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 (MAPbBr₃). 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 CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>, CH₃NH₃PbI₃, and their meso-structured analogues. High mobilities close to 30 cm²/(V s) and microsecond-long lifetimes are found in thin films of CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>, compared to lifetimes of only a few hundred nanoseconds in CH₃NH₃PbI₃ 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 × 10¹⁴ cm^–3 in CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>, 6 × 10¹⁶ cm^–3 in the CH₃NH₃PbI₃ thin film and ca. 10¹⁵ cm^–3 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 TiO₂‑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 TiO₂ 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 TiO₂ 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 TiO₂, improving electron transport rates and photovoltage. Importantly, the primary recombination pathway between the TiO₂ and the hole transporting material is blocked by the perovskite itself. This understanding helps to rationalize the high voltages attainable on mesoporous TiO₂-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(NH₂)₂SnI₃ 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 (FASnI₃) 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 cm² V^–1 s^–1 favoring efficient transport. In addition, FASnI₃ 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 (MAPbI₃) and formamidinium lead iodide (FAPbI₃) 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 FAPbI₃ films compared to MAPbI₃ 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 (CH₃NH₃PbI₃) 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 Pb²⁺ 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 CH₃NH₃PbI₃ 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