The Nanoscale Structure and Stability of Organic Photovoltaic Blends Processed with Solvent Additives.

Controlling the nanomorphology in bulk heterojunction photoactive blends is crucial for optimizing the performance and stability of organic photovoltaic (OPV) technologies. A promising approach is to alter the drying dynamics and consequently, the nanostructure of the blend film using solvent additives such as 1,8-diiodooctane (DIO). Although this approach is demonstrated extensively for OPV systems incorporating fullerene-based acceptors, it is unclear how solvent additive processing influences the morphology and stability of nonfullerene acceptor (NFA) systems. Here, small angle neutron scattering (SANS) is used to probe the nanomorphology of two model OPV systems processed with DIO: a fullerene-based system (PBDB-T:PC71BM) and an NFA-based system (PBDB-T:ITIC). To overcome the low intrinsic neutron scattering length density contrast in polymer:NFA blend films, the synthesis of a deuterated NFA analog (ITIC-d52) is reported. Using SANS, new insights into the nanoscale evolution of fullerene and NFA-based systems are provided by characterizing films immediately after fabrication, after thermal annealing, and after aging for 1 year. It is found that DIO processing influences fullerene and NFA-based systems differently with NFA-based systems characterized by more phase-separated domains. After long-term aging, SANS reveals both systems demonstrate some level of thermodynamic induced domain coarsening.

Influence of the coagulation bath on the nanostructure of cellulose films regenerated from an ionic liquid solution.

Cellulose membranes were prepared from an EMIMAc ionic liquid solution by nonsolvent-induced phase separation (NIPS) in coagulation baths of water-acetone mixtures, ethanol-water mixtures and water at different temperatures. High water volume fractions in the coagulation bath result in a highly reproducible gel-like structure with inhomogeneities observed by small-angle neutron scattering (SANS). A structural transition of cellulose takes place in water-acetone baths at very low water volume fractions, while a higher water bath temperature increases the size of inhomogeneities in the gel-like structure. These findings demonstrate the value of SANS for characterising and understanding the structure of regenerated cellulose films in their wet state. Such insights can improve the engineering and structural tuning of cellulose membranes, either for direct use or as precursors for carbon molecular sieve membranes.

Recent Advances in Poly(Ionic Liquid)-Based Membranes for CO2 Separation.

Poly(ionic liquid)-based membranes have been the subject of intensive research in the last 15 years due to their potential for the separation of CO2 from other gases. In this short review, different types of PIL-based membranes for CO2 separation are described (neat PIL membranes; PIL-IL composite membranes; PIL-polymer blend membranes; PIL-based block copolymer membranes, and PIL-based mixed matrix membranes), and their state-of-the-art separation results for different gas pairs (CO2/N2, CO2/H2, and CO2/CH4) are presented and discussed. This review article is focused on the most relevant research works performed over the last 5 years, that is, since the year 2017 onwards, in the field of poly(ionic liquid)-based membranes for CO2 separation. The micro- and nano-morphological characterization of the membranes is highlighted as a research topic that requires deeper study and understanding. Nowadays there is an array of advanced structural characterization techniques, such as neutron scattering techniques with contrast variation (using selective deuteration), that can be used to probe the micro- and nanostructure of membranes, in length scales ranging from ~1 nm to ~15 µm. Although some of these techniques have been used to study the morphology of PIL-based membranes for electrochemical applications, their use in the study of PIL-based membranes for CO2 separation is still unknown.

Graded Morphologies and the Performance of PffBT4T-2OD:PC71BM Devices Using Additive Choice.

The impact of several solvent processing additives (1-chloronaphthalene, methylnaphthalene, hexadecane, 1-phenyloctane, and p-anisaldehyde), 3% v/v in o-dichlorobenzene, on the performance and morphology of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2',5',22033,5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)-based polymer solar cells was investigated. Some additives were shown to enhance the power conversion efficiency (PCE) by ~6%, while others decreased the PCE by ~17-25% and a subset of the additives tested completely eliminated any power conversion efficiency and the operation as a photovoltaic device. Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) revealed a clear stepwise variation in the crystallinity of the systems when changing the additive between the two extreme situations of maximum PCE (1-chloronaphthalene) and null PCE (hexadecane). Small-Angle Neutron Scattering (SANS) revealed that the morphology of devices with PCE ~0% was composed of large domains with correlation lengths of ~30 nm, i.e., much larger than the typical exciton diffusion length (~12 nm) in organic semiconductors. The graded variations in crystallinity and in nano-domain size observed between the two extreme situations (1-chloronaphthalene and hexadecane) were responsible for the observed graded variations in device performance.

