A THEORETICAL AND EXPERIMENTAL STUDY OF CHARGE TRANSPORT IN ORGANIC THERMOELECTRIC MATERIALS AND CHARGE TRANSFER STATES IN ORGANIC PHOTOVOLTAICS

Applications of organic electronics have increased significantly over the past two decades. Organic semiconductors (OSC) can be used in mechanically flexible devices with potentially lower cost of fabrication than their inorganic counterparts, yet in many cases organic semiconductor-based devices suffer from lower performance and stability. Investigating the doping mechanism, charge transport, and charge transfer in such materials will allow us to address the parameters that limit performance and potentially resolve them. In this dissertation, organic materials are used in three different device structures to investigate charge transport and charge transfer. Chemically doped π-conjugated polymers are promising materials to be used in thermoelectric (TE) devices, yet their application is currently limited by their low performance. Blending two polymers is a simple way to change the TE properties of the film. Here we use an analytical model to calculate the TE properties of polymer blends, which takes into account energetic disorder, energetic offsets between mean energy of states of the two polymers, and localization length. These calculations show that the TE performance of polymer blends can exceed the individual polymers when there is a small (e.g., 0.1-0.2 eV) offset between the mean of the density of states (DOS) distributions of the two polymers, the polymer with the higher energy DOS has a wider DOS distribution and a larger localization length (mobility), and the polymers are homogeneously mixed. We show these improvements are achievable by experimentally testing TE properties of selected polymer blends. These sets of polymers are selected with variations in electrical mobility, ionization energy and degree of crystallinity to cover a range of possibilities explored in the calculations. Further, to investigate the effect of dopant size in polymers, we use organic electrochemical transistors to investigate the effect of anion size on polaron delocalization and the thermoelectric properties of single polymers. This device structure allows us to control the charge carrier concentration with minimizing the effects on the film morphology. Another application of OSC is in organic photovoltaics (OPVs), where they can potentially provide a cheap and flexible source of solar energy, yet they currently suffer from low performance and stability. In OPVs, fluorination of donor molecules is a proven strategy for increasing the performance of OPV donor materials. Herein, we investigate the charge transfer state energy between the electron donor anthradithiophene (ADT) and the electron acceptor C60 upon halogenation of the ADT molecule. Interfacial energetics and charge transfer state energies between donor and acceptor are crucial to the PV performance of these devices. We probe interfacial energetics of donor/acceptor interfaces with ultraviolet photoemission spectroscopy (UPS), charge transfer state energies with sensitive external quantum efficiency (EQE) both in bilayer and bulk heterojunction device structures. These measurements coupled with DFT calculations allow us to explain that in bulk-heterojunction OPVs the halogenated ADT derivatives will likely increase charge recombination due to lower energy CT states present in the mixed phase. Therefore, the less favorable energy landscapes observed upon halogenation suggest that the benefits of fluorination observed in many OPV material systems may be more due to morphological factors

\u3ci\u3en\u3c/i\u3e-Type Charge Transport in Heavily \u3ci\u3ep\u3c/i\u3e-Doped Polymers

It is commonly assumed that charge-carrier transport in doped π-conjugated polymers is dominated by one type of charge carrier, either holes or electrons, as determined by the chemistry of the dopant. Here, through Seebeck coefficient and Hall effect measurements, we show that mobile electrons contribute substantially to charge-carrier transport in π-conjugated polymers that are heavily p-doped with strong electron acceptors. Specifically, the Seebeck coefficient of several p-doped polymers changes sign from positive to negative as the concentration of the oxidizing agents FeCl3 or NOBF4 increase, and Hall effect measurements for the same p-doped polymers reveal that electrons become the dominant delocalized charge carriers. Ultraviolet and inverse photoelectron spectroscopy measurements show that doping with oxidizing agents results in elimination of the transport gap at high doping concentrations. This approach of heavy p-type doping is demonstrated to provide a promising route to high-performance n-type organic thermoelectric materials

Global age-sex-specific mortality, life expectancy, and population estimates in 204 countries and territories and 811 subnational locations, 1950–2021, and the impact of the COVID-19 pandemic: a comprehensive demographic analysis for the Global Burden of Disease Study 2021

