Research

Reducing waste through engineering design

The generation and accumulation of solid waste is a growing problem world-wide. We aim to develop ways to minimize waste through engineered systems that are designed for enhanced recoverability or reuse. In this sense, we develop materials, components, and devices that are more ecological efficient, or eco-efficient. Our efforts encompass reconfiguring microstructure to enable reuse or "re-programming", control of chemistry to enhance sustainability, and implementation of low-energy processes that open avenues for low-cost reprocessing.

Learn more about our work on this project from our publications in Macromolecules, PCCP, and Advanced Functional Materials (coming soon).

Charge transport in polymer semiconductors

Our work has demonstrated the role of crystallization kinetics, crystallinity, and liquid crystalline order on charge conduction in conjugated polymers. We show that faster crystallization can lead to more tie chains between crystalline domains, thereby providing high-mobility pathways. Systematic comparisons of local order across different systems reveals that some polymers are limited by intra-crystalline disorder, while charge transport in other polymers depends on the connectivity between crystalline domains. We have also developed an analytic description of how molecular weight and liquid crystalline order enhances charge mobilities in polymer semiconductors.

Learn more about our work on this project from our publications in Macromolecular Rapid Communications, Macromolecules, and ACS Central Science.

Role of miscibility and mesostructure on the morphology of the active layer and device performance of organic photovoltaics

Recent evidence has demonstrated that amorphous mixed phases are ubiquitous within mesostructured polythiophene-fullerene mixtures. Nevertheless, the role of mixing within nanophases on charge transport of organic semiconductor mixtures is not fully understood. To this end, we examine the electron mobility in amorphous blends of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester. Our studies reveal that the miscibility of the components strongly affects electron transport within blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. As a consequence, partial miscibility may be important for efficient charge transport in polythiophene-fullerene mixtures and organic solar cell performance.

Learn more about our work on this project from our publications in Macromolecules, Chemical Communications, Physical Review Letters, Chemistry of Materials, and Advanced Energy Materials.

Fully conjugated block copolymer photovoltaics

Organic electronic materials have the potential to impact almost every aspect of modern life including how we access information, light our homes, and power personal electronics. Nevertheless, weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. We have demonstrated control of donor−acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. X-ray scattering results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor−acceptor interfaces.

Learn more about our work on this project from our publications in Nano Letters, Advanced Functional Materials, Macromolecules (2015 and 2016), Journal of Physical Chemistry C, and Journal of Materials Chemistry A (2017 and 2018) .

Contact doping with strong polyelectrolytes for organic photovoltaics

Barriers to charge transfer at electrode-semiconductor contacts are ubiquitous and limit the applicability of organics in electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers, but the soft nature of organic materials allows for small molecule dopants to diffuse and migrate, degrading the performance of the device and limiting effective interfacial doping. We have demonstrated that contact doping in organic electronics is possible through ionic polymer dopants, which resist diffusion or migration due to their large size. Sub-monolayer deposition of non-conjugated strong polyelectrolytes, e.g. sulfonated poly(sulfone)s, at the anode-semiconductor interface of organic photovoltaics enables efficient hole extraction at the anode. The performance of contact-doped organic photovoltaics nearly matches the performance of devices comprised of traditional hole transport layers such as PEDOT:PSS. We find that the degree of sulfonation of the dopant polymer and the thickness of the ionic dopant layer is critical to optimizing doping and the efficiency of the device.

Learn more about our work on this project from our publications in Advanced Energy Materials, ACS Applied Materials & Interfaces, and Macromolecules.

Advanced X-ray scattering of biological systems

The development of new X-ray scattering tools allows the study of complex systems with unprecedented sensitivity and fidelity. For example, resonant soft X-ray scattering couples spectroscopy and scattering to enhance contrast based on differences in elemental composition or chemical bonding. We have applied this technique to examine the shape and size of proteins in solution as well as the structure within plant cell walls. In the latter, we further examine texturing of crystalline cellulose, and find that this texturing is universal in a variety of plant species.

Learn more about our work on this project from our publications in Scientific Reports, MRS Communications and Structure.

Pushing the limits of transmission electron microscopy of soft materials

Recent instrumental developments, although often not designed for the study of soft materials, has the potential to revolutionize imaging at the nanoscale. For example, monochromated sources can enable imaging based on differences in the valence electronic structure, thereby revealing heterogeneity in local electronic properties. Furthermore, improvements in detectors can push the sensitivity of elemental mapping with nanometer resolution of organic and polymeric materials .

Learn more about our work on this project from our publications in Macromolecules, Advanced Functional Materials, and MRS Communications.

Composites from cold sintering

Controlling the microstructure in composites is crucial for optimizing mechanical, optical, and electrical properties. Achieving this control is challenging, and is often limited by the incompatible processing requirements of organics and hard materials. A recent approach developed at Penn State has demonstrated the sintering of ceramics at temperatures near 100 ˚C. This remarkable achievement opens the door for co-sintering of soft materials and ceramics, as well as many other organic-inorganic composites, in a way that can open new avenues for microstructure control.

Learn more about our work on this area from our publications in the Journal of Applied Polymer Science, ACS Macro Letters and Physical Chemistry Chemical Physics.

Connecting the nanoscale morphology of water filtration membranes to transport properties

The development of membrane materials is limited by our lack of tools to characterize their complex microstructure. We have demonstrated how a combination of careful sample preparation, electron tomography, and quantitative analysis of 3D models can provide unique insights into the morphology of polyamide active layers used in reverse osmosis membranes. Extracting the 3D morphology is required to obtain accurate estimates of the top surface area and internal void fraction. Furthermore, mapping the internal heterogeneity shows that polymer density is highest near the top surface and and our work thus suggests new models of how microstructure and membrane performance is connected. Complementary to electron microscopy, we have also pioneered the use of resonant soft X-ray scattering to quantify the microstructure in membrane materials.

Learn more about our work in this project from our publications in ACS Macro Letters and PNAS.

Elucidating the fundamental factors that govern charge transport in molecular semiconductors

Organic field-effect transistors (OFETs) continue to attract widespread interest because of their potential to enable low-cost flexible electronic devices. The strong dependence of electrical transport on long-range order in small molecule semiconductors imparts significant sensitivity of device performance on molecular structure and processing conditions. Thus, we are interesting in perturbing the molecular structure and processing conditions to achieve high mobilities in organic devices. Furthermore, charge transport in OFETs is limited by the dielectric constant of the gate insulator, likely due to energetic disorder caused by poorly organized polar groups of the dielectric layer. We have demonstrated approaches to control chain conformations of polymers used as gate insulators, and thereby achieve high mobilities and high charge densities simultaneously.

Learn more about our work in this project from our publications in Journal of Materials Chemistry C (2015, 2018) and Advanced Materials.