Yang Yang, Ph.D. - US grants

Abstract Program: Physical Foundations of Enabling Technologies CAREER Competition Proposal Number: ECS-9733355 Principle Investigator: Yang Yang Title: Ionic polarized polymer light-emitting diodes fabricated by ink-jet printing technology Conjugate polymers are a novel class of semiconductor materials that combine the optical and electronic properties of semiconductors with advantages of simple processing and flexible mechanical properties of polymers. Due to its low cost and ease of fabrication this unique class of materials will revolutionize many existing electronics and bio-medical technologies. This CAREER proposal will engage in research and education activities to lay down the foundation for a consolidated polymer electronics research center at UCLA. In particular, the research is aimed developing new and novel polymer light-emitting diodes using an innovative ink-jet printing technique. the particular innovation of this proposal is the novel and potentially low-cost fabrication of large-area displays.

Recently, we observed that organic semiconductor device shows strong bistable states with remarkably differing electrical conductivities when it is structure right. The transition from an electrically insulating state to a conducting state in the device is accompanied by a drastic increase in injection current by as much as six orders of magnitude. The retention of the high conductivity state was observed even after switching off the power. Furthermore, the low conductive states can be re-established by applying a negative voltage pulse. These discoveries pave the way for potential applications such as low-cost, large-area, electrically addressable high-density data storage devices, organic switches, and sensors. This newly invented organic device is significant for two reasons. First, this device uses organic insulators as the active material, thereby providing new options for organic electronic devices, which have been traditionally associated with organic semiconductors. Second, electronic memory is a very important component in all electronic devices such as computers, cell phones, PDAs etc. It is anticipated that the successful development of this device as memory cells will have a tremendous impact in the electronic industry. Unfortunately, the mechanism of this device, for example the sudden change in electrical conductivity at ~3V; and the reason behind the retention of the high conductivity state even after switching off the power, is not yet clearly understood. Our goal of this project is to gain the understanding of the organic bistable device, from both experimental and theoretical modeling approaches. Based on the obtained results, we will try to further improve device performance and to realize other applications.

Because of the relentless march of the silicon-based electronics technology as predicted by Moore's Law, computation, storage, and communication are now woven into the fabrics of our lives. The emerging technology of flexible electronics, where electronics components such as transistors and wires are built on a thin flexible material, offers a similar opportunity to weave computation, storage, and communication into the fabric of the very clothing that we wear. The implications of seamlessly integrating a large number of communicating computation and storage resources, mated with sensors and actuators, in close proximity to the human body will transform many aspects of biomedical research and practice. For example, one can imagine biomedical applications where biometric and ambient sensors are woven into the garment of a patient or a person in a medically-critical or hazardous environment to trigger or modulate the delivery of a drug.
To realize this vision outside the laboratory, radical innovation is required in the area of system-level information technology. These systems will not scale to widespread use if they are viewed simply as traditional chips or motherboards based on a different, flexible form factor. Rather, a rethinking of the architecture and the design methodology for all layers of these systems is needed. The reasons are two-fold. First, the underlying technology of electronics in flexible materials has characteristics and computation-communication cost trade-offs that are very different from that of silicon and PCB-based electronics. Second, the natural applications of these systems have environmental dynamics, physical coupling, resource constraints, infrastructure support, and robustness requirements that are very different from those faced by traditional systems.
One of the challenges in developing the needed information technology architecture and design methodology for these systems is that one needs to both conduct experimental work and develop a conceptual understanding of the problem domain. This research studies:
Application: Use as a driver application capability, reconfigurable fabric (R-Fabric) based on a combination of (i) the technology of flexible electronics using organic materials, and (ii) computing, communication, and sensing elements implemented as E-Buttons.
Architecture: Develop the general architecture concepts and cost/performance optimization techniques. The issues that we will focus on will include (i) appropriate primitives for composing the architecture, (ii) system interconnect network optimized for the electrical characteristics of the organic electronics, (iii) techniques to cope with the high ration of communication to computation cost, and (iv) architecture level self-configuration and re-configuration for robust operation.
Programming: Develop techniques and primitives for programming a system composed of hundreds of computation, storage, sensing, and actuation elements that are individually resource constrained and are connected by a structured but fault-prone high-cost interconnect network.
Processors: Develop domain-specific processor architecture optimized for these power-constrained, physically coupled applications.

