1,720,962 research outputs found
Numerical analysis of the effect of grain size and defects on the performance of CIGS solar cells
No full textThis work shows two-dimensional numerical simulations aimed at providing indications for the development of optimized thin-film CIGS solar cells. The poly-crystalline nature of CIGS and the presence of defects and Cu-poor regions at the grain boundaries and at the CIGS surface are taken into account, together with the possible displacement of the p-n junction from the CdS/CIGS heterojunction
Sensorless Control of BLDC Motors at Low Speed Based on Differential BEMF Measurement
No full textBLDC drives are widely employed in industry due to their intrinsic robustness and high torque-to-weight ratio. The availability of cheap embedded processing power in recent years paved the way for the widespread use of sensorless control techniques; the removal of speed and position sensors leads to substantial increase of robustness and cost savings. Nevertheless, traditional sensorless BLDC control techniques suffer from low efficiency at low speed due to degraded signal to noise ratio on feedback signals and low dynamic response.
This paper describes a novel sensorless control algorithm based on a differential measurement, which allows higher dynamic performance and lower minimum operating speed with respect to traditional techniques without requiring dedicated hardware solutions or complex calculations
On the combined effects of window/buffer and buffer/absorber conduction-band offsets, buffer thickness and doping on thin-film solar cell performance
No full textIn this study we perform an extensive campaign of numerical simulations of thin-film solar cell structures aimed at investigating how the conduction band offsets at buffer/window (ΔBW) and buffer/absorber (ΔAB) heterojunctions and the thickness and doping of the buffer layer combine to affect the performance parameters (Jsc, Voc, FF and η). For the two scenarios of ideal (i.e., without traps) and non-ideal (with traps) buffer/absorber interface, we vary ΔAB and ΔBW in the range -0.5 eV to 0.5 eV, and analyze, for each combination, the physical mechanisms limiting the cell performance and the way to optimize it by choosing optimal buffer doping and thickness. We show that assuming ΔAB as the main indicator of the performance potential of a cell can be misleading because ΔBW can heavily influence performance, even when ΔAB is positive (conduction band higher in the buffer than in the absorber) and near to its theoretical optimal value (0.3eV). However, we also show that ΔAB 0. We verify our findings by simulating several examples of CIGS-based solar cells with different buffer layers (CdS, Zn1-xMgxO, In2S3, Zn(O,S)) taken from the literature; these comparisons confirm the validity of our results and suggest that the combination of ΔAB and ΔBW is the predominant factor in the design of high-efficiency solar cells. We finally propose simple quantitative guidelines for thin-film solar cell design and optimization
Simulation of Current Transport in Polycrystalline CdTe Solar Cells
No full textPolycrystalline thin-film CdTe solar cells have demonstrated laboratory efficiency
exceeding 17% and are nowadays a commercial technology (albeit with
somewhat lower efficiencies). The standard process features a poorly understood
recrystallization step, obtained by annealing with a source of chlorine.
This study uses two-dimensional numerical modeling to investigate current
transport inside the polycrystalline CdTe absorber with and without recrystallization
effects [increase of grain size and donor ClTe states at grain
boundaries (GBs)]. Solving the Poisson equation and the drift–diffusion model
for transport with Fermi statistics, while treating the optical problem by the
one-dimensional transfer matrix method and complex refractive indexes, this
study shows that: (i) in a columnar absorber (i.e., one where only vertical GBs
exist), the presence of ClTe donor traps at GBs results in a dip in the band
profiles that effectively serves as an electron collector, significantly increasing
the short-circuit current and efficiency compared with nondecorated GBs; (ii)
while the same dip acts as a hole barrier and thus can be expected to block
holes from flowing when horizontal GBs are present, under illuminated conditions
electron collection at GBs reduces the dip enough to allow substantial
hole flow, and the cell performance is only moderately affected
A numerical study of the design of ZnMgO window layer for Cadmium-free thin-film CIGS solar cells
No full textThe paper analyzes results of numerical simulation with the aim of providing theoretical indications for Cd-free CIGS solar cell. Thickness, composition and doping of the new-material ZnMgO window layer are taken into account, together with the effects of interface states at the heterojunction ZnMgO/CIGS
Modeling of thin-film Cu(In,Ga)Se2 solar cells
No full textWe show results of numerical and compact modeling of poly-crystalline CIGS thin-film solar cells. We use numerical simulations as a benchmark to develop a simple, physics-based compact model of the behavior of the cell in the dark. We show that, while the single-crystal cell behavior can be accurately described by a two-diode model, when grain boundaries are present and active a four-diode model is required. Finally, we show results of the application of numerical simulations to the study of the cell degradation under damp heat stress conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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