Background: Estimates of demographic metrics are crucial to assess levels and trends of population health outcomes. The profound impact of the COVID-19 pandemic on populations worldwide has underscored the need for timely estimates to understand this unprecedented event within the context of long-term population health trends. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 provides new demographic estimates for 204 countries and territories and 811 additional subnational locations from 1950 to 2021, with a particular emphasis on changes in mortality and life expectancy that occurred during the 2020–21 COVID-19 pandemic period. Methods: 22 223 data sources from vital registration, sample registration, surveys, censuses, and other sources were used to estimate mortality, with a subset of these sources used exclusively to estimate excess mortality due to the COVID-19 pandemic. 2026 data sources were used for population estimation. Additional sources were used to estimate migration; the effects of the HIV epidemic; and demographic discontinuities due to conflicts, famines, natural disasters, and pandemics, which are used as inputs for estimating mortality and population. Spatiotemporal Gaussian process regression (ST-GPR) was used to generate under-5 mortality rates, which synthesised 30 763 location-years of vital registration and sample registration data, 1365 surveys and censuses, and 80 other sources. ST-GPR was also used to estimate adult mortality (between ages 15 and 59 years) based on information from 31 642 location-years of vital registration and sample registration data, 355 surveys and censuses, and 24 other sources. Estimates of child and adult mortality rates were then used to generate life tables with a relational model life table system. For countries with large HIV epidemics, life tables were adjusted using independent estimates of HIV-specific mortality generated via an epidemiological analysis of HIV prevalence surveys, antenatal clinic serosurveillance, and other data sources. Excess mortality due to the COVID-19 pandemic in 2020 and 2021 was determined by subtracting observed all-cause mortality (adjusted for late registration and mortality anomalies) from the mortality expected in the absence of the pandemic. Expected mortality was calculated based on historical trends using an ensemble of models. In location-years where all-cause mortality data were unavailable, we estimated excess mortality rates using a regression model with covariates pertaining to the pandemic. Population size was computed using a Bayesian hierarchical cohort component model. Life expectancy was calculated using age-specific mortality rates and standard demographic methods. Uncertainty intervals (UIs) were calculated for every metric using the 25th and 975th ordered values from a 1000-draw posterior distribution. Findings: Global all-cause mortality followed two distinct patterns over the study period: age-standardised mortality rates declined between 1950 and 2019 (a 62·8% [95% UI 60·5–65·1] decline), and increased during the COVID-19 pandemic period (2020–21; 5·1% [0·9–9·6] increase). In contrast with the overall reverse in mortality trends during the pandemic period, child mortality continued to decline, with 4·66 million (3·98–5·50) global deaths in children younger than 5 years in 2021 compared with 5·21 million (4·50–6·01) in 2019. An estimated 131 million (126–137) people died globally from all causes in 2020 and 2021 combined, of which 15·9 million (14·7–17·2) were due to the COVID-19 pandemic (measured by excess mortality, which includes deaths directly due to SARS-CoV-2 infection and those indirectly due to other social, economic, or behavioural changes associated with the pandemic). Excess mortality rates exceeded 150 deaths per 100 000 population during at least one year of the pandemic in 80 countries and territories, whereas 20 nations had a negative excess mortality rate in 2020 or 2021, indicating that all-cause mortality in these countries was lower during the pandemic than expected based on historical trends. Between 1950 and 2021, global life expectancy at birth increased by 22·7 years (20·8–24·8), from 49·0 years (46·7–51·3) to 71·7 years (70·9–72·5). Global life expectancy at birth declined by 1·6 years (1·0–2·2) between 2019 and 2021, reversing historical trends. An increase in life expectancy was only observed in 32 (15·7%) of 204 countries and territories between 2019 and 2021. The global population reached 7·89 billion (7·67–8·13) people in 2021, by which time 56 of 204 countries and territories had peaked and subsequently populations have declined. The largest proportion of population growth between 2020 and 2021 was in sub-Saharan Africa (39·5% [28·4–52·7]) and south Asia (26·3% [9·0–44·7]). From 2000 to 2021, the ratio of the population aged 65 years and older to the population aged younger than 15 years increased in 188 (92·2%) of 204 nations. Interpretation: Global adult mortality rates markedly increased during the COVID-19 pandemic in 2020 and 2021, reversing past decreasing trends, while child mortality rates continued to decline, albeit more slowly than in earlier years. Although COVID-19 had a substantial impact on many demographic indicators during the first 2 years of the pandemic, overall global health progress over the 72 years evaluated has been profound, with considerable improvements in mortality and life expectancy. Additionally, we observed a deceleration of global population growth since 2017, despite steady or increasing growth in lower-income countries, combined with a continued global shift of population age structures towards older ages. These demographic changes will likely present future challenges to health systems, economies, and societies. The comprehensive demographic estimates reported here will enable researchers, policy makers, health practitioners, and other key stakeholders to better understand and address the profound changes that have occurred in the global health landscape following the first 2 years of the COVID-19 pandemic, and longer-term trends beyond the pandemic

