Stephen R. Forrest - US grants

Our objective is to understand the conductivity switching processes in polymers, and to apply this phenomenon to the demonstration of reliable, and very high density write once read many times (WORM) memory devices in a hybrid organic/inorganic semiconductor architecture. Recently, and in collaboration with our industrial partners, we have shown that the electrochromic polymer, PEDOT, undergoes an irreversible reduction in conductivity when a large current is passed through it. When placed in series with a nonlinear electrical element (e.g. a Si p-n junction diode), the PEDOT behaves as a fuse in an active matrix array of cross-points, forming a WORM memory; a key component in any electronic technology.

Intellectual merit and Broader Impacts: This work will explore both the conductivity mechanisms in electrochromic polymers such as PEDOT, and the nature of organic/inorganic heterojunctions - both fundamental questions explore the nature of organic semiconductor materials. This work can lead to a range of electronic devices employing widely disparate hybrid semiconductor materials systems (i.e. organics with inorganic semiconductors) which can impact the functionalities of both materials systems. The project will engage a diverse pool of students. Coupled to courses already being taught at Princeton by Prof. Forrest in the field of organic thin film devices and materials, and by Prof. Register in polymer chemistry and physics, the team will provide an opportunity for learning in the rapidly expanding field of organic electronics. Our collaboration with our industrial partner, Universal Display Corp., further ensures a rich experience for training students, while providing an immediate technology transfer pathway leading to rapid commercialization.

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

The objective of this project is to acquire an electron-beam lithography system for the University of Michigan. This system will be used to facilitate research on a broad array of new technology, materials, structures, and devices, including nanolithography, negative-index materials, polymer-based photovoltaics, nanotube applications, quantum devices, nanomechanical terahertz antennas, fluidic systems for disease diagnostics, and chemical gas analyzers.

The electron-beam lithography system will enable research in the emerging areas of science and engineering. Research on new materials will include tunable light sources, spintronics, and high efficiency photovoltaics. Research in nanobiotechnology will include DNA analysis, protein patterning, and biophotonic flow cytometry. Research on nanodevices will include 250Gb/cm2 crossbar memories, nanoscale gas analyzers, and terahertz antennas. These discoveries will push our understanding of materials and devices well beyond current levels into the new frontier of atomic-scale nanotechnology.

This system allows generation of new sources for secure broadband communications, high-efficiency lighting, renewable energy, wristwatch-size sensors for global environmental monitoring and security, and breakthroughs in prostheses for deafness, blindness, paralysis and Parkinson's disease. The research enabled by this tool thus tackles some of the most critical challenges in energy, security, environmental quality, and health care facing us in the 21st century. In addition, it will also be utilized in undergraduate and graduate classes, in technical workshops for engineers, in outreach activities to convey the excitement of science and engineering careers to students at the pre-college level, and as a resource to researchers from academia and industry through the NSF National Nanofabrication Infrastructure Network.

This award on solar energy research is co-funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences of the Directorate for Mathematical and Physical Sciences. A collaboration of chemistry, materials science, mathematics, physics, and engineering groups at the University of Michigan and the University of Southern California will develop a unique, new, thin film solar cell based on polymer-wrapped carbon nanotubes (CNTs). These films will be used in donor-acceptor heterojunctions employing a range of new organic materials and device structures, including polymers and small molecules. The use of CNTs extends the optical sensitivity from the blue into the near infrared, allowing organic-based devices to approach nearly thermodynamically-limited power conversion efficiencies. Simulated excited state (exciton) flow and charge transport through the CNT network uses new treecode algorithms and semi-classical hydrodynamical models. Efficient, multi-dimensional optimization methods are used to develop novel aperiodic dielectric stacks that couple the broad solar spectrum into very thin films used as the active device region in solar cells. A diverse range of undergraduate and graduate students and postdoctoral fellows are engaged in this interdisciplinary research in renewable energy. These students are provided with opportunities to influence policy decisions regarding energy choices through coursework in energy policy and geopolitics on their respective university campuses. The group is also involved in the University of Michigan's Saturday Morning Physics lecture series, providing the public with insights into the latest science and technologies.

Abstract

The Center of Excellence in Materials Research and Innovation* (CEMRI) at the University of Michigan's Photonic and Multiscale Nanomaterials (C-PHOM) Center is focused on developing novel multiscale materials for nanophotonics. Light-matter interactions can be controlled both by engineering the electronic properties of the material on the single-nanometer scale, and by incorporating materials into structures that control the propagation of light on the scale of a fraction of the optical wavelength and longer. Materials that are engineered to simultaneously access these multiple length scales simultaneously are referred to as multiscale materials; their development constitutes one of the most exciting frontiers in materials research and in photonics today. Center activities are focused on two interdisciplinary research groups (IRGs) and a coordinated program in diversity, education, and human resources development (EHRD).