Fragilidad del adulto mayor en tres Unidades de Salud Familiar del Paraguay en 2019

RESUMEN Introducción: la fragilidad es un deterioro progresivo de los sistemas fisiológicos, relacionado con la edad, que disminuye las reservas de capacidad intrínseca, lo que confiere mayor vulnerabilidad a factores de estrés. Si bien tiende a empeorar espontáneamente, con intervenciones adecuadas y precoces se pueden revertir, enlentecer o aminorar sus consecuencias. Objetivos: determinar la frecuencia de fragilidad del adulto mayor según los criterios de Fried en tres Unidades de Salud Familiar de Areguá, Paraguay, en 2019. Metodología: estudio observacional, descriptivo, prospectivo, multicéntrico. Se incluyeron varones y mujeres ≥60 años asistidos en las Unidades de Salud Familiar de Yuquyty, Costa Fleytas y Caacupemí (Areguá, Paraguay) en 2019. Se midieron variables antropométricas y clínicas. La fragilidad se determinó con el cuestionario de Fried, previo consentimiento informado. Resultados: se incluyeron 81 sujetos, con edad media 71±7 años, con predominio del sexo femenino (65%). Las comorbilidades más frecuentes fueron la hipertensión arterial, artropatías y diabetes mellitus. La depresión fue autorreportada en 9,8%. Se detectó fragilidad en 53%. El déficit más frecuente de los criterios de Fried fue la lentitud en la marcha. El factor de riesgo asociado a la fragilidad fue el sexo femenino (p 0,006). Conclusiones: la fragilidad se detectó en 53%, con predominio en el sexo femenino.

ABSTRACT Introduction: Frailty is a progressive deterioration of physiological systems, related to age, which reduces the reserves of intrinsic capacity, which confers greater vulnerability to stressors. Although it tends to worsen spontaneously, with appropriate and early interventions, its consequences can be reversed, slowed or reduced. Objectives: To determine the frequency of frailty of the elderly according to Fried criteria in three Family Health Units of Areguá, Paraguay, in 2019. Methodology: Observational, descriptive, prospective, multicenter study. Men and women aged ≥60 years who attended the Family Health Units of Yuquyty, Costa Fleytas and Caacupemí (Areguá, Paraguay) in 2019 were included. Anthropometric and clinical variables were measured. Frailty was determined with Fried questionnaire, after informed consent. Results: Eighty-one subjects were included, with a mean age of 71±7 years, with a predominance of women (65%). The most frequent comorbidities were high blood pressure, arthropathies, and diabetes mellitus. Depression was self-reported in 9.8%. Frailty was detected in 53% and the most frequent deficit in Fried criteria was slow gait. The risk factor associated with frailty was female sex (p=0.006). Conclusions: Frailty was detected in 53%, with a predominance of women.

Cellulose-Based Carbon Molecular Sieve Membranes for Gas Separation: A Review.

In the field of gas separation and purification, membrane technologies compete with conventional purification processes on the basis of technical, economic and environmental factors. In this context, there is a growing interest in the development of carbon molecular sieve membranes (CMSM) due to their higher permeability and selectivity and higher stability in corrosive and high temperature environments. However, the industrial use of CMSM has been thus far hindered mostly by their relative instability in the presence of water vapor, present in a large number of process streams, as well as by the high cost of polymeric precursors such as polyimide. In this context, cellulosic precursors appear as very promising alternatives, especially targeting the production of CMSM for the separation of O2/N2 and CO2/CH4. For these two gas separations, cellulose-based CMSM have demonstrated performances well above the Robeson upper bound and above the performance of CMSM based on other polymeric precursors. Furthermore, cellulose is an inexpensive bio-renewable feed-stock highly abundant on Earth. This article reviews the major fabrication aspects of cellulose-based CMSM. Additionally, this article suggests a new tool to characterize the membrane performance, the Robeson Index. The Robeson Index, θ, is the ratio between the actual selectivity at the Robeson plot and the corresponding selectivity-for the same permeability-of the Robeson upper bound; the Robeson Index measures how far the actual point is from the upper bound.

