombination through dynamic exciton: charge transfer (CT) state at the interface. This can efficiently convert the incident photons to triplets without relying on the ISC, whose rate is typically accelerated by the heavy-atom effect. As a result, a solid-state UC system is achieved with an external efficiency of two orders of magnitude higher than those of the conventional systems. Using this result, efficient UC, from near-infrared to visible light, cab be realized on flexible organic thin films under a weak light-emitting diode-induced excitation, observable by naked eyes. Reference: S. Izawa and M. Hiramoto, Nat. Photon., DOI: org/10.1038/s41566-021-00904-w.

Charge transfer (CT) state is a key intermediate to understand recombination processes in organic solar cells (OSCs). In this study, we measured the recombination emission from the CT state under different applied voltages in OSCs and a photocurrent density flowing on the circuit simultaneously. We proposed a “photoluminescence (PL)–voltage (V) plot” that is the voltage dependence of PL intensity of the CT state. The PL–V plot includes information only from the CT state recombination at the donor/acceptor interface and is complementary to the “current density (J)–V plot” that is the most important information for evaluating OSCs. The results demonstrated that the fill factor (FF) of the PL–V plot is higher than that of the J–V plot, predicting the ideal FF of the device. Our result demonstrated that the simultaneous measurement of photocurrent and recombination emission could be a strong tool for evaluating photoconversion characteristics in OSCs.

Reducing operating voltage is the remaining frontier for organic light-emitting diodes (OLEDs) because their quantum efficiency (QE) of electroluminescence has been already maximized. Herein, we report an efficient OLED in which a blight emission equivalent to a luminance of a display is achieved by only a 1.5 V battery. The OLED is based on upconversion (UC) emission utilizing triplet-triplet annihilation occurring near donor/acceptor (D/A) interface. We found that a character of a charge transfer state that is key intermediate for the UC emission could be controlled by D/A interfacial interaction. As a result, parasitic loss processes for UC were greatly suppressed from over 90% to about 10%, and two order of magnitude higher QE than the previous UC-OLED was achieved. Our result demonstrated that the efficient UC could be realized by the management of the energy transfer steps at the D/A interface and utilizing UC emission can be one of the possible candidate for efficient OLED with extremely low driving voltage.

In the field of organic solar cells, lateral junction is a new concept. Moreover, the photovoltaic behavior is firstly observed for surprising long organic lateral junctions reaching 1.8 cm and the new operation mechanism dominated by trap-assisted recombination is proposed.

In the field of organic solar cells, lateral junction is a new concept. Moreover, the photovoltaic behavior is firstly observed for surprising long organic lateral junctions reaching 1.8 cm and the new operation mechanism dominated by trap-assisted recombination is proposed.

An experimental study is performed to investigate the flow structures near the edges of rotating disks with different edge shapes. By changing the rotational speed, the Reynolds number is changed from 277 to 356. It is found that the fluid motion near the sharp edge differs depending on whether a rotating shaft is at the disk surface. A flow developing on the flat side of the disk surface changes its direction toward the curved outer surface on the shaft side, and the flow goes straight outward, leaving the disk on the flat side. The existence of the supporting shaft increases the radial velocity of the flow while decreasing the azimuthal velocity. The effects of the edge shape of the disk on the flow fields are also investigated by changing the shape of the disk edges. Rounded and chamfered edges have no noticeable effect on the azimuthal velocity on the curved outer surface, whereas changing the edge shape enhanced the velocity in the disk-thickness direction. In the FFT analyses of the azimuthal velocity measured at the edge of the curved outer surface when the disk edge is rounded, an increase in power across a range of frequencies is observed. Only in the chamfered edge disk case, a peak in the spectrum of the velocity that corresponds to a wavenumber which appears in the transitional boundary layers is observed.

Perylene has had a tremendous impact in the history of material research for the molecular semiconductors. Among numerous derivatives of this polyaromatic hydrocarbon, perylene diimide (PDI) represents a promising class of organic materials envisioned as non-fullerene acceptors (NFAs) for the practical organic photovoltaic (OPV) applications due to their enhanced photo- and thermal stability and remarkably high electron affinity, some of which realize band-like transport properties. The present review guides some of the representative achievements in the development of rationally designed PDI systems, highlighting synthetic methodologies based on bay-functionalization strategies for creating well-designed molecular nanostructures and structure-performance relationship of perylene-based small molecular acceptors (SMAs) for the photovoltaic outcomes.

