Jiangeng Xue, Ph.D. - US grants

Background: Finding sufficient supplies of clean energy to replace the depleting and polluting fossil fuels will be one of society's foremost challenges for the next half-century. Solar energy is clean, abundant and renewable, yet it is vastly under-utilized in the current world. One major solar energy utilization approach is the direct conversion of sunlight to electricity using photovoltaic cells; however this has only constituted a negligible portion of the overall energy supply, mainly due to the high manufacturing and installation costs of photovoltaic modules. Devices based on organic materials can potentially provide very low cost solar energy conversion due to their low material cost, ease of processing, and compatibility with flexible substrates. There have been considerable interests in organic-based photovoltaic (OPV) cells in the last two decades, and the power conversion efficiency of OPV cells has steadily improved to the current record of approximately 5%. However, not only such efficiencies are still far lower than the theoretical limits, but it is imperative to greatly improve the efficiency to really make this technology suitable for future large-scale commercial applications.

Intellectual Merit: This CAREER proposal focuses on developing very high efficiency, organic-based photovoltaic cells, which have the potential to provide low cost solar energy conversion due to many technological advantages of organic electronic materials. Utilizing his knowledge and expertise in device physics, device fabrication, material processing, and characterization, the PI will address several fundamental device issues in organic photovoltaic cells by employing novel nanostructures and approaches to manage the photons, excitons, and electrons. The goal is to thrust the power conversion efficiency of these devices from the current 5% mark to the 15-20% regime. The research activities of this program consist of four components: controlled fabrication of an interdigitated nanostructured donor-acceptor heterojunction conducive to very efficient exciton dissociation and charge transport; study of the mechanisms contributing to the dark current and the energetics at the material interface to increase the voltage output; harvest of near-infrared photons with low-gap organic materials and inorganic nanocrystals; and application of the host/guest system to achieve long exciton diffusion lengths without sacrificing the coverage of the solar spectrum.

Broader Impacts: This program addresses fundamental engineering science research with strong technological relevance to the electronics/photonics. In particular, direct conversion of solar energy to electricity using low-cost, high efficiency organic-based photovoltaic cells will have a tremendous socio-economical impact on the world's energy supply. Moreover, the PI is committed to closely integrate research and education, and to promote student's involvement at all levels. A strategic educational program has been designed to include student training, curriculum contribution, and outreach to K-12 students and teachers. The PI will also work with the local utility company to educate and outreach to the general public.

Technical Summary:

X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS, respectively) provide information on chemical composition, chemical states, and electronic properties of surfaces and interfaces, which are critical to assist the understanding and further development of materials for a broad range of applications. The UPS capability coupled with the C60 ion sputtering source further expands the instrumentation capability to 'soft materials' such as organic, polymeric, and biological materials, which have received great scientific interests in recent decades. Examples of current NSF sponsored projects at the University of Florida that will greatly benefit from this instrument include: (1) characterization of organic-based semiconductors and interfaces for electronic and optoelectronic applications; (2) surfaces and interfaces of metal phosphonate and cyanometallate materials; (3) molecular approaches to directional growth of nanostructures for nanoelectronics; (4) microstructure-informed design methodology for advanced magnesium alloys; (5) graphene and other carbon-based materials and interfaces; (6) self-assembled colloidal nanoparticles for magnetic/biosensing applications. The Major Analytical Instrumentation Center (MAIC) is a user facility providing analytical microscopy and spectrometry instrumentation support to all disciplines at the university and the surrounding community. The state-of-the-art XPS/UPS instrument requested in this proposal replaces a 25-year-old XPS system, which not only accelerates the progress in various research programs through the vastly increased efficiency, but also provides new information through spatial imaging, high resolution chemical profiling, and vast expansion of the realm of materials to be characterized. The broader impacts of this instrumentation are also realized through a number of educational and outreach activities at the University of Florida and beyond.

Non-Technical Summary:

The advancement of many modern technologies, such as various energy solutions (storage, conversion, and conservation), nanotechnology, electronics and photonics, biomaterials and biotechnology, and transportation, rests upon the fundamental understanding of surfaces of the relevant materials and/or interfaces between different material components. One of the most informative surface analytical techniques is the x-ray or ultraviolet photoelectron spectroscopy (XPS and UPS, respectively), in which an x-ray or ultraviolet light is shone on the targeted materials and the types of atoms and the local chemical environment and physical properties of these atoms in the material near the surface are revealed by analyzing the energy of electrons ejected from the material surface. Buried material interfaces can also be probed when combining XPS/UPS with appropriate material deposition or removal techniques. The state-of-the-art XPS/UPS instrument requested in this proposal replaces a 25-year-old XPS system at the Major Analytical Instrumentation Center (MAIC), a user facility at the University of Florida which hosts an array of analytical instrumentation for the study of various types of materials. The new instrument provides vastly increased efficiency, new information through spatial imaging, higher energy resolution, and vast expansion of the realm of materials to be characterized. Furthermore, the new instrument provides unique opportunities for the cross-disciplinary research training and education of many graduate and undergraduate students, as well as opportunities for outreach to K-12 students and teachers and the general science and engineering community.

