Organic solar cells consisting of stacked amine–thiophene copolymer and 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole layers

Organic solar cells were fabricated using a new amine–bithiophene copolymer as an electron donor layer and 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole (PV) as an electron acceptor layer. The amine–thiophene copolymer, poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-bis(4-tert-butylphenyl)benzidine-N,N′-bis(phenylene-4,4′-diyl)]-co-(2,2′-bithiophene-5,5′-diyl)} (PF8-TPD-T2), had a glass transition temperature (T_g) at about 77 °C, and exhibited liquid crystalline states and a high hole mobility. The rigid bithiophene units in the polymer chain are probably responsible for the formation of the liquid crystalline states and the high hole mobility. A solar cell made of the PF8-TPD-T2 copolymer and PV layers showed a photocurrent density of 0.99 mA/cm², an open-circuit voltage of 0.61 V, and an energy conversion efficiency of 0.332%. The photocurrent of the solar cells was generated at both the copolymer and PV layers, and the copolymer layer was the main contributor to photocurrent when the thickness of the polymer was about 17 nm. After annealing the solar cells at temperatures well above the glass transition temperature (T_g) of the copolymer, the photocurrent action spectra of the solar cells were broadened and the performance was improved. The changes were mostly due to the increased contribution of the PV layer to the photocurrent by the annealing.     

Electrochemical oxidation behavior of Mg–Li–Al–Ce–Zn and Mg–Li–Al–Ce–Zn–Mn in sodium chloride solution

Mg–Li–Al–Ce–Zn and Mg–Li–Al–Ce–Zn–Mn alloys were prepared using a vacuum induction melting method. Their electrochemical oxidation behavior in 0.7 M NaCl solution was investigated by means of potentiodynamic polarization, potentiostatic oxidation, electrochemical impedance technique and scanning electron microscopy examination. Their utilization efficiencies and performances as anode of metal–hydrogen peroxide semi-fuel cell were determined. The Mg–Li–Al–Ce–Zn–Mn exhibited higher discharge activity and utilization efficiency than Mg–Li–Al–Ce–Zn, and gave improved fuel cell performance. The utilization efficiency of Mg–Li–Al–Ce–Zn–Mn is comparable with that of the state-of-the-art magnesium alloy anode AP65. The magnesium–hydrogen peroxide semi-fuel cell with Mg–Li–Al–Ce–Zn–Mn anode presented a maximum power density of 91 mW cm⁻² at room temperature. Scanning electron microscopy and electrochemical impedance studies indicated that the alloying element Mn prevented the formation of dense oxide film on the alloy surface and facilitated peeling off of the oxidation products.     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.     

Thermal–electrical–luminous model of multi-chip polychromatic LED luminaire

This paper proposed a thermal–electrical–luminous dynamic model of red–green–blue (RGB) light-emitting diode (LED) luminaire for lighting control. The thermal–electrical–luminous model consists of three parts, namely, electrical–thermal (E–T), electrical–luminous (E–L), and thermal–luminous (T–L) models. Using step response method, the electrical–thermal (E–T) model G(s) is derived as a first-order bi-proper system. The electrical–luminous (E–L) and thermal–luminous (T–L) models are zeroth order model with a constant gain since the luminous response to electric or thermal input is much faster. The thermal–electrical–luminous model shows that the luminous intensity is proportional to input power and inversely proportional to junction temperature. The dynamic response of luminous intensity is dominated by the electrical–thermal model G(s).The whole thermal–electrical–luminous model can be further divided into a constant gain and a first-order bi-proper system. The constant gain causes the instantaneous response at power switch on; the first-order system represents the luminous variation due to junction temperature change which is mainly related to the heat sink design. The complete model can accurately describe luminous dynamic behavior and be used in control system design of RGB LED lighting luminaire.     