Clarifying critical differences in free charge generation and recombination processes between inorganic and organic semiconductors is important for developing efficient organic photoconversion devices such as solar cells (SCs) and photodetector. In this study, we analyzed the dependence of doping concentration on the photoconversion process at the organic pn-homojunction interface in a single organic semiconductor using the temperature dependence of J&ndash, V characteristics and energy structure measurements. Even though the organic pn-homojunction SC devices were fabricated using a single host material and the doping technique resembling an inorganic pn-homojunction, the charge generation and recombination mechanisms are similar to that of conventional donor/acceptor (D/A) type organic SCs, that is, the charge separation happens from localized exciton and charge transfer (CT) state being separated by the energy offset between adjacent molecules, and the recombination happens from localized charge carrier at two adjacent molecules. The determining factor for photoconversion processes is the localized nature of charges in organic semiconductors. The results demonstrated that controlling the delocalization of the charges is important to realize efficient organic photoconversion devices.

In this study, the operation of donor/acceptor photovoltaic cells fabricated on homoepitaxially grown p-doped rubrene single-crystal substrates is demonstrated. The photocurrent density is dominated by the sheet conductivity (&sigma, □) of the p-type single-crystal layer doped to 100 ppm with an iron chloride (Fe2Cl6) acceptor. A 65 mm thick p-type rubrene single-crystal substrate is expected to be required for a photocurrent density of 20 mA&middot, cm&minus, 2. An entire bulk doping technique for rubrene single crystals is indispensable for the fabrication of practical organic single-crystal solar cells.

Homoepitaxial growth of organic semiconductor single crystals is a promising methodology toward the establishment of doping technology for organic opto-electronic applications. In this study, both electronic and crystallographic properties of homoepitaxially grown single crystals of rubrene were accurately examined. Undistorted lattice structures of homoepitaxial rubrene were confirmed by high-resolution analyses of grazing-incidence X-ray diffraction (GIXD) using synchrotron radiation. Upon bulk doping of acceptor molecules into the homoepitaxial single crystals of rubrene, highly sensitive photoelectron yield spectroscopy (PYS) measurements unveiled a transition of the electronic states, from induction of hole states at the valence band maximum at an adequate doping ratio (10 ppm), to disturbance of the valence band itself for excessive ratios (&ge, 1000 ppm), probably due to the lattice distortion.

Direct numerical simulations are carried out to investigate the role of the turbulent region in a self-sustaining system with a spiral vortex structure in the three-dimensional boundary layer over a rotating disk by solving the full Navier–Stokes equations. Two computational domains with two different azimuthal sizes, $2\unicode[STIX]{x03C0}/68$ and $2\unicode[STIX]{x03C0}/32$, are used to deal with different initially dominant wavenumbers. An artificial disturbance is introduced by short-duration strong suction and blowing on the disk surface. After the flow field reaches a steady state, a turbulent region forms downstream of $Re=640$. The turbulent region is then removed using two methods: a sponge region, and application of a slip condition at the wall. In both cases, the turbulent region disappears, leaving the spiral vortex structure upstream unaffected. The results suggest that the downstream turbulent region is not related to the velocity fluctuations that grow by the global instability. In addition, when the area where the slip condition is applied is changed from $Re>630$ to $Re>610$, the velocity fluctuations decay. The results indicate that the vibration source of the velocity fluctuations which grow by the global instability is located between $Re=611$ and $Re=630$.

We report the fabrication and operation of organic photovoltaic cells with lateral junctions and separated carrier-generating and carrier-transporting layers. Significant photocurrent increase was observed by inserting carrier generating layer having strong visible absorption. For the lateral tandem cells, an increase in the number of unit cells increased the photocurrent while keeping the photovoltage constant. It is easier to increase the number of unit cells in lateral tandem cells compared to conventional vertical tandem cells because the adjustment of photocurrent density flowing in each unit cell is no longer necessary.