With this award the Macromolecular, Supramolecular and Nanochemistry Program supports Professor Ronald Castellano and his collaborators at the University of Florida to study semiconducting carbon-based (organic) materials. For many downstream applications - displays, lighting, solar cells, transistors, and sensors - the three-dimensional (3-D) arrangement of organic molecules in processed thin (10-1,000 nanometer) films dictates device performance. This study is developing a general self-assembly approach to allow functional organic molecules to form useful superstructures in a self-guided way. In the broadest sense, the work contributes to a general understanding of how to control the 3-D structure of single and multi-component organic matter at the nano-/mesoscale. Broader impacts with respect to education and training come from (1) graduate and undergraduate students being exposed to multidisciplinary science that includes synthetic and physical organic chemistry, spectroscopy, computation, materials processing/characterization, and device fabrication; (2) the student participants, together with the UF Chemistry Club, disseminating standards-aligned experimental modules to K-8 classrooms that embed concepts of the chemical/materials sciences and their own research; and (3) broadening the participation of underrepresented groups in STEM research activities.

While it is known how to tailor the electronic and optical properties of individual pi-conjugated molecules as well as to program the arrangements of the molecules in solution, a major bottleneck for their usage is predicting and controlling three-dimensional morphological structure in thin films. This research examines a hierarchical self-assembly approach based on hydrogen bonding to create nanoscale to mesoscale order in thin films of pi-conjugated oligomers irrespective of pi-chromophore structure. The design involves covalently linking typical pi-conjugated components to molecular recognition units capable of directing their supramolecular assembly into predictably shaped aggregates. The resulting topologies are expected to uniquely support the natural segregation of appropriately shaped additives in thin film blends, and allow desirable optoelectronic properties to be achieved. The proposed bottom-up self-assembly approach effectively decouples the morphological structure of organic semiconductor thin films from the intrinsic optoelectronic structure of the constituent pi-conjugated chromophores, allowing these aspects to be independently optimized through rational/theory-guided material design. The specific aims of this collaborative project include modular design and synthesis of pi-conjugated oligomers, evaluation of optoelectronic structure and (supra)molecular ordering in solution and in thin films, and characterization of the optoelectronic properties of and bulk ordering within the thin films using diagnostic devices.

Non-technical Description. In recent years a new class of materials, called organometallic halide perovskite materials, has emerged with a potential to improve a solar cell's performance while fabricated at low cost. Concerned with the adverse effects to environment and health due to the use of lead in the existing perovskite materials, which can be dissolved in water, the research team aims to discover and develop new lead-free organometallic halide perovskite materials that are environmentally friendly and stable. The expected outcome of this project enables a sustainable solar cell technology that is safe for commercial deployment to generate low-cost solar electricity. The research team participates in multiple educational and outreach activities to recruit, train and mentor graduate, undergraduate, and high school students, particularly from underrepresented groups, who are interested in materials science and engineering. In addition, the project team intends to openly disseminate the computer codes used to design these materials and to predict their structures and properties.

Technical Description. The goal of this project is to use a computation-guided approach to study new lead-free organometallic halide perovskite materials with mixed metal ions. The lead-containing organometallic halide perovskite materials have shown great promise for offering superior optoelectronic properties to enable high performance photovoltaic action while allowing to be easily processed in solution or high vacuum. In this project, the research team seeks to replace the lead in these perovskite materials with ion pairs with the same or different valence states in specific ratios. The scientific tasks include: (1) computationally screening a large number of material candidates to identify promising candidates for further studies by employing high-throughput framework of MPInterfaces; (2) computational simulation of structure and properties of the identified candidates using density functional theory calculations; (3) solution-based synthesis of these perovskite materials in both thin film and bulk crystal forms; and (4) characterization of the electronic, optical, photovoltaic, and multiferroic behavior of these new perovskite materials. The ultimate goal is to explore the large family of lead-free, perovskite-structured organic-inorganic hybrid materials and to identify the most promising, environmentally-safe materials with improved performance for photovoltaics applications.

Carbon-based (organic) materials hold enormous potential for use in everyday applications including displays, lighting, solar cells, transistors, and sensors. In these contexts, it is often the three-dimensional (3-D) arrangement of the molecules in thin (10-1,000 nanometer) films that ultimately affects efficiency, stability, and overall utility. This research borrows an approach used by nature, called self-assembly, to encourage organic materials to adopt profitable arrangements in thin films largely independent of their size and shape. Central to the universal approach of self-assembly are hydrogen bonding interactions that are also responsible for "sticking" water molecules or the DNA bases together. In the broadest sense, the research contributes to a fundamental understanding of how to control the 3-dimensional structure of organic matter at the nano-/mesoscale to accelerate development of next-generation organic-based electronic materials. Broader impacts with respect to education and training come from graduate and undergraduate students being exposed to multidisciplinary science that includes synthetic and physical organic chemistry, spectroscopy, computation, and materials processing/characterization. Professors Castellano and Xue take advantage of the University of Florida (UF) Bridge to Doctorate Fellowship program, a collaboration between the Graduate School at UF and the Florida-Georgia Louis Stokes Alliance for Minority Participation (FGLSAMP), to help recruit female and minority students contribute to research and receive mentorship to encourage degree completion. Through the team's participation in U-FUTuRES, physical science content is delivered to Florida science teachers (grades 4?8).

The prevailing interactions between pi-conjugated organic molecules in thin films do not yield large energy differences among competing solid-state packing arrangements, leading to kinetically-driven spatial structures that are susceptible to minor changes to molecular structure and processing. The structure-property studies in this research expose the thermodynamic and kinetic drivers of hydrogen bond (H-bond) guided thin film assembly to allow the charge mobility and optical characteristics to be independently addressed through rational design and synthesis. This project specifically examines the interplay of kinetics and thermodynamics in film formation through variation of programmed intermolecular interactions and deposition conditions. The research also introduces a bioinspired structural scan to relate H-bonding configuration to optoelectronic properties and examines alternative molecular designs involving repositioned H-bonding units within the pi-framework to improve assembly orientational control in the pi-stacking direction and with respect to the substrate.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.