12 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
Biexciton Auger Recombination Differs in Hybrid and Inorganic Halide Perovskite Quantum Dots
We use time-resolved photoluminescence
measurements to determine
the biexciton Auger recombination rate in both hybrid organic–inorganic
and fully inorganic halide perovskite nanocrystals as a function of
nanocrystal volume. We find that the volume scaling of the biexciton
Auger rate in the hybrid perovskites, containing a polar organic A-site
cation, is significantly shallower than in the fully inorganic Cs-based
nanocrystals. As the nanocrystals become smaller, the Auger rate in
the hybrid nanocrystals increases even less than expected, compared
to the fully inorganic nanocrystals, which already show a shallower
volume dependence than other material systems such as chalcogenide
quantum dots. This finding suggests there may be differences in the
strength of Coulombic interactions between the fully inorganic and
hybrid perovskites, which may prove to be crucial in selecting materials
to obtain the highest performing devices in the future, and hints
that there could be something “special” about the hybrid
materials
Correlating Photoluminescence Heterogeneity with Local Electronic Properties in Methylammonium Lead Tribromide Perovskite Thin Films
We
conduct correlated laser scanning confocal photoluminescence
(PL) microscopy, scanning kelvin probe microscopy, and conductive
atomic force microscopy (c-AFM) to understand the origins and effects
of local heterogeneity in films of the hybrid organic–inorganic
perovskite semiconductor methylammonium lead tribromide (MAPbBr<sub>3</sub>). We compare PL between perovskite films deposited on glass
and on hole-transporting contacts. In both systems, we observe heterogeneous
PL, but this heterogeneity is due to different mechanisms. On glass
substrates, we observe that the PL maps are dominated by lateral carrier
diffusion, and on hole-transporting contacts, we observe an anticorrelation
between PL and local hole injected current as measured by c-AFM. We
conclude that the local variations are due to heterogeneous electronic
coupling at the perovskite–electrode interface. We also show
that correlated PL and AFM studies are expected to play a key role
in studying the electronic heterogeneities in the perovskite itself,
which are currently screened by the perovskite–contact interfaces.
Our results suggest the need for new selective contacts to improve
the charge transfer at the perovskite–contact interfaces
Charge Carriers in Planar and Meso-Structured Organic–Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States
Efficient solar cells have been obtained
using thin films of solution-processed
organic–inorganic perovskites. However, there remains limited
knowledge about the relationship between preparation route and optoelectronic
properties. We use complementary time-resolved microwave conductivity
(TRMC) and photoluminescence (PL) measurements to investigate the
charge carrier dynamics in thin planar films of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>, CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, and their meso-structured
analogues. High mobilities close to 30 cm<sup>2</sup>/(V s) and microsecond-long
lifetimes are found in thin films of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>,
compared to lifetimes of only a few hundred nanoseconds in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and meso-structured perovskites.
We describe our TRMC and PL experiments with a global kinetic model,
using one set of kinetic parameters characteristic for each sample.
We find that the trap density is less than 5 × 10<sup>14</sup> cm<sup>–3</sup> in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>, 6 × 10<sup>16</sup> cm<sup>–3</sup> in the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> thin film and ca. 10<sup>15</sup> cm<sup>–3</sup> in both meso-structured perovskites. Furthermore, our results imply
that band-to-band recombination is enhanced by the presence of dark
carriers resulting from unintentional doping of the perovskites. Finally,
our general approach to determine concentrations of trap states and
dark carriers is also highly relevant to other semiconductor materials
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
Radiative Monomolecular Recombination Boosts Amplified Spontaneous Emission in HC(NH<sub>2</sub>)<sub>2</sub>SnI<sub>3</sub> Perovskite Films
Hybrid
metal-halide perovskites have potential as cost-effective
gain media for laser technology because of their superior optoelectronic
properties. Although lead-halide perovskites have been most widely
studied to date, tin-based perovskites have been proposed as a less
toxic alternative. In this Letter, we show that amplified spontaneous
emission (ASE) in formamidinium tin triiodide (FASnI<sub>3</sub>)
thin films is supported by an observed radiative monomolecular charge
recombination pathway deriving from its unintentional doping. Such
a radiative component will be active even at the lowest charge-carrier
densities, opening a pathway for ultralow light-emission thresholds.