Design Methodology: Develop techniques and hybrid emulation platform for systematic architecture exploration, simulation, optimization, and reconfiguration of these systems.

TECHNICAL SUMMARY: The intersection between the emerging fields of conducting polymers and nanoscience offers exciting opportunities to make very sensitive sensors, high-density memory storage devices and artificial muscles. Our newly developed process for making ultra-small diameter fibers of the conducting polymer polyaniline provides the basis for this project. The nanofibers will be decorated with functional molecules, nanoparticles and polymers at the nanometer scale. Coating processes will be developed to form uniform nanofiber films. These films will be used to make sensors that exploit the rapid change in electrical conductivity possible with conducting polymer nanofibers. Suitable additives will be dispersed into polyaniline nanofiber networks to tailor the interaction between nanofibers and analytes, greatly enhancing their sensitivity and selectivity for sensing toxic chemicals. Non-volatile molecular memory devices that can write, read, store and erase information based on polyaniline nanofibers decorated with metal nanoparticles will be explored. An ordinary camera flash has been found to cause a film of nanofibers to weld together. This process, called flash welding, will be used to weld conventional polymers together, to create composites between conducting and traditional polymers, to form patterned structures and to create "artificial muscles". Artificial muscles are a type of mechanical actuator that responds to a chemical or electrochemical stimulus by expanding or contracting. Disseminating information to the public and effective training of students at all levels (K-12, undergraduate, graduate and postdoctoral) are the most important ways in which this proposal will have broad impact. Our plan is to have students learn every aspect of developing conducting polymer nanofibers from synthesis and characterization to building and testing devices. Our interdisciplinary research involving scientists and engineers from not only UCLA but also industry, the national labs and international collaborations will provide our students with exceptional educational opportunities that will serve them well in their future endeavors. All students and faculty involved in this project will bring their enthusiasm for science to the public through outreach activities.


NON-TECHNICAL SUMMARY: The intersection between the emerging fields of conducting polymers and nanoscience offers exciting opportunities to make very sensitive sensors, high-density memory storage devices and artificial muscles. Our newly developed process for making ultra-small diameter fibers of the conducting polymer polyaniline provides the basis for this project. Sensors will be constructed that exploit the rapid change in electrical conductivity possible with conducting polymer nanofibers. Additives will be dispersed into the polyaniline nanofiber networks to enhance their selectivity for sensing toxic chemicals. Non-volatile molecular memory devices that can write, read, store and erase information based on polyaniline nanofibers decorated with metal nanoparticles will be explored. An ordinary camera flash has been found to cause a film of nanofibers to weld together. This process, called flash welding, will be used to weld conventional polymers together, to create composites between conducting and traditional polymers, to form patterned structures and to create artificial muscles.
Disseminating information to the public and effective training of students at all levels (K-12, undergraduate, graduate and postdoctoral) are the most important ways in which this proposal will have broad impact. Our plan is to have students learn every aspect of developing conducting polymer nanofibers from synthesis and characterization to building and testing devices. Our interdisciplinary research involving scientists and engineers from not only UCLA but also industry, the national labs and international collaborations will provide our students with exceptional educational opportunities that will serve them well in their future endeavors. All students and faculty involved in this project will bring their enthusiasm for science to the public through outreach activities.

