Zhen-Gang Zhu

EXACT SOLUTIONS OF NONLINEAR EQUATIONS

Abstract Based on the homogeneous balance method, a general method is suggested to obtain several kinds of exact solutions for some kinds of nonlinear equations. The validity and reliability of the method is tested by applying it to the Bousseneq equation.

Achieving High-Performance Ternary Organic Solar Cells through Tuning Acceptor Alloy

Acceptor alloys based on n-type small molecular and fullerene derivatives are used to fabricate the ternary solar cell. The highest performance of optimized ternary device is 10.4%, which is the highest efficiency for one donor/two acceptors-based ternary systems. Three important parameters, JSC , VOC , and FF, of the optimized ternary device are all higher than the binary reference devices.

Highly efficient light management for perovskite solar cells

Organic-inorganic halide perovskite solar cells have enormous potential to impact the existing photovoltaic industry. As realizing a higher conversion efficiency of the solar cell is still the most crucial task, a great number of schemes were proposed to minimize the carrier loss by optimizing the electrical properties of the perovskite solar cells. Here, we focus on another significant aspect that is to minimize the light loss by optimizing the light management to gain a high efficiency for perovskite solar cells. In our scheme, the slotted and inverted prism structured SiO2 layers are adopted to trap more light into the solar cells, and a better transparent conducting oxide layer is employed to reduce the parasitic absorption. For such an implementation, the efficiency and the serviceable angle of the perovskite solar cell can be promoted impressively. This proposal would shed new light on developing the high-performance perovskite solar cells.

Slow dynamics of the α and α' relaxation processes in poly(methyl methacrylate) through the glass transition studied by mechanical spectroscopy

In this paper, low-frequency mechanical spectroscopy has been used to study poly(methyl methacrylate) (PMMA) melt around the glass transition temperature Tg for shedding light on its unique relaxation behaviors. The mechanical spectra show an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. The shoulder peak corresponds to the α relaxation due to the local segmental motion; while the maximum peak is assigned to the α′ relaxation due to the slow motion of longer chain segments or chains. The α relaxation mode has a stronger temperature dependence of relaxation time than the α′ mode, causing the α′ mode to merge with the α mode with decreasing temperature toward Tg. Time-temperature superposition (TTS) breaks down in the entire temperature range due to the different friction coefficients of the α and α′ relaxations. For only the α′ process, TTS is found to hold for PMMA melt, where the high-frequency decay of the mechanical loss d...

Longer-scale segmental dynamics of amorphous poly(ethylene oxide)/poly(vinyl acetate) blends in the softening dispersion

To study the influence of poly(ethylene oxide) (PEO) on the relaxation process of poly(vinyl acetate) (PVAc) in the glass to rubber softening dispersion, the mechanical loss behaviour of PEO/PVAc blends (with PEO contents up to 20 wt%) were measured as a function of temperature and frequency. It is shown that the PEO component has a plasticizing effect on the α and α′ relaxation modes of PVAc, where the α mode is ascribed to the local segmental mode and the α′ mode is related to the motion of longer chain segments in the softening dispersion, composed of the sub-Rouse modes and the Rouse modes. Both the α and α′ modes shifted to lower temperatures with increase of PEO concentration. Time–temperature superposition breaks down for PEO/PVAc blends in the entire temperature range due to the different friction coefficients of the α and α′ modes. A single master curve of the α′ mode for pure PVAc and the PEO/PVAc blends is obtained, suggesting that the size of chain segments containing on the order of 10 to 50 or more backbone bonds does not change upon blending. Furthermore, the dynamics of the α′ mode of the PVAc component in the blends is found to show a characteristic crossover through the temperature dependence of relaxation time and relaxation strength. The crossover temperature TB decreases with increasing PEO content, while the crossover relaxation time for the blends is constant with a value about 0.08 s, much longer than 10−7 ± 1 s for the α mode. According to the coupling model, the crossover is suggested to be due to the variation of intermolecular coupling at TB.

Time-dependent spin-polarized transport through a resonant tunneling structure with multiterminals

The spin-dependent transport of the electrons tunneling through a resonant tunneling structure with ferromagnetic multi-terminal under dc and ac fields is explored by means of the nonequilibrium Green function technique. A general formulation for the time-dependent current and the time-averaged current is established. As its application the systems with two and three terminals in noncollinear configurations of the magnetizations under dc and ac biases are investigated, respectively. The asymmetric factor of the relaxation times for the electrons with different spin in the central region is uncovered to bring about various behaviours of the TMR. The present three-terminal device is different from that discussed in literature, which is coined as a spin transistor with source. The current-amplification effect is found. In addition, the time-dependent spin transport for the two-terminal device is studied. It is found that the photonic sidebands provide new channels for the electrons tunneling through the barriers, and give rise to new resonances of the TMR, which is called as the photon-asisted spin-dependent tunneling. The asymmetric factor of the relaxation times is observed to lead to additional resonant peaks besides the photon-asisted resonances.

Mechanical relaxation studies of α and slow β processes in Nd65Fe15Co10Al10 bulk metallic glass

The relaxation dynamics of the primary α and secondary β processes in Nd65Fe15Co10Al10 bulk metallic glass has been investigated by using low-frequency mechanical spectroscopy, differential scanning calorimetry (DSC) and x-ray diffraction. From the shift of internal friction peaks by the frequency change, the activation energy of β relaxation (Eβ) is found to be about 1.01 eV, and a correlation between Eβ and glass transition temperature (Tg) is found Eβ ≈ 24 RTg, indicating that the β relaxation is intrinsic in metallic glasses. According to the coupling model, the uncoupled activation energy of α relaxation (Eα*) is found to be about 1.38 eV, still larger than the value of Eβ corresponding to a simple Debye relaxation process. The possible mechanism and the connection between α and β relaxations are discussed.

Tuning Voc for high performance organic ternary solar cells with non-fullerene acceptor alloys

Open circuit voltage (Voc) is a critical parameter for ternary organic solar cells, while its mechanism is obscure. Here we employed two non-fullerene molecules TPE-4PDI and FT-2PDI (perylenediimide, a PDI-based small molecule) to form acceptor alloys with ITIC-Th (indacenodithieno, an IDDT-based small molecule) for ternary systems. The results demonstrated that the experimental Voc values fit the simulation data accurately based on the equation not only in our new ternary systems but also in other reported small molecular alloy based ternary systems. More importantly, TPE-4PDI is more efficient to enhance the Voc of ternary solar cells as the third component than FT-2PDI, since TPE-4PDI possesses a larger quasi frontier orbital density (Ne) value. Thus, upon tuning the weight ratio of the TPE-4PDI:ITIC-Th acceptor alloy, high performance ternary solar cells with an efficiency over 11% were achieved. This contribution has shed light on understanding the mechanisms of ternary solar cells and demonstrated a method for enhancing Voc efficiently to achieve high performance solar cells.

Thermally Driven Pure Spin and Valley Currents via the Anomalous Nernst Effect in Monolayer Group-VI Dichalcogenides.

The spin and valley-dependent anomalous Nernst effects are analyzed for monolayer MoS_{2} and other group-VI dichalcogenides. We find that pure spin and valley currents can be generated perpendicular to the applied thermal gradient in the plane of these two-dimensional materials. This effect provides a versatile platform for applications of spin caloritronics. A spin current purity factor is introduced to quantify this effect. When time reversal symmetry is violated, e.g., two-dimensional materials on an insulating magnetic substrate, a dip-peak feature appears for the total Nernst coefficient. For the dip state it is found that carriers with only one spin and from one valley are driven by the temperature gradient.

Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy

Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.