Using time-resolved THz photoconductivity analysis, we further show
that the material has an unprecedentedly high charge-carrier mobility
of 22 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> favoring
efficient transport. In addition, FASnI<sub>3</sub> exhibits strong
radiative bimolecular recombination and Auger rates that are over
an order of magnitude lower than for lead-halide perovskites. In combination,
these properties reveal that tin-halide perovskites are highly suited
to light-emitting devices
Direct Observation and Quantitative Analysis of Mobile Frenkel Defects in Metal Halide Perovskites Using Scanning Kelvin Probe Microscopy
Ion migration is
seen as a primary stability concern of halide
perovskite-based photovoltaic and optoelectronic devices. Here, we
provide experimental studies of long-distance, reversible ion migration
in methylammonium lead iodide (MAPbI<sub>3</sub>) and formamidinium
lead iodide (FAPbI<sub>3</sub>) films. We use time-resolved scanning
Kelvin probe microscopy on insulator-coated lateral electrodes to
probe the dynamic redistribution of charged Frenkel defects over micrometer
distances after application of an electric field. We combine these
dynamic measurements with drift–diffusion simulations that
yield self-consistent pictures of the sign, distribution, mobility,
and activation energy of the associated, mobile Frenkel defects. This
comprehensive approach is applied to study the impact of an organic
cation on ionic mobility in metal halide perovskites, which we find
to be significantly reduced in the case of FAPbI<sub>3</sub> films
compared to MAPbI<sub>3</sub> films
Orientation of Ferroelectric Domains and Disappearance upon Heating Methylammonium Lead Triiodide Perovskite from Tetragonal to Cubic Phase
We
study the spontaneous polarization of the archetypal semiconducting
halide perovskite methylammonium lead triiodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) that is currently being investigated for
use in thin film solar cells and light-emitting diodes. Using both
lateral and vertical piezoresponse force microscopy (PFM) to image
polycrystalline thin films, we observed domains in the piezoresponse
that reversibly appear and disappear below and above the tetragonal-to-cubic
phase transition temperature. Importantly, we observe these domains
to exhibit a piezoresponse that is predominantly in-plane for films
with the (110) plane oriented parallel to the substrate, providing
a measure of the polarization associated with specific crystal planes.
We characterize the polarization and its temporal response using both
local switching spectroscopy and time-dependent PFM spectra. These
data show hysteresis loops with the polarization switching with bias
but relaxing back on time scales of several minutes. Our results suggest
the existence of ferroelectric behavior due to off-center displacement
of the Pb<sup>2+</sup> cation, although the local polarization response
is complicated by the presence of local ionic and electronic conductivity.
Understanding the nature of these domains paves the way for further
optimization of optoelectronic devices using CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite material
The Potential of Multijunction Perovskite Solar Cells
Metal
halide perovskite semiconductors offer rapid, low-cost deposition
of solar cell active layers with a wide range of band gaps, making
them ideal candidates for multijunction solar cells. Here, we combine
optical and electrical models using experimental inputs to evaluate
the feasible performances of all-perovskite double-junction (2PJ),
triple-junction (3PJ), and perovskite–perovskite–silicon
triple-junction (2PSJ) solar cells. Using parameters and design constraints
from the current state-of-the-art generation of perovskite solar cells,
we find that 2PJs can feasibly approach 32% power conversion efficiency,
3PJs can reach 33%, and 2PSJs can surpass 35%. We also outline pathways
to improve light harvesting and demonstrate that it is possible to
raise the performances to 34%, 37%, and 39% for the three architectures.
Additionally, we discuss important future directions of research.
Finally, we perform energy yield modeling to demonstrate that the
multijunction solar cells should not suffer from reduced operational
performances due to discrepancies between the AM1.5G and real-world
spectrum over the course of a year