49 research outputs found
To form a union without having a child. The lengthening of the initial period of life in union before parenthood. A study based on European FFS data
Comunicació presentada a l'European Population Conference: "Migration and Migrants in Europe"(Sessió 53). Organitzat per l'European Association for Population Studies (EAPS); Centre d'Estudis Demogràfics. Barcelona, del 9 al 12 de juliol de 2008.The authors of the Second Demographic Transition scheme single out the postponement of the age at first childbearing as the main effect of the changes in habits of young adults associated with this transition. This postponement is accompanied by an increase in the length of the initial period of life in partnership when the couple has no plan yet to have children. This change is made possible by the use of contraceptive means by people living in partnerships in order to delay first childbearing. This is in sharp contrast with the First Demographic Transition, which was also characterized by the extension of the use of contraceptive means, but only after the birth of children. So contraception was used then to control fertility, when it is used nowadays to extend the period of life when no irreversible decisions, like having a child, have been made yet. In this work, we study various dimensions of this postponement of childbearing by couples. First, we try to quantify the magnitude of the increase in the duration of this initial period, when the couple delays parenthood. Second we have a look at the way this change in fertility behaviours in the first years of union change the duration model that was typical at the end of the First Demographic Transition. Finally, we investigate the possible determinants of the increase of this initial period using data on time spent by women studying and working, and of the transition from cohabitation to marriage. We use data from Fertility and Families Surveys for 17 countries and apply life table techniques and proportional hazard modelling.Un dels principals canvis associats a la Segona Transició Demogràfica és el retard en l'edat de tenir el primer fill, allargant-se el període d'unió sense plans de tenir-los. El retard és possible gràcies a la utilització dels mitjans anticonceptius, fet que contrasta amb la Primera Transició Demogràfica, a on els mitjans anticonceptius s'empraven després del naixement dels fills. En aquest estudi s'analitzen les diverses dimensions d'aquest ajornament. En primer lloc, es quantifica l'augment temporal d'aquest període inicial sense fills; en segon lloc s'analitza la forma en què aquest canvi modifica el model de la Primera Transició Demogràfica; finalment, s'apunten possibles determinants, emprant dades sobre el temps dedicat per les dones a estudiar i a treballar, i de la transició de la cohabitació al matrimoni. La font bàsica d'informació és la Fertility and Families Surveys , per a 17 països.Uno de los principales cambios asociados a la Segunda Transición Demográfica, es el aplazamiento en la edad de tener el primer hijo, ampliándose el período de unión sin planes de tenerlos. La demora es posible gracias a la utilización de los medios anticonceptivos, hecho que contrasta con la Primera Transición Demográfica, donde los medios anticonceptivos se utilizaban después del nacimiento de los hijos. En este estudio se analizan las dimensiones de este aplazamiento. En primer lugar, se cuantifica el aumento temporal de esta etapa inicial sin hijos; en segundo lugar, se analiza la forma en que este cambio modifica el modelo de la Primera Transición Demográfica; finalmente, se apuntan posibles determinantes utilizando datos sobre el tiempo dedicado por las mujeres a estudiar y a trabajar, y de la transición de la cohabitación al matrimonio. La fuente básica de información es la Fertility and Families Surveys, para 17 países
Charge Transport Limitations in Self-Assembled TiO<sub>2</sub> Photoanodes for Dye-Sensitized Solar Cells
Solid-state dye-sensitized solar cells offer the possibility
of
high-power conversion efficiencies due to theoretically lower fundamental
losses in dye regeneration. Despite continuous progress, limitations
in charge diffusion through the mesoporous photoanode still constrain
the device thickness and hence result in reduced light absorption
with the most common sensitizers. Here we examine block copolymer-assembled
photoanodes with similar surface area and morphology but a large variation
in crystal size. We observe that the crystal size has a profound effect
on the electron transport, which is not explicable by considering
solely the ratio between free and trapped electrons. Our results are
consistent with the long-range mobility of conduction band electrons
being strongly influenced by grain boundaries. Therefore, maximizing
the crystal size while maintaining high enough surface area will be
an important route forward
Oligothiophene Interlayer Effect on Photocurrent Generation for Hybrid TiO<sub>2</sub>/P3HT Solar Cells
A series
of conjugated 3-hexylthiophene derivatives with a cyanoacrylic
acid group has been prepared with conjugation length from one up to
five thiophene units (<b>1T</b>–<b>5T</b>). The
UV–vis spectra, photoluminescence spectra, electrochemical
data and DFT calculations show lowering of LUMO energies and red-shift
of absorption into the visible as the thiophene chain length increases.
