16 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
Minimal Effect of the Hole-Transport Material Ionization Potential on the Open-Circuit Voltage of Perovskite Solar Cells
Hole-transport material optimization
is an important step toward
maximizing the efficiency of perovskite solar cells. Here, we investigate
the role of one hole-transport material property, the ionization potential,
on the performance of perovskite solar cells. We employ a device architecture
that allows us to systematically tune the ionization potential while
avoiding any impact to other device parameters, and we find that for
a wide range of ionization potentials the photovoltaic performance
is minimally affected. This finding relaxes the requirement for the
development of hole-transport materials with particular ionization
potentials, allowing for the optimization of hole-transport materials
that can improve performance in differing ways such as through increased
stability or decreased parasitic absorption
Mechanism of Tin Oxidation and Stabilization by Lead Substitution in Tin Halide Perovskites
The recent development
of efficient binary tin- and lead-based
metal halide perovskite solar cells has enabled the development of
all-perovskite tandem solar cells, which offer a unique opportunity
to deliver high performance at low cost. Tin halide perovskites, however,
are prone to oxidation, where the Sn<sup>2+</sup> cations oxidize
to Sn<sup>4+</sup> upon air exposure. Here, we identify reaction products
and elucidate the oxidation mechanism of both ASnI<sub>3</sub> and
ASn<sub>0.5</sub>Pb<sub>0.5</sub>I<sub>3</sub> (where A can be made
of methylammonium, formamidinium, cesium, or a combination of these)
perovskites and find that substituting lead onto the B site fundamentally
changes the oxidation mechanism of tin-based metal halide perovskites
to make them more stable than would be expected by simply considering
the decrease in tin content. This work provides guidelines for developing
stable small band gap materials that could be used in all-perovskite
tandems
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
Hole Transport Materials with Low Glass Transition Temperatures and High Solubility for Application in Solid-State Dye-Sensitized Solar Cells
We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs
Barrier Design to Prevent Metal-Induced Degradation and Improve Thermal Stability in Perovskite Solar Cells
Metal-contact-induced
degradation and escape of volatile species
from perovskite solar cells necessitate excellent diffusion barrier
layers. We show that metal-induced degradation limits thermal stability
in several perovskite chemistries with Au, Cu, and Ag gridlines even
when the metal is separated from the perovskite by a layer of indium
tin oxide (ITO). Channels in a sputtered ITO layer that align with
perovskite grain boundaries are pathways for metal and halide diffusion
into or out of the perovskite. Planarizing the perovskite morphology
with a spin-cast organic charge-transport layer results in a subsequently
deposited ITO layer that is uniform and impermeable. We show that
it is critical to seal the edges of the active layers to prevent escape
of volatile species. We demonstrate 1000 h thermal stability at 85
°C in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells
with complete-coverage silver contacts. Our barrier layer design enables
long-term thermal stability of perovskite solar cells, a critical
step to commercialization
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
Cross-Linkable, Solvent-Resistant Fullerene Contacts for Robust and Efficient Perovskite Solar Cells with Increased <i>J</i><sub>SC</sub> and <i>V</i><sub>OC</sub>
The active layers
of perovskite solar cells are also structural
layers and are central to ensuring that the structural integrity of
the device is maintained over its operational lifetime. Our work evaluating
the fracture energies of conventional and inverted solution-processed
MAPbI<sub>3</sub> perovskite solar cells has revealed that the MAPbI<sub>3</sub> perovskite exhibits a fracture resistance of only ∼0.5
J/m<sup>2</sup>, while solar cells containing fullerene electron transport
layers fracture at even lower values, below ∼0.25 J/m<sup>2</sup>. To address this weakness, a novel styrene-functionalized fullerene
derivative, <b>MPMIC</b><sub><b>60</b></sub>, has been
developed as a replacement for the fragile PC<sub>61</sub>BM and C<sub>60</sub> transport layers. <b>MPMIC</b><sub><b>60</b></sub> can be transformed into a solvent-resistant material through
curing at 250 °C. As-deposited films of <b>MPMIC</b><sub><b>60</b></sub> exhibit a marked 10-fold enhancement in fracture
resistance over PC<sub>61</sub>BM and a 14-fold enhancement over C<sub>60</sub>. Conventional-geometry perovskite solar cells utilizing
cured films of <b>MPMIC</b><sub><b>60</b></sub> showed
a significant, 205% improvement in fracture resistance while exhibiting
only a 7% drop in PCE (13.8% vs 14.8% PCE) in comparison to the C<sub>60</sub> control, enabling larger <i>V</i><sub>OC</sub> and <i>J</i><sub>SC</sub> values. Inverted cells fabricated
with <b>MPMIC</b><sub><b>60</b></sub> exhibited a 438%
improvement in fracture resistance with only a 6% reduction in PCE
(12.3% vs 13.1%) in comparison to those utilizing PC<sub>61</sub>BM,
again producing a higher <i>J</i><sub>SC</sub>
Compositional Engineering for Efficient Wide Band Gap Perovskites with Improved Stability to Photoinduced Phase Segregation
Metal halide perovskites
are attractive candidates for the wide
band gap absorber in tandem solar cells. While their band gap can
be tuned by partial halide substitution, mixed halide perovskites
often have lower open-circuit voltage than would be expected and experience
photoinduced trap formation caused by halide segregation. We investigate
solar cell performance and photostability across a compositional space
of formamidinium (FA) and cesium (Cs) at the A-site at various halide
compositions and show that using more Cs at the A-site rather than
more Br at the X-site to raise band gap is more ideal as it improves
both <i>V</i><sub>OC</sub> and photostability. We develop
band gap maps and design criteria for the selection of perovskite
compositions within the Cs<sub><i>x</i></sub>FA<sub>1–<i>x</i></sub>Pb(Br<sub><i>y</i></sub>I<sub>1–<i>y</i></sub>)<sub>3</sub>, space. With this, we identify perovskites
with tandem-relevant band gaps of 1.68 and 1.75 eV that demonstrate
high device efficiencies of 17.4 and 16.3%, respectively, and significantly
improved photostability compared to that of the higher Br-containing
compositions
Employing PEDOT as the p‑Type Charge Collection Layer in Regular Organic–Inorganic Perovskite Solar Cells
Organic–inorganic halide perovskite
solar cells have recently
emerged as high-performance photovoltaic devices with low cost, promising
for affordable large-scale energy production, with laboratory cells
already exceeding 20% power conversion efficiency (PCE). To date,
a relatively expensive organic hole-conducting molecule with low conductivity,
namely spiro-OMeTAD (2,2′,7,7′-tetrakis(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenyl-amine) 9,9′-
spirobifluorene), is employed widely to achieve highly efficient perovskite
solar cells. Here, we report that by replacing spiro-OMeTAD with much
cheaper and highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT)
we can achieve PCE of up to 14.5%, with PEDOT cast from a toluene
based ink. However, the stabilized power output of the PEDOT-based
devices is only 6.6%, in comparison to 9.4% for the spiro-OMeTAD-based
cells. We deduce that accelerated recombination is the cause for this
lower stabilized power output and postulate that reduced levels of
p-doping are required to match the stabilized performance of Spiro-OMeTAD.
The entirely of the materials employed in the perovskite solar cell
are now available at commodity scale and extremely inexpensive