Fast and slow lifetime degradation in boron‐doped Czochralski silicon described by a single defect

A demonstration that boron–oxygen related degradation in boron‐doped Czochralski silicon could be caused by a single defect with two trap energy levels is presented. In this work, the same two‐level defect can describe the fast and slow lifetime decay with a capture cross‐section ratio of electrons and holes for the donor level of σ_n/σ_p = 19 ± 4. A model is proposed for the multi‐stage degradation involving a single defect, in which the product of the slow reaction is a reactant in the fast reaction. After thermal processing, a population of interstitial oxygen (O_i) exists in a certain state (the precursor state) that can rapidly form defects (fast degradation) and another population of O_i exists in a state that is required to undergo a slow transformation into the precursor state before defect formation can proceed (slow degradation). Kinetic modelling is able to adequately reproduce the multi‐stage degradation for experimental data. Dark annealing is also shown to impact the extent of ‘fast’ degradation. By decreasing the dark annealing time on pre‐degraded wafers, a more severe ‘fast’ degradation of the samples can be enabled during subsequent illumination, consistent with this theory. The paper then discusses possible candidates for the chemical species involved. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Light‐induced degradation in indium‐doped silicon

Light‐induced degradation of charge carrier lifetime was observed in indium‐doped silicon. After defect formation, an annealing step at 200 °C for 10 min deactivates the defect and the initial charge carrier lifetime is fully recovered. The observed time range of the defect kinetics is similar to the well known defect kinetics of the light‐induced degradation in boron‐doped samples. Differences between defect formation in boron‐ and indium‐doped silicon are detected and discussed. A new model based on an acceptor self‐interstitial A_Si–Si_i defect is proposed and established with experimental findings and existing ab‐initio simulations.

     

Thermal deactivation of lifetime‐ limiting grown‐in point defects in n‐type Czochralski silicon wafers

In this study, we uncover a recombination‐active grown‐in defect reducing the minority carrier lifetime of Czochralski grown n‐type silicon from 5 ms to below 2 ms. We also demonstrate that the defect can be de‐activated by annealing between 300 °C and 360 °C. Our experimental findings suggest that vacancy‐related pairs incorporated during ingot growth may be responsible for the decreased minority carrier lifetime. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A new‐generation color converter for high‐power white LED: transparent Ce3+:YAG phosphor‐in‐glass

Currently, the major commercial white light‐emitting diode (WLED) is the phosphor‐converted LED made of the InGaN blue‐emitting chip and the Ce³⁺:Y₃Al₅O₁₂ (Ce:YAG) yellow phosphor dispersed in organic epoxy resin or silicone. However, the organic binder in high‐power WLED may age easily and turn yellow due to the accumulated heat emitted from the chip, which adversely affects the WLED properties such as luminous efficacy and color coordination, and therefore reduces its long‐term reliability as well as lifetime. Herein, an innovative luminescent material: transparent Ce:YAG phosphor‐in‐glass (PiG) inorganic color converter, is developed to replace the conventional resin/silicone‐based phosphor converter for the construction of high‐power WLED. The PiG‐based WLED exhibits not only excellent heat‐resistance and humidity‐resistance characteristics, but also superior optical performances with a luminous efficacy of 124 lm/W, a correlated color temperature of 6674 K and a color rendering index of 70. This easy fabrication, low‐cost and long‐lifetime WLED is expected to be a new‐generation indoor/outdoor high‐power lighting source.     

Protecting Quantum State in Time‐Dependent Decoherence‐Free Subspaces Without the Rotating‐Wave Approximation

In this paper, we propose a scheme to protect quantum state by utilizing the time‐dependent decoherence‐free subspaces (TDFSs) theory without the rotating‐wave approximation (RWA). A coherent control is designed to drive the quantum system into the TDFSs, moreover, the singularities of the designed coherent control can be avoided by appropriately choosing the control parameters. From an experimental view point, the influences of variations of the control parameters and the imperfect initial state are discussed in detail. Numerical simulations confirm that the scheme can protect the quantum information from both the environmental decoherence and the control errors. In addition, by comparing with the scheme employing RWA, we show that the weak coherent control field is not suitable to create the TDFS, the counter‐rotating terms in the strong coherent control are helpful to protect the quantum information.     

