Vol. 64, No. 3 (2015)
2015-02-05
SPECIAL ISSUE — New generation solar cell
2015, 64 (3): 033301.
doi: 10.7498/aps.64.033301
Abstract +
Perovskite solar cells with a solid-state thin film structure have attracted great attention in recent years due to their simple structure, low production cost and superb photovoltaic performance. Because of the boost in power conversion efficiency (PCE) in short intervals from 3.8% to 19.3% at present, this hybrid cells have been considered as the next generation photovoltaic devices. It is expected that the efficiencies of individual devices could ultimately achieve 25%, which is comparable to the single-crystal silicon solar cell.In this article, the perovskite absorber, its basic device structure, and operating principles are briefly introduced. Since most of the high efficiency perovskite solar cells employ hole transporting materials (HTM), they could benefit the hole transport and improve the metal-semiconductor interface in the cells. This perspective gives analyses of some effective hole transporting materials for perovskite solar cell application. The hole transporting materials used in perovskite solar cell are classified into six categories according to their structures, including triphenylamine-based small molecule HTM, small molecule HTM containing N atom, sulfur-based small molecule HTM, sulfur-based polymer HTM, polymer HTM containing N atom and inorganic HTM. Emphasis is placed on the interplay of molecular structures, energy levels, and charge carrier mobility as well as device parameters. A critial look at various approaches applied to achieve desired materials and device performance is provided to assist in the identification of new directions and further advances.
2015, 64 (3): 038103.
doi: 10.7498/aps.64.038103
Abstract +
This review surveys the application of graphene and its derivatives in organic solar cells, used as interfacial layers: including anode interfacial layers, cathode interfacial layers, and intermediate layers in a tandem device. Research work has be done for increasing the electroconductivity by reducing the oxide to partially oxidized graphene, as well as chemically modifying or making composite interfacial layer. Additionally, the researches on graphene derivatives and combined interfacial layers used as a cathode interfacial layer or an intermediate layer in the tandem device are discussed. Finally, this review suggests that graphene and its derivatives are potential to be used in perovskite solar cells.
2015, 64 (3): 038104.
doi: 10.7498/aps.64.038104
Abstract +
Perovskite solar cell, which is prepared by using the organic-inorganic hybrid halide CH3NH3PbX3 (X = I, Cl and Br), receives widespread attention because of its solution processability and high photon-to-electron conversion efficiency. The highest reported photon-to-electron conversion efficiency is that using CH3NH3PbI(3-x)Clx as an absorber. It is reported that the diffusion length is greater than 1 micrometer in this mixed halide perovskite. The method most commonly used in preparing CH3NH3PbI(3-x)Clx film is the one-step pyrolysis method, which has a complex reaction mechanism. In this paper, we review the work about CH3NH3PbI(3-x)Clx perovskite, in which emphasis is put on the importance of the preparation process, and analyze the role of CH3NH3I in the one-step pyrolysis method for fabricating the CH3NH3PbI(3-x)Clxperovskite layer. Scanning electron microscope images show that CH3NH3I can improve the coverage and crystallinity of the perovskite layer for precursors in low CH3NH3I concentrations (CH3NH3I/PbCl2=2.0 and 2.5). For precursors in high CH3NH3I concentrations (CH3NH3I/PbCl2=2.75 and 3), this change is not obvious. X-ray photoelectron spectroscopy confirms the change of coverage, and indicates that the content of Cl in CH3NH3PbI(3-x)Clx will be less than 5% for precursors with high CH3NH3I concentrations (CH3NH3I/PbCl2>2.5). Perovskite solar cells based on CH3NH3PbI(3-x)Clx with different Cl dopant concentrations are studied by photoelectric measurements. Photocurrent density-photovoltage curves show that the performance of the devices increases with the increase of CH3NH3I concentration in precursors. And the incident-photon-to-current conversion efficiency (IPCE) measurements indicate that the devices fabricated by using precursors with high CH3NH3I concentration have a relatively high external quantum efficiency. These results imply that only CH3NH3PbI(3-x)Clx with very low Cl dopant concentration will be effective material for photovoltaic application. To further understand the difference between these devices during working condition, transient photovoltage/photocurrent measurements are carried out to investigate the carrier dynamics in the device. Transient photovoltage decay curves indicate that high Cl dopant concentration may decrease the photoelectron lifetime in CH3NH3PbI(3-x)Clx, and results in a relative low open-circuit photovoltage in the corresponding photovoltaic devices. The charge transport time in the devices of various Cl concentrations are studied by transient photocurrent decay method. CH3NH3PbI(3-x)Clx with low Cl dopant concentration has relative short transport time, which can avoid the recombination process and increase the charge collection efficiency.
2015, 64 (3): 038401.
doi: 10.7498/aps.64.038401
Abstract +
The development of highly efficient and low-cost solar cells is the key to large-scale application of solar photovoltaic technology. In recent years, the solution-processed organic-inorganic perovskite solar cells attracted considerable attention because of their advantages of high energy conversion efficiency, low cost, and ease of processing. The ambipolar semiconducting characteristic of perovskite enables the construction of planar heterojunction architecture to be possible in perovskite-based solar cells. This kind of architecture avoids the use of mesoporous metal oxide film, which simplifies the processing route and makes it easier to fabricate flexible and tandem perovskite-based solar cells. Planar heterojunction perovskite solar cells can be divided into n-i-p type and p-i-n type according to the charge flow direction. Two interfaces are formed between perovskite film and hole/electron transport layer, where efficient charge separation can be realized. Hole and electron transport layers can form separated continuous paths for the transport of holes and electrons, thus beneficial to improving exciton separation, charge transportation, and collection efficiency. In addition, this planar architecture avoids the use of high temperature sintered mesoporous metal oxide framework; this is beneficial to expanding the choice of the charge transport materials. In this paper, we review the recent progress on the planar heterojunction perovskite solar cells. First, we introduce the material properties of perovskite, the evolution of device architecture, and the working principle of p-i-n type and n-i-p type planar heterojunction perovskite solar cells. Then, we review the recent progress and optimization of planar heterojunction perovskite solar cells from every aspect of perovskite preparation and the selection of electron/hole transport materials. Finally, we would like to give a perspective view on and address the concerns about perovskite solar cells.
2015, 64 (3): 038402.
doi: 10.7498/aps.64.038402
Abstract +
Analysis of the DC current-voltage (I-V) characteristics is an effective approach to investigate the charge transport properties in a solar cell. The perovskite solar cell attracted wide research interest in the past two years due to their outstanding photovoltaic capacity. However, the charge transport characteristics and working principles of this kind of cells have not been clearly clarified. In this work, the I-V characteristics of the perovskite solar cell have been investigated from the experimental and theoretical perspective views. Moreover, the S-shaped I-V feature coming from the limitation of interfacial charge transport was focused on. With a series connected diode model, the I-V characteristics of the solar cell are investigated and simulated. It is found that the charge accumulation appears gradually when the interfacial charge transport velocity is decreased, lowering the output of the cell. When the interfacial charge transport decreases gradually, the short-circuit current density and the fill factor of the cell also decrease obviously. In experiments, limitations of charge transport at the front and back contacts of the cell have been designed, successfully producing varied S-shaped I-V features. It is found that both in the hole transport material-free and in the p-i-n perovskite solar cells, the S-shaped I-V characteristics can appear. Moreover, the origins of these features in various experimental conditions have also been discussed, which can be the energy barriers or large charge transport resistances in the cell. These energy barriers and resistances will lower the charge transport velocity and may cause charge accumulation, thus leading to the appearence of the S-shaped features. Meanhiwle, the emerging S-shaped I-V curves all have their own features, which may be due to the specific interfacial energy band structures. Thus, to promote the cell performance, the charge transport and interface energy barrier should be attached importance to and carefully designed. This work directly shows the interface factors that can significantly affect the cell performance, and gives a theoretical guide in cell design. By considering these limiting factors, the cell fabrication has been carefully designed with the control on the thickness of the mesoporous layer and the perovskite absorber film deposition, and a forward-swept efficiency of 15.5% can be achieved without any modification of the cell.