Promising window layer of thin film Si solar cell with p–i–n structure prepared by using SiH2Cl2

The fabrication process for a-Si:H solar cells with p–i–n structure contains a problem of damage to the SnO₂ substrate particularly at higher process temperatures. We have reported that the suppression of darkening and wide optical gap (E_opt) are obtained by using SiH₂Cl₂ instead of SiH₄ as a source gas (Mater. Res. Soc. Symp. Proc. 609 (2000), in press). In this paper, p-type a-Si:H:(Cl) was investigated. Comparable E_opt and dark conductivity (σ_dark) to those of conventional a-SiC:H were obtained. Solar cells using this a-Si:H:(Cl) show higher current density (J_sc) and higher collection efficiency in all wavelength regions as compared to a p-layer not using chlorine processes. The newly developed p-layer has been applied to solar cells with p–i–n structure fabricated at higher substrate temperatures (T_s). Although the a-Si:H material deposited at higher substrate temperatures has been reported as being more stable against light soaking (21st IEEE PVSC Proceeding, Florida, USA, 1990, p. 1656), the high temperature processing is difficult to apply to the a-Si:H p–i–n structure because of the significant darkening of SnO₂ at higher T_s. With an a-Si:H:(Cl) buffer layer, a-Si:H solar cells can be fabricated at higher T_s (300°C) with reasonable cell performance. The best stabilized efficiency was 7.5% obtained at a T_s of 250°C.     

Development of membranes and a study of their interfaces for rechargeable lithium–air battery

This paper describes an investigation with an objective to screen and select high performance membrane materials for a working, rechargeable lithium–air battery. Membrane laminates comprising glass–ceramic (GC) and polymer–ceramic (PC) membranes were assembled, evaluated and analyzed. A superionic conducting GC membrane with a chemical composition of Li_1+xAl_xGe_2−x(PO₄)₃ (x = 0.5) was used. Polymer membranes comprising of PC(BN), PC(AlN), PC(Si₃N₄) and PC(Li₂O) electrochemically coupled the GC membrane with the lithium anode. The cell and membrane laminates were characterized by determining cell conductivity, open circuit voltage and carrier concentration and its mobility. The measurements identified Li₂O and BN as suitable dopants in polymer matrix which catalyzed anodic charge transfer reaction, formed stable SEI layer and provided high lithium ion conductivity.     

Analysis of photocurrent in the i-layer of GaAs-based n–i–p solar cell

A numerical investigation of the intrinsic layer effect on the improvement of GaAs n–i–p solar cell performances is presented. Solution of Poisson's equation together with continuity equations for electrons and holes allows the determination of carrier's density, electric field and recombination profiles within the i-layer. The analysis examines the effect of i-layer thickness on the electric field, recombination rate and collection efficiency. It is found that increasing the i-layer thickness increases the absorption while it reduces the electric field and increases the recombination rate. The three competing parameters have to be monitored simultaneously so as to obtain an optimal thickness. To achieve this, the variation of the total photocurrent is used as indicator. The photocurrent shows a sharp increase in the domain of very thin i-layers (<0.5 μm) then a saturation is reached for thicker layers (>1 μm), the simulation is performed for thicknesses up to 2 μm.     

Optimal optical generation profiles in a-Si:H p–i–n solar cells

The dependence of the maximum power output P_M and short-circuit current J_SC on the form (relative variation with position) of the optical generation rate profile in an a-Si:H p–i–n solar cell has been investigated computationally. It was found that there was an optimal form for the generation profile, and that P_M increased from 4.64 to 5.29 mWcm⁻², an increase of about 14%, when this optimal generation profile was used in the simulation. Optimal doping of the i-layer of the cell with phosphorous led to a P_M of 5.60 mWcm⁻², and when the optimal generation profile for this P-profiled cell was found, it yielded a P_M of 7.86 mWcm⁻², an increase of about 40%. This suggests that the combination of P-profiling and optimal generation could lead to significant improvements in cell performance. Moreover, it was found that for both cells the form of the optimal generation profile could be associated with the position of the peak in the external quantum efficiency, obtained from the spectral response. The possibility of using band-gap grading to achieve an optimal generation rate profile has been suggested.     

