Search

This paper describes and presents the direct numerical simulation of a volumetric solar receiver under concentrated irradiation. The simulations systematically analyze the receiver performance and thermal loss mechanisms for operating conditions when outlet temperatures are in the 1000–1500 °C range. The implemented numerical method fully considers interactions between radiation, convection, and conduction using a discrete ordinates approach as a radiation model. The power over air mass (POM) and air mass flux were modified for six receiver channels with different cell sizes. The receiver efficiency decreased when the air mass flux increased beyond a certain criterion for constant POM. For a 3.8 mm cell, the receiver efficiency decreased to

To perform thermochemical cycles using non-volatile metal oxides to split water and produce hydrogen, a directly irradiated fluidized bed reactor is designed and fabricated for beam-down configuration. As the main aim of this investigation is to analyze the heat transfer and particulate flow of the reactor, chemically inert particles are used. A transient 3D heat and mass transfer model is formulated by the combined approach of discrete element method and computational fluid dynamics. The radiative transfer is solved using the discrete ordinate radiation model. Experimental validation is accomplished by the measured temperatures, obtained with the fluidized bed reactor prototype tested under 30 kWth high-flux solar simulator. The model is applied to analyze the granular flow characteristics and efficiency of the reactor for various superficial gas velocities and bed masses. The results indicate that higher gas flow rate increases the velocity and convection loss of the bed and decreases the bed temperature and efficiency of the reactor.

Gasification of carbonaceous materials using concentrated solar thermal radiation has been considered as one of the promising renewable pathways to produce syngas. In this study, thermal performance of a recently developed solar thermochemical reactor is presented. To analyze the gas–solid flow and heat transfer characteristics of the reactor, a transient three dimensional numerical model has been developed using discrete element method and computational fluid dynamics. Particle collision dynamics has been solved by the spring-dashpot model based on the soft-sphere method. To perform model verification, experiments have been performed using 30kWth fluidized bed reactor prototype under high flux solar simulator. The particulate and thermal characteristics of spout, annulus and fountain of the fluidized bed are analyzed for different irradiation power, loaded powder and gas flow rates. The results indicate that large and small size particles govern the bottom and fountain part of the bed respectively due to gravitational force, and the peak temperature is moved from fountain core to the fountain periphery of the bed when increasing the gas flow rate.

The surface electronic properties of the light absorber and band alignment at the p/n heterointerface are key issues for high-performance heterojunction solar cells. We investigated the band alignment of the heterointerface between cadmium sulfide (CdS) and Ge-incorporated Cu2ZnSnSe4 (CZTGSe), with Ge/(Ge + Sn) ratios ( x) between 0 and 0.4, by X-ray photoelectron, ultraviolet, and inversed photoemission spectroscopies (XPS, UPS, and IPES, respectively). In particular, we used interface-induced band bending in order to determine the conduction band offset (CBO) and valence-band offset (VBO), which were calculated from the core-level shifts of each element in both the CdS overlayer and the CZTGSe bottom layer. Moreover, the surface electronic properties of CZTGSe were also investigated by laser-irradiated XPS. The CBO at the CdS/CZTGSe heterointerface decreased linearly, from +0.36 to +0.20 eV, as x was increased from 0 to 0.4; in contrast, the VBO at the CdS/CZTGSe heterointerface was independent of Ge content. Both UPS and IPES revealed that the Fermi level at the CZTGSe surface is located near the center of the band gap. The hole concentration at the CZTGSe surface was on the order of 1011 cm-3, which is much smaller than that of the bulk (∼1016 cm-3). We discuss the differences in hole deficiencies near the surface and in the bulk on the basis of laser-irradiated XPS and conclude that hole deficiencies are due to defects distributed near the surface with densities that are lower than in the bulk, and the Fermi level is not pinned at the CZTGSe surface.

A combined numerical and experimental investigation of hydrodynamics and heat transfer of internally circulating fluidized bed reactor for solar gasification of coal coke to produce syngas is reported. As the main objective of this study is to study the thermal performance of the reactor, chemically inert quartz particles have been used. A numerical model has been developed by the combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The radiation transfer equation has been solved by the discrete ordinate (DO) model. The experimental data have been used for model validation. The thermal performance of the reactor and the spout-annulus flow have been predicted by the model and other hydrodynamic parameters such as internal solid volume fraction, velocity and temperature of the particles are analyzed as a function of superficial gas velocity (gas flow rate) at different initial conditions.