Thiophene- and Carbazole-Substituted N-Methyl-Fulleropyrrolidine Acceptors in PffBT4T-2OD Based Solar Cells.

The impact of fullerene side chain functionalization with thiophene and carbazole groups on the device properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2';5',2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) as donor and C60 or C70 fulleropyrrolidines as acceptors. The photovoltaic performance clearly depended on the molecular structure of the fulleropyrrolidine substituents although no direct correlation with the surface morphology of the photoactive layer, as determined by atomic force microscopy, could be established. Although some fulleropyrrolidines possess favorable lowest unoccupied molecular orbital levels, when compared to the standard PC71BM, they originated OPV cells with inferior efficiencies than PC71BM-based reference cells. Fulleropyrrolidines based on C60 produced, in general, better devices than those based on C70, and we attribute this observation to the detrimental effect of the structural and energetic disorder that is present in the regioisomer mixtures of C70-based fullerenes, but absent in the C60-based fullerenes. These results provide new additional knowledge on the effect of the fullerene functionalization on the efficiency of organic solar cells.

Different agglomeration properties of PC61BM and PC71BM in photovoltaic inks - a spin-echo SANS study.

Fullerene derivatives are used in a wide range of applications including as electron acceptors in solution-processable organic photovoltaics. We report agglomeration of fullerene derivatives in optically opaque solutions of PC61BM and PC71BM, with concentrations ranging from 30 mg mL-1 up to 90 mg mL-1, in different solvents with relevance to organic photovoltaics, using a novel neutron scattering technique, Spin-Echo Small Angle Neutron Scattering (SESANS). From SESANS, agglomerates with correlation lengths larger than 1 µm are found in some PC61BM solutions, in contrast no agglomerates are seen in PC71BM solutions. These results clearly show that PC71BM is fundamentally more soluble than PC61BM in the solvents commonly used in photovoltaic inks and corroborating similar observations previously achieved using other experimental techniques. Computer models are presented to study the energetics of solution and agglomeration of both species, ascribing the difference to a kinetic effect probably related to the larger anisotropy of PC71BM. Also, this work showcases the power of SESANS to probe agglomerates of fullerene derivatives in completely opaque solutions for agglomerates of the order of one to several microns.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted N-Methyl-Fulleropyrrolidine Acceptors.

Novel C60 and C70 N-methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PC61BM and PC71BM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PC71BM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C70 yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C70 derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.

Determination of the Thin-Film Structure of Zwitterion-Doped Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate): A Neutron Reflectivity Study.

Doping poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is known to improve its conductivity; however, little is known about the thin-film structure of PEDOT:PSS when doped with an asymmetrically charged dopant. In this study, PEDOT:PSS was doped with different concentrations of the zwitterion 3-( N, N dimethylmyristylammonio)propanesulfonate (DYMAP), and its effect on the bulk structure of the films was characterized by neutron reflectivity. The results show that at a low doping concentration, the film separates into a quasi-bilayer structure with lower roughness (10%), increased thickness (18%), and lower electrical conductivity compared to the undoped sample. However, when the doping concentration increases, the film forms into a homogeneous layer and experiences an enhanced conductivity by more than an order of magnitude, a 20% smoother surface, and a 60% thickness increase relative to the pristine sample. Atomic force microscopy (AFM) and profilometry measurements confirmed these findings, and the AFM height and phase images showed the gradually increasing presence of DYMAP on the film surface as a function of the concentration. Neutron reflectivity also showed that the quasi-bilayer structure of the lowest concentration-doped PEDOT:PSS is separated by a graded rather than a well-defined interface. Our findings provide an understanding of the layer structure modification for doped PEDOT:PSS films which should prove important for device applications.

Recent Developments in the Optimization of the Bulk Heterojunction Morphology of Polymer: Fullerene Solar Cells.

