A novel profile with high efficiency for hydrogen-circulating Roots pumps used in FCVs

Low flow rate is one of the primary disadvantages of Roots pumps when they are applied in hydrogen fuel cell vehicles. A novel profile for Roots pumps was developed in this paper to increase the working volume and reduce the internal leakage. The available range of the design parameters of the Roots pump with the new profile was determined analytically, and an improvement in the working volume was validated. The flow dynamics inside the traditional and new pumps were investigated by experiments and numerical simulations. The analytical results indicated that the maximum area utilization of the new profile was approximately 10.4% higher than that of the traditional profile at the same lobe number. The numerical results demonstrated the superiority of the proposed profile in high flow rate and sealing. The tip concentric arcs of the new profile reduced the internal leakage via the radial gap. The characteristic of multipoint meshing within a certain range of rotational angles reduced the interlobe leakage.     

Development of heat transfer coefficient correlation for concentric helical coil heat exchanger

The present study deals with developing a Correlation for heat transfer coefficient for flow between concentric helical coils. Existing Correlation is found to result in large discrepancies with the increase in gap between the concentric coils when compared with the experimental results. In the present study experimental data and CFD simulations using Fluent 6.3.26 are used to develop improved heat transfer coefficient correlation for the flue gas side of heat exchanger. Mathematical model is developed to analyze the data obtained from CFD and experimental results to account for the effects of different functional dependent variables such as gap between the concentric coil, tube diameter and coil diameter which affects the heat transfer. Optimization is done using Numerical Technique and it is found that the new correlation for heat transfer coefficient developed in this investigation provides an accurate fit to the experimental results within an error band of 3–4%.     

Low band gap polymers for organic photovoltaics

Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies on alternating copolymers of donor and acceptor groups where strategies for band gap design are possible. Current challenges for OPV such as chemical stability and energy level alignment are discussed. We finally provide a compilation of the most studied classes of low band gap materials and the results obtained in photovoltaic applications and give a tabular overview of rarely applied materials.     

Band gap engineering in huge-gap semiconductor SrZrO3 for visible-light photocatalysis

Using SrZrO₃ (SZO, the intrinsic band gap being 5.6 eV) as an example, we have investigated the design principles for huge-gap semiconductors with band gap larger than 5 eV for the application of efficient visible-light driven photocatalysts for splitting water into hydrogen. Based on the hybrid density function calculations, the electronic structures of mono-doped and co-doped SZO are investigated to obtain design principles for improving their photocatalytic activity in hydrogen generation. The cationic–anionic co-doping in SZO could reduce the band gap significantly and its electronic band position is excellent for the visible-light photocatalysis. This work reports a new type of candidate material for visible-light driven photocatalysis, i.e., huge-gap semiconductors with band gap larger than 5 eV. Furthermore, based on the present results we have proposed the design principles for band gap engineering that provides general guideline for other huge-gap semiconductors.     

The effect of temperature on corrosion behavior of SS316L in the cathode environment of proton exchange membrane fuel cells

The effect of temperature on the corrosion behavior of SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments has been systematically studied. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to characterize the corrosion behavior. Atomic force microscope (AFM) is used to examine the surface morphology and X-ray photoelectron spectroscopy (XPS) analysis is used to identify the composition and the depth profile of the passive film. Photo-electrochemical (PEC) measurements are also performed to determinate the band gap energy of the passive film semiconductor. Interfacial contact resistances (ICR) between polarized SS316L and carbon paper are also measured. The experimental results show that corrosion resistance decreases with temperatures even though the thickness of passive film increases with temperature, at a given cell potential, the corrosion behavior of SS316L can be significantly different at different temperatures in PEMFC cathode environments, and the band gap of passive films decrease with temperature. The results also show that within the temperature range studied (25-90 °C), after different passivation time, the corrosion current densities of SS316L are all lower than the US DOE 2015 target value of 1 μA cm⁻², but the ICR between the carbon paper and polarized SS316L does not satisfy the US DOE 2015 target.     

Bi-Te基热电材料的能带结构计算

采用基于密度泛函理论的自洽赝势方法，计算了Bi—Te基热电材料不同化学配比下的电子结构。介绍了Bi2Te3材料的能带以及态密度，并计算了不同配比材料的载流子有效质量。计算结果显示：随着碲含量的增加，Bi—Te基热电材料从N型半导体向P型转变，在导电性质确定的情况下，塞贝克系数随着碲含量的增加而升高。     

A simple model for the photocurrent density of a graded band gap CIGS thin film solar cell

A simple model for the photocurrent density of a linearly graded band gap Cu(In,Ga)Se₂ solar cell is presented. Both generation and recombination mechanisms in the space charge region and absorber region of the cell are considered. The carrier collection function and effective absorption coefficient are introduced in the calculations to obtain a more realistic model. The results show that photocurrent density of the graded band-gap solar cell is higher than that with a constant averaged band gap. There is an optimum for grading strength or band gap widening of the absorber region. Recombination current reduces the photocurrent density with a lower reduction in the absorber material than in the depletion region. For longer diffusion lengths (or greater values of carrier collection factor), a higher photocurrent density is obtained except where collection probability is already unity everywhere in the absorber.     