A stacked layer of ultrathin hydrogenated silicon oxide (SiOx:H) and hydrogenated amorphous silicon (a-Si:H) has been developed to passivate the crystalline silicon (c-Si) surface (see graphical abstract). Silicon oxide has the advantage of excellent optical and passivation properties. The SiOx:H layer is deposited on the c-Si surface by atomic layer deposition (ALD), with its thickness precisely controlled below 2 nm. The a-Si:H layer is deposited on the SiOx:H layer by plasma-enhanced chemical vapor deposition (PECVD) with a specific doping property, i.e. intrinsic, n- or p-type. The samples are then annealed in the range of 100 °C to 950 °C to study the fundamental passivation properties. We find that a combination of an ultrathin SiOx:H and (p) a-Si:H layers shows a favorable passivation compared to a neat (p) a-Si:H layer. The effective minority carrier lifetime, measured by quasi-steady-state photoconductance (QSSPC), is ~ 0.5 ms after low temperature annealing at 300 °C. The passivation property is discussed in terms of hydrogen concentration, bond configurations, stoichiometry x of SiOx:H, and material microstructure, characterized by Fourier transform infra-red (FTIR) and Raman spectroscopy. It is suggested that a reorganization of both the SiOx:H, and the (p) a-Si:H layers, associated with hydrogen diffusion, plays an important role in improving the passivation.

Considerable attention is being paid in recent years to develop and improve alternative technologies to replace fossil fuels due to pollution problems and limited conventional energy sources. Solar thermal energy has been considering as one of the most attractive renewable sources to provide solution to the aforementioned problems. In recent years, latent thermal energy storage systems using phase change materials (PCMs) have been developed to aid energy supply and demand of solar thermal power plants. As one of the obstacles of these systems is the low thermal conductivity of PCMs, ceramic fin has been used in this study to enhance heat transfer rate between the PCM and heat transfer fluid. To assess the heat transfer characteristics of the triplex-tube thermal energy storage system with ceramic fin, a numerical model has been developed and the effect of fin for various Stefan numbers has been studied numerically. The heat transfer and solidification rates are significantly enhanced when connecting the fins between the inner and outer tubes.

The major issues of thin-film silicon solar cells have been the light-induced metastability of hydrogenated amorphous silicon (a-Si:H) and the weak infrared light absorption of hydrogenated microcrystalline silicon (μc-Si:H). In order to overcome these challenges, we have developed a novel deposition process for a-Si:H and an improved light trapping scheme for μc-Si:H light absorbers, leading to four record independently-confirmed conversion efficiencies with single-junction and multijunction device structures. In this article, we review some technological progresses that led to the notable improvement in thin-film silicon solar cells, and address some limitations imposed by the absorber layer quality as well as the cell design.

This study analyzes the effect of cut-back inlets on the conjugate heat transfer of honeycomb-channel solar receivers. Two-way coupling simulations are reported for a plane-surface inlet, and five kinds of cut-back inlet receivers. The receivers are based on a 1.9-mm × 1.9-mm square cell with a 1.0-mm wall thickness. The cut-back inlet receivers have different amounts of material removed from the walls at the channel inlets. Numerical simulations demonstrate that receivers with a 2.4-mm cut from the channel inlet increased the exit temperature by 20.0 K or more, and decreased the pressure drop relative to the plane-surface receiver. Therefore, the results indicate that the cut-back inlet receiver’s thermal, and hydrodynamic performance is better for industrial use than is the plane-surface receiver. This performance is then explained with detailed examinations of the individual heat-transfer processes in the model. These detailed comparisons indicate that the top cuts reduce shadow effects as light irradiates the channel walls, allowing more direct irradiation to reach the wall surface, thus improving the overall performance.