2015, 64 (3): 038403.
doi: 10.7498/aps.64.038403
Abstract +
Due to their high efficiency and low cost, organic-inorganic hybrid perovskite solar cells are attracting growing interest recently. For the most commonly studied perovskite CH3NH3PbI3, optimization of the morphology and crystallinity of CH3NH3PbI3 thin films can greatly improve the efficiency of perovskite solar cells. A homogenous and uniform perovskite film can prevent direct contact between the hole transport layer and the electron transport layer, and thus can significantly reduce charge recombination. And the high crystallinity perovskite film facilitates fast charge transportation and injection. Various studies have proved that solvent has a critical influence on both the morphology and the crystallinity of perovskite thin films. In this work, we thoroughly studied the influence of the normally used N, N-Dimethylformamide (DMF) and r-butyrolactone (GBL) solvents on perovskite morphology, crystallinity, as well as the solar cells efficiency. When using DMF as the solvent, the efficiency is only 2.8%, while the efficiency of the cell obtained based on GBL can reach 10.1%. SEM and HRTEM are employed to study the morphology and crystallinity of these two kinds of perovskite films. The perovskite film prepared using solvent DMF shows a rough capping layer consisting of strip-like perovskite crystals, and the filling of meso-TiO2 is poor. Compared with DMF, the GBL perovskite film shows a better capping layer structure consisting of large perovskite domains, and the filling of meso-TiO2 is improved as well. This great difference in capping layer morphology and meso-TiO2 filling is one reason for the different performance. Besides morphology, different defect concentrations in these two kinds of perovskite films are another crucial issue. By Combined XRD and UV techniques, the mechanisms how perovskite precipitats from DMF and GBL solutions can be disclosed. In DMF, because of its low spoiling point of 153 ℃, most of DMF solvent volatilize by spin-coating, and an intermediate MOF structure of PbI2: MAI: xDMF is formed. During thermal annealing, the unstable MOF structure breaks down and a large amount of dislocations form in perovskite films, which highly restrict the charge transport. However, the spoil point of GBL (206 ℃) is higher than that of DMF, which makes it hard to be fully volatilized by spin-coating. During the following thermal treatment, the solubility of perovskite is lowered with increasing temperature. So perovskite crystallites precipitate from the GBL first and then gradually grow up with the volatilization of the excess solvent. We finally find that coordination between the solvent and the PbI2 plays a big role on the morphology and the crystallinity of the solution-processed perovskite film, and this is responsible for the difference of the device performance.
2015, 64 (3): 038404.
doi: 10.7498/aps.64.038404
Abstract +
Preparation of Perovskite solar cell, an emerging low-cost photovoltaic technology in rapid development, has provided a ray of hope to solve the energy problem. However, its low reproducibility and stability limit the wide application of this potential technology. In this review, we summarize the recent progress with a focused discussion on some key issues in the development of perovskite solar cells. Starting from the analysis of basic structure and working principles, we first discuss the perovskite-based light harvesting layer and the general strategy to control its spectrum response. We also demonstrate the effect of film morphology on the device performance and the reproducibility which requires very uniform thin films. Then we discuss the major function of electron transporting layer and hole blocking layer, and point out the importance of compact hole blocking layer with less nano-scaled pinholes. For the hole transporting layer, we focus the discussion on the stability problem induced by widely used dopants that can improve the hole conductivity in the hole transporting layer while the dopants' deliquescent behavior also can induce the decomposition of perovskite-based light harvesting layer with a rapid degradation of the whole device. The potential approaches to solve this stability problem, such as using a dopant-free hole transporting material or making device without any hole transporting materials, are also discussed. Finally, we are in prospect of overcoming the main challenges in the future research for high performance perovskite solar cells.
2015, 64 (3): 038405.
doi: 10.7498/aps.64.038405
Abstract +
Recently solid-state organic-inorganic hybrid solar cells based on perovskite structured materials have evidenced a great breakthrough due to their perfect light absorption and charge transfer optoelectronic properties. The power conversion efficiencies have exceeded 20.1% during the last 5 years, since the first report on perovskite solar cells with an efficiency of 3.8% in 2009. Remarkably, perovskite solar cells with a planar-heterojunction structure have achieved an efficiency of 19.3%, and the perovskite solar cells with conventional mesoporous structure have achieved a certified efficiency above 16.7%. This review article first introduces the development of the third generation of solar cells from dye-sensitized solar cells to the perovskite solar cells, and then focuses on the optical and physical properties of the perovskite materials and their application in solid-state solar cells. We discuss the performance characteristics and advantages of the perovskite solar cells having mesoporous, planar heterojunction, flexibility, and hole-conductor-free structure respectively, and the charge collection layer which is applied in perovskite solar cells, such as semiconductor oxide (TiO2, Al2O3, ZnO and NiO) and PEDOT:PSS, etc. More over this review article introduces the charge transport materials, including P3HT, spiro-OMeTAD, PTAA, and PCBM, as well as different photoabsorption material, such as CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3PbI3-xClx, etc. aiming to analyze their performance characteristic in the perovskite solar cells with different configurations; and the main factor related to the performance. Finally, this review elaborates the perspective and understanding of the perovskite solar cells and points out the critical point and expectation for improving the performance of perovskite solar cells further.
2015, 64 (3): 038406.
doi: 10.7498/aps.64.038406
Abstract +
Recently, antimony selenide (Sb2Se3) has been proposed as an alternative earth-abundant absorber material for thin film solar cells. Sb2Se3 is a simple V2-VI3 binary compound with an orthorhombic crystal structure and a space group of Pnma 62. It is a staggered layered compound consisting of parallel 1D (Sb4Se6)n ribbons held together by weak van der Waals forces. Sb2Se3 has a direct band gap of approximately 1.15 eV with a large absorption coefficient (>105 cm-1, at short wavelength) and a low grain growth temperature (～300^{o}C), facilitating the fabrication of low-cost thin film solar cells. Moreover, it is a simple binary compound in single phase with a fixed composition, which provides a much simpler growth chemistry than the multicomponent Cu2ZnSn(S,Se)4. In addition, it is stable upon exposure to the ambient air, thus having a better prospect for long-term stability than the organic-inorganic halide perovskite solar cells. Theoretical analysis indicates that the efficiency limit is >30% for single junction Sb2Se3 solar cells. Various approaches, including vacuum evaporation, electrodeposition, spray pyrolysis, and chemical bath deposition (CBD), have been explored to produce Sb2Se3 thin films; however, it is only in these years that Sb2Se3 solar cells have been reported by our group as well as by others. Seok's group presented the deposition of Sb2Se3 on mesoporous TiO2 films by thermal decomposition of Sb2Se3 single-source precursors, and fabricated Sb2Se3-sensitized inorganic-organic heterojunction solar cells with a remarkable efficiency of 3.21%. Tena-Zaera's group fabricated the FTO/TiO2/Sb2Se3/CuSCN/Au heterojunction device and achieved 2.1% device efficiency; their Sb2Se3 was obtained by an electrodeposition route and CuSCN served as a hole conducting layer. Different from the above Sb2Se3-sensitized solar cells reported by other groups, our group is the first in the world working on Sb2Se3 thin film solar cells so far as wu know. We have fabricated a hydrazine solution-processed TiO2/Sb2Se3 heterojunction solar cell, achieving 2.26% device efficiency (Voc = 0.52 V, Jsc = 10.3 mA/cm2 and m FF = 42.3%). In addition to the solution processing method, thermal-evaporated substrate and superstrate CdS/Sb2Se3 thin film solar cells with 2.1% and 1.9% efficiencies respectively were also demonstrated by our group. Recently, we have further improved the superstrate device performance to 3.7% (Voc=0.335 V, Jsc=24.4 mA/cm2, and m FF=46.8%$) by using a post selenization step. Selenization can compensate the Se loss during thermal evaporation, attenuate selenium vacancy-related recombination loss and hence improve the device performance. In summary, this paper summarizes the recent research progress in Sb2Se3-related researches, including material properties of Sb2Se3, synthesis of Sb2Se3 nanomaterials and thin films, theoretical studies on electrical properties, device configuration and efficiency improvement of Sb2Se3 sensitized and thin film solar cells. This review also presents a perspective on future development of Sb2Se3 solar cells.