Calculation of band gap in long alkyl-substituted heterocyclic-thiophene-conjugated polymers with electron donor–acceptor fragment

Since the interaction between alternating donors and acceptors results in a diminished band gap, a low band gap (<1.8 ev) will be expected in polymers containing donor–acceptor (D–A) repeating units. In order to predict the band gaps for guiding the synthesis of novel materials with low band gaps, we apply quantum-chemical techniques to calculate the band gaps in several polythiophene (PT) copolymers with D–A repeating units: poly{5,7-bis(3-octyl thiophen-2-yl)thieno-[3,4-b]pyrazine}(OTP), poly{5,7-di(thiophen-2-yl)thieno[3,4-b]-pyrazine}(TP), poly{4-(4-hexyl-5-(3-hexylthiophen-2-yl)thiophen-2-yl)benzo-[c][1, 2, 5]thiadiazole(HH-OTB), and poly{4-(5-(thiophen-2-yl)thiophen-2-yl) benzo[c]-[1,2,5]thiadiazole(TB). The geometries of the oligomers were optimized using semi-empirical AM1. The band gap calculations on these oligomers were performed by density functional theory (DFT) (B3LYP/3–21G*) and DFT (B3LYP/6–31G*). Band gaps of the corresponding polymers were obtained by extrapolating oligomers gaps to infinite chain lengths. The results indicate that calculated band gaps are in good agreement with the experimental values, in particular for long alkyl-substituted copolymer (HH-OTB/OTP). In addition, long alkyl side chain can induce steric hindrance, which leads to destroyed chain coplanar and increased band gap (>1.8 ev) in thiophene copolymers with alternate D–A units.     

Two dimensional MoSSe/BSe vdW heterostructures as potential photocatalysts for water splitting with high carrier mobilities

Production of hydrogen fuel from water splitting driven by solar energy is an effective technique to overcome the energy crisis and environmental problems in coming decades. To explore low cost and efficient photocatalysts is highly desired. In this work, we study the electronic, optical and photocatalytic properties of MoSSe/BSe (Model-1 and Model-2) vdW heterostructures by PBE and HSE06 functionals using first-principle calculations. The stabilities of these heterostructures are confirmed through phonon spectra and ab initio molecular dynamic simulations. The Model-1 and Model-2 heterostructures have indirect band gaps of 1.95 and 1.54 eV respectively by HSE06 hybrid functional. Interestingly, the transition from indirect to direct band gap occurs in Model-1 after including spin-orbit coupling effect. Remarkably, the high carrier mobilities are quantitatively explored by means of deformation potential theory. Furthermore, the transition from type-I to type-II band alignment happens at compressive strain in both Model-1 and Model-2, which effectively slows down the recombination of electron-hole pairs. Compared to the isolated monolayers, the MoSSe/BSe (Model-1 and Model-2) heterostructures harvest maximum portion of visible spectrum, revealing the outstanding paybacks of high efficiency utilization of solar spectrum. Most intriguingly, the band edges of MoSSe/BSe vdW heterostructures meet the redox potential requirements for water splitting. Our results will be valuable for easing the investigation and applications of MoSSe/BSe heterostructures for optoelectronics and photocatalytic water splitting.     

Quantum size effects in amorphous Si superlattice solar cells

Amorphous silicon/alloy superlattices provide advantages in solar cell design, such as (a) effective band gap widening (b) effective mass separation (c) increased open-circuit voltage. The latter increases via Fermi level control, due to p-doping of potential barriers, pushing EF towards the valence bands, with simultaneous widening of the effective band gap, thus leading to potentially higher collection incident wavelengths. The density of gap states in the heavily doped layer is modeled as an exponential whose parameter kT* can be varied by the doping concentrations, while its activation energy saturates at some value. This communication provides (i) a general formulation of the problem at finite temperatures as well as numerical results for specific realizable contacts (ii) detailed treatment of gap states (iii) the neutrality condition (iv) a relation between Fermi level position and open-circuit voltage in the nitride region (superlattice p-region). For a p-(a-SiN: H/a-Si: H)-i (a-Si: H)-n (a-Si: H) sample, we compute the Fermi level position relative to the a-Si: H valence band edge. For low and wide gap thin layers of the order of 2.5–3.5 nm, open-circuit voltage values are predicted in excess of 1.05 V, and efficiencies are predicted in excess of 12%.     