Solar gasification is one of the promising techniques to convert the carbonaceous materials to clean chemical fuels, which offers the advantages of being transportable as well as storable for extended period of time. In this study, thermal performance of a recently developed 5 kWth fluidized bed reactor for solar gasification has been investigated and reported. Discrete element method (DEM) has been used for modeling the granular flow, and computational fluid dynamics (CFD) method has been used for modeling the multiphase flow. To validate the developed model, experiments were preformed and compared with modeling results. Discrete ordinate radiation model has been used to solve the radiative transfer equation. The thermal performance of the reactor and particulate flow behavior have been predicted and the effect of particle size, particle size distribution and gas flow rate are analyzed. The results indicate that the performance of the bed increases when fluidizing the annulus region particles as the high porosity increases the diffusion rate of radiation throughout the bed.

A loop thermosiphon thermal collector was developed for the waste heat recovery power generation with electric capacity of 500 W. The heat collector with heat transfer area of 0.159 m2 (500 mm width and 300 mm depth) was connected to the condenser with a shrunken heat transfer area by a loop. The thermal performance of the apparatus was declined when increasing the water filling rate to 90% as the working fluid occupied the internal volume. In the range of water filling rate between 30% and 60%, the effective thermal conductivity was around100 times of the conductivity of copper.

A new device design of microcrystalline silicon thin-film solar cell allows for approaching the geometric light-trapping limit. The solar cell is based on triangular textured surfaces in combination with optimized front and back contacts with very low optical losses. In comparison to crystalline silicon solar cells with record energy conversion efficiency the material usage of the thin-film solar cells is reduced to 1–2%, while exhibiting the potential to achieve short circuit current densities of more than 80% of their counterparts. The short circuit current density of the thin-film solar cells is approaching the geometric light-trapping limit commonly known as the Yablonovitch limit under perpendicular incidence. The design of the solar cell is described considering the electrical and optical properties of the textured solar cell.

The feasibility of using surfactant-free semiconducting silicon-tin alloy nanocrystals with quantum confinement effect for photovoltaics is demonstrated. Synthetized nanoparticles with average of 3 nm in diameter and optical bandgap of 0.81 eV at room temperature were obtained. X-ray diffraction measurements with synchrotron radiation have confirmed a Si0.88Sn0.12 alloyed composition that corresponds to a ratio of about eight Si atoms for every Sn atom. Fourier transform infrared spectroscopy revealed a reduced surface oxygen concentration compared with elemental silicon nanocrystals. Furthermore the potential of silicon-tin nanocrystals as a photovoltaic material is assessed and an enhancement of the solar cells performance is demonstrated due to the extended spectral range and increased absorption. In particular, the short circuit current density has shown improvements as the concentration of silicon-tin nanocrystals is increased.

The particles flow and heat transfer characteristics of a high temperature solar thermochemical fluidized bed reactor have been studied for solar beam-down concentrating systems. A numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model since it is an effective approach for studying the gas-solid flow. The discrete ordinate model has been used to solve the radiation heat transfer. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. The effect of gas superficial velocity, bed mass and inlet gas temperature on the flow pattern and temperature characteristics of the bed have been investigated. The results showed that the maximum and average temperature of the bed, depends on the top layer position and focal point of the concentrated radiation, decreased when increasing the total mass of the bed.

This paper describes a volumetric solar receiver that is vertically integrated with beam-down optics for condensed light irradiation. The heat-transfer performance of a silicon carbide honeycomb receiver was investigated using a 30-kW th solar simulator and numerical simulation. The experiments achieved an air temperature of 840 K at the receiver outlet by varying the operational parameters. Numerical simulations were performed for a vertical honeycomb block with beam-down irradiation and a horizontal honeycomb block with tower-type irradiation to elucidate the effects of buoyancy. Three blocks with different sizes were simulated for a variety of operational parameters. When the block was oriented vertically, the flow and temperature fields remained nearly symmetric in and near the receiver. In contrast, when it was oriented horizontally, the flow and temperature became asymmetric, with the hot spot moving toward the receiver's side wall and the stream in the receiver being reversed. The vertical orientation's robustness to buoyancy effects prevented any reduction in the receiver efficiency or outlet temperature and suppressed the thermal leakage.

In this study, a numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model to study the particle-fluid flow of the fluidized bed reactor for solar beam-down concentrating system. The contact forces between the particles have been calculated by the spring-dashpot model, based on the soft-sphere method. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. To investigate the influence of fluid flow rate and particle size on the flow pattern of the reactor, simulations have been performed for various conditions. The results indicate that the large size particles induce three-dimensional effects as they are accumulated at the central axis region. The average bed height of the left tower increased by 23.4% when increasing the flow rate about 70%.