IRG1 is dedicated to wide-bandgap nanostructured materials for quantum light emitters. The program will develop wide-gap materials, particularly GaN-based nanostructures, establishing inorganic semiconductor nanophotonic structures with large bandgap and high exciton binding energy for high-efficiency visible light emitters, lasers, energy conversion, and novel quantum devices. The proposed research includes the epitaxy and synthesis of GaN-based nanostructures, their structural, electrical, and optical characterization, their application in laser spectroscopy and quantum optical studies, investigation of strong coupling phenomena, polariton lasing, high-efficiency visible LEDs, and microcavity lasers. This effort is centered at the University of Michigan, with partners at the University of Illinois Urbana Champaign and Queens College CUNY.

IRG2 is focused on advanced electromagnetic metamaterials (MM's) and near-field tools. Metamaterials are nanostructured mixtures that behave as homogeneous optical materials with electromagnetic properties unattainable with naturally existing materials, such as negative refraction, cloaking, plasmonic hot spots, and super-resolution. This IRG will investigate MM's - particularly chiral, quasiperiodic and hyperbolic MM's - and MM-inspired structures with unusual properties such as near-field plates and hyperlenses, and develop understanding leading to potential applications in communication, sensing, and imaging (notably sub-wavelength imaging). The IRG consists of a partnership between the University of Michigan and Purdue University, and additional collaborations with Wayne State University and the University of Texas at Austin. Both IRG's include extensive interactions with national labs (Sandia and Argonne) and with overseas institutions.

A key feature of the center mode of research in a CEMRI is that it provides a framework for the close integration of research with educational and outreach initiatives, at the high school, undergraduate, graduate, and postdoctoral levels. Specific programs to be implemented include undergraduate involvement in CEMRI research via both a Michigan and an international ("City of Light") research experience for undergraduates (REU) program, a focused program for regional high school students from schools with large underrepresented group enrollment, a coordinated system for recruiting underrepresented groups into higher education, and an entrepreneurship program to train PhD students and postdocs and encourage translation of technology developed in the center.

*An NSF Materials Research Science and Engineering Center (MRSEC)

Technical: This collaborative research project at CUNY Queens College and University of Michigan aims to develop a new class of nonlinear optical materials that combine the advantages of organic, inorganic and metallic systems. Composite structures comprising hybridized excitons that have the desirable nonlinear optical properties of large oscillator strength (organic like), low saturation power (inorganic like), and quasiparticles (exciton-plasmon polaritons) that form through the strong interaction between inorganic excitons and plasmons of metal nanoparticles are investigated. The research project is expected to realize these hybridized materials systems through (i) dipole-dipole interaction of the Frenkel and Wannier-Mott excitons at the organic-inorganic interface and (ii) strong coupling between inorganic excitons and plasmons of metal nanoparticles using layered nanocomposite geometry. Nonlinear optical properties and morphology of the materials are investigated using a variety of spectroscopic and structural characterization techniques.
Non-technical: The project addresses basic research issues in a topical area of materials science with high technological relevance. A successful outcome of this research project will make substantial contributions to the field of nonlinear optics by exploring a new class of engineered nonlinear optical materials. Besides potential applications such as efficient all-optical switching elements, imaging, spectroscopy and second harmonic generation, these materials can potentially contribute to the interdisciplinary field of quantum informatics. The collaborative project also trains, creates research opportunities, and helps instill interest in science and engineering for graduate, undergraduate and high school students, from diverse backgrounds and ethnicities.

Non-technical Description: Energy transfer between molecular systems lies at the heart of several natural and artificial energy harvesting systems as well as in high-performance light emitters and sensors. Thus understanding and controlling the energy transfer process are of significant importance to both fundamental research and technological applications. This collaborative project develops artificial organic-inorganic hybrid systems where the energy transfer process is controlled by engineering the surrounding optical environment. The results of the project will provide guidelines for the design of efficient energy transferring organic/inorganic molecular systems geared towards applications such as next-generation solar cells, light emitting diodes and sensors. The project provides training in an interdisciplinary environment to graduate and undergraduate students at City College of New York and University of Michigan. In addition, the project integrates with outreach efforts such as high-school student participation in the research and public lectures/demonstrations to local-area high-school students. These are excellent recruiting and training tools for future scientists and engineers.

Technical Description: The primary goal of this project is to elucidate the role of exciton wavefunction delocalization in the energy transfer process between donor and acceptor molecules and to develop methods to control it using artificially engineered systems. In this context researchers develop artificially engineered energy transfer systems through strongly coupled organic-inorganic hybrid excitons. By exploiting the fundamentally different nature of excitons in organic and inorganic systems and hybridizing them provides a whole new control parameter for energy transfer. Here the coupling strengths and exciton wavefunctions are controlled via light, morphology and dimensionality. Specific materials systems investigated include inorganic excitons of zinc oxide (ZnO) and cadmium sulfide (CdS) quantum dots and the organic excitons of polycrystalline organic 3,4,7,8 napthalenetetracarboxylic dianhydride (NTCDA) and anthracene. These studies are designed to form the basis for developing highly efficient energy transfer systems for practical applications.