2015, 64 (3): 038802.
doi: 10.7498/aps.64.038802
Abstract +
Ever since the first organic-inorganic hybrid halogen perovskite solar cell was first used as a photo-voltaic material in 2009, reports on this type of solar cell have grown exponentially over the years. Up till May 2014, the photo-energy conversion efficiency of the perovskite solar cell have already achieved an efficiency approaching 20%. Surpassing the efficiency achieved by organic and dye synthesized solar cell, the perovskite solar cell is in good hope of reaching the efficiency compatible with that of mono-crystalline silicon solar cell, thus it is going to be the star in photo-voltaic industry. In a perovskite solar cell, the film-formation and electron-mobility in the electron transfer layer can dramatically affect its efficiency and life-span. Especially in the up-right structured device, the mesoscopic structures of the electron-transfer layer will directly influence the growth of the perovskite layer. The present researches of electron transport materials mainly focus on three aspects: (1) How to improve the instability in mesoporous TiO2-mesosuperstructured solar cells, that arises from light-induced desorption of surface-adsorbed oxygen. (2) How to obtain TiO2 or other electron transport materials at low temperature (sub 150 ℃) in order to be applicatable in flexible devices. (3) How to substitute the mesoporous TiO2 or compact TiO2 transport layer by organic or composite materials. This article devides the materials that are used to make the electron-transfer layer into three distinct groups according to their chemical composition: i.e. metal oxides, organic small molecules, and composite materials, and introduces about the role they play and the recent development of them in constructing the perovskite solar cell.
2015, 64 (3): 038803.
doi: 10.7498/aps.64.038803
Abstract +
In 2009, organic-inorganic hybrid perovskite was first used as the light-absorbing material for solar cells. The rapidly increased efficiency, simple preparation process, and low cost have aroused widespread concern. The last five years have witnessed the increase of the power conversion efficiency in the organic-inorganic hybrid perovskite solar cells from 3.8% to 19.3%. At present, most researches focus on how to improve the photoelectric conversion efficiency rather than the stability. With the improvement of the power conversion efficiency, people have realized that the long-term stability is also one of the key issues in practical applications.The present preliminary researches indicate that there are two main factors connected with the stability. One is the stability of the perovskite materials, including thermal stability and humidity stability; the other is the stability of solar devices, mainly related to the design and optimization of devices' structure. To solve the problems of stability of perovskite materials, the main point is its crystal structure. Based on the tolerance factor related to the stability of the perovskite lattice structure, choosing a more suitable size of the moiety can reduce its sensitivity to humidity and improve its stability. To design the device structure, we should try to select a hydrophobic material to protect the perovskite materials from being affected by the surrounding environment. Researches have so far showed that by optimizing the design of the solar cell structure via combining the elements utilized and the bonding interface work, the stability of the hybrid perovskites solar cell is supposed to be entirely solved, and this will determine the practical process of hybrid perovskite photovoltaic materials. However, by the moment, the study on stability of perovskite solar cells is far from being sufficient.
2015, 64 (3): 038804.
doi: 10.7498/aps.64.038804
Abstract +
Hybrid polymer-based solar cells (HPSCs) that use conjugate polymers as electron donor (D) and inorganic semiconductor nanocrystals as electron acceptor (A) are novel photovoltaic devices. HPSCs integrate the properties of organic polymer (flexibility, ease of film formation, high absorption coefficient) and inorganic nanostructures (high electron mobility, high electron affinity, and good stability), and have the extra advantages, such as the rich sources of synthesized nanostructures by wet chemistry, tunable and complementary properties of assembled components, solution-processibility on a large scale at low cost and light-weight, etc. Amongst various inorganic semiconductor materials, the nanostructured metal oxides are the promising electron acceptors for HPSCs, because they are environment-friendly, transparent in visible spectrum and easy to be synthesized. After a brief introduction to the current research status, working principles, device architecture, steady-state and dynamic characterizations of HPSCs, this paper mainly reviews our recent research advances in the HPSCs using ZnO and TiO2 nanostructures as main electron acceptor and transporter, with emphasis on the theoretical models for charge carrier transport dynamics, design and preparation of efficient materials and devices, and the device performance related with nanostructural characteristics. Finally, the main challenges in the development of efficient HPSCs in basic researches and practical applications are also discussed. The main conclusions from our studies are summarized as follows: (i) IMPS and IMVS are powerful dynamic photoelectrochemical methods for studying the charge transport dynamics in HPSCs, and our theoretical models enable the IMPS to serve as an effective tool for the mechanistic characterization and optimization of HPSC devices. (ii) Using a multicomponent photoactive layer with complementary properties is an effective strategy to achieve efficient HPSCs. (iii) Using the complementary property of components, enhancing the dissociation efficiency of excitons, and improving the transport properties of the acceptor channels with reduced energy loss to increase collection efficiency all are the effective measures to access a high photocurrent generation in HPSCs. (iv) The band levels of components in the photoactive layer of HPSCs are aligned into type II heterojunctions, in which the nanostructured component with the lowest conduction band edge acts as the main acceptor/transporter; the maximum open-circuit voltage (Voc) in HPSCs is determined by the energy difference between the highest occupied molecular orbital (HOMO) level of conjugated polymer and the conduction band edge of the main acceptor, but the Voc in practical devices correlates strongly with the quasi-Fermi levels of the electrons in the main acceptor and the holes in the polymer. While passivating the surface defects on the main acceptor, increasing spatial e-h separation, and enhancing the electron density in conduction band of the main acceptor will facilitate the increase in Voc. (v) There is no direct correlation among Voc, photogenerated voltage (Vph) and electron lifetime (τe), and they may change in the same or the opposite trend when the same or different factors affect them, therefore one should get insight into the intrinsic factors that influence them when discussing the changes in Voc, V_{ph} and τe that are subject to nanostructural characteristics.
2015, 64 (3): 038805.
doi: 10.7498/aps.64.038805
Abstract +
The efficiency of solar cells based on organic-inorganic hybrid perovskite materials has a rapid growth from 3.8% in 2009 to 19.3%. The perovskite material (CH3NH3PbX3) exhibits advantages of high absorbing coefficient, low cost, and easily synthesised, which achieved extremely rapid development in recent years and gains great concern from the academic circle. As we know, perovskite materials not only serve as light absorption layer, but also can be used as either electron or hole transport layer. Consequently, various structures are designed based on the function of the perovskite, such as the solid-state mesoscopic heterojunction, meso-superstructured planar-heterojunction, HTM-free and the organic structured layers. Besides, it is also attractive for its versatility in fabrication techniques: one-step precursor solution deposition, two-step sequential deposition, dual-source vapor deposition, and vapor-assisted solution processing etc. This review mainly introduces the development and mechanism of the perovskite solar cells performance and the fabrication methods of peroskite films, briefly describes the specific function and improvement of each layer, and finally discusses the challenges we are facing and the development prospects, in order to have a further understanding of perovskite solar cells and lay a solid foundation for the preparation of new structures of the perovskite solar cells.
2015, 64 (3): 038806.
doi: 10.7498/aps.64.038806
Abstract +
Quantum dot sensitized solar cells (QDSCs) appear to be one of the promising photovoltaic candidates, due to the lower cost of obtaining materials and assembling processes, as well as the advantages of their QD sensitizers which exhibit properties of tailoring the absorbance spectrum to near-infrared (NIR) regions, the multiple exciton generation (MEG), hot electron extraction, etc. However, the difficulty of QDs penetrating into TiO2 mesoporous film remains to be an obstacle for the development of QDSCs, which comes from (1) their larger size (1-10 nm) compared with dye molecules, (2) steric hindrance from the long chain organic ligands on the surface, and (3) the lack of terminal functional group of the ligand with affinity to TiO2. These issues imply the importance of implementing an efficient QD deposition method in the fabrication process. Based on summarizing the advantages and shortcomings, this review demonstrates the development of the QD deposition approaches in direct growth deposition methods: the chemical bath deposition (CBD) method, the successive ionic layer adsorption and reaction (SILAR) method, and the pre-synthesized QD deposition methods: linker-assisted deposition (LA), direct absorption (DA) and electrophoretic deposition (EPD). As an overall comparison to be taken for all these deposition approaches, the pre-synthesized QD deposition method has outperformed the direct growth deposition method due to the use of pre-synthesized high quality QD sensitizers for better performance in surface chemistry. Especially, the LA approach in this method exhibits its excellence of fast and uniform QD deposition with high coverage, as well as in building high efficiency QDSC devices. Specifically, the improved structure of the sensitizers such as the inverted type-I, type-II core/shell structures and alloyed configuration through surface ion-exchange, has been employed to boost the charge injection and depress the charge recombination, benefited from LA pre-synthesized QDs deposition method. The advantages of the LA method are fully illustrated by the examples of the most recent work in the achievement of reaching the record efficiency of QDSCs. Finally, outlooks have been given on possible approaches to realize further improvement of fabricating the QDSCs with excellent performance at higher levels.