We determined that the conduction band offset (CBO) and the valence band offset (VBO) at the CdS/ Cu2ZnSnSe4 (CZTSe) heterointerface are +0.56 and +0.89eV, respectively, by using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inversed photoemission spectroscopy (IPES). A positive CBO value, so-called “spike” structure, means that the position of conduction band becomes higher than that of absorber layer. The evaluated CBO of +0.56 eV suggests that the conduction band alignment at CdS/CZTSe interface is enough to become an electron barrier. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7 % could be obtained for the CdS/CZTSe heterojunction solar cells.

Cyclic dehydrogenation-rehydrogenation (D-R) experiments are performed on thin oxide passivated contact (TOPCon) p-type poly-Si/SiOx stack to investigate the role of hydrogen atoms for crystalline silicon solar cell. The dehydrogenation steps are performed by thermal annealing at 450 °C for 1h under vacuum condition, and the rehydrogenation steps by hydrogen plasma treatment (HPT) at 300 °C for 1min. The ultrathin SiOx layer of 0.8±0.1 nm is realized by atomic layer deposition (ALD) technique allowing a precise control at atomic scale. The effective lifetime is enhanced to 1.4 ms by the HPT. The D-R method underlines the contribution of H atoms to the c-Si surface passivation that diffuses into the SiOx/c-Si interface. Besides, the lifetime recovery is well described by a stretched exponential function that indicates the dispersive nature of the p-type poly-Si/SiOx stack.

A transient three dimensional numerical model of the heat transfer and fluid flow of a windowed fluidized bed reactor for solar thermochemical conversions is formulated and solved using discrete element method coupled to computational fluid dynamics. Radiation transfer equation is solved by discrete ordinate radiation model and the particle collision dynamics is solved by spring-dashpot model based on soft-sphere method. The instantaneous granular flow behavior of the irradiated bed is presented along with the incident radiation and particle size distribution. The results indicate that as time progresses the average velocity of the particle increases due to high temperature and bed expansion effect.

Electronic defects in semiconductor devices play important roles in device performance and reliability. These defects are created during fabrication using plasma-based processing technology. The authors investigate the generation and annihilation of defects in amorphous silicon by measuring the photocurrent during processing with hydrogen and argon plasmas. Unfavorable species for defect generation and favorable species for defect recovery are distinguished. These results are beneficial for improving semiconductor plasma processing, which is used to make state-of-the-art devices such as FinFETs, computer memory, and solar cells.

The effects of pre-annealing the stacked metallic precursor (Zn/Cu/Sn/Mo/glass) on the structural and morphological properties of the resultant Cu 2 ZnSnSe 4 (CZTSe) thin films and their device properties were investigated. The crystalline quality and morphology of the CZTSe thin films were enhanced by the pre-annealing treatment. The pre-annealed CZTSe solar cells showed an improved conversion efficiency η of 7.02% compared with those that were not pre-annealed (η=5.65%). This enhancement in solar cell performance was mainly attributed to an improvement in shunt resistance ( R sh ) from 80 to 150 Ω cm 2 , which resulted in improvements in open-circuit voltage and fill factor.

The relationship between light trapping and microstructure of high-efficiency microcrystalline silicon thin-film solar cells on honeycomb textured substrates is investigated by experiments, optical simulations and analytical calculations. The solar cells realized on the honeycomb textured substrates exhibit short circuit current of 30 mA/cm2 and energy conversion efficiencies exceeding 11%. The microstructure of the solar cells is limited by the formation of so called “cracks” in the film, which negatively affect the short circuit current, fill factor and open circuit voltage. The formation of cracks is studied by transmission electron microscopy, optoelectrical measurements and simulations of the 3D morphology of the solar cell. Furthermore, a simple analytical model is presented to calculate the critical thickness at which cracks are formed. Both models are compared to experimental results. Guidelines are provided on how to avoid the formation of cracks in microcrystalline silicon films on the textured substrates while maximizing the light trapping properties.