Electrical characterization and Poole–Frenkel effect in sol–gel derived ZnO:Al thin films

Electrical properties of ZnO:Al thin films, prepared by sol–gel dip-coating technique were studied in the range of 0.32% to 1.62% Al concentrations in the films. Room temperature electrical conductivity was found in the range of 0.08 to 1.39 S/cm for different aluminium concentrations in the films. I–E characteristics of the films at a constant temperature showed non-linearity, while non-linearity becomes more and more pronounced with increase in temperature and this could be explained by Poole–Frenkel model of thermionic emission. Presence of adsorbed oxygen and excess Al atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trapped electrons and created a potential barrier across the grain boundaries. In the presence of an external field, the barrier height was attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (φ_t) were calculated for different doping concentrations in the films. The thermoelectric power (TEP) measurement confirmed the n-type nature of the films and the room temperature Seebeck coefficient was found to be −91.65 μV/K.     

Optical and solar parameters of irradiated lead–alkali–silicate glass

Lead–alkali–silicate glass that is used for a shielding window of hot cells in nuclear technology has been irradiated by a ⁶⁰Co radioisotope source between 0.998 and 35.939 kGray dose levels. Gamma rays can affect glass and change its several optical and solar parameters such as secondary internal heat transfer factor (q_i), direct solar transmittance (τ_e), solar factor (g) and shading coefficient via the absorbed dose. It is aimed to investigate the performance of the glass in terms of the shading coefficient, which is the most important parameter to view clearly inside of the hot cell. Furthermore, a comparative evaluation has been done with respect to the unexposed lead–alkali–silicate glass. Change in the shading coefficient with respect to absorbed dose is extremely important.     

Optical and electrochemical characteristics of sol–gel deposited iron oxide films

Homogenous, crack free iron oxide films are prepared by the sol–gel spin coating technique from a solution of iron iso-propoxide and isopropanol. The films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible (UV–Vis) spectroscopy and cyclic voltammetry (CV). XRD of the films showed that they had an amorphous structure. The optical constants refractive index (n) and extinction coefficient (k) were measured by scanning spectrometer in the wavelength range of 390–990 nm. The n and k values were found n =2.3±0.01 and k =0.2±0.002 at 650 nm. The electrochemical behavior investigated in 0.5 M LiClO₄ propylene carbonate (PC) electrolyte-CV examinations showed good rechargeability of the Li⁺/e⁻ insertion extraction process beyond 300 cycles. Spectroelectrochemistry showed that these films exhibit weak cathodic coloration in the spectral range of 350–800 nm.     

LSCF–SDC core–shell high-performance durable composite cathode

Core–shell type La_0.6Sr_0.4Co_0.2Fe_0.8O_3−d (LSCF)–Sm_0.2Ce_0.8O_2−d (SDC) powders are synthesized to achieve a high-performance durable cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The SDC core size is controlled so that all core particles are surrounded by the LSCF particles with no unattached spots. Such a core–shell composite cathode develops an ideal microstructure with improved phase contiguity, homogeneity, and maximized triple-phase boundary density. The cathode that involves an SDC core of 500 nm exhibits the lowest interfacial polarization resistance (0.265 Ω cm² at 650 °C), as well as long-term stability during both thermo-cyclic and electrochemically accelerated tests.     

Magnesium–titanium alloy thin-film switchable mirrors

In this paper, we show gasochromic and electrochromic switching properties of Pd top capped magnesium–titanium (Mg–Ti) thin films prepared by DC magnetron sputtering. These films show excellent switchable mirror properties. By exposing to 4% H₂ in Ar, Pd (4 nm)/Mg_0.82Ti_0.18 (40 nm) film changed from the metallic state to the transparent state drastically within 5 s. By exposing to air, it goes back to the metallic state within 60 s. The transmittance spectrum in the hydride state is quite flat in the wavelength range from 400 to 2500 nm. It looks complete color neutral and its chromaticity coordinates are x=0.326 and y=0.340. Simple electrochromic device of Mg–Ti thin film using a liquid electrolyte works very well. It can be switched between the mirror state and the color-neutral transparent state.     