High-quality narrow gap (1.52 eV) a-Si:H with improved stability fabricated by excited inert gas treatment

Narrow band gap (1.5 eV) hydrogenated amorphous silicon (a-Si:H) were fabricated by a chemical annealing technique using noble gases (Ar, He, Ne). Although hydrogen content in the film was reduced to 1 atm% and band gap was decreased to 1.52 eV, high photoconductivity and large mobility–lifetime products were maintained and no marked changes in the short-range structure was found. Using these narrow band gap a-Si:H for photoactive layer in n-i-p solar cells, reasonable photovoltaic performances were obtained, i.e., open-circuit voltage of 0.71 V and fill factor of 57%. Also enhanced red response was observed with the 1.58 eV band gap i-layer solar cell prepared on textured substrate.     

Study of optical properties of some sol–gel derived films of ZnO

The optical transmission and reflection data for aluminum doped zinc oxide films have been analyzed. The optical absorption coefficient (α) and hence the refractive index (n), extinction coefficient (k) and the band gap (E_g) have been determined for these films using different methods. The films are prepared by sol–gel technique and are optically transparent. It is observed that the band gap increases with aluminum doping from 3.19 to 3.24 eV, but is less than the bulk ZnO crystal. A qualitative explanation has been put forward for the band gap widening with doping. The width of the band tail states which is connected to localized states in the band gap is least for 1 at% doped aluminum film. The porosity of the films as deduced from the refractive index seems to be of the same order in all the cases, but relatively higher for 1 at% aluminum doped films.     

Influence of using amorphous silicon stack as front heterojunction structure on performance of interdigitated back contact-heterojunction solar cell (IBC-HJ)

Interdigitated back contact-heterojunction (IBC-HJ) solar cells can have a conversion efficiency of over 25%. However, the front surface passivation and structure have a great influence on the properties of the IBC-HJ solar cell. In this paper, detailed numerical simulations have been performed to investigate the potential of front surface field (FSF) offered by stack of n-type doped and intrinsic amorphous silicon (a-Si) layers on the front surface of IBC-HJ solar cells. Simulations results clearly indicate that the electric field of FSF should be strong enough to repel minority carries and cumulate major carriers near the front surface. However, the over-strong electric field tends to drive electrons into a-Si layer, leading to severe recombination loss. The n-type doped amorphous silicon (n-a-Si) layer has been optimized in terms of doping level and thickness. The optimized intrinsic amorphous silicon (i-a-Si) layer should be as thin as possible with an energy band gap (E_g) larger than 1.4 eV. In addition, the simulations concerning interface defects strongly suggest that FSF is essential when the front surface is not passivated perfectly. Without FSF, the IBC-HJ solar cells may become more sensitive to interface defect density.     

Theoretical analysis of the optimum energy band gap of semiconductors for fabrication of solar cells for applications in higher latitudes locations

In this work some results of theoretical analysis on the selection of optimum band gap semiconductor absorbers for application in either single or multijunction (up to five junctions) solar cells are presented. For calculations days have been taken characterized by various insolation and ambient temperature conditions defined in the draft of the IEC 61836 standard (Performance testing and energy rating of terrestrial photovoltaic modules) as a proposal of representative set of typical outdoor conditions that may influence performance of photovoltaic devices. Besides various irradiance and ambient temperature ranges, these days additionally differ significantly regarding spectral distribution of solar radiation incident onto horizontal surface. Taking these spectra into account optimum energy band gaps and maximum achievable efficiencies of single and multijunction solar cells made have been estimated. More detailed results of analysis performed for double junction cell are presented to show the effect of deviations in band gap values on the cell efficiency.     

Band gap engineering of Gd and Co doped BiFeO3 and their application in hydrogen production through photoelectrochemical route