2015, 64 (3): 038807.
doi: 10.7498/aps.64.038807
Abstract +
This paper introduces the history and current research status of the novel ferroelectric-semiconductor coupled photovoltaic devices, in which a ferroelectric field of polarized dipoles from nanoparticles separates the photogenerated carriers. Fabrication of such devices by combining a CdS nanodipole and a CdTe absorber via a feasible method is described, which involves a phase segregation process of CdS from a CdS-CdTe pseudobinary system. An irregular behavior is observed on this type of devices, i.e. the hysteresis of open circuit voltage due to external bias of direct-current (DC) electric field. Other macroscopic and microscopic evidences of the dipole field photovoltaic effect are also described. Meanwhile, similar photovoltaic mechanism observed in other types of solar cells are also discussed, such as organic photovoltaic devices and quantum dot devices with photo-induced dipole polarization field, piezo-phototronic devices, ferroelectric photovoltaic devices, as well as perovskite solar cells. It is apparent that the polarization field of dipoles not only exists in the various types of photovoltaic devices, but also may dominate the behavior of devices. Therefore, we propose that a new concept of dipole field semiconductor devices could be properly used to explain the photovoltaic behavior of both junctional and un-junctional devices. The junctional devices could function with either pn junction or Schottky junction, while the un-junctional devices include all the devices mentioned above. We expect that various innovation should be inspired by this concept in photovoltaic community.
REVIEW
EDITOR'S SUGGESTION
2015, 64 (3): 034101.
doi: 10.7498/aps.64.034101
Abstract +
The real-time observation of atomic motion in space and time is of great importance for natural science research. Ultrafast electron diffraction (UED) technique, which is equipped with both the high temporal resolution of femtosecond laser pulses and the high spatial resolution of electron diffraction, can provide an effective approach to study the structural change of matter in atomic scale. In this review, we make an introduction of the development history, experimental methods, related applications and future prospects of UED technique.
GENERAL
2015, 64 (3): 030301.
doi: 10.7498/aps.64.030301
Abstract +
We investigate the effects of inhomogeneous magnetic field and magnetic impurity on the quantum correlation in spin-1 system by means of negativity and measurement-induced disturbance. Results show that the increase of the inhomogeneous magnetic field not only decreases entanglement, but also can induce the entanglement, and increases the value of critical nonlinear coupling Kc. The critical magnetic field for measurement-induced disturbance is higher than that for negativity, and the measurement-induced disturbance (MID) will not disappear with the decrease of nonlinear coupling |K|, so it can reveal all the properties of quantum correlation. Results also show that the effects of different magnetic impurity on MID are independent of each other. Under the magnetic impurity, the entanglement exists only if the couplings |J| are less than the nonlinear couplings |K|, while there will be the MID when the couplings |J| are greater than the nonlinear couplings |K|. It is just the minimum point for MID when |J| equals to |K|. Moreover, the size of the Chain will influence the quantum correlation also.
2015, 64 (3): 030302.
doi: 10.7498/aps.64.030302
Abstract +
We consider the ground-state structure of a rotating Bose-Einstein condensate with spin-orbit coupling which is confined in a harmonic plus qurtic potential. Combined effects of spin-orbit coupling and rotation on the ground-state structure of such a system are investigated in detail. Results show that a large number of novel ground-state structures, such as stripe, two rows, snakeskin piebald, and so on, can be produced under the combination of anisotropic spin-orbit-coupling and rotation.
2015, 64 (3): 030501.
doi: 10.7498/aps.64.030501
Abstract +
Taking Chua's circuit with a smooth cubic flux-controlled memristor as an example, the impulsive control synchronization method for two identical memristor-based chaotic systems is studied. Based on the Lyapunov stability theory, the asymptotic stability condition for the impulsive synchronization of the memristor-based chaotic systems is given. Combining with the maximum conditional Lyapunov exponent spectrum of the error system, effects of the system initial values on the performances of impulsive synchronization are discussed, and corresponding simulation experiments are performed. Results indicate that using impulsive synchronization for the two identical memristor-based chaotic systems is feasible and effective with appropriate impulsive control parameters; the initial values of the memristor-based chaotic systems have some effects on the performances of impulsive synchronization, which can be inhibited by increasing the impulsive coupling strength.
2015, 64 (3): 030502.
doi: 10.7498/aps.64.030502
Abstract +
Human behaviors are usually determined by some social and/or economic trend. In the past few years, many attempts have been made, in the field of complex scientific systems, to describe the dynamics of these behaviors quantitatively and have an accurate understanding of the corresponding mechanisms. In this paper, a generalized potential, that is, a migration desire function defined by the age of the migrating people, the migrating distance, and the so-called economic-population density of the emigration area, is proposed. It can be transformed into Hamilton-Jacobi equation by using a random dynamical method, Langevin equation, so that the decision-making behavior can be investigated, based on a statistic framework during a group migration process. By taking use of the multi-dimensional steepest descent method, the Hamilton-Jacobi equation is solved; the solution shows that the information entropy of the system varies, leading by a single peak, as the age of the migrating people increases. It also demonstrates that the second derivative of the migrating distance to the information entropy has a change of zero-crossing (which actually means a phase change). The third characteristic of the solution is that the information entropy follows another single peak as the economic-population density increases. A deeper analysis reveals the significance behind these findings and the corresponding mechanisms. So some new understandings of the group human behaviors can be obtained, and some worthy references can be provided for some related administrative offices.
2015, 64 (3): 030503.
doi: 10.7498/aps.64.030503
Abstract +
This paper presents the mathematical model of membrane current of neuron resulting from electromagnetic radiation based on the foundation of neuronal energy theory; and the effect of electromagnetic radiation on the dynamic behaviors of single neuron and the firing activities of two neurons coupled with gap connection are investigated. Results show that the neuronal firing rate is lowered as the radiation intensity increases, and finally reaches a stable value. As the radiation intensity increases, the periodical spiking of neuron is transformed into bursting firing, which is well explained based on the dynamic bifurcation theory. It turns out that the bursting firing induced by the electromagnetic radiation could spread out in neuronal network through an electrical gap junction.
2015, 64 (3): 030504.
doi: 10.7498/aps.64.030504
Abstract +
For the chaos existing in a single-mode Haken-Lorenz laser system, a scheme of intermittent feedback with one variable is proposed. Numerical simulation results show that with the increase of appropriate control intensity, the Haken-Lorenz chaotic system can be suppressed to period 1, period 2, period 4, period 6, and period 8. Meanwhile, the synchronization of Haken-Lorenz chaotic systems is achieved based on the scheme of intermittent feedback.
2015, 64 (3): 030505.
doi: 10.7498/aps.64.030505
Abstract +
Parameter estimation for fractional-order chaotic systems is a multi-dimensional optimization problem, which is one of the important issues in fractional-order chaotic control and synchronization. With the characteristic of quantum parallel, a new quantum parallel particle swarm optimization algorithm is proposed for solving the problem of parameter estimation in fractional-order chaotic systems. A new method of quantum coding is presented with quantum parallel characteristic which can make the calculation number of each generation increase exponentially. On the basis of this method, a particle evolution equation composed of quantum current rotation angle, individual optimal rotation angle, and global optimum rotation angle is proposed, which can restraint the behavior of particles in quantum space, and also can improve the search capability of the algorithm. Numerical simulations of the fractional-order Lorenz system and the fractional-order Chen system are conducted and the results demonstrate the effectiveness, robustness and versatility of the proposed algorithm.