Global burden of 288 causes of death and life expectancy decomposition in 204 countries and territories and 811 subnational locations, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021

BACKGROUND Regular, detailed reporting on population health by underlying cause of death is fundamental for public health decision making. Cause-specific estimates of mortality and the subsequent effects on life expectancy worldwide are valuable metrics to gauge progress in reducing mortality rates. These estimates are particularly important following large-scale mortality spikes, such as the COVID-19 pandemic. When systematically analysed, mortality rates and life expectancy allow comparisons of the consequences of causes of death globally and over time, providing a nuanced understanding of the effect of these causes on global populations. METHODS The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 cause-of-death analysis estimated mortality and years of life lost (YLLs) from 288 causes of death by age-sex-location-year in 204 countries and territories and 811 subnational locations for each year from 1990 until 2021. The analysis used 56 604 data sources, including data from vital registration and verbal autopsy as well as surveys, censuses, surveillance systems, and cancer registries, among others. As with previous GBD rounds, cause-specific death rates for most causes were estimated using the Cause of Death Ensemble model-a modelling tool developed for GBD to assess the out-of-sample predictive validity of different statistical models and covariate permutations and combine those results to produce cause-specific mortality estimates-with alternative strategies adapted to model causes with insufficient data, substantial changes in reporting over the study period, or unusual epidemiology. YLLs were computed as the product of the number of deaths for each cause-age-sex-location-year and the standard life expectancy at each age. As part of the modelling process, uncertainty intervals (UIs) were generated using the 2·5th and 97·5th percentiles from a 1000-draw distribution for each metric. We decomposed life expectancy by cause of death, location, and year to show cause-specific effects on life expectancy from 1990 to 2021. We also used the coefficient of variation and the fraction of population affected by 90% of deaths to highlight concentrations of mortality. Findings are reported in counts and age-standardised rates. Methodological improvements for cause-of-death estimates in GBD 2021 include the expansion of under-5-years age group to include four new age groups, enhanced methods to account for stochastic variation of sparse data, and the inclusion of COVID-19 and other pandemic-related mortality-which includes excess mortality associated with the pandemic, excluding COVID-19, lower respiratory infections, measles, malaria, and pertussis. For this analysis, 199 new country-years of vital registration cause-of-death data, 5 country-years of surveillance data, 21 country-years of verbal autopsy data, and 94 country-years of other data types were added to those used in previous GBD rounds. FINDINGS The leading causes of age-standardised deaths globally were the same in 2019 as they were in 1990; in descending order, these were, ischaemic heart disease, stroke, chronic obstructive pulmonary disease, and lower respiratory infections. In 2021, however, COVID-19 replaced stroke as the second-leading age-standardised cause of death, with 94·0 deaths (95% UI 89·2-100·0) per 100 000 population. The COVID-19 pandemic shifted the rankings of the leading five causes, lowering stroke to the third-leading and chronic obstructive pulmonary disease to the fourth-leading position. In 2021, the highest age-standardised death rates from COVID-19 occurred in sub-Saharan Africa (271·0 deaths [250·1-290·7] per 100 000 population) and Latin America and the Caribbean (195·4 deaths [182·1-211·4] per 100 000 population). The lowest age-standardised death rates from COVID-19 were in the high-income super-region (48·1 deaths [47·4-48·8] per 100 000 population) and southeast Asia, east Asia, and Oceania (23·2 deaths [16·3-37·2] per 100 000 population). Globally, life expectancy steadily improved between 1990 and 2019 for 18 of the 22 investigated causes. Decomposition of global and regional life expectancy showed the positive effect that reductions in deaths from enteric infections, lower respiratory infections, stroke, and neonatal deaths, among others have contributed to improved survival over the study period. However, a net reduction of 1·6 years occurred in global life expectancy between 2019 and 2021, primarily due to increased death rates from COVID-19 and other pandemic-related mortality. Life expectancy was highly variable between super-regions over the study period, with southeast Asia, east Asia, and Oceania gaining 8·3 years (6·7-9·9) overall, while having the smallest reduction in life expectancy due to COVID-19 (0·4 years). The largest reduction in life expectancy due to COVID-19 occurred in Latin America and the Caribbean (3·6 years). Additionally, 53 of the 288 causes of death were highly concentrated in locations with less than 50% of the global population as of 2021, and these causes of death became progressively more concentrated since 1990, when only 44 causes showed this pattern. The concentration phenomenon is discussed heuristically with respect to enteric and lower respiratory infections, malaria, HIV/AIDS, neonatal disorders, tuberculosis, and measles. INTERPRETATION Long-standing gains in life expectancy and reductions in many of the leading causes of death have been disrupted by the COVID-19 pandemic, the adverse effects of which were spread unevenly among populations. Despite the pandemic, there has been continued progress in combatting several notable causes of death, leading to improved global life expectancy over the study period. Each of the seven GBD super-regions showed an overall improvement from 1990 and 2021, obscuring the negative effect in the years of the pandemic. Additionally, our findings regarding regional variation in causes of death driving increases in life expectancy hold clear policy utility. Analyses of shifting mortality trends reveal that several causes, once widespread globally, are now increasingly concentrated geographically. These changes in mortality concentration, alongside further investigation of changing risks, interventions, and relevant policy, present an important opportunity to deepen our understanding of mortality-reduction strategies. Examining patterns in mortality concentration might reveal areas where successful public health interventions have been implemented. Translating these successes to locations where certain causes of death remain entrenched can inform policies that work to improve life expectancy for people everywhere. FUNDING Bill & Melinda Gates Foundation