In this study, the photovoltaic behavior of centimeter-long lateral organic junctions, reaching 1.8 cm, is reported. The organic junctions are formed using organic semiconductor films with high mobilities of holes and electrons. The lateral diffusion lengths of photogenerated electrons and holes are 4.7 and 5.5 mm, respectively. The photovoltaic behavior in the centimeter-long lateral junctions is controlled by the trap-assisted recombination between the electrons and holes.

Preparation of highly crystalline organic semiconductor films is vital to achieving high performance in electronic devices. Here we report that surface segregated monolayers (SSMs) on top of phenyl-C61-butyric acid methyl ester (PCBM) thin films induce crystal growth in the bulk, resulting in a dramatic change in the structure to form a new crystal phase. Highly ordered crystalline films with large domain sizes of several hundreds of nanometers are formed with uniaxial orientation of the crystal structure perpendicular to the substrate. The molecular rearrangements in SSMs trigger the nucleation at a lower temperature than that for the spontaneous nucleation in PCBM. The vertical charge mobility in the SSM-induced crystal domains of PCBM is five times higher than in the ordinary polycrystalline domains. Using surface monolayers may be a new strategy for controlling crystal structures and obtaining high-quality organic thin films by post-deposition crystallization.

The effect of thermal annealing on the energy levels of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 °C induces reductions of both the ionization potential (IP) and the electron affinity (EA) with a narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and a 2.54% reduction in the film thickness. Precise measurements of both the IP and EA enabled an evaluation of the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.

Recent research on cellular circadian rhythms suggests that the coupling of transcription-translation feedback loops and intracellular redox oscillations is essential for robust circadian timekeeping. For clarification of the molecular mechanism underlying the circadian rhythm, methods that allow for the dynamic and simultaneous detection of transcription/translation and redox oscillations in living cells are needed. Herein, we report that the cyanobacterial circadian redox rhythm can be electrochemically detected based on extracellular electron transfer (EET), a process in which intracellular electrons are exchanged with an extracellular electrode. As the EET-based method is non-destructive, concurrent detection with transcription/translation rhythm using bioluminescent reporter strains becomes possible. An EET pathway that electrochemically connected the intracellular region of cyanobacterial cells with an extracellular electrode was constructed via a newly synthesized electron mediator with cell membrane permeability. In the presence of the mediator, the open circuit potential of the culture medium exhibited temperature-compensated rhythm with approximately 24 h periodicity. Importantly, such circadian rhythm of the open circuit potential was not observed in the absence of the electron mediator, indicating that the EET process conveys the dynamic information regarding the intracellular redox state to the extracellular electrode. These findings represent the first direct demonstration of the intracellular circadian redox rhythm of cyanobacterial cells.

A dyad molecule was synthesized, with oligo( p -phenylenevinylene) (OPV) as the electron donor and naphthalene diimide (NDI) as the acceptor. The optical, thermal, and X-ray diffraction measurements of the film structure showed that the OPV and NDI moieties of the dyad crystallized separately in the spin-coated films. This is in striking contrast to the previously reported OPV and C 70 fullerene dyad, in which the introduction of C 70 severely disrupted the OPV packing. The separated crystallization in the OPV–NDI dyad was attributed to the planar shape of the acceptor and the differences in the crystalline structures of planar NDI and rod-like OPV.

A wind tunnel experiment was performed to investigate the suitable conditions in which a localized turbulent region can easily be generated in a flat-plate boundary layer. An artificial disturbance of zero-mass flux was introduced upstream using a combination of a short-duration jet and suctions to prepare a potentially unstable environment. The disturbed region by itself decayed downstream. Another jet was then ejected downstream at several different timings and two different spanwise locations relative to the passage of the locally disturbed region to promote transition to turbulence. Although the jet was too weak to trigger the turbulence transition by itself, an isolated turbulent region, the so-called turbulent spot, was generated when ejected against the disturbed region. The optimum conditions were found when the jet was ejected between the high- and low-speed areas of the convecting unstable region.