TiO<sub>2</sub>/P3HT solar cells were prepared with prior functionalization
of the TiO<sub>2</sub> surface by <b>1T</b>–<b>5T</b> and studies include cells using undoped P3HT and using P3HT doped
with H-TFSI. Without H-TFSI doping, photocurrent generation occurs
from both the oligothiophene and P3HT. Doping the P3HT with H-TFSI
quenches photocurrent generation from excitation of P3HT, but enables
very effective charge extraction upon excitation of the oligothiophene.
In this case, photocurrent generation increases with the light harvesting
ability of <b>1T</b>–<b>5T</b> leading to a highest
efficiency of 2.32% using <b>5T</b>. Overall, we have shown
that P3HT can act in either charge generation or in charge collection,
but does not effectively perform both functions simultaneously, and
this illustrates a central challenge in the further development of
TiO<sub>2</sub>/P3HT solar cells
Well-Defined Nanostructured, Single-Crystalline TiO<sub>2</sub> Electron Transport Layer for Efficient Planar Perovskite Solar Cells
An
electron transporting layer (ETL) plays an important role in
extracting electrons from a perovskite layer and blocking recombination
between electrons in the fluorine-doped tin oxide (FTO) and holes
in the perovskite layers, especially in planar perovskite solar cells.
Dense TiO<sub>2</sub> ETLs prepared by a solution-processed spin-coating
method (S-TiO<sub>2</sub>) are mainly used in devices due to their
ease of fabrication. Herein, we found that fatal morphological defects
at the S-TiO<sub>2</sub> interface due to a rough FTO surface, including
an irregular film thickness, discontinuous areas, and poor physical
contact between the S-TiO<sub>2</sub> and the FTO layers, were inevitable
and lowered the charge transport properties through the planar perovskite
solar cells. The effects of the morphological defects were mitigated
in this work using a TiO<sub>2</sub> ETL produced from sputtering
and anodization. This method produced a well-defined nanostructured
TiO<sub>2</sub> ETL with an excellent transmittance, single-crystalline
properties, a uniform film thickness, a large effective area, and
defect-free physical contact with a rough substrate that provided
outstanding electron extraction and hole blocking in a planar perovskite
solar cell. In planar perovskite devices, anodized TiO<sub>2</sub> ETL (A-TiO<sub>2</sub>) increased the power conversion efficiency
by 22% (from 12.5 to 15.2%), and the stabilized maximum power output
efficiency increased by 44% (from 8.9 to 12.8%) compared with S-TiO<sub>2</sub>. This work highlights the importance of the ETL geometry
for maximizing device performance and provides insights into achieving
ideal ETL morphologies that remedy the drawbacks observed in conventional
spin-coated ETLs
Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>
The organic–inorganic hybrid
perovskites methylammonium
lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) and the
partially chlorine-substituted mixed halide CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> emit strong and broad photoluminescence (PL) around their
band gap energy of ∼1.6 eV. However, the nature of the radiative
decay channels behind the observed emission and, in particular, the
spectral broadening mechanisms are still unclear. Here we investigate
these processes for high-quality vapor-deposited films of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> using time- and excitation-energy dependent photoluminescence
spectroscopy. We show that the PL spectrum is homogenously broadened
with a line width of 103 meV most likely as a consequence of phonon
coupling effects. Further analysis reveals that defects or trap states
play a minor role in radiative decay channels. In terms of possible
lasing applications, the emission spectrum of the perovskite is sufficiently
broad to have potential for amplification of light pulses below 100
fs pulse duration
ZrO<sub>2</sub>/TiO<sub>2</sub> Electron Collection Layer for Efficient Meso-Superstructured Hybrid Perovskite Solar Cells
Since
the first reports of efficient organic–inorganic perovskite
solar cells in 2012, an explosion of research activity has emerged
around the world, which has led to a rise in the power conversion
efficiencies (PCEs) to over 20%. Despite the impressive efficiency,
a key area of the device which remains suboptimal is the electron
extraction layer and its interface with the photoactive perovskite.
Here, we implement an electron collection “bilayer”
composed of a thin layer of zirconia coated with titania, sitting
upon the transparent conductive oxide fluorine-doped tin oxide (FTO).
