Vicente Hernández Díaz

Bio: Vicente Hernández Díaz is an academic researcher from Technical University of Madrid. The author has contributed to research in topics: Wireless sensor network & Node (networking). The author has an hindex of 11, co-authored 34 publications receiving 469 citations. Previous affiliations of Vicente Hernández Díaz include Universiti Putra Malaysia & Autonomous University of Aguascalientes.

A GaAs solar cell with an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns

Abstract: A GaAs solar cell without prismatic covers, with the highest efficiency known to the authors in the range of 1000-2000 suns for a single junction, is presented. Low temperature liquid phase epitaxy is used for its growth. In addition to improvements such as the achievement of a good quality material or a low contact resistance, this solar cell exhibits specific enhanced aspects. Among the most noticeable are: (1) an innovative design; (2) a double and gradual emitter layer; (3) a small size: 1 mm/sup 2/, (4) a finger width of the front metal grid of 3 /spl mu/m; and (5) a tailored ARC deposition based on a nondestructive and accurate AlGaAs window layer characterization. As a consequence, an efficiency of 26.2% at 1000 suns and 25.0% at 2000 suns AM1.5D (standard conditions) is achieved thanks mainly to a short-circuit current density at 1000 suns of 26.8 A/cm/sup 2/ (and 53.6 A/cm/sup 2/ at 2000 suns) with a simultaneous series resistance of 3 m/spl Omega//spl middot/cm/sup 2/.

Probability Prediction-Based Reliable and Efficient Opportunistic Routing Algorithm for VANETs

Abstract: In the vehicular ad hoc networks (VANETs), due to the high mobility of vehicles, the network parameters change frequently and the information that the sender maintains may outdate when it wants to transmit data packet to the receiver, so for improving the routing efficiency and reliability, we propose the probability prediction-based reliable and efficient opportunistic routing (PRO) algorithm for VANETs. The PRO routing algorithm can predict the variation of signal-to-interference-plus-noise ratio (SINR) and packet queue length (PQL) of the receiver. The prediction results are used to determine the utility of each relaying vehicle in the candidate set. The calculation of the vehicle’s utility is the weight-based algorithm, and the weights are the variances of SINR and PQL. The relaying priority of each relaying vehicle is determined by the value of its utility. By these innovations, the PRO can achieve better routing performance (such as the packet delivery ratio, the end-to-end delay, and the network throughput) than the SRPE, ExOR (street-centric), and greedy perimeter stateless routing algorithms.

Influence of series resistance on guidelines for manufacture of concentrator p-on-n GaAs solar cells

Abstract: This paper deals with the determination of the main factors influencing series resistance in p-on-n GaAs solar cells working at concentration levels of 1000 suns or higher. Prior to this analysis, a comparison between different front metal grid geometries is presented to show the strong influence that the front grid component of series resistance exerts on its global value. Once the inverted square grid geometry is selected, a detailed analysis of the different components of series resistance is carried out. For this purpose, a multidimensional optimisation of the whole GaAs solar cell (antireflecting coatings, series resistance and semiconductor structure) has been used for the first time. In order to orient the manufacture of very high concentrator GaAs solar cells, recommendations on the threshold values of solar cell size, specific p- and n-contact resistances, thickness of the front metal grid and both doping level and thickness of the substrate are formulated. Several traditional ideas on the influence of these parameters are questioned. Copyright © 2000 John Wiley & Sons, Ltd.

Distributed Power Control for Interference-Aware Multi-User Mobile Edge Computing: A Game Theory Approach

Abstract: The computation task offloading and resource management in mobile edge computing (MEC) become attractive in recent years. Many algorithms have been proposed to improve the performance of the MEC system. However, the research on power control in MEC systems is just starting. The power control in the single-user and an interference-free multi-user MEC systems has been investigated; but in the interference-aware multi-user MEC systems, this issue has not been learned in detail. Therefore, a game theory-based power control approach for the interference-aware multi-user MEC system is proposed in this paper. In this algorithm, both the interference and the multi-user scenario are considered. Moreover, the existence and uniqueness of the Nash Equilibrium (NE) of this game are proved, and the performance of this algorithm is evaluated via theoretical analysis and numerical simulation. The convergence, the computation complexity and the price of anarchy in terms of the system-wide computation overhead are investigated in detail. The performance of this algorithm has been compared with the traditional localized optimal algorithm by simulation. The simulation results demonstrate that the proposed algorithm has more advantages than the traditional one.