2015, 64 (3): 030506.
doi: 10.7498/aps.64.030506
Abstract +
A short-term wind speed time series prediction is studied. First, 0-1 test method for chaos is used to identify the short-term wind speed time series that has chaotic characteristics. Through phase space reconstruction, the delay time is determined by using C-C algorithm; and the embedding dimension is determined by using G-P algorithm. Then a least square support vector machine with parameters online modified is proposed, so that an improved particle swarm optimization algorithm may be used for the prediction of parameters optimization. Simulation experiment shows that the present method for its prediction accuracy, prediction error, and prediction effect is better than other prediction methods. Thus the proposed prediction method is effective, and feasible.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (3): 033101.
doi: 10.7498/aps.64.033101
Abstract +
HfO2, as a gate dielectric material for the charge trapping memory, has been studied extensively due to its merits such as high k value, good thermal stability, and conduction band offset relative to Si, etc.. In order to understand the reason why the charge trapping efficiency is improved by high k capture layer with respect to charge trapping type memory, the variation of HfO2 crystal texture induced by oxygen vacancy and the influences of it are investigated using the first principle calculation based on density functional theory. Results show that the distance of the nearest neighbor oxygen atom from oxygen vacancy is markedly reduced after optimization, whereas the decrease of distances between the next nearest neighbor oxygen atom from oxygen vacancy and hafnium is less. The change of local crystal lattice is caused by optimized oxygen vacancy for it significantly changes the local lattice, but rarely influences the far lattice. Deep energy level and density of electron states in conduction band are contributed by Hf atoms, while the density of electron states in valence band is contributed by O atoms. The local density of electron states in each element and the total density of electron states in the optimization system are all larger than those in the system without optimization, and the sum of the local densities of electron states is less than the total density of electron states. The trapped charges are moving mainly around the oxygen vacancy and the adjacent atoms of oxygen in the optimization system, but the charges are without optimization throughout the system. The local energy of charge is increased in optimized defect system, while the local energy of charge is conspicuously reduced in the system without optimization, i.e. lattice variation without saturation characteristic has a large effect on the local energy of charge. Results further prove that the change of crystal lattice induced by oxygen vacancy has strong ability to capture charge, which helps improve the features of memory.
2015, 64 (3): 033401.
doi: 10.7498/aps.64.033401
Abstract +
Research on cavitation is very significant for preventing cavitation erosion and for making use of bubbles effectively. Characteristics of cavitation in canonical ensemble are studied by molecular dynamics simulation. Effects of temperature and numerical density on cavitation are analyzed. Comparison with lattice Boltzmann method is also conducted. Simulation results indicate that the temperature and numerical density may affect cavitation remarkably. The formation of cavitation bubbles becomes unstable as the temperature increases, and even hard to occur. A lower numerical density makes cavitation bubble form easier. Moreover, as numerical density reduces, the temperature effect on cavitation becomes less.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (3): 034102.
doi: 10.7498/aps.64.034102
Abstract +
A new two-dimensional left-handed material based on double-triangled sructure is proposed. The simulation software HFSS is used to analyse the structure and its derivative structure. Results show that this type of structure can present left-handed properties when electromagnetic waves are incident on either perpendicularly or parallelly the plane of the substrate. And the general rule for this type structure is that: When the incident electromagnetic wave is in the perpendicular or parallel direction, and for each additional metal strip in the middle of the metal wires, the resonant frequency of the structure moves to higher frequencies around 0.5 or 4 GHz. These can be of reference values in further development of multi-dimensional left-handed material.
2015, 64 (3): 034201.
doi: 10.7498/aps.64.034201
Abstract +
With the advances in technology, how to solve the problem of contradiction between the large field of view and high resolution of images becomes one of the research focus of many scientific researchers. In this paper we present the concept of dually foveated imaging optical system, based on the traditional singly foveated imaging system which simulates the human eye, by introducing a reflective liquid crystal spatial light modulator for modulating the aberrations of two fields of a view, so improving the corresponding aberrations and achieving high-resolution image of the two different fields of view, while the remaining fields of the view are of low-resolution image. In this way it can solve the contradiction between the large field of view and high resolution image. In this paper, we design a dually foveated imaging optical system with the following parameters: reference wavelength is 587 nm, the field of view is 60° (i.e., ± 30°), F/8, the focal length is 60 mm. Simulation is conducted by CODE V, achieving a 5° and 17° dual field high-resolution image, and the remaining field being of low-resolution image; and calculation shows the high diffraction efficiency of the system with sampling resolution of 32 × 32, verifying the scientificness and accuracy of the design method.
2015, 64 (3): 034202.
doi: 10.7498/aps.64.034202
Abstract +
The location and intensity of non-resolved space object can be obtained in space-based surveillance, however, the material, size, and status of the object are lost. Spectrum represents the inherent property difference of the object, which can be used as an important means for feature extraction and recognition of non-resolved space object. According to the influence factors, including material, structure, background and orbit, a mathematical model for spectral properties of space object is established. Based on the model, inverse calculation method for feature extraction and recognition of space object is proposed. Taking the HuanJing-1 satellite scale model as an example, experimental verification for feature extraction and recognition of space object in typical parameters is made. Experimental results demonstrate the validity of the modeling method.
2015, 64 (3): 034203.
doi: 10.7498/aps.64.034203
Abstract +
In this work, a zero index magnetic metamaterial (ZIMM) is designed based on the two-dimensional array of ferrite rods periodically arranged in the air. By calculating the photonic band structures within the framework of multiple scattering theory and retrieving the effective electric permittivity εeff and effective magnetic permeability μeff, the structure parameters can be optimized and then the effectively matched zero index with εeff = μeff = 0 is achieved. Within this matched ZIMM, electromagnetic (EM) wave can propagate without any phase delay, resulting in the manipulation of phase pattern in space. By simulating the electric field patterns of a Gaussian beam incident on ZIMM slabs with different thickness, zero phase delay inside the slab can be observed. By designing various outgoing interfaces a plane EM wavefront can be transformed into a cylindrical one, or even into a more general wavefront. In addition, the focusing and beam splitting effects are demonstrated as well. Besides, since the permeability of magnetic materials can be controlled by an external magnetic field or a temperature, the EM features of ZIMM can be flexibly tuned, enabling a promising prospect in designing EM devices and potential applications.
Control of topological structure in high-order optical vortices by use of noncanonical helical phase
2015, 64 (3): 034204.
doi: 10.7498/aps.64.034204
Abstract +
This paper proposes a method for controlling the topological structures in high-order optical vortices by employing a noncanonical phase structure. The control of the evolutions in high-order optical vortices by using a noncanonical phase structure with a nonuniform azimuthal gradient is studied numerically and experimentally. Results show that the propagation of high-order optical vortices along with a noncanonical phase structure becomes a decayed optical distribution with multiple one-charged singularities along a line. In addition, the control from the noncanonical phase structure can suppress random evolutions of topological structures resulted from the phase noise. These conclusions may indicate a new method to control the decay of high-order optical vortices, and promising potential applications in many fields, such as optical vortices-based optical communications and optical tweezers.
2015, 64 (3): 034205.
doi: 10.7498/aps.64.034205
Abstract +
This paper introduces the concept of non-uniform partially coherent beams and investigates the spatial coherence change during beam propagation. Results show that non-uniform spatial coherence would make significantly changes during propagation, which is completely different from the classical beams. These changes have relationships with propagation distance, spatial coherence, and intensity distribution of the source. The points adjoining self-coherence area would mutate when beams propagate from the source; the high coherence area in the source may decrease after the beams propagate a short distance. Curves of spatial coherence would not overlap after propagation, while the spatial coherence tends to be uniform with increasing propagation distance.
EDITOR'S SUGGESTION
2015, 64 (3): 034206.
doi: 10.7498/aps.64.034206
Abstract +
The asymmetry of the absorption curve of electromagnetically induced transparency by a resonant coupling field in -type three-level systems for Rb87 cold atoms is investigated. We find that it is the other excited state, separated by an interval of 814 MHz, that induces this phenomenon. The primary cause of it is the stimulated Raman scattering. We also find that the ratio between the heights of the absorption peaks on the two sides of the transparency window is proportional to the intensity of the coupling beam. The theoretical result agrees well with the experimental data.