With this double collection layer we have reached up to 17.9% power
conversion efficiency, delivering a stabilized power output (SPO)
of 17.0%, measured under simulated AM 1.5 sunlight of 100 mW cm<sup>–2</sup> irradiance. Finally, we propose a mechanism of the
charge transfer processes within the fabricated architectures in order
to explain the obtained performance of the devices
Band Gaps of the Lead-Free Halide Double Perovskites Cs<sub>2</sub>BiAgCl<sub>6</sub> and Cs<sub>2</sub>BiAgBr<sub>6</sub> from Theory and Experiment
The recent discovery of lead-free
halide double perovskites with
band gaps in the visible represents an important step forward in the
design of environmentally friendly perovskite solar cells. Within
this new family of semiconductors, Cs<sub>2</sub>BiAgCl<sub>6</sub> and Cs<sub>2</sub>BiAgBr<sub>6</sub> are stable compounds crystallizing
in the elpasolite structure. Following the recent computational discovery
and experimental synthesis of these compounds, a detailed investigation
of their electronic properties is warranted in order to establish
their potential as optoelectronic materials. In this work, we perform
many-body perturbation theory calculations and obtain high accuracy
band gaps for both compounds. In addition, we report on the synthesis
of Cs<sub>2</sub>BiAgBr<sub>6</sub> single crystals, which are stable
in ambient conditions. From our complementary theoretical and experimental
analysis, we are able to assign the indirect character of the band
gaps and obtain both experimental and theoretical band gaps of these
novel semiconductors that are in close agreement
The Importance of Perovskite Pore Filling in Organometal Mixed Halide Sensitized TiO<sub>2</sub>‑Based Solar Cells
Emerging from the field of dye-sensitized
solar cells, organometal
halide perovskite-based solar cells have recently attracted considerable
attention. In these devices, the perovskite light absorbers can also
be used as charge transporting materials, changing the requirements
for efficient device architectures. The perovskite deposition can
vary from merely sensitizing the TiO<sub>2</sub> electron transporting
scaffold as an endowment of small nanoparticles, to completely filling
the pores where it acts as both light absorber and hole transporting
material in one. By decreasing the TiO<sub>2</sub> scaffold layer
thickness, we change the solar cell architecture from perovskite-sensitized
to completely perovskite-filled. We find that the latter case leads
to improvements in device performance because higher electron densities
can be sustained in the TiO<sub>2</sub>, improving electron transport
rates and photovoltage. Importantly, the primary recombination pathway
between the TiO<sub>2</sub> and the hole transporting material is
blocked by the perovskite itself. This understanding helps to rationalize
the high voltages attainable on mesoporous TiO<sub>2</sub>-based perovskite
solar cells
Enhanced Hole Extraction in Perovskite Solar Cells Through Carbon Nanotubes
Here, we report the use of polymer-wrapped
carbon nanotubes as
a means to enhance charge extraction through undoped spiro-OMeTAD.
With this approach a good solar cell performance is achieved without
the implementation of conventional doping methods. We demonstrate
that a stratified two-layer architecture of sequentially deposited
layers of carbon nanotubes and spiro-OMeTAD, outperforms a conventional
blend of the hole-conductor and the carbon nanotubes. We also provide
insights into the mechanism of the rapid hole extraction observed
in the two-layer approach
Unraveling the Function of an MgO Interlayer in Both Electrolyte and Solid-State SnO<sub>2</sub> Based Dye-Sensitized Solar Cells
The coating of n-type mesoporous metal oxides with nanometer
thick
dielectric shells is a route that has proven to be successful at enhancing
the efficiency of some families of dye-sensitized solar cells. The
primary intention is to introduce a “surface passivation layer”
to inhibit recombination between photoinduced electrons and holes
across the dye-sensitized interface. However, the precise function
of these dielectric interlayers is often ambiguous. Here, the role
of a thin MgO interlayer conformally deposited over mesoporous SnO<sub>2</sub> in liquid electrolyte and solid-state dye-sensitized solar
cells is investigated. For both families of devices the open-circuit
voltage is increased by over 200 mV; however, the short-circuit photocurrent
is increased for the solid-state cells, but reduced for the electrolyte
based devices. Through electronic and spectroscopic characterization
we deduce that there are four distinct influences of the MgO interlayer:
It increases dye-loading, slows down recombination, slows down photoinduced
electron transfer, and results in a greater than 200 mV shift in the
conduction band edge, with respect to the electrolyte redox potential.
The compilation of these four factors have differing effects and magnitudes
in the solid-state and electrolyte DSCs but quantitatively account
for the difference in device performances observed for both systems
with and without the MgO shells. To the best of our knowledge, this
is the most comprehensive account of the role of dielectric shells
in dye-sensitized solar cells and will enable much better interfacial
design of photoelectrodes for DSCs