With this award from the Major Research Instrumentation Program and support from the Chemistry Research Instrumentation Program, Professor Jennifer Ogilvie from the University of Michigan and colleagues Stephen Forrest, Kevin Kubarych, Theodore Norris and Vanessa Sih will develop a multispectral multidimensional non-linear (MMDS) spectrometer to enable studies of dynamical processes in atomic, molecular, and material systems spanning femtoseconds to seconds, from the ultraviolet (UV) to the infrared (IR) regimes. This will allow insight into DNA photodamage and repair mechanisms and metalloenzyme function. The ability to mix and match infrared, visible and ultraviolet wavelengths will be a novel and potentially very powerful technique. The investigators will create modules that will serve for outreach activities to K-12 students. Importantly, participation of students and postdoctoral associates in this project will serve to educate a new generation of instrumentalists.

The award is aimed at enhancing research and education at all levels, especially in areas such as the study of complex condensed phase systems. The instrument will also be used to probe the dynamics of the building blocks of biological matter: amino and nucleic acids, to gain insight into DNA photodamage and repair mechanisms and metalloenzyme function and will provide a tool for studying the physics of nanostructures. It is also noted that the instrument would address key questions relevant to improving the efficiency of organic photovoltaic devices.

Non-technical abstract: We are reaching the inevitable end of Moore's Law: the scaling law that says the number of transistors on a dense integrated circuit will double every 18 months to 2 years. This trend has been the engine of productivity growth of modern technological societies for at least 50 years. To extend this trend, an exciting class of two-dimensional materials is emerging as a major opportunity. Atomically thin layered materials, often called two dimensional materials, represent a radical departure from conventional semiconductors such as silicon that comprise current electronic devices, mimicking sheets of paper as opposed to large three dimensional blocks. This two dimensionality leads to unusual properties such as exceptionally low resistance along the sheet, yet poor conduction perpendicular to it. This makes it ideal for use in extremely high performance optical and electrical circuits. But, as in all electronic devices, junctions between materials play a central role in the overall functioning of the optical and electronic devices out of which they are made. In fact the junction is often the weakest link in the device performance chain. In this project, the research team is investigating the photophysics and energy transport at interfaces of dissimilar materials with different dimensionality. Specifically, the team explores junctions between organic semiconductors, traditional inorganic semiconductors such as silicon and gallium arsenide, and the new class of two dimensional compounds. The goal is to understand and enhance the energy and charge transport across the junctions, ultimately with the goal of vastly improving the performance of electronic and optical circuits. The potential applications of such hybrid materials include solar energy harvesting, light emitting diodes and secure quantum information technologies. This research project has a strong educational component that involves graduate and undergraduate student training, as well as summer research opportunities for underrepresented minority high school students.

Technical Abstract: Understanding energy and charge transfer across interfaces between widely dissimilar semiconductor materials is key to realizing devices that exploit the unique advantages of the different contacting materials. The properties of interest that can be shared, or optimized in such materials combinations include ultrahigh optical oscillator strengths and mechanical flexibility of organics, along with the very large charge mobilities and quantum delocalization found in limited dimensional inorganic semiconductors. It is precisely these aspects that make organic molecules, two dimensional transition metal dichalcogenides and inorganic quantum wells attractive for optoelectronic applications. However, much less is known about interfaces that form between these material systems and their emergent properties. The fundamental nature of three dimensional organic and inorganic semiconductors forming junctions with two dimensional van der Waals solids presents an ideal platform to investigate interface physics. The team investigates three unique classes of heterointerfaces between systems of different composition and dimensionality: (i) organic semiconductor - two dimensional materials, (ii) inorganic semiconductor - two dimensional materials and (iii) lateral heterojunctions between dissimilar two dimensional materials. The combination of steady state and time resolved spectroscopic measurements including near field microscopies along with transport measurements are used to gain a fundamental appreciation of the physics governing the interplay of photons and electrons at these largely unexplored interfaces with the goal to develop quantum mechanical models grounded on observation of the energy and charge transfer processes across the interfaces, the formation of hybrid excited states and their transport as well as nonlinear optical properties. The anticipated outcomes include the ultimate exploitation of combinations of materials and dimensionalities through engineering of materials, interface properties, structures, and film morphologies and their tuning to achieve optimized performance for a particular application.