Clear transparency all-solid-state switchable mirror with Mg–Ti thin film on polymer sheet

We applied magnesium–titanium (Mg–Ti) thin film as the optical switching layer to all-solid-state switchable mirror on plastic sheet (polyethylene terephthalate, PET) in the viewpoint of clear transparency at the transparent state. For the switching speed from the reflective to the transparent states, the PET-device showed a little slower switching speed than the glass-device, and it depended on the sheet resistance of indium tin oxide (ITO). When Mg–Ti thin film was applied to the PET-device, absorption at the visible ray was reduced, resulting in near-colorless state at the transparent state. The PET-device with Mg–Ti thin film showed chromaticity coordinates of x=0.341 and y=0.339 and a luminous transmittance of 42.4% at the transparent state. However, the PET-device had lower durability than that of the glass-device. It seemed to be related with the degradation mechanism of the optical switching layer. When Mg–Ti thin film after the durability test was analyzed by XPS, though the glass-device had much oxidized state of magnesium, the PET-device had mixture states of oxide and hydroxide in the optical switching layer. We suggest that the hydroxide in the layer will be formed by including water in the PET sheet, and the hydroxide might significantly affect the rapid degradation of the PET-device.     

Modelling of stratified gas–liquid two-phase flow in horizontal circular pipes

This paper reports numerical and experimental investigation of stratified gas–liquid two-phase flow in horizontal circular pipes. The Reynolds averaged Navier–Stokes equations (RANS) with the k–ω turbulence model for a fully developed stratified gas–liquid two-phase flow are solved by using the finite element method. A smooth interface surface is assumed without considering the effects of the interfacial waves. The continuity of the shear stress across the interface is enforced with the continuity of the velocity being automatically satisfied by the variational formulation. For each given interface position and longitudinal pressure gradient, an inner iteration loop runs to solve the non-linear equations. The Newton–Raphson scheme is used to solve the transcendental equations by an outer iteration to determine the interface position and pressure gradient for a given pair of volumetric flow rates. Favorable comparison of the numerical results with available experimental results indicates that the k–ω model can be applied for the numerical simulation of stratified gas–liquid two-phase flow.     

Performance of solar driven methanol–water combined ejector–absorption cycle in the Athens area

The performance of a solar driven CH₄O-H₂O combined ejector– absorption unit, operating in conjunction with intermediate temperature solar collectors in Athens, is predicted along the five months (May–September) in case of the unit working as heat pump in an industrial area. The operation of the unit and the related thermodynamics are simulated by suitable computer codes and the required local climatological data are determined by statistical processings over a considerable number of years. It is found that the heat gain factor varies in the range from 2.1330 to 2.4481 for the above period of time. The maximum HGF of about 2.4481 is obtained in July at 14.25 hrs with corresponding specific heat gain power 915 W/m². The maximum Q_gain of about 1086 W/m² is obtained in June at 12.54 hrs with corresponding HGF 2.3572. Also the maximum value of HGF was estimated by correlation of three temperatures: generator temperature (85.0°C–97.2°C), condenser temperature (43.3°C–47.6°C) and evaporator temperature (12.6°C–25.4°C).     

Comparative study of a three-bucket Savonius rotor with a combined three-bucket Savonius–three-bladed Darrieus rotor