2015, 64 (3): 034207.
doi: 10.7498/aps.64.034207
Abstract +
To Obtain an appropriate coherent population trapping (CPT) signal through a miniature atomic vapor cell is one of the key steps to implement a chip-scale atomic clock (CSAC). In the present experiment, the high S/N differential CPT spectral line has been achieved with a miniature atomic vapor cell through modulating the laser and extracting 87Rb atom laser interacting signal using the phase sensitive demodulation technique. With the spectral line, the dependence of CPT signal on the working parameters and the dependence of frequency stability of CSAC on the quality of the CPT signal have been studied; the obtained experimental results agree well with the theoretical prediction, which can be used as the recommended working parameters for CSAC. The methods exploited in the experiment can be implemented in the resource of a CSAC, therefore this experiment explores for CSAC the practical approaches of performance optimization.
2015, 64 (3): 034208.
doi: 10.7498/aps.64.034208
Abstract +
Based on the extended Huygens-Fresnel principle and non-Kolmogorov spectrum, the analytical expressions for beam width and M2-factor of partially coherent Hermite-cosh-Gaussian beams going through a non-Kolmogorov turbulence are derived by means of second moments for the Wigner distribution function. Results show that the relative beam width and normalized M2-factor of partially coherent Hermite-cosh-Gaussian beams going through a non-Kolmogorov turbulence will increase when propagating in the turbulent atmosphere, and will be less affected by turbulent atmosphere with a larger beam, smaller coherent length, smaller Ch-part parameter. The relative beam width has a maximum value for increasing waist width, and normalized M2-factor has a minimum value for increasing waist width in a specific extent of coherent length. The relative beam width and normalized M2-factor both have maximum values according to the generalized power law, but decrease with increasing inner scale, and have nearly no change with increasing outer scale.
2015, 64 (3): 034209.
doi: 10.7498/aps.64.034209
Abstract +
The mechanism of generating double defect modes in a photonic crystal with double local states is analyzed based on the tight-binding method. The optical transmission characteristic of a one-dimensional photonic crystal is studied using the transfer matrix method. And the relationship between the transmission spectrum and structural parameters of the photonic crystal is obtained. The mesoscopic calender effect is discussed when the photonic crystal is exerted on by a homotaxial stress on the basis of these theories. Therefore, a multichannel filter with a simple structure which can be modulated by the stress in the near infrared band is designed. Numerical simulation results show that the defect modes may produce a red shift with the increase of the dielectric layers' refractive index or thickness. When the uniaxial tensile stress is applied on the ployphyly photonic crystal with double local states, all defect peaks will move to the direction of the long wavelength, and the values of these defect peaks remain unchanged basically. Through numerical fitting, the relationship between these defect peaks' central wavelengths and the size of the strain produced by the homotaxial tensile stress on the photonic crystal is linear. This kind of photonic crystal filter with a simple structure has good tunability and has practical application value in the manufacture of a series of exquisite photonic crystal lasers, wavelength division multiplexers and other precision instruments.
2015, 64 (3): 034210.
doi: 10.7498/aps.64.034210
Abstract +
This paper studies the work which is done by periodical external force to the gain-noise model for a single-mode laser. Result shows that the work exhibits a maximum as the angular frequency of the periodical external force varies, and the energetic stochastic resonance occurs; and the influence of system parameter and noise intensity on the resonance peak is discussed in detail.
2015, 64 (3): 034211.
doi: 10.7498/aps.64.034211
Abstract +
Three kinds of bilayer films, i.e. aluminum (Al)/fluorine-doped tin oxide (FTO), copper (Cu)/FTO and silver (Ag)/FTO, are prepared by coating a commercial FTO glass with sputtered metal layers, and subsequently thermally annealed. Then all the as-annealed bilayer films are irradiated using a 532 nm nanosecond pulsed laser. X-ray diffraction (XRD) analysis confirms that all the laser-irradiated films have underwent laser annealing, resulting in an improvement in their photoelectric properties. More significantly, after laser irradiation, the as-annealed Ag/FTO film exhibits the highest increment in average transmittance (400–800 nm) that is increased from 72.6% to 80.5%. This should be attributed mainly to the formation of laser-induced grating structures that have anti-reflection effect on their surfaces. It is also found that the laser irradiation decreases the sheet resistance of the as-annealed Ag/FTO film from 5.6 to 5.3 Ω/sq. The annealing caused by thermal effect of laser irradiation gives rise to an increase in grain size, thereafter reduces carrier scattering at grain boundaries and enhances carrier mobility, which should be responsible for the improvement in conductivity. The calculated results show that after laser irradiation the figure of merit of the as-annealed Ag/FTO film is greatly increased from 0.73×10-2 to 2.16×10-2Ω-1, indicating a significant enhancement in the overall photoelectric property of the film. Laser irradiation can simultaneously achieve fabrication of grating structures and laser annealing, providing a new idea for performance optimization of metal-layer-composited transparent conductive films.
2015, 64 (3): 034212.
doi: 10.7498/aps.64.034212
Abstract +
Using the coupled nonlinear Schrödinger (CNLS) equation containing the Raman gain, the Duffing equation with Raman gain is derived, which is represented by the Stokes parameters, and then the elliptic integrals are used to compute the analytical solutions. This article focuses on the analysis of the Raman gain influence on the evolution of the polarization state when the optical waves are propagating in a low-birefringence fiber. Results show that the transmission cycle of the polarization evolution state can be changed due to Raman gain, if the input power and motion constants have a certain relationship.
2015, 64 (3): 034213.
doi: 10.7498/aps.64.034213
Abstract +
In this paper, the spontaneous Raman spectra of K(H1-xDx)2PO4 (DKDP) crystals with different deuteration degrees in the Z(XX)Y scattering geometry are measured. And the Raman spectroscopy parameters including Raman shift, full-width at half maximum, and scattering intensity, which are related to the transverse stimulated Raman scattering (TSRS) gain coefficients, are analyzed in detail. Using the Raman scattering from water as a reference, the TSRS gain coefficients of DKDP crystals with different deuteration degrees are derived. It is found that with increasing deuteration degree in DKDP crystal the TSRS gain coefficient first decreases to about 40.1% of the KDP crystal, then increases to about 68.9%. We regard the change of the full-width at half maximum as the main reason for the dependence of TSRS gain coefficient on the deuteration degree.
2015, 64 (3): 034214.
doi: 10.7498/aps.64.034214
Abstract +
Ultrashort femtosecond pulse laser (pulse duration 2 nano-micron films (thickness 150–200 nm) are prepared by spin-coating method. Optical limiting test results show that for the ultrashort pulse laser, this film shows increasing transparency at low light intensity, while it shows decreasing transparency at a higher light intensity (optical limiting). It is capable of changing the optical limiting threshold by changing the incident wavelength which can be used for transparency enhancement and protection against damage of concentrator solar cells. Using this method to have a commercial GaAs solar cells coated found the conversion efficiency reduction 50%.
2015, 64 (3): 034215.
doi: 10.7498/aps.64.034215
Abstract +
Properties of nonlinear microstructured fiber fabricated in our laboratory are theoretically analyzed using the finite element method. This fiber has a high nonlinearity and phase matching for the dispersion wave generation. To achieve all-fiber nonlinearity in microstructured fiber, the dependence of dispersion wave on the pump power is investigated. When changing the pump power at 1032 nm with a femtosecond fiber laser, the near-infrared dispersion waves cover a region from 753 to 789 nm. The central wavelength and bandwidths alter obviously, and the fiber length has a remarkable impact on pulse broadening and frequency spectrum. Results coincide with the analyses. These results could be a reference for all-fiber nonlinearity of microstructured fiber, and lay a foundation for biological and medical applications, especially some researches on the near-infrared source for nonlinear light microscopy.
2015, 64 (3): 034216.
doi: 10.7498/aps.64.034216
Abstract +
Experimental program is designed to analyze the influence of background radiation on the accuracy of passive ranging based on oxygen spectral absorption; an acousto-optic tunable hyper spectral imaging spectrometer is used as the measuring device and a halogen light as the target. Firstly, the basic principles and experimental program of passive ranging technology based on oxygen absorption are introduced; then the halogen light spectral distribution at different distances during the night is collected using the acousto-optic tunable hyper spectral imaging spectrometer. Oxygen absorption rate is calculated and the relationship model between the oxygen absorption rate and the path is established according to the principle of oxygen spectrum absorption passive ranging. Then the oxygen absorption rate of the target is collected and calculated at the distance of 2360 m for different time. The measuring ranges during the day are solved by the model and the errors are analyzed, the influence of background radiation on the passive ranging is gained finally. Results show that according to the model, the maximum ranging error is 6.74% during the daytime, and the error becomes smaller with the elevation angle of the sun becoming smaller and the background darker. The results give 1.10% ranging error during the nighttime.