Processing Dependent Influence of the Hole Transport Layer Ionization Energy on Methylammonium Lead Iodide Perovskite Photovoltaics

Organometal halide perovskite photovoltaics typically contain both electron and hole transport layers, both of which influence charge extraction and recombination. The ionization energy (IE) of the hole transport layer (HTL) is one important material property that will influence the open-circuit voltage, fill factor, and short-circuit current. Herein, we introduce a new series of triarylamino­ethynylsilanes with adjustable IEs as efficient HTL materials for methyl­ammonium lead iodide (MAPbI₃) perovskite based photovoltaics. The three triarylamino­ethynylsilanes investigated can all be used as HTLs to yield PV performance on par with the commonly used HTLs PEDOT:PSS and Spiro-OMeTAD in inverted architectures (i.e., HTL deposited prior to the perovskite layer). We further investigate the influence of the HTL IE on the photovoltaic performance of MAPbI₃ based inverted devices using two different MAPbI₃ processing methods with a series of 11 different HTL materials, with IEs ranging from 4.74 to 5.84 eV. The requirements for the HTL IE change based on whether MAPbI₃ is formed from lead acetate, Pb­(OAc)₂, or PbI₂ as the Pb source. The ideal HTL IE range is between 4.8 and 5.3 eV for MAPbI₃ processed from Pb­(OAc)₂, while with PbI₂ the PV performance is relatively insensitive to variations in the HTL IE between 4.8 and 5.8 eV. Our results suggest that contradictory findings in the literature on the effect of the HTL IE in perovskite photovoltaics stem partly from the different processing methods employed

Processing Dependent Influence of the Hole Transport Layer Ionization Energy on Methylammonium Lead Iodide Perovskite Photovoltaics

Organometal halide perovskite photovoltaics typically contain both electron and hole transport layers, both of which influence charge extraction and recombination. The ionization energy (IE) of the hole transport layer (HTL) is one important material property that will influence the open-circuit voltage, fill factor, and short-circuit current. Herein, we introduce a new series of triarylamino­ethynylsilanes with adjustable IEs as efficient HTL materials for methyl­ammonium lead iodide (MAPbI₃) perovskite based photovoltaics. The three triarylamino­ethynylsilanes investigated can all be used as HTLs to yield PV performance on par with the commonly used HTLs PEDOT:PSS and Spiro-OMeTAD in inverted architectures (i.e., HTL deposited prior to the perovskite layer). We further investigate the influence of the HTL IE on the photovoltaic performance of MAPbI₃ based inverted devices using two different MAPbI₃ processing methods with a series of 11 different HTL materials, with IEs ranging from 4.74 to 5.84 eV. The requirements for the HTL IE change based on whether MAPbI₃ is formed from lead acetate, Pb­(OAc)₂, or PbI₂ as the Pb source. The ideal HTL IE range is between 4.8 and 5.3 eV for MAPbI₃ processed from Pb­(OAc)₂, while with PbI₂ the PV performance is relatively insensitive to variations in the HTL IE between 4.8 and 5.8 eV. Our results suggest that contradictory findings in the literature on the effect of the HTL IE in perovskite photovoltaics stem partly from the different processing methods employed