Theoretical Design of a Pump‐Free Ultrahigh Efficiency All‐Optical Switching Based on a Defect Ring Optical Waveguide Network

A theoretical design of a defect ring optical waveguide network is proposed to construct a pump‐free ultrahigh efficiency all‐optical switch. This switch creates ultrastrong photonic localization and causes the nonlinear dielectric in the defect waveguide to intensely respond. At its ON state, this material defect without Kerr response helps to produce a pair of sharp pass bands in the transmission spectrum to form the dual channel of the all‐optical switch. When it is switched to its OFF state, the strong Kerr response induced refractive index change in the high nonlinear defect waveguide strongly alters the spectrum, leading to a collapse of the dual channels. Network equation and generalized eigenfunction method are used to numerically calculate the optical properties of the switch and obtain a threshold control energy of about 2.90 zJ, which is eight orders of magnitude lower than previously reported. The switching efficiency/transmission ratio exceeds 3× 10¹¹, which is six orders of magnitude larger than previously reported. The state transition time is nearly 108 fs, which is approximately two orders of magnitude faster than the previously reported shortest time. Furthermore, the switch size can be much smaller than 2.6 µm and will be suitable for integration.     

Assembly‐Induced Emission of Cellulose Nanocrystals for Hiding Information

Fluorescent‐labeled cellulose nanocrystal (CNC) films have been used to record and protect information in paper materials, whereas the fluorescent materials usually suffer photobleaching. Herein, a strategy of solid‐state emission induced by the vertical assembly of CNCs was established. The assembly‐induced emission starts from the structural diffraction of CNC, whose wavelength is adjusted into the ultraviolet (UV) region for hiding information under natural light. The small diameter (≈10 nm) of CNCs then promotes the resonance between the vertically assembled CNCs and the UV light, leading to a strong blue emission with an emitting quantum efficiency as high as 13.90%. By introducing the vertical‐assembly film with a specific pattern into paper materials, an anti‐counterfeiting image is obtained under a UV radiation. Since CNCs are a kind of cellulose with high crystallinity, this material can be a wear‐resistant anti‐counterfeiting material for banknotes or other paper applications.     

Acceleration and mitigation of carrier‐induced degradation in p‐type multi‐crystalline silicon

Recently, a new carrier‐induced defect has been reported in multi‐crystalline silicon (mc‐Si), and has been shown to be particularly detrimental to the performance of passivated emitter and rear contact (PERC) cells. Under normal conditions, this defect can take years to fully form. This Letter reports on the accelerated formation and subsequent passivation of this carrier‐induced defect through the use of high illumination intensity and elevated temperatures resulting in passivation within minutes. The process was tested on industrial mc‐Si PERC solar cells, where degradation after a 100 hour stability test was suppressed to only 0.1% absolute compared to 2.1% for non‐treated cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A combined transient and steady state approach for robust lifetime spectroscopy with micrometer resolution

We present the combination of two complementary micro‐photoluminescence spectroscopic techniques operating in transient and steady state condition, respectively. Introducing the time domain into the well‐established micro‐photoluminescence mapping approach operating under steady state conditions demonstrates a distinct improvement of the robustness and reliability in the determination of charge carrier lifetime measured with micrometer spatial resolution. Lifetimes from 50 ns to above ms are accessible. We elaborate a calibration procedure and apply the combined all‐photoluminescence setup to high‐performance multicrystalline silicon. A lifetime image obtained from the established photoluminescence imaging technique is reconstructed from the microscopic map by considering lateral diffusion and optical blurring, revealing a more detrimental influence of small angle grain boundaries as well as a higher lifetime within grains as may be deduced from the standard imaging technique. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Measuring picosecond excited‐state lifetimes at synchrotron sources

A new analysis method for the short excited‐state lifetime measurement of photosensitive species in crystals is described. Based on photocrystallographic techniques, this method is an alternative to spectroscopic methods and is also valid for non‐luminescent excited species. Two different approaches are described depending on the magnitude of the lifetime τ. For very short lifetimes below the width of the synchrotron pulse, an estimated τ can be obtained from the occurrence of the maximal system response as a function of the pump–probe delay time Δt. More precise estimates for both short and longer lifetimes can be achieved by a refinement of a model of the response as a function of the pump–probe delay time. The method also offers the possibility of the structure determination of excited species with lifetimes in the 40–100 ps range.     

p‐Type Dye‐Sensitized Solar Cells with a CdSeS Quantum‐Dot‐Sensitized NiO Photocathode for Outstanding Short‐Circuit Current

CdSeS quantum dots (QDs) are firstly introduced into a NiO photocathode for photocathodic dye‐sensitized solar cells (p‐DSCs). The optimized sample exhibits a short‐circuit density (14.68 mA cm^?2) and power conversion efficiency (1.02%) that are almost one order of magnitude higher than the reported value of p‐QDSCs. Steady‐state photoluminescence and time‐resolved photoluminescence measurements indicate that the photoexcited holes can be almost completely injected from CdSeS QDs into the valence band of NiO. At the same time, it can be observed from electrochemical impedance spectra measurements.     