Carrier trapping in hydrogenated amorphous silicon (a-Si:H) has been investigated by means of an optical pump-probe technique. The trapped carriers (electrons) at the conduction band tail are detected as an increment of the photocurrent, and their density is quantitatively determined under the assumption of carrier generation and recombination kinetics. We find that carrier trapping strongly depends on the band tail distribution in addition to the pump light intensity. Specifically, the trapped electron density increases with the Urbach energy that characterizes the valence band tail broadening. Under the condition of a pump light intensity of 10 mW/cm2 operated at 532 nm, the trapped electron density is determined to be ≈ 4 × 1017 cm− 3 for an intrinsic a-Si:H film with an Urbach energy of 45 mV. The effects of carrier trapping on the device performance are studied in single-junction a-Si:H p-i-n solar cells. The results suggest that carrier trapping causes a reduction in the fill factor of the solar cells.

This paper describes the thermal collector type loop thermosiphon for low-temperature waste heat recovery. Water is used as the working fluid for heat transport at temperatures 100 °C and higher. The loop thermosiphon comprises of a thermal receiver, a condenser, and riser and downcomer tubes. The thermal receiver is made of a cupper plate brazed by meandering heat transfer tube. This receiver collects the thermal radiation from the electric heater at the heat transfer area of 1000 cm 2 (40 cm × 25 cm), and transports the heat by vaporization of water to the condenser having heat transfer area of 62 cm 2 . In the no inclination mode, the thermosiphon is upright so that the thermal receiver and the condenser are placed vertically. In this mode, the effective thermal conductivity exceeds 60 kW/(m K) when the thermal receiver temperature was higher than 125 °C for the water filling ratio α = 30–70%. Although the effective thermal conductivity is deteriorated for the higher filling ratio α = 80% and 90%, the effects from the filling ratio are tiny for α = 30–70%. The experimental tests were also made for the negatively inclined mode where the inlet and exit ports of the receiver were directed downward and for the positively inclined modes where they were directed upward. The tests revealed that the negative inclination almost halted the heat transport. However, the tests also indicated that the positive inclination showed the performance comparable to the no inclination mode for the cases up to the inclination angle 90°.

Ge-incorporated Cu2ZnSnSe4 (CZTGSe) thin films were prepared, and their materials and device properties were examined. The CZTGSe thin films were prepared using a two-step process comprising co-evaporation of each element and a subsequent annealing step. Ge atoms were successfully incorporated into the Cu2ZnSnSe4 thin films, and the band-gap (Eg) of CZTGSe was controlled via the full Ge/(Sn+Ge) ratio range of 0–1. In addition, the annealing environment containing GeSe2 led to CZTGSe thin films with flat surfaces, dense morphologies, and large grains comparable to their thickness. The highest efficiency achieved with the fabricated CZTGSe solar cells was 10.03%, with an open circuit voltage (VOC) of 0.54 V. The CZTGSe thin-film solar cells exhibited an improved VOC deficit (Eg/q−VOC, q: electron charge) of 0.647 V, which is comparable to that of high-efficiency Cu2ZnSn(SxSe1−x)4 solar cells.

Power conversion efficiency of thin-film microcrystalline silicon solar cells has been remarkably improved recently: from 10.1% to 11.8%. Front transparent conductive oxide (TCO) films have played an important role in this efficiency improvement. In this study, the impact of the front TCO films was investigated by comparing microcrystalline silicon solar cells with In2O3:Sn films grown by sputtering and ZnO:B films grown by metal-organic chemical vapor deposition (MOCVD). Improvement mechanisms in open-circuit voltages and fill factors in solar cells are investigated and discussed.

Direct numerical simulation was performed for the entrance region of a channel with repeated ribs under iso-thermal wall heating condition. Heat transfer performance was discussed for the case of the Reynolds number of 20,460 which was based on the mean velocity and the hydraulic diameter. Numerical results indicated that turbulence are generated in the shear layer separated from each rib top surface to excite the thermal transport in the cavity between ribs. This turbulence promotion becomes prominent when the rib-pitch to rib-height ratio increases in Pi/H = 4, 8 and 16. On the other hand, the spatial mean Nusselt number is the highest in the case of the smallest rib pitch of Pi/H = 4 in the inlet area of 0

Double-sided, front and rear, tunnel oxide passivated contact (TOPCon) of crystalline silicon (c-Si) solar cells on textured wafer is presented. The double-sided TOPCon structure is composed of (p) poly-Si/SiOx/(n) c-Si/SiOx/(n) poly-Si, where the silicon oxide (SiOx) layer is formed by atomic layer deposition (ALD). With a 0.6 ± 0.1 nm-thick SiOx layer, an 18.8 %-efficiency solar cell is fabricated, where an excellent short circuit current above 39 mA/cm2 is confirmed for a front grid structure. The solar cell performance is improved by optimizing the boron and hydrogen diffusion profiles near the p-side SiOx/c-Si interface.

