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"Collins, Brian A."
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Structural control of mixed ionic and electronic transport in conducting polymers
Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), PEDOT:PSS, has been utilized for over two decades as a stable, solution-processable hole conductor. While its hole transport properties have been the subject of intense investigation, recent work has turned to PEDOT:PSS as a mixed ionic/electronic conductor in applications including bioelectronics, energy storage and management, and soft robotics. Conducting polymers can efficiently transport both holes and ions when sufficiently hydrated, however, little is known about the role of morphology on mixed conduction. Here, we show that bulk ionic and electronic mobilities are simultaneously affected by processing-induced changes in nano- and meso-scale structure in PEDOT:PSS films. We quantify domain composition, and find that domain purification on addition of dispersion co-solvents limits ion mobility, even while electronic conductivity improves. We show that an optimal morphology allows for the balanced ionic and electronic transport that is critical for prototypical mixed conductor devices. These findings may pave the way for the rational design of polymeric materials and processing routes to enhance devices reliant on mixed conduction.
Conducting polymers are promising materials for applications including bioelectronics and soft robotics, but little is known about how morphology affects mixed conduction. Here, the authors show how bulk ionic/electronic transport is affected by changes in nano- and meso-scale structure in PEDOT:PSS films.
Journal Article
Quantitative relations between interaction parameter, miscibility and function in organic solar cells
Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction–function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous–amorphous interaction parameter, χaa(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative ‘constant-kink-saturation’ relation between χaa and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χaa is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.
Journal Article
Probing the pathways of free charge generation in organic bulk heterojunction solar cells
The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.
Contradictory models are being debated on the dominant pathways of charge generation in organic solar cells. Here Kurpiers et al. determine the activation energy for this fundamental process and reveal that the main channel is via thermalized charge transfer states instead of hot exciton dissociation.
Journal Article
Access, quality, and home language support for emergent bilingual children in NYC’s universal Pre-K programs
Providing support for children’s home languages is a critical component of high-quality Early Childhood Education and Care (ECEC). We examine the types and availability of Pre-K programs for emergent bilingual children and their families under New York City’s (NYC) Universal Pre-K program. Currently, in the hyper-diverse linguistic landscape of NYC where the majority of children come from homes where a language other than English (LOTE) is spoken, bilingual Pre-Ks only represent 6% of all programs. This study describes Pre-K program types available in NYC that offer Dual Language (DL) or Enhanced Language Support (ELS) to the large populations of emergent bilinguals. We analyze the number of programs within each borough and compare these with census tract data on the percentage of children who speak a LOTE at home. Our findings show that the number of Pre-K bilingual programs offered is grossly inadequate and, in most cases, is not significantly correlated to the percentage of LOTE children in the corresponding Pre-K neighborhoods. Only within the borough of Manhattan are bilingual DL programs significantly associated with neighborhoods with high percentages of LOTE children. In the other boroughs of Brooklyn and Queens, the higher percentages of LOTE children are served by ELS. Lastly, we compare the ratings of quality across Pre-K programs in each of the NYC boroughs. We present data demonstrating the inequity of access to high-quality programs that meet the community needs and discuss policy implications and recommendations.
Journal Article
Label-free characterization of organic nanocarriers reveals persistent single molecule cores for hydrocarbon sequestration
Self-assembled molecular nanostructures embody an enormous potential for new technologies, therapeutics, and understanding of molecular biofunctions. Their structure and function are dependent on local environments, necessitating in-situ/operando investigations for the biggest leaps in discovery and design. However, the most advanced of such investigations involve laborious labeling methods that can disrupt behavior or are not fast enough to capture stimuli-responsive phenomena. We utilize X-rays resonant with molecular bonds to demonstrate an in-situ nanoprobe that eliminates the need for labels and enables data collection times within seconds. Our analytical spectral model quantifies the structure, molecular composition, and dynamics of a copolymer micelle drug delivery platform using resonant soft X-rays. We additionally apply this technique to a hydrocarbon sequestrating polysoap micelle and discover that the critical organic-capturing domain does not coalesce upon aggregation but retains distinct single-molecule cores. This characteristic promotes its efficiency of hydrocarbon sequestration for applications like oil spill remediation and drug delivery. Such a technique enables operando, chemically sensitive investigations of any aqueous molecular nanostructure, label-free.
In-situ methods are important for investigating the local structure and function in molecular nanostructures but such investigations often involve laborious labeling methods that can disrupt behavior or are not fast enough to capture stimuli-responsive phenomena. Here, the authors use X-rays resonant with molecular bonds to demonstrate an in-situ nanoprobe that eliminates the need for labels and enables data collection times within seconds.
Journal Article
Resonant Soft X-Ray Scattering Reveals Chromophore Domains in Polymer Doped with Disperse Orange 11 Dye
Chromophore domains were proposed in a previous work as the mediators of self-healing of optical properties in dye-doped polymers. A statistical mechanical model based on domains matches all observed self-healing dynamics as a function of dye concentration, temperature and light intensity. This suggests that domains are responsible. However, there is no direct observation of domains, nor has their physical morphology been determined. This work reports the first observation of domains in a self-healing polymer using resonant soft X-ray scattering (RSoXS), which gives a domain size in the range of 39.3 Å to 62.8 Å. This range includes the domain model’s prediction of an average domain size of roughly 30 molecules, which is about 56 Å, if the molecules form a loosely packed ball. X-ray scattering of samples of concentration spanning from neat polymer to the saturation limit of disperse orange 11 (DO11) dye in poly(methyl methacrylate) (PMMA) polymer shows domains in the expected size scales, with the mode of the effective scattering width varying little with concentration. However, for constant domain shape, the mode peak would decrease in q with increasing concentration, according to the domain model. This work suggests that the domain shape might change with concentration, which warrants further investigations of domain topology and geometry. The important evidence presented in this work is the direct experimental observation of domains, which is central to self-healing models.
Journal Article
The influence of molecular orientation on organic bulk heterojunction solar cells
In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (−1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT–DTBT, PBnDT–DTBT or PBnDT–TAZ.
X-ray scattering experiments indicate that the molecular orientation at the interfaces of bulk heterojunction organic solar cells influences the cells’ fill factor and short-circuit current.
Journal Article
Animal Behavior for Shelter Veterinarians and Staff
A comprehensive resource to understand the behavioral considerations for intake, management, and rehoming of dogs and cats Animal Behavior for Shelter Veterinarians and Staff provides readers with comprehensive information addressing the behavior of both animals and humans associated with the intake, management, and rehoming of dogs and cats.
eBook
Teacher-Child Relationship and Behavior Problem Trajectories in Elementary School
The present study examined associations between the quality of teacher-child relationships and behavior problems among elementary school students using data from the NICHD Study of Early Child Care and Youth Development, a study of 1,364 children from birth through adolescence. There were two main findings. First, high-quality teacher-child relationships predicted low levels of externalizing behaviors. Second, high-quality relationships acted as protective factors, helping to prevent children with high levels of internalizing behaviors in early childhood from developing trajectories of long-term internalizing behavior problems. Teacher-child relationships may be proximal phenomena that can be targeted in interventions to help prevent behavior problems in middle childhood.
Journal Article