Methodology for the solutions of some reduced Fokker‐Planck equations in high dimensions

In this paper, a new methodology is formulated for solving the reduced Fokker‐Planck (FP) equations in high dimensions based on the idea that the state space of large‐scale nonlinear stochastic dynamic system is split into two subspaces. The FP equation relevant to the nonlinear stochastic dynamic system is then integrated over one of the subspaces. The FP equation for the joint probability density function of the state variables in another subspace is formulated with some techniques. Therefore, the FP equation in high‐dimensional state space is reduced to some FP equations in low‐dimensional state spaces, which are solvable with exponential polynomial closure method. Numerical results are presented and compared with the results from Monte Carlo simulation and those from equivalent linearization to show the effectiveness of the presented solution procedure. It attempts to provide an analytical tool for the probabilistic solutions of the nonlinear stochastic dynamics systems arising from statistical mechanics and other areas of science and engineering.     

Mechanism of sulfur transfer from 1,2,4‐dithiazolidine‐3,5‐diones to triphenylphosphines

The mechanism of sulfurization of substituted triphenylphosphines with 4‐(3‐ and 4‐substituted)‐1,2,4‐dithiazolidine‐3,5‐diones in acetonitrile, dichloromethane, tetrahydrofuran and toluene at 25 °C was studied. The reaction pathway involves rate‐limiting initial nucleophilic attack of the phosphorus at sulfur followed by fast decomposition of the phosphonium intermediate to the corresponding phosphine sulfide, phenylisocyanate and carbonylsulfide. From the Hammett correlations and from the solvent dependency, it was concluded that the transition‐state structure is very polar and resembles the zwitter‐ionic intermediate. The extent of P–S bond formation and S–S bond cleavage is very similar in the solvents series, but the latter gradually decreases with the decreasing polarity of the solvent. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermal activation and deactivation of grown‐in defects limiting the lifetime of float‐zone silicon

By studying the minority carrier lifetime in recently manufactured commercially available n‐ and p‐type float‐zone (FZ) silicon from five leading suppliers, we observe a very large reduction in the bulk lifetime when FZ silicon is heat‐treated in the range 450–700 °C. Photoluminescence imaging of these samples at the wafer scale revealed concentric circular patterns, with higher recombination occurring in the centre, and far less around the periphery. Deep level transient spectroscopy measurements indicate the presence of recombination active defects, including a dominant center with an energy level at ～E_v + 0.5 eV. Upon annealing FZ silicon at temperatures >1000 °C in oxygen, the lifetime is completely recovered, whereby the defects vanish and do not reappear upon subsequent annealing at 500 °C. We conclude that the heat‐treatments at >1000 °C result in total annihilation of the recombination active defects. Without such high temperature treatments, the minority carrier lifetime in FZ silicon is unstable and will affect the development of high efficiency (>24%) solar cells and surface passivation studies.     

Theoretical studies on the electronic and optical properties of arene‐ versus fluoroarene‐thiophene co‐oligomer

A series of regiochemically varied and core size extension‐modulated arene‐ and fluoroarene‐thiophene co‐oligomers and the unsubstituted sexithiophene α6T were investigated theoretically to explore their electronic and optical properties. These phenylene‐thiophene oligomers show great potential for application in organic light‐emitting diodes (OLEDs), organic diode lasers, and organic thin‐film transistors (OTFTs) because of their feasible tuning of optical and electronic properties by the various structural tunings. Density functional theory (DFT) and the ab initio HF were employed to investigate the geometric and electronic structures of the oligomers in the ground state, and the singles configuration interaction (CIS) methods were used to study the lowest singlet excited state. The lowest excitation energies (E_gs), the radiative lifetime τ, and the maximal absorption/emission wavelength of the oligomers were studied within time‐dependent DFT (TDDFT). All calculations were performed using the 6‐31G(d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IPs), electron affinities (EAs), and reorganization energies are significantly affected by the various structural tunings in these co‐oligomers, which is important for the improvement of the hole and electron injection into OLEDs. Interestingly, the LUMO energy of 1b , 2b , and 3b is lower than that of α6T and 1a , 2a , 3a by about 0.12 ～ 0.47 eV, indicating that the fluorophenyl‐substitution has significantly improved the electron injection properties of the oligomers. Copyright © 2008 John Wiley & Sons, Ltd.     

Focusing femtosecond X‐ray free‐electron laser pulses by refractive lenses

The possibility of using a parabolic refractive lens with initial X‐ray free‐electron laser (XFEL) pulses, i.e. without a monochromator, is analysed. It is assumed that the measurement time is longer than 0.3 fs, which is the time duration of a coherent pulse (spike). In this case one has to calculate the propagation of a monochromatic wave and then perform an integration of the intensity over the radiation spectrum. Here a general algorithm for calculating the propagation of time‐dependent radiation in free space and through various objects is presented. Analytical formulae are derived describing the properties of the monochromatic beam focused by a system of one and two lenses. Computer simulations show that the European XFEL pulses can be focused with maximal efficiency, i.e. as for a monochromatic wave. This occurs even for nanofocusing lenses.     

