1,720,983 research outputs found
Next-generation kesterite thin-film solar cells: development, characterization and modeling
No full textIn order to cope with the ever-growing energy demand while cutting down
greenhouse gas emissions, solar power must be deployed at the largest
extent possible. This requires the widespread installation of photovoltaic
technologies beyond utility-scale to comply with the greater integrability
and adaptability needed in emerging markets. Thin-film solar cells
based on chalcogenide compounds hold highly encouraging promises for
such applications, due to their versatile architecture and tuneable properties.
Among them, kesterite materials with the Cu2ZnSn(S,Se)4 crystalline
structure provide an interesting alternative based on Earth-abundant elements
and compatible with low-cost processes, making them a promising
solution with reduced carbon footprint and cost. Yet, their performance
must be increased to align their competitiveness with commercial technologies.
Reaching this objective entails to resolve their still too significant
deficit in open-circuit voltage, which is the main culprit for their limited efficiency
around 15% for the highest-performing devices. The physical origin
of these losses is manifold: control of the kesterite phase and composition
in a uniform fashion, regulation of its crystalline quality to favour low disorder,
monitoring of the growth environment and conditions enabling weakly
defective materials and interfaces, as well as preserving all these aspects
when tuning the absorber bandgap for relevant applications, among others.
This thesis aims at tackling these challenges on different levels.
First of all, it is demonstrated through an extensive review of the literature
that germanium (Ge) alloying constitutes a promising strategy to
enhance the quality of kesterite thin films by overcoming intrinsic electronic
limitations related to tin (Sn) while promoting superior structural
morphology. On top of this, this approach also allows to control the absorber
bandgap, which contributes to a greater versatility of the kesterite
compounds. Still, most studies have focused on low to moderate proportions
of Ge corresponding to a narrow bandgap range not yet matching the
specifications of emerging applications. This highlights the need to pursue
the exploration of the broad Ge compositional domain. The importance of
the kesterite deposition process is also detailed, showing the greater potential
of solution-based routes allowing finer control of composition and
phases at the lab scale, thus providing conditions for high efficiency. In
comparison, samples from the presented starting baseline, deposited via
sequential physical processes, exhibit limited performance with symptoms
of low opto-electronic kesterite quality and device non-ideality.
Following this, a molecular ink chemical route in ambient environment
is developed to allow flexible Ge alloying in high-quality single-phase thinfilm kesterite absorbers. Remarkably, this updated baseline enables to
tune the kesterite bandgap without compromising material quality. In
particular, narrow band tails associated to mitigated open-circuit voltage
radiative losses are ensured for a broad bandgap range in which the device
performance potential is therefrom augmented. Resolving this prerequisite
for photovoltaic absorbers is however only one of the main steps in
the development of a solar cell technology. Indeed, non-radiative losses
leading to low minority carrier lifetime remain an even more critical challenge
to be tackled. It originates from the numerous intrinsic point defects
in the kesterite lattice, the exact nature, location and dominance of
which remain partly undetermined. It is therefore essential to pinpoint
the underlying mechanism responsible for defect-assisted recombination
and the associated performance losses. This is especially relevant for the
next-generation devices based on multinary-alloyed solution-processed
kesterite absorbers which are presently leading the way towards future
efficiency breakthroughs.
Eventually, based on the study of temperature- and light intensitydependent
current-voltage measurements confronting analytical models
and SCAPS-1D simulations, the closer-to-ideal behaviour of state-of-theart
kesterite devices is demonstrated as a consequence of weaker band
tailing and bandgap and potential fluctuations. The close agreement with
the single diode formalism combined with the observed dark and light reconciliation
allow to gauge the various contributions to the open-circuit voltage
deficit. The dominant loss mechanism is hypothesized to be a defectrich
kesterite layer at the interface with the buffer, highlighting the diode
ideality factor and saturation current density as the primary causes of
restrained performance. Light is also shed on carrier trapping-detrapping
via shallow defect states as the mechanism behind shunt leakage currents,
also contributing to lower efficiency. The applicability of this whole analysis
extends across various samples, which emphasizes its potential to
support further enhancements of next-generation kesterite solar cells.Doctoral dissertation submitted to obtain the degrees of-Doctor of Engineering Technology | UHasselt-Docteur en Sciences de l'ingénieur et technologie | UCLouvai
Low-temperature admittance spectroscopy for defect characterization in Cu(In,Ga)(S,Se)2 thin-film solar cells
No full textWe present a methodology to use low-temperature admittance measurements for characterizing defects in thin-film Cu(ln,Ga)(S,Se)2 solar cells, which is a major step towards increased performance. We develop the theory behind admittance spectroscopy at both room and low temperature, focusing on the so-called “loss-map” graphical representation. It allows to distinguish the entangled responses of different loss mechanisms and, combined with SCAPS 1- D simulations, leads to a refined interpretation of experimental admittance measurements. Using this methodology on experimental measurements, we identify the likely presence of an interface defect, and extract its activation energy (EA=0.093eV) and capture cross-section (σ=2.88⋅10−18cm2)
Comparative study of the interface passivation properties of LiF and Al2O3 using silicon MIS capacitor
No full text
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
Band gap reduction in highly-strained silicon beams predicted by first-principles theory and validated using photoluminescence spectroscopy
Full text linkA theoretical study of the band gap reduction under tensile stress is performed and validated through experimental measurements. First-principles calculations based on density functional theory (DFT) are performed for uniaxial stress applied in the [001], [110] and [111] directions. The calculated band gap reductions are equal to 126, 240 and 100 meV at 2% strain, respectively. Photoluminescence spectroscopy experiments are performed by deformation applied in the [110] direction. Microfabricated specimens have been deformed using an on-chip tensile technique up to ∼1% as confirmed by back-scattering Raman spectroscopy. A fitting correction based on the band gap fluctuation model has been used to eliminate the specimen interference signal and retrieve reliable values. Very good agreement is observed between first-principles theory and experimental results with a band gap reduction of, respectively, 93 and 91 meV when the silicon beam is deformed by 0.95% along the [110] direction
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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