The improved silicon heterojunction solar cell efficiency by employing porous a-Si:H passivation layer deposited using Si 2 H 6 precursor is presented. By introducing Si 2 H 6 precursor into the commonly-used SiH 4 plasma, it is demonstrated that the porosity of a-Si:H passivation layer can be increased significantly. A stack of the porous a-Si:H passivation layer with overlying dense a-Si:H capping layer preserves excellent interface passivation even after doped layer deposition. In the finished solar cell, improvements in V OC from 0.691 to 0.714 V and efficiency from 20.0 to 21.0% were obtained as a consequence of using porous a-Si:H passivation layer deposited under high Si 2 H 6 concentration.

P-type hydrogenated nanocrystalline silicon (nc-Si:H(p)) films as emitter layers in silicon heterojunction solar (SHJ) cells by VHF-PECVD (65 and 100 MHz) and standard RF-PECVD (13.56 MHz) were investigated. We demonstrate that the use of VHF-PECVD is advantageous in fast nucleation in a very thin layer (

Interplay between intrinsic bi-layers and overlying doped layers in a-Si:H/c-Si heterojunction solar cells was investigated. Insertion of the more porous intrinsic a-Si:H layer at the a-Si:H/c-Si interface is beneficial to mitigate the degradation of the surface passivation after p-layer deposition. This interfacial layer was also applied to SHJ solar cells, and a notably high V$_{OC}$ of 0.742 as well as a high conversion efficiency of 22.2% was achieved.

In this research, heat transfer and hydrodynamics of a solar thermochemical fluidized bed reactor filled with ceria particles have been studied numerically and experimentally for beam-down solar concentrating system. A CFD-DEM model has been developed, in which the contact forces between the particles have been calculated by the spring-dashpot model, based on the soft-sphere method. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. Simulation results have been compared with experimental data to validate the CFD-DEM model. Results indicate that the model can predict the particle-fluid flow of the two-tower fluidized bed reactor. Using this model, the key operating parameters can be optimized.

The first performance test of a novel windowed solar fluidized bed reactor was carried out for a two-step water splitting cycle with ceria particles by using a 30-kWth sun-simulator. O2 and H2 were successfully produced by the reactor system. During the thermal reduction step of the two-step water splitting, a fluidized bed of ceria particles was created by passing N2 gas through the perforated bottom plate of the reactor, and about 30 kWth of high flux visible light from 19 xenon-arc lamps of the sun-simulator was directed inside the reactor through a quartz window to directly irradiate the top of the fluidized bed. The central bed temperature reached the temperature of 1400 to 1500°C in the thermal reduction step. The subsequent water decomposition or hydrolysis step was performed at the central bed temperature of 800 - 900°C. About 6 - 12 Ndm3 of hydrogen was produced by one cycle.

We fabricated 20-layer-stacked Ge/Si 1− x C x quantum dots (QDs) by solid source molecular beam epitaxy, for the realization of Si-based, strain-compensated Ge QDs. We inserted 2-nm-thick Si mediating layers between the Ge QDs and the Si 1− x C x strain-compensating spacer layers (SCLs) and evaluated their influence on the structural and optical properties of the obtained system. High density QDs with a size fluctuation of approximately 14% were successfully maintained independent of the number of stacked layers in the case of QDs with Si mediating layers, while the QD size monotonically increased with a number of stacked layers in the case of QDs without Si mediating layers. The sheet density of QDs with Si mediating layers reached 1.2×10 11 cm −2 . A strong photoluminescence (PL) emission with a line-width of 96.7 meV was obtained for the QDs with Si mediating layers. The improvement of the aforementioned properties was caused by both the absence of C bonds at the surface and the improved surface roughness obtained by introducing Si mediating layers. The QD size increased for the 20-layer-stacked Ge/Si QDs samples grown with 10-nm-thick spacer layers, due to the accumulation of internal strains. By contrast, the size distribution of QDs was almost constant for the range of spacer layer thicknesses used in the present work (between 10 nm and 40 nm), implying that the tensile-strained Si 1− x C x SCLs compensated a certain fraction of the strain field created by the compressive-strained Ge QDs.