The vertical axis wind turbines are simple in construction, self-starting, inexpensive and can accept wind from any direction without orientation. A combined Savonius–Darrieus type vertical axis wind rotor has got many advantages over individual Savonius or individual Darrieus wind rotor, such as better efficiency than Savonius rotor and high starting torque than Darrieus rotor. But works on the combined Savonius–Darrieus wind rotor are very scare. In view of the above, two types of models, one simple Savonius and the other combined Savonius–Darrieus wind rotors were designed and fabricated. The Savonius rotor was a three-bucket system having provisions for overlap variations. The Savonius–Darrieus rotor was a combination of three-bucket Savonius and three-bladed Darrieus rotors with the Savonius placed on top of the Darrieus rotor. The overlap variation was made in the upper part, i.e. the Savonius rotor only. These were tested in a subsonic wind tunnel available in the department. The various parameters namely, power coefficients and torque coefficients were calculated for both overlap and without overlap conditions. From the present investigation, it is seen that with the increase of overlap, the power coefficients start decreasing. The maximum power coefficient of 51% is obtained at no overlap condition. However, while comparing the power coefficients (C_p) for simple Savonius-rotor with that of the combined configuration of Savonius–Darrieus rotor, it is observed that there is a definite improvement in the power coefficient for the combined Savonius–Darrieus rotor without overlap condition. Combined rotor without overlap condition provided an efficiency of 0.51, which is higher than the efficiency of the Savonius rotor at any overlap positions under the same test conditions.     

Simple preparation of Pd–Pt nanoalloy catalysts for methanol-tolerant oxygen reduction

Carbon-supported Pd–Pt bimetallic nanoparticles of different atomic ratios (Pd–Pt/C) have been prepared by a simple procedure involving the complexing of Pd and Pt species with sodium citrate followed by ethylene glycol reduction. As-prepared Pd–Pt alloy nanoparticles evidence a single-phase fcc disordered structure, and the degree of alloying is found to increase with Pd content. Both X-ray diffraction and transmission electron microscopy characterizations indicate that all the Pd–Pt/C catalysts possess a similar mean particle size of ca. 2.8 nm. The highest mass and specific activity of the oxygen reduction reaction (ORR) using the Pd–Pt/C catalysts are found with a Pd:Pt atomic ratio of 1:2. Moreover, all Pd–Pt alloy catalysts exhibit significantly enhanced methanol tolerance during the ORR than the Pt/C catalyst, ensuring a higher ORR performance while diminishing Pt utilization.     

A strong viscous–inviscid interaction model for rotating airfoils

Two‐dimensional (2D) and quasi‐three dimensional (3D), steady and unsteady, viscous–inviscid interactive codes capable of predicting the aerodynamic behavior of wind turbine airfoils are presented. The model is based on a viscous–inviscid interaction technique using strong coupling between the viscous and inviscid parts. The inviscid part is modeled by a 2D panel method, and the viscous part is modeled by solving the integral form of the laminar and turbulent boundary‐layer equations with extension for 3D rotational effects. Laminar‐to‐turbulent transition is either forced by employing a boundary‐layer trip or computed using an eⁿ envelope transition method. Validation of the incompressible 2D version of the code is carried out against measurements and other numerical codes for different airfoil geometries at various Reynolds numbers, ranging from 0.9 ? 10⁶ to 8.2 ? 10⁶. In the quasi‐3D version, a parametric study on rotational effects induced by the Coriolis and centrifugal forces in the boundary‐layer equations shows that the effects of rotation are to decrease the growth of the boundary‐layer and delay the onset of separation, hence increasing the lift coefficient slightly while decreasing the drag coefficient. Copyright © 2013 John Wiley & Sons, Ltd.     

n–p Junction formation in p-type silicon by hydrogen ion implantation

Hydrogen ion implantations at an energy of 250 keV and a dose of 3×10¹⁶ cm⁻² were applied to float zone, Czochralski grown silicon wafers and to multicrystalline samples. It was found that after annealing at 350°C<T<550°C for 1 h a n–p junction is formed and a photovoltaic behaviour is observed. Spectral responses show that the photocurrent in the near infrared part of the spectrum is comparable to that given by a standard silicon solar cell. The depth of the junction is about 2 μm and C–V measurements show that the junction is graduated. Hydrogen plasma immersion leads to similar results. The conversion of p- to n-type silicon is explained by the formation of shallow donor levels associated to a high concentration of hydrogen.