Processing Dependent Influence of the Hole Transport Layer Ionization Energy on Methylammonium Lead Iodide Perovskite Photovoltaics

Organometal halide perovskite photovoltaics typically contain both electron and hole transport layers, both of which influence charge extraction and recombination. The ionization energy (IE) of the hole transport layer (HTL) is one important material property that will influence the open-circuit voltage, fill factor, and short-circuit current. Herein, we introduce a new series of triarylamino­ethynylsilanes with adjustable IEs as efficient HTL materials for methyl­ammonium lead iodide (MAPbI₃) perovskite based photovoltaics. The three triarylamino­ethynylsilanes investigated can all be used as HTLs to yield PV performance on par with the commonly used HTLs PEDOT:PSS and Spiro-OMeTAD in inverted architectures (i.e., HTL deposited prior to the perovskite layer). We further investigate the influence of the HTL IE on the photovoltaic performance of MAPbI₃ based inverted devices using two different MAPbI₃ processing methods with a series of 11 different HTL materials, with IEs ranging from 4.74 to 5.84 eV. The requirements for the HTL IE change based on whether MAPbI₃ is formed from lead acetate, Pb­(OAc)₂, or PbI₂ as the Pb source. The ideal HTL IE range is between 4.8 and 5.3 eV for MAPbI₃ processed from Pb­(OAc)₂, while with PbI₂ the PV performance is relatively insensitive to variations in the HTL IE between 4.8 and 5.8 eV. Our results suggest that contradictory findings in the literature on the effect of the HTL IE in perovskite photovoltaics stem partly from the different processing methods employed

Processing Dependent Influence of the Hole Transport Layer Ionization Energy on Methylammonium Lead Iodide Perovskite Photovoltaics

Organometal halide perovskite photovoltaics typically contain both electron and hole transport layers, both of which influence charge extraction and recombination. The ionization energy (IE) of the hole transport layer (HTL) is one important material property that will influence the open-circuit voltage, fill factor, and short-circuit current. Herein, we introduce a new series of triarylamino­ethynylsilanes with adjustable IEs as efficient HTL materials for methyl­ammonium lead iodide (MAPbI₃) perovskite based photovoltaics. The three triarylamino­ethynylsilanes investigated can all be used as HTLs to yield PV performance on par with the commonly used HTLs PEDOT:PSS and Spiro-OMeTAD in inverted architectures (i.e., HTL deposited prior to the perovskite layer). We further investigate the influence of the HTL IE on the photovoltaic performance of MAPbI₃ based inverted devices using two different MAPbI₃ processing methods with a series of 11 different HTL materials, with IEs ranging from 4.74 to 5.84 eV. The requirements for the HTL IE change based on whether MAPbI₃ is formed from lead acetate, Pb­(OAc)₂, or PbI₂ as the Pb source. The ideal HTL IE range is between 4.8 and 5.3 eV for MAPbI₃ processed from Pb­(OAc)₂, while with PbI₂ the PV performance is relatively insensitive to variations in the HTL IE between 4.8 and 5.8 eV. Our results suggest that contradictory findings in the literature on the effect of the HTL IE in perovskite photovoltaics stem partly from the different processing methods employed

Effect of Halogenation on the Energetics of Pure and Mixed Phases in Model Organic Semiconductors Composed of Anthradithiophene Derivatives and C₆₀

Halogenation, particularly fluorination, is commonly used to manipulate the energetics, stability, and morphology of organic semiconductors. In the case of organic photovoltaics (OPVs), fluorination of electron donor molecules or polymers at appropriate positions can lead to improved performance. In this contribution, we use ultraviolet photoelectron spectroscopy, external quantum efficiency measurements of charge-transfer (CT) states, and density functional theory calculations to systematically investigate the effects of halogenation on the bulk solid-state energetics of model anthradithiophene (ADT) materials, their interfacial energetics with C₆₀, and the energetics of various ADT:C₆₀ blend compositions. In agreement with previous work, nonhalogenated ADT molecules show higher energy CT states in blends with C₆₀ and lower energy CT states in the ADT/C₆₀ bilayers. However, this trend is reversed in the halogenated ADT/C₆₀ systems, wherein the CT state energies of ADT:C₆₀ blends are lower than those in the bilayers. In bulk-heterojunction photovoltaics, the lower energy CT states present in the mixed phase with the halogenated ADT derivatives will likely decrease the probability of charge separation and increase charge recombination. The less favorable energy landscapes observed upon halogenation suggest that the benefits of fluorination observed in many OPV material systems may be more due to morphological factors