DESIRS: a state‐of‐the‐art VUV beamline featuring high resolution and variable polarization for spectroscopy and dichroism at SOLEIL

DESIRS is a new undulator‐based VUV beamline on the 2.75 GeV storage ring SOLEIL (France) optimized for gas‐phase studies of molecular and electronic structures, reactivity and polarization‐dependent photodynamics on model or actual systems encountered in the universe, atmosphere and biosphere. It is equipped with two dedicated endstations: a VUV Fourier‐transform spectrometer (FTS) for ultra‐high‐resolution absorption spectroscopy (resolving power up to 10⁶) and an electron/ion imaging coincidence spectrometer. The photon characteristics necessary to fulfill its scientific mission are: high flux in the 5–40 eV range, high spectral purity, high resolution, and variable and well calibrated polarizations. The photon source is a 10 m‐long pure electromagnetic variable‐polarization undulator producing light from the very near UV up to 40 eV on the fundamental emission with tailored elliptical polarization allowing fully calibrated quasi‐perfect horizontal, vertical and circular polarizations, as measured with an in situ VUV polarimeter with absolute polarization rates close to unity, to be obtained at the sample location. The optical design includes a beam waist allowing the implementation of a gas filter to suppress the undulator high harmonics. This harmonic‐free radiation can be steered toward the FTS for absorption experiments, or go through a highly efficient pre‐focusing optical system, based on a toroidal mirror and a reflective corrector plate similar to a Schmidt plate. The synchrotron radiation then enters a 6.65 m Eagle off‐plane normal‐incidence monochromator equipped with four gratings with different groove densities, from 200 to 4300 lines mm^?1, allowing the flux‐to‐resolution trade‐off to be smoothly adjusted. The measured ultimate instrumental resolving powers are 124000 (174 µeV) around 21 eV and 250000 (54 µeV) around 13 eV, while the typical measured flux is in the 10¹⁰–10¹¹ photons s^?1 range in a 1/50000 bandwidth, and 10¹²–10¹³ photons s^?1 in a 1/1000 bandwidth, which is very satisfactory although slightly below optical simulations. All of these features make DESIRS a state‐of‐the‐art VUV beamline for spectroscopy and dichroism open to a broad scientific community.     

Graphene‐Based Phosphorus‐Doped Carbon as Anode Material for High‐Performance Sodium‐Ion Batteries

Graphene‐based phosphorus‐doped carbon (GPC) is prepared through a facile and scalable thermal annealing method by triphenylphosphine and graphite oxide as precursor. The P atoms are successfully doped into few layer graphene with two forms of P–O and P–C bands. The GPC used as anode material for Na‐ion batteries delivers a high charge capacity 284.8 mAh g^?1 at a current density of 50 mA g^?1 after 60 cycles. Superior cycling performance is also shown at high charge?discharge rate: a stable charge capacity 145.6 mAh g^?1 can be achieved at the current density of 500 mA g^?1 after 600 cycles. The result demonstrates that the GPC electrode exhibits good electrochemical performance (higher reversible charge capacity, super rate capability, and long‐term cycling stability). The excellent electrochemical performance originated from the large interlayer distance, large amount of defects, vacancies, and active site caused by P atoms doping. The relationship of P atoms doping amount with the Na storage properties is also discussed. This superior sodium storage performance of GPC makes it as a promising alternative anode material for sodium‐ion batteries.     

Bioinspired Nanozymes with pH‐Independent and Metal Ions‐Controllable Activity: Field‐Programmable Logic Conversion of Sole Logic Gate System

In nature, different environmental factors make organisms to be programmed into different forms. However, it is difficult and significant to realize the field‐programmable logic conversion of sole logic system for molecular logic gates. Here, the concept of pH‐programmable “logic conversion” on the single logic gate based on the peculiar enzyme‐mimicking activity is presented. Inspired by natural enzymes with high pH‐stability and metal ions‐stimulated activity, pH‐independent and metal ions‐controllable catalase mimics (Co₃O₄ nanozymes) are designed by protein‐directed method. Although pH cannot directly change the activity of nanozymes, pH can change the existence state of metal ions and then the electron transfer rate on nanozymes. So, versatile roles of metal ions for catalase‐like and electrocatalytic activities are discovered on the premise of pH‐independency. For the proof‐of‐concept, OR/INHIBIT‐ and INHIBIT/AND‐switchable logic gates are facilely constructed using pH as the environmental stimulus and metal ions as inputs. Hence, the transformation of logic gate functions is realized without the change of logic gate elements and input molecules. This contribution may not only broaden the species and application area of nanozymes, but also open novel avenues for the molecular logic conversion and the metal ions sensor.