Organic photovoltaic (OPV) devices, made with semiconducting polymers, have recently attained a power conversion efficiency (PCE) over 14% in single junction cells and over 17% in tandem cells. These high performances, together with the suitability of the technology to inexpensive large-scale manufacture, over lightweight and flexible plastic substrates using roll-to-roll (R2R) processing, place the technology amongst the most promising for future harvesting of solar energy. Although OPVs using non-fullerene acceptors have recently outperformed their fullerene-based counterparts, the research in the development of new fullerenes and in the improvement of the bulk-heterojunction (BHJ) morphology and device efficiency of polymer:fullerene solar cells remains very active. In this review article, the most relevant research works performed over the last 3 years, that is, since the year 2016 onwards, in the field of fullerene-based polymer solar cells based on the copolymers PTB7, PTB7-Th (also known as PBDTTT-EFT) and PffBT4T-2OD, are presented and discussed. This review is primarily focused on studies that involve the improvement of the BHJ morphology, efficiency and stability of small active area devices (typically < 15 mm²), through the use of different processing strategies such as the use of different fullerene acceptors, different processing solvents and additives and different thermal treatments.

Effect of fullerene acceptor on the performance of solar cells based on PffBT4T-2OD.

We have studied bulk-heterojunction (BHJ) solar cells composed of the polymer PffBT4T-2OD as electron donor and three different electron accepting fullerenes, namely PC71BM, PC61BM and indene-C60-bis-adduct (ICBA) in order to understand the impact of different fullerenes on the morphology and efficiency of the corresponding photovoltaic devices. Despite PffBT4T-2OD:ICBA devices being characterised by higher values of Voc, they display the lowest power conversion efficiency (PCE) due to their lower Jsc and FF values. We find that although all blend films have similar morphologies, X-ray scattering indicates a reduced degree of order within the fullerene domains in the ICBA-based film. Due to the high LUMO level of ICBA, the corresponding blends are characterised by a lower initial exciton dissociation and this associated with the reduced ordering within the ICBA domains results in increased geminate recombination of the photogenerated electrons in the fullerene-rich domains and a consequently reduced PCE of the corresponding devices.

Does 1,8-diiodooctane affect the aggregation state of PC71BM in solution?

1,8-Diiodooctane (DIO) is an additive used in the processing of organic photovoltaics and has previously been reported, on the basis of small-angle X-ray scattering (SAXS) measurements, to deflocculate nano-aggregates of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene. We have critically re-examined this finding in a series of scattering measurements using both X-rays and neutrons. With SAXS, we find that the form of the background solvent scattering is influenced by the presence of DIO, that there is substantial attenuation of the X-rays by the background solvent and that there appears to be beam-induced aggregation. All three factors call into question the suitability of SAXS for measurements on these samples. By contrast, small-angle neutron scattering (SANS) measurements, performed at concentrations of 15 mg ml-1 up to and including 40 mg ml-1, show no difference in the aggregation state for PC71BM in chlorobenzene with and without 3% DIO; we find PC71BM to be molecularly dissolved in all solvent cases. In situ film thinning measurements of spin-coated PC71BM solution with the DIO additive dry much slower. Optical imaging shows that the fullerene films possess enhanced molecular mobility in the presence of DIO and it is this which, we conclude, improves the nanomorphology and consequently solar cell performance. We propose that any compatible high boiling solvent would be expected to show the same behaviour.

Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices.

We demonstrate that the inclusion of a small amount of the co-solvent 1,8-diiodooctane in the preparation of a bulk-heterojunction photovoltaic device increases its power conversion efficiency by 20%, through a mechanism of transient plasticisation. We follow the removal of 1,8-diiodooctane directly after spin-coating using ellipsometry and ion beam analysis, while using small angle neutron scattering to characterise the morphological nanostructure evolution of the film. In PffBT4T-2OD/PC71BM devices, the power conversion efficiency increases from 7.2% to above 8.7% as a result of the coarsening of the phase domains. This coarsening process is assisted by thermal annealing and the slow evaporation of 1,8-diiodooctane, which we suggest, acts as a plasticiser to promote molecular mobility. Our results show that 1,8-diiodooctane can be completely removed from the film by a thermal annealing process at temperatures ≤100 °C and that there is an interplay between the evaporation rate of 1,8-diiodooctane and the rate of domain coarsening in the plasticized film which helps elucidate the mechanism by which additives improve device efficiency.