Design and optimization of very high power density monochromatic GaAs photovoltaic cells

Abstract: This paper deals with the structure optimization of very high power density monochromatic GaAs photovoltaic cells and the theoretical prediction of their performance at irradiances ranging from 0.1 to 100 W/cm/sup 2/. A multifaceted optimum design including the front metal grid, device size and the semiconductor layer structure is presented. The variation in efficiency depending on emitter thickness, base thickness, emitter doping and base doping is also addressed. The objective of this is the configuration of a structure suitable for working up to 100 W/cm/sup 2/ without the detrimental influence of series resistance. For this, a detailed analysis of the effect of series resistance and the quantitative determination of its different components is carried out. The optimum wavelength is 830 nm at 300 K for all the analyzed light intensities, in which a 63% peak efficiency under an irradiance of 100 W/cm/sup 2/ for a p/n structure is obtained. The temperature effect on device performance in the 273-350 K range is also studied. Finally, the influence of device processing is analyzed.

Enhanced electron extraction using SnO 2 for high-efficiency planar-structure HC(NH 2 ) 2 PbI 3 -based perovskite solar cells

Abstract: Planar structures for halide perovskite solar cells have recently garnered attention, due to their simple and low-temperature device fabrication processing. Unfortunately, planar structures typically show I–V hysteresis and lower stable device efficiency compared with mesoporous structures, especially for TiO2-based n-i-p devices. SnO2, which has a deeper conduction band and higher electron mobility compared with traditional TiO2, could enhance charge transfer from perovskite to electron transport layers, and reduce charge accumulation at the interface. Here we report low-temperature solution-processed SnO2 nanoparticles as an efficient electron transport layer for perovskite solar cells. Our SnO2-based devices are almost free of hysteresis, which we propose is due to the enhancement of electron extraction. By introducing a PbI2 passivation phase in the perovskite layer, we obtain a 19.9 ± 0.6% certified efficiency. The devices can be easily processed under low temperature (150 ∘C), offering an efficient method for the large-scale production of perovskite solar cells. Planar structured perovskite solar cells often show hysteresis and lower efficiency than mesoporous ones. Jiang et al. show that using a SnO2 electron transport layer improves the performance of planar devices, reporting a certified efficiency of 19.9%, and enables a lower processing temperature.

Solar hydrogen production with nanostructured metal oxides

Abstract: The direct conversion of solar energy into hydrogen represents an attractive but challenging alternative for photo-voltaic solar cells. Several metal oxide semiconductors are able to split water into hydrogen and oxygen upon illumination, but the efficiencies are still (too) low. The operating principles of photo-electrochemical devices for water splitting, their main bottlenecks, and the various device concepts will be reviewed. Materials properties play a key role, and the advantages and pitfalls of the use of interfacial layers and dopants will be discussed. Special attention will be given to recent progress made in the synthesis of nanostructured metal oxides with high aspect ratios, such as nanowire arrays, which offers new opportunities to develop efficient photo-active materials for solar water splitting.

GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies

Abstract: Although compound semiconductors like gallium arsenide have a substantial performance advantage over silicon in photovoltaic and optoelectronic applications, these do not outweigh the costly process of growing large, high-quality layers of these materials and transferring them to flexible or transparent substrates for use in devices such as solar cells, night vision cameras and wireless communication systems. But now John Rogers and his team demonstrate a new fabrication approach that may remove this disadvantage. They grow films of GaAs and AlGaAs in thick, multilayered assemblies in a single deposition sequence, then release the individual layers and distribute them over foreign substrates by printing. The technological potential of this strategy to large-area applications is illustrated with the fabrication of GaAs devices such as field-effect transistors on glass and photovoltaic modules on sheets of plastic. Although compound semiconductors like gallium arsenide (GaAs) offer advantages over silicon for photovoltaic and optoelectronic applications, these do not outweigh the costly process of growing large layers of these materials and transferring them to appropriate substrates. However, a new fabrication approach is now demonstrated: films of GaAs and AlGaAs are grown in thick, multilayered assemblies in a single sequence; the individual layers are then released and distributed over foreign substrates by printing. Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices1,2 to radio-frequency electronics3,4 and most forms of optoelectronics5,6. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

III–V multijunction solar cells for concentrating photovoltaics

Abstract: Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III–V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.

Empirical low-field mobility model for III-V compounds applicable in device simulation codes

Abstract: A Caughey–Thomas-like mobility model with temperature and composition dependent coefficients is used in this work to describe the dependence of electron and hole mobilities on temperature, doping concentration, and alloy composition. Appropriate parameter sets are given for a large number of III–V binary and ternary compounds, including: GaAs, InP, InAs, AlAs, GaP, Al0.3Ga0.7As, In0.52Al0.48As, In0.53Ga0.47As, and In0.49Ga0.51P. Additionally, physically justifiable interpolation schemes are suggested to find the mobilities of various ternary and quaternary compounds (such as AlxGa1−xAs, In1−xGaxP, In1−xGaxAs, In1−xAlxAs, and In1−xGaxAsyP1−y) in the entire range of composition. The models are compared with numerous measured Hall data in the literature and very good agreement is observed. The limitations of the present model are also discussed. The results of this work should be extremely useful in device simulation packages, which are currently lacking a reliable mobility model for the above materials.