The present report deals with the synthesis of Gd and Co doped BiFeO₃ (BFO) i.e. Bi_1-xGd_xFe_1-yCo_yO₃ (BGFCO, x = 0.0, 0.1; y = 0, 0.05, 0.10, 0.20, 0.25) nanoparticles by sol–gel method. The co-doping leads to band gap engineering of BiFeO₃ with the band gap varying from 2.23 eV to 1.77 eV. The band gap engineering coupled with UV–Vis spectroscopy has been used to find the optimum material. The significant lowering in the band gap of the doped BFO is attributed to the deformation produced in Fe–O octahedron geometry as well as rearrangement in its molecular orbitals. The band gap engineering leads to materials with improved solar spectral response which in turn results in better harvesting of solar energy. X-ray diffraction (XRD) patterns indicate the formation of pure phase of BiFeO₃ and its doped variants. The surface morphologies and particle sizes of different compositions have been investigated through scanning electron microscope (SEM). The as synthesized BFO as well as its doped variants have been used as photoanodes for hydrogen production through photoelectrochemical (PEC) splitting of water. The optimum material Bi_0.9Gd_0.1Fe_0.75Co_0.25O₃ (BGFCO-25) with band gap of 1.77 eV has been used as photoanode having PEC configuration of 1 mol/L NaOH as the electrolyte solution and the Pt as cathode using 1.5 AM UV–Vis illumination. This has produced the photocurrent density of 2.03 mA/cm² and hydrogen production rate of 74.57 μmol cm^?2 h^?1. The maximum photo-conversion efficiency has been found to be 2.29% for BGFCO-25 which is higher than that of BFO in which it is 0.76%. This noteworthy enhancement in the photoelectrochemical properties is ascribed to narrowing of the band gap which improves the solar spectral response and allows the absorption of higher density of photons. The stability test of the photoanode has been done through chronoamperometry technique.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

MgH2 and LiH metal hydrides crystals as novel hydrogen storage material: Electronic structure and optical properties

We have performed a comprehensive theoretical investigation of the electronic band structure, density of states, electronic charge density and optical properties of the novel hydrogen storage material MgH₂ and LiH compounds. The all electron full potential linear augmented plane wave method was employed. The local density approximation (LDA), the generalized gradient approximation (GGA) and the Engle Vosko generalize gradient approximation (EVGGA) were used to treat the exchange-correlation potential. The calculations show that the MgH₂ compound is indirect gap semiconductor as the conduction band minimum (CBM) situated at R point of the Brillouin zone (BZ), while the valence band maximum (VBM) located between Λ and Γ points of the BZ, whereas LiH is a direct gap material as the CBM and the VBM located at X point of BZ. The values of the calculated energy band gap of MgH₂ (LiH) compounds are 3.372 (2.769), 3.735 (3.067) and 5.104 (4.488) eV for LDA, GGA, and EVGGA, respectively. From the partial density of states and the electronic charge density in (0 0 1) and (1 0 1) crystallographic planes we conclude that there exists strong ionic bonds. The bond lengths were calculated and compared with the available experimental and theoretical results, our results show better agreement with the experimental values than the other theoretical results. The frequency dependent dielectric function's dispersions were calculated and analyzed so as to obtain further insight into the electronic structure. The calculated dielectric function's dispersions confirm the semiconducting nature of MgH₂ and LiH compounds.     

Energy band structure parameters and their data, derived from the measurements of minority carrier current density in heavily doped emitters of silicon devices

In the n(p)-type heavily doped emitter region (HDER) of silicon devices, at room temperature, we have investigated the minority-carrier lifetime and the energy band structure parameters such as band-gap narrowing, apparent band-gap narrowing, unperturbed Fermi energy shift, optical gap, and reduced interacting density-of-majority conduction (valence) band states effective mass. As used in our previous paper, the present treatment is also based on the two assumptions for minority-carrier transport parameters. Gaussian impurity density profile, and accurate expression for minority-carrier mobility. Our empirical models for minority-carrier lifetime and band-gap narrowing are proposed and determined such that their curves versus the impurity concentration lie in between 3heir existing experimental data. Then, from a conjunction between electrical and optical phenomena, it is found that our theoretical values of such the energy band structure parameters are in good accordance with our own corresponding data, derived from the measurements of minority-carrier saturation current density in the n(p)-type HDER of silicon devices.     

Effects of interface wettability on microscale flow by molecular dynamics simulation

Non-equilibrium molecular dynamic simulations have been carried out to study the effect of the interface wettability on the pressure driven flow of a Lennard-Jones (LJ) fluid in a nanochannel. The results show that the hydrodynamic boundary condition at the solid-liquid interface depends on both the interface wettability and the magnitude of the driving force. For a LJ fluid in a nanochannel with hydrophilic surfaces, the velocity profiles have the traditional parabolic shape. The no-slip boundary condition may break down when the driving force exceeds a critical value that overcomes the interfacial resistance. In such a case, the MD results show a pattern of an adsorbing layer sliding along the solid wall. For a LJ fluid in a nanochannel with hydrophobic interfaces, the results show that a gap exists between the liquid and the surface, resulting in almost frictionless resistance; the velocity shows a plug flow profile and the slip length is not constant but depends on the driving force. Furthermore, it is found that the non-uniform temperature and pressure profiles near the solid walls are owing to the effect of interface wettability.