Harmonic analysis filtering methods are used to decompose 3D forced and decaying turbulent fields into coherent and incoherent parts. The filtering methods are the curvelets, wavelet and dual-tree complex wavelet filtering techniques. The methods are applied against the velocity fields of forced and decaying homogeneous isotropic turbulence. The velocity fields are generated using the Lattice Boltzmann Method (LBM) with a resolution of 128^3 where the Taylor micro-scale Reynolds numbers are 72 and 29 for forced and decaying fields, respectively. The main objective of this study is to use other different filtering methods rather than depending only on wavelet and Fourier decompositions. Using different filtering methods may help in turbulence modeling, improving our knowledge about the structure of turbulent flow, choosing the appropriate filtering method and extracting general characteristics of the flow under study. Results show that the coherent and the random incoherent fields contribute to all scales in the inertial range, but the statistical behavior is different for each field. The three filtering methods approximately identify the coherent fields with the same geometrical shapes of the organized vortical structures. The statistical properties such as flatness, skewness and spectrum of the extracted fields (coherent and incoherent parts) are also investigated for each filtering method. The joint PDFs between the third and second invariants of the velocity gradient tensor are also studied. The joint PDFs for the coherent parts are found similar to previous topological studies of isotropic turbulence; however the incoherent PDFs show different topological features.

The streamwise interfaces of an isolated turbulent region in a flat plate boundary layer are investigated by performing a wind tunnel experiment. A bimorph-type piezoceramic actuator is used to generate a trapezoidal turbulent region whose spanwise width is larger than those of a typical turbulent spot. The experimental results show that the traveling speeds of the leading and trailing ends of the turbulent region are almost same as those of the turbulent spots with an arrow-head shape. This result indicate that the spanwise width have little influence to the streamwise growth of an isolated turbulent region. It is also found that the traveling speed of the trailing end of the turbulent region exceeds that of local flow velocity in the near-wall region, which is η < 1.3, and vice versa. This result shows that a relaminarization is in progress in the near-wall region, where the turbulent fluid is moving across the trailing end turning laminar. The critical wall-normal distance of η = 1.3 corresponds to about 10 in the wall unit (y+), suggesting that the relaminarization process is taking place in the viscous sublayer.

Most filtering methods for turbulent flow depend on wavelet and Fourier decompositions; however, this paper employs curvelets and total variation (TV) filtering methods. These two methods are investigated to extract the coherent and incoherent parts of a forced homogeneous isotropic turbulent field. The Lattice Boltzmann method with resolutions of 128 3 and 256 3 is used to simulate the turbulent fields, and the Q -identification method is applied to extract the elongated vortical structures. Most of the previous efforts apply filtering techniques to the velocity or vorticity fields; however, this paper applies the two filtering methods to the Q -field. 3D curvelet filtering is applied using a nonlinear thresholding technique. The 3D total variation method is applied using the split Bregman regularization method. The results indicate that the two filtering methods identify the coherent and incoherent parts smoothly. The results of the two filtering methods tend to identify coherent vortices and remove incoherent noise without any change in the physical structure of the turbulent fields. The results are compared with previous efforts using wavelet and Fourier decompositions.

The streamwise interface of an isolated turbulent region in a laminar flat plate boundary layer is investigated by performing a wind tunnel experiment. In the experiment, bimorph-type piezoceramic actuators are used to generate a trapezoidal-shaped turbulent region that has a wide interface in the spanwise direction. With the use of the piezoceramic actuators, the interference of the spanwise interface with the streamwise interface of the turbulent region is eliminated. Therefore, the streamwise interface can be considered as an independent interface. The experimental results show that the rms value of the velocity fluctuation and the turbulent fluctuation rapidly changes at the leading edge of the turbulent region. On the other hand, the rms value of the velocity fluctuation remains high after the turbulent fluctuation disappears at the trailing edge of the turbulent region. It is also shown that the traveling speed of the trailing edge of the turbulent region is faster than the ensemble-averaged streamwise velocity at the trailing edge of the turbulent region near the wall (η≤1.3). In addition, the velocity profile at the trailing edge of the turbulent region indicates the existence of the acceleration of the turbulent fluid near the wall. From the above results, it is found that the inverse transition occurs at the trailing edge of the turbulent region near the wall due to the acceleration of the turbulent fluid.