2015, 64 (3): 034217.
doi: 10.7498/aps.64.034217
Abstract +
The airborne imaging DOAS (differential optical absorption spectroscopy) method which can obtain two-dimensional distributions of trace gas is introduced in this paper. Based on this method, pollution source in a wide range of area can be found quickly, and a map of trace gas distribution can be obtained to study the pollutant diffusion tendency. In this paper we report the flying experiment over Tianjin and Tangshan. The airborne imaging system is introduced in detail and an image of two-dimensional NO2 distributions is obtained with a resolution of about 30 m×80 m. Combined with wind field data, the emission rate of a point source on the flight path is calculated to be 1570 kg/h.
2015, 64 (3): 034218.
doi: 10.7498/aps.64.034218
Abstract +
Optical en/decoder is one of the most crucial factors affecting the performance of the optical code division multiple access (OCDMA). The autocorrelation peak to maximum wing ratio (P/W) and cross-correlation ratio (P/C) are two important parameters to quantitatively evaluate the performance of the en/decoder.Based on the SOI parallel-cascaded coupled micro-ring reflector, a mathematical model of two-dimensional coherent optical en/decoder is established. Influences of the structure parameters including the coupling coefficient, the loss factor, the array distance, and the channel spacing on P/W and P/C ratios are discussed in detail. Results show that for the micro-ring with a radius of 50 μm, when the ring-bus and ring-ring coupling coefficients are 0.6–0.7 and 0.1–0.2, respectively, the propagation loss is lower than 2 dB/cm, the array distance is greater than 3 mm, and when the channel spacing is given between 25–36 GHz, the proposed structure can have an optimal performance of en/decoding.
2015, 64 (3): 034301.
doi: 10.7498/aps.64.034301
Abstract +
The technology of microwave-induced thermo-acoustic tomography that transmits electromagnetic wave pulses to the object and makes it absorb energy, can cause a rapid temperature rise in it. At the same time, a pressure wave will be generated instantaneouly, corresponding to generating an ultrasonic signal which can be detected by an ultrasonic sensor. After the ultrasonic signal is sampled and an image reconstructed, the image can reflect the characteristics of the electromagnetic energy absorbed by the object. The method combines a microwave imaging of high contrast and high resolution ultrasound imaging characteristics, hence verifies theoretically the feasibility of the thermo-acoustic imaging techniques for early breast cancer detection. In this study, we use S-band microwave pulse radiation source to radiate the biological tissue, and also make use of the circling mechanical motion systems to scan the tissue. In order to verify the imaging performance of the simulation experiments, we use both tumors, body and actual biological tissue as the samples of the experiments. The imaging reconstruction and comparative analysis can verify that the experimental system detects and distinguishes the tumor phantoms and the real biological tissue effectively. Results of the performance of high-resolution images and high contrast by the methods can provide further theoretical support for early detecting of breast cancer.
2015, 64 (3): 034501.
doi: 10.7498/aps.64.034501
Abstract +
A meshfree method based on polynominal point interpolation method (PIM) is proposed for dynamic analysis of a rotating cantilever beam; PIM is used to describe the deformation of the flexible beam. Its longitudinal and transverse deformations are both taken into consideration, and the coupling term of the deformation which is caused by the transverse deformation is included in the longitudinal deformation of the beam. The rigid-flexible coupling dynamic equations of the system are derived via employing Lagrange's equations of the second kind. Compared with the finite element method (FEM), only individual nodal data are required in the current method so that its preprocessor is easier, and the shape functions constructed have high-order continuities due to more nodes to be interpolated. Simulation results of the meshfree method are compared with those obtained by using FEM and assumption of modes method. It is demonstrated that the meshfree method can be extended to the field of multibody system dynamics.
2015, 64 (3): 034502.
doi: 10.7498/aps.64.034502
Abstract +
In this paper, the problem of exact invariants and adiabatic invariants for nonholonomic system in non-Chetaev's type based on the El-Nabulsi dynamical model is studied. First, the El-Nabulsi-d'Alembert-Lagrange principle is deduced and the differential equations of motion of the system are established. Then, the relation between the Noether symmetry and the exact invariant that is led directly by the symmetry for undisturbed nonholonomic system in non-Chetaev's type is given. Furthermore, by introducing the concept of high-order adiabatic invariant of a mechanical system, the conditions that the perturbation of symmetry leads to the adiabatic invariant and its formulation are studied for the nonholonomic system in non-Chetaev's type under the action of small disturbance. As a special case, the problem of the exact invariants and the adiabatic invariants for the nonholonomic system in Chetaev's type in El-Nabulsi model is discussed. At the end of the paper, two examples for the nonholonomic systems in non-Chetaev's type constraints and also the Chetaev's type constraints are given respectively to show the application of the methods and the results of this paper.
2015, 64 (3): 034601.
doi: 10.7498/aps.64.034601
Abstract +
The physical meaning of ductile spall fracture corresponding to the typical properties in free surface velocity curve has been discussed. A correlation between the macroscopic responses and the evolution of mic-damage is established; the pullback signal the found to correspond to the condition for nucleation of voids, and the rate at which the velocity rises to the first peak beyond the minima corresponds to the rate of damage evolution, and by using the velocity period after the the pullback signal we can distinguish the state of damage.
2015, 64 (3): 034701.
doi: 10.7498/aps.64.034701
Abstract +
Based on the constructal theory, the constructal optimization of a complex fin is carried out by taking the minimum equivalent thermal resistance, which is defined according to entransy dissipation rate, as the optimization objective. Optimal constructal of the complex fin is obtained by tsking into consideration the entransy dissipation performance caused by heat conduction and heat convection. Comparisons between the optimal constructal with different shapes and optimization objectives of the fins are performed. Results show that there exist the optimal ratios of the height to the length of the elemental fin, central cavity and fin tip which lead to the triple minimum equivalent thermal resistance of the complex fin. By comparing the optimal constructal of the complex fin with that of the T-Y shaped fin, the structure of the complex fin will greatly improve its global heat transfer performance. When the heat transfer of the fin is two-dimensional and the root of the fin is broader, the more non-uniform the temperature at the fin root, the bigger difference of the optimal constructs the complex fin obtains, based on the minimizations of the equivalent thermal resistance and maximum thermal resistance. For the optimal design of the fin in pracuice, when the thermal safety of the fin is ensured, the constructal design scheme of the fin with minimum equivalent thermal resistance can be adopted to reduce temperature difference in the average heat transfer and improves the global heat transfer performance. This paper provides some guidelines for the optimal design of the complex fin from the point of view of heat transfer optimization.
2015, 64 (3): 034702.
doi: 10.7498/aps.64.034702
Abstract +
The Lamb vector is known to be of profound importance for fluid dynamics generally and for the dynamics of turbulence in particular. Based on the new developments of the statistical theory of isotropic turbulence, some new analytical indications are found that the Lamb vector has a contribution in the potential and solenoidal parts, which are almost equal for some typical parameters.