Mature solar technologies, in particular silicon, are now providing energy in a growing number of locales across the US at costs well below even that of fossil fuels. In short, solar energy is delivering on its promise of over 60 years as a source of low cost, clean and literally infinitely renewable energy. However, basing all solar solutions on this single materials system is still far from an optimal solution. To be impactful, new solutions must have the objective of making solar power generation a ubiquitous presence, ultimately filling all of our ever-expanding energy needs. Recently there have been dramatic increases in organic photovoltaics (OPVs) that have led to the demonstration of >15% solar power conversion efficiencies (PCE) in cells containing new non-fullerene acceptors (NFAs) and unusual combinations of liquid and solution-processed organic and metallic materials that leverage our increased understanding of charge and exciton transport in films with heterogeneous phases at the nanometer scale. The primary impact of the research will be to understand and improve OPVs to ultimately provide ultralow cost solar power in situations where established, mature solar technologies are less effective, such as in solar power generating windows and building-integrated photovoltaics, and generation at very low light levels to scavenge waste illumination power. The project will support the education of a diverse group of graduate and undergraduate students in experimental and material design and synthesis, spectroscopic measurement, data-taking and analysis, and scientific communication. The PIs will seek to host a Conference for Undergraduate Women in Physics (CUWiP) at the University of Michigan in 2021. They will also develop strong ties to local minority serving institutions to give seminars and short lectures discussing state of the art OPV design and spectroscopic measurements.

The project will combine extensive expertise in OPV design and characterization with state-of-the-art and emerging multidimensional coherent spectroscopies (MDCS) to understand the energy loss mechanisms that currently limit single junction OPV device efficiencies. The research aims to improve our fundamental understanding of the mechanisms governing charge and excited state transport and sources of energy loss in NFA-based OPVs. This will be facilitated by the development and use of new spectroscopic and visualization tools that can precisely quantify, on femtosecond - second time scales across the UV and into the infrared, the dynamics of photogenerated charge transfer across heterogeneous interfaces and transport away from their points of origin. The principles derived from these fundamental studies will provide molecular design rules to guide the improvement of NFA-based OPVs towards their thermodynamically limited efficiencies of ~25%.

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.

Non-technical Description. Solar technologies are increasingly providing energy across the US at costs well below even that of fossil fuels. In short, solar energy is delivering on its promise as a source of low cost, clean and renewable energy. However, solar solutions have largely been based on silicon, which is far from an optimal solution. New solutions must have the objective of making solar power ubiquitous, helping to fulfill our ever-expanding energy needs. These include solar power generating windows and building-integrated photovoltaics as well as devices that operate at very low light levels to scavenge waste illumination power. This is an urgent and fundamental technological challenge. Recently, there have been dramatic increases in the efficiency of potentially low-cost organic solar cells to over 19%, approaching that of silicon cells. This project is directed at determining the ultimate power conversion efficiency of organic solar cells. The investigators will study new organic materials with state-of-the-art optical spectroscopy to understand the power generating mechanisms that limit the efficiency of organic solar cells. The principles derived from these studies can provide molecular design rules and guide the improvement of organic solar cells towards their theoretical limit of ~25% efficiency. The project supports training of a diverse workforce through the education of graduate and undergraduate students in materials design, synthesis, and characterization, coupled with device engineering and scientific communication. The PIs will recruit and retain a diverse next generation of students in STEM fields through diversity, equity and inclusion efforts at the University of Michigan, including outreach to underrepresented groups and hosting a Conference for Undergraduate Women in Physics.

Technical Description. The primary goal of this project is to understand and improve organic photovoltaic (OPV) devices through improved materials and device design strategies based on quantum mechanical models. Dramatically reduced energy losses in the charge photogeneration process may ultimately provide a pathway towards ultralow cost solar power in situations where established, mature solar technologies are less effective. Beyond solar energy harvesting, these systems open new avenues for engineering materials for charge and energy transport at the atomistic level, and for their exploitation in applications as light emission, energy and charge transfer over exceptional distances, and may even result in extending electronic technology well beyond its current limits. This project combines the investigators’ extensive expertise in OPV materials, design and characterization with state-of-the-art and emerging multidimensional spectroscopies to understand the energy loss mechanisms that currently limit single junction organic solar cell device efficiencies. The principles derived from these fundamental studies provide molecular design rules to guide the improvement of cell efficiencies towards their thermodynamic limit of ~25%. The work significantly expands the spectroscopic toolbox for probing OPVs, providing transformative opportunities for understanding the mechanisms of charge generation and concomitant energy losses. The research has the following primary goals: (i) Gain a fundamental understanding of the mechanisms governing charge generation and energy loss at organic heterojunctions (HJs) to increase the solar-to-electrical power conversion efficiency to near the thermodynamic limit; (ii) Map the complete HJ charge photogeneration process using multidimensional spectroscopy to probe the mechanisms of charge generation and the origins of energy loss; (iii) Exploit ultrastrong coupling in unique light harvesting architectures to realize exciton-polariton transfer with near-zero energy loss.

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.