We examined the influence of the processing temperature on hydrogenated amorphous Si/crystalline Si0.3Ge0.7 heterojunction solar cells with a bandgap of 1.0 eV, which were grown on Si(001) by solid-source molecular beam epitaxy. Stepwise, compositionally graded buffer Si1−xGex layers were applied to prepare Si0.3Ge0.7 films with low dislocation density. We studied the influence of the annealing temperature applied at the buffer layer on the diode characteristics. The diode factor was minimized to 1.45 at 800 °C, and the saturation current density monotonically decreased from 7.5×10−4 A/cm2 for an annealing temperature of 650 °C to 5.0×10−5 A/cm2 for that of 900 °C, as a result of the promotion of dislocation annihilation. Consequently, the Si0.3Ge0.7 heterojunction solar cell showed a maximum conversion efficiency of 1.50% when the annealing temperature was 850 °C. We also studied the influence of the growth temperature of the p-Si0.3Ge0.7 active layer (absorber) on the cell performance. The short circuit current density increased from 16.03 mA/cm2 for 520 °C to 20.95 mA/cm2 for 600 °C. This result corresponded to an improvement of quantum efficiency response in the longer wavelength region due to the reduction of crystal defects caused by oxygen contamination. However, the Si0.3Ge0.7 active layer processed at a higher temperature showed an accumulation of threading dislocations and roughening of the surface, resulting in the degradation of both the open circuit voltage and the fill factor.

Mechanisms for dissimilarities between momentum and heat transport in a disturbed turbulent channel flow of air were studied through direct numerical simulations. A square rib protrusion with an aspect ratio of up to 64 was used to minimize constraints that the side boundary imposed on the wake and the related heat transfer. Channel walls were heated to maintain isothermal conditions. The frictional Reynolds number was set to either 150 or 300. The maximum number of grid points was 96,993,280. A grid convergence test showed that numerical results were independent of grid resolution. The mean pressure obtained from numerical simulation agreed with existing experimental data. Heat transfer characteristics were evaluated using the spatial mean Nusselt number over the ribbed wall and over rib surfaces. This Nusselt number was compared with that for a smooth channel at the same pumping power; the comparison showed that heat transfer increased by as much as 10%. This increase was caused by the simultaneous increase in heat transfer and reduction in skin friction behind the rib over a wall length of 30 times the rib height. Thus, the dissimilarity between local heat and momentum transport involved improving bulk heat transfer and reducing losses to skin friction. Mechanisms leading to the transport dissimilarity were studied via instantaneous structures, turbulence statistics, and spectral analyses. The results suggest that spanwise vortices shed from the rib contributed to the transport dissimilarity. Spanwise vortices entrained fresh fluid into the near-wall region, increasing heat transfer and reducing skin friction simultaneously. This behavior is consistent with reports in the literature obtained by visualization (Yao et al.) and by RANS simulation (Inaoka et al.). The novelty in the present study was that turbulent spanwise vortices and related thermal fields could be reproduced without using a turbulence model. This approach enabled us to know structures of the vortices and the flow three-dimensionality.

We have fabricated superstrate In2S3/ZnCuInS2 (Zn2xCu1-xIn1-xS2, ZCIS) solar cells as the top cell of multi-junction solar cells. The ZCIS and In2S3 thin films were deposited by spray pyrolysis deposition (SPD) and an efficiency of 4.4% was achieved with bandgap of 1.8 eV. However, in the structure of superstrate type solar cells, interdiffusion have been caused between the buffer layer and the ZCIS layer. In order to suppress the interdiffusion, we have fabricated In2S3/ZCIS substrate type solar cells. The thickness of the interdiffusion region decreased from 70 nm of the superstrate cell to 15 nm of the substrate cell. As a result, the substrate cell with bandgap of 1.8 eV showed VOC of 0.71 V and FF of 0.55 higher than those of the superstrate cells. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present our latest results toward high-efficiency thin-film silicon solar cells. Owing to the superior light trapping capability of periodic textures combined with other technologies, a new world record was achieved for the efficiency of single-junction microcrystalline silicon solar cells, with a conversion efficiency of 11.0%, independently confirmed by the Advanced Industrial Science and Technology (AIST) Characterization, Standards, and Measurement (CSM) team.