2015, 64 (3): 034703.
doi: 10.7498/aps.64.034703
Abstract +
Since the lateral jet in a horizontal stream can reduce the friction of bionic jet surface, a bionic jet surface model is established by using the SST k- turbulence model in numerical simulation of bionic jet surface for jet hole with different shape, and experimental verification of the numerical simulation results is done. Results show that, when the flow length and span length of the jet hole are kept constant, the drag reduction of the third model with broken-line jet hole is the best; the broken-line jet hole is simplified to an arc-shaped hole, when its radius r=35mm, the drag reduction rate increases with jet velocity; furthermore, the best drag reduction can be obtained when r = 4 mm, the maximum drag reduction rate is 9.51%. Drag reduction is produced because the jet fluid injected to the lateral mainstream field through jet holes, would change the flow field structure of boundary layer near jet surface, and make the thickness of the underlying viscous sublayer in boundary layer increase. As a result, the gradient of normal velocity, perpendicular to jet surface, is decreased, and thus reduces the wall shear stress. Meanwhile, the low speed jet fluid is blocked at the boundary layer, reducing the sweep of high speed fluid on the wall, which contributes to the drag reduction.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (3): 036801.
doi: 10.7498/aps.64.036801
Abstract +
Ion-beam surface bombardment modification technology is successfully used to manufacture different kinds of nano-textures on the surface of silicon substrate. Relationship between the morphology and arrangement patterns of nano-textures and the bombarding parameters is studied. Results show that the ion-beam bombardment has a significant impact on surface morphology. Different kinds of textures on silicon substrate can be formed according to the variation of bombardment time. The nanodot array texture is observed on the surface of silicon substrate when the duration of argon ion-beam bombardment is 15 minutes. Simultaneously, the tetrahedral amorphous carbon film is deposited on the silicon substrates that have different kinds of nano-texture. The microstructure of ta-C film deposited on unprocessed and nano-textured silicon substrate is analyzed by X-ray photoelectron spectroscopy. Results indicate that the content of sp3 bonds decreases with increasing bombardment time, and thereafter keeps a steady value. The ta-C films deposited on the nanodot-textured substrate shows the lowest sp3 fraction. It is also observed by friction and wear test that the wear time is enhanced from 10 to 70 min. The tribological properties is highly improved when the coating is deposited on the nano-textured substrate.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (3): 037501.
doi: 10.7498/aps.64.037501
Abstract +
Spinel ferrite is one of the very important magnetic materials, having the unique physical properties, chemical properties, magnetic properties, and electronic properties. CoFe2O4 is widely used due to their good electromagnetic properties. We have studied the electronic structure and magnetic properties of CoRE0.125Fe1.875O4 (RE = Nd, Eu, Gd)by first-principles plane-wave pseudopotential method based on density functional theory (DFT), combined with the generalized gradient approximation (GGA + U) in this paper. Results show that the lattice constants of the compunds CoFe1.875RE0.125O4 (RE=Nd, Eu and Gd) will decrease due to the decreasing ionic radius of RE as the atomic number increases. Their magnetic properties depend on the unpaired 4f electrons of RE3+ ions, and the net magnetic moment of CoFe2O4 will increase with Eu and Gd doping, mainly because there are more unpaired 4f electrons in Eu3+ and Gd3+. Thus the doping of Eu3+ and Gd3+ may have a greater impact on the magnetic properties of cobalt ferrite. The contribution from the doping of Nd is not remarkable on the magnetic properties, since the Nd3 + ion, having a larger ionic radius, could distort the crystal structure of CoFe2O4.
2015, 64 (3): 037801.
doi: 10.7498/aps.64.037801
Abstract +
In order to overcome the problem of poor accuracy of resonant interferometer method in the measurement of thermodynamic sound speed and hypersound speed of liquids an experimental setup for measuring the sound speed of liquids is established based on the principle of spontaneous Brillouin light scattering. A Fabry-Perot interferometer is used to filter the scattered light and a data acquisition card as well as a photon counting head is used to detect and analyze the scattered light, then a data acquisition and analysis method of scattered light is presented. This method overcomes the limitation of the signal distortion in conventional Brillouin light scattering and increases the measuring accuracy of the sound speed of liquids remarkably. The sound speed of saturated liquid CCl4 is measured in the frequency range of 308.6 to 906.2 MHz at 298.15 K. Results agree well with the data reported in the literature, and show that the experimental method is feasible. In addition, the method for measuring the ultrasonic speed is proposed by adding several free spectral ranges to the measured Brillouin frenquency-shift. The ultrasonic speed of CCl4 measured is in the frequency range of 5406.1–5521.0 MHz. It is shown that the thermodynamic sound speed does not change with the sound frequency, while the hypersound speed increases with the increase of sound frequency and it is much greater than the thermodynamic sound speed, which proves the dispersion phenomena of CCl4.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (3): 038101.
doi: 10.7498/aps.64.038101
Abstract +
A miniaturization dual-band metamaterial (MM) model with a unit cell of hybrid-aligned hairpin split ring resonator (SRR) is proposed in this letter. The unit cell of this MM structure is a hairpin SRR, and the proposed dual-band MM is designed for security applications of wireless local-area networks (WLAN) at 2.4 GHz and worldwide interoperability for microwave access (WiMAX) at 3.5 GHz. Furthermore, a flexible substrate is adopted to improve the flexibility and practicability of the MM. Both simulated and measured results show that the center frequencies of the dual-band MM can be allocated by properly choosing the dimension parameters of the SRR. In addition, the MM are simulated at different angles of incidence, and the results reveal that the MM can operate quite well over a range of angles of incidence. Finlly, the current distribution in the MM has also been investigated to explain the mechanism of the dual-band resonance prodnced.
EDITOR'S SUGGESTION
2015, 64 (3): 038102.
doi: 10.7498/aps.64.038102
Abstract +
This article reports a novel color tuning technology based on micro-nano structure and metal nanolayer. On the basis of theoretical analysis, a color tuning model is established. Aluminum(Al) metal nanolayers are magnetron-sputtered on the surfaces of porour alumina (PA) templates with the pore-depths of 250 nm and 410 nm, and their pictures and reflective interference spectra show clearly green and red colors, respectively. These results indicate that different colors can be achieved just by controlling the pore-depth in PA templates. As comparison, a nanolayer of chromium(Cr) metal is magnetron-sputtered on the surface of PA template about 410 nm in pore-depth, the reflective interference spectra show that color tuning can also be achieved in the visible spectrum by changing the material and the thickness of the metal nanolayer. Moreover, a color pattern is further prepared based on mask and local sputtering method. Theoretical and experimental results validate the feasibility of this color tuning method.
2015, 64 (3): 038201.
doi: 10.7498/aps.64.038201
Abstract +
Study on simulation method for the thin film growth processes on atomic scale is currently a hot research field. The simulation method mainly aiming at nanometer scale model demands huge computational cost and memory cost. In order to solve the problem, a cellular method combined with Monte Carlo method is presented in this article to simulate the growth processes of thin film on micron scale. Based on cellular method for model representation and evolutionary computation, we greatly reduce the memory requirements and improve the efficiency of computation, and the Monte Carlo method is used to determine the particle migration. Moreover, specific research on the growth process of silicon nitride thin film is implemented, and the simulation results are compared with the experimental data and the molecular dynamics simulation results of the surface morphology and composition, so as to verify the effectiveness of this method.
2015, 64 (3): 038501.
doi: 10.7498/aps.64.038501
Abstract +
Strained Ge attracts attention of researchers for its high mobility and compatibility with Si technology. Based on the valence band model for compressively strained Ge/(001)Si1-xGex, the relationships between hole scattering, mobility, and Ge content (x) are established in this paper, including ionized impurity, acoustic phonon, non-polar optical phonon, total scattering rates, and the averaged and directional mobility of holes. Our quantitative data gained within the models can provide valuable references for the research of modified Ge materials physics and the design of the related devices.
2015, 64 (3): 038801.
doi: 10.7498/aps.64.038801
Abstract +
To investigate the catalytic activity of transition metals in hydrogenation process, the density-functional method has been performed to study the hydrogen interaction with metal-doped Al (111) surfaces. Results indicate that Al (111) surfaces doped with Sc, V, Fe, or Ti atom can effectively enhance hydrogenation reaction. H2 dissociation barriers on Sc, V, Fe and Ti doped surfaces are 0.54 eV, 0.29 eV, 0.12 eV, and 0.51 eV respectively, while diffusion barrier for H atom away from the Sc, V, and Ti doped surfaces are 0.51 eV, 0.66 eV, and 0.57 eV correspondently. Especially, V doped Al (111) surface has shown an amazing catalytic hydrogenation performance for the lower activating energy and diffusion barrier. Moreover, the metal atoms tend to be uniformly distributed on the Al (111) surface. And increasing the number of doping metal atoms, the catalytic performance are similar to that of the isolated transition metal atom doped Al (111) surface. This research may provide a reference to study the metal activity of hydrogen reuptake for NaAlH4.