Cu 2 Sn 1− x Ge x Se 3 (CTGSe) thin film solar cells were fabricated by coevaporation with post-annealing treatment. The CTGSe solar cells showed a remarkably improved conversion efficiency of 3.0% compared to a Cu 2 SnSe 3 (CTSe) solar cells ( η =0.045%). This enhancement of the solar cell properties is attributed to improvement of the film morphology (e.g., grain size and densification), reduction of the carrier concentration, and bandgap widening with Ge incorporation into the CTSe. The optical band gap of CGTSe ( x =0.58) is 0.77 eV, which is close to the 0.8 eV optimal bandgap of bottom cells in multi-junction solar cells.

Numerical simulation was made for high-temperature solar and thermal receivers of pressurized air for solar micro gas turbine system. The solar / biomass hybrid gas turbine was considered to generate 30kW to 100kW power. The gas turbine system was provided with the concentrated solar light from the dish reflector at the solar receiver and the combustion heat from the biomass synthesis gas at the thermal receiver. Numerical model was developed to the solar receiver and the thermal receiver to reveal their thermal potential. The solar receiver was a close loop concentric annuli to receive highly condensed solar light of 1,000kW/m2. The inner cylinder was made of high-temperature resistance ceramic irradiated by the condensed light on the inner side. The liner was inserted between the inner cylinder and the outer shell. The pressurized air passes the many holes of the liner to impinge the outer surface of the irradiation wall. These impinging jets caused high heat transfer coefficient on the irradiation wall and alleviates the thermal distribution in the receiver aisle. The liner and the outer shell were made by the high temperature resistance INCONEL alloy. The thermal receiver was also a close loop annuli. This uses the same part as the solar receiver and the biomass gas combustor combined to it. The combustor comprises of the liner and the center tube, installed to the inside of the ceramic cylinder. The biomass gas was provided to the gap between liner and the center tube, and the oxidant air to the outer side of the liner. The biomass gas was spouted from the many holes of the liner and mixed with the oxidant air. The resulted hot combustion gas impinged directly to the inner side of the ceramic cylinder. The impingement of the hot combustion gas thinned the thermal boundary layer and enhanced the heat transfer on the ceramic wall. The thermal receiver was designed to attain the preferable heat transfer performance by the inner impinging jet of the hot combustion gas as well as the outer impinging jet of the pressurized air. Three dimensional numerical model was developed to the solar receiver and the thermal receiver considered in the present study using ANSYS FLUENT. Parameter study showed that the exit air from the solar receiver was heated above 1200K or higher presently, and was continued to search better condition and better configuration of the system to obtain higher temperature. The numerical simulation revealed that the distance from the jet nozzle (linear holes) and the heat transfer surface is critical to the thermal distribution. The concept of the new solar and thermal receivers was confirmed on their usefulness; the multiple impinging jet effectively enhanced the heat transfer on the ceramic wall of the solar receiver and the thermal receiver to reduce the thermal inhomogeneity near the heat transfer surface with pressure loss of order 800Pa.

Potential of a-Si:H/c-Si heterojunction cells with very thin wafers was examined from the optical and electrical points of view. Optical characterization of dummy c-Si cell structures shows that a realistic light trapping scheme, i.e., pyramidally textured Si wafers with a dielectric anti-reflection coating and a back reflector, realizes an efficient quasi-Lambertian light absorption enhancement, even for very thin wafers. The potential photocurrent densities evaluated using absorption spectra suggests that J sc reduction by decreasing the wafer thickness can be mitigated by using the realistic light trapping scheme, suppose that the parasitic absorption loss is minimized. The potential of higher V OC in thin c-Si cells was investigated using thin c-Si wafers passivated with intrinsic a-Si:H thin layers. A steady increase of implied V OC with decreasing the wafer thickness was experimentally confirmed in a wide range of wafer thickness down to 30 $\mu \mathrm{m}$ . The V OC enhancement was experimentally confirmed in a SHJ cell with a thickness below 60 $\mu \mathbf{m}$ .