In this award, funded by the Experimental Physical Chemistry Program, Prof. Yang Yang of the University of California-Los Angeles will carry out research that aims to achieve high performance polymer solar cells. This will be achieved via design and synthesis of novel n- and p-type organic materials with desired properties to enhance the optical absorption, appropriate energy levels to optimize the open circuit voltage; and high carrier mobility to enhance the short circuit current. These new materials will be applied to two categories of photovoltaic devices including (1) Polymer solar cells using new designed p-type materials as electron donors and PCBM as the electron acceptor, (2) Non-fullerene based polymer solar cell using new synthesized n-type materials as electron acceptors and new designed or commercially available p-type materials as electron donors. The intellectual merit of the research will be embodied in both basic scientific research with important technological impacts. In this international collaborative program with Prof. Yongfan Li in the Chinese Academy of Sciences (CAS), Prof. Yang will collaborate with Li's group closely by weekly Internet meetings, exchange students, and frequent visits by the PIs, resulting in a productive situation for education and research. In addition to the broader scientific impact in this proposed research, Prof. Yang aims to have six to eight PhDs produced in this field. It is also our intention to include 6 undergraduate students to work with our PhD students to become familiar with the field of organic electronics. Finally, two mini-symposiums will be held, one in the second year (UCLA) and one in the third year (Beijing), to enhance the intellectual exchange in the organic photovoltaic area.

This international collaborative research project is supported jointly by the NSF and the National Science Foundation of China.

This award is funded under the American Recovery and Reinvestment Act of 2009(Public Law 111-5). This Integrative Graduate Education and Research Traineeship (IGERT) award supports a program at the University of California, Los Angeles on the topic of clean energy for green industry. An interdisciplinary approach to graduate education is designed to train U.S. Ph.D. scientists and engineers for leadership roles in the clean energy sector. The technical thrusts merge three scientific areas of energy harvesting, storage and conservation with policy and business to address complex clean energy issues and identify areas for transformational research. Through this foundational structure, this IGERT addresses the urgent societal challenge of meeting increasing energy needs without further negatively affecting the environment. The development of such solutions is only feasible through university-industry-local government partnerships with highly-skilled, broadly-educated, globally-minded leadership. Such partnerships have high potential for economic development in urban areas primed for growth in this sector, with a well-trained workforce, a supportive government and visionary industrial foundations. Close interaction with industry is fostered through industrial innovation partners, intellectual property development and rapid technology transfer for job creation. Emphasis is placed on economic expansion through clean energy research, new business, highly trained workforce development, equity and inclusion. Program strategies include cross-disciplinary, integrated research and education training, modular clean energy curriculum for both on-campus and on-line dissemination, diversity and inclusion. The overarching theme of energy harvesting, storage, conservation and policy is echoed in integrated research, education and service components. IGERT is an NSF-wide program intended to meet the challenges of educating U.S. Ph.D. scientists and engineers with the interdisciplinary background, deep knowledge in a chosen discipline, and the technical, professional, and personal skills needed for the career demands of the future. The program is intended to catalyze a cultural change in graduate education by establishing innovative new models for graduate education and training in a fertile environment for collaborative research that transcends traditional disciplinary boundaries.

Abstract

Research Objectives and Approaches - The objective of this research is to advance the solution processed hybrid triple-junction photovoltaic technology to 15% efficiency. The approaches are comprehensive and include: design of organic donor and acceptor materials with high efficiency and spectrum-matched absorption; a solution processed inorganic solar cell to cover the near-IR spectrum; novel interlayer and hybrid tandem solar cells; and novel transparent electrodes to realize all solution processed hybrid tandem solar cells. 15% efficiency will be a milestone for low-cost high efficiency solar cells.

Intellectual merit - Renewable energy is crucial for sustainable economic development and environmental protection. The proposed research represents a new front in solar cell research and integrates both chemistry and materials science to generate new perspectives in science and offer solutions to challenging sustainable energy issues. We aim to contribute to our national energy security and a greener economy by achieving a breakthrough in solution processed high efficiency solar cells. This project will not only give us device success, but enrich our understanding of fundamental physics.

Broader impacts - The proposed research will impact the national and global effort in developing new sustainable energy strategies and technologies, thus impact our nation?s economy and improve environmental quality by reducing the use of fossil fuels. This project offers students (from high school to graduate-level) the opportunity to learn the skills to face future scientific challenges. The team has strong partnerships with the private sector through patent licensing, which will continue and greatly benefit the US economy.

The NSF Sustainable Energy pathways (SEP) Program, under the umbrella of the NSF Science, Engineering and Education for Sustainability (SEES) initiative, will support the research program of Prof. Luping Yu and co-workers at the University of Chicago, Prof. Yang Yang and co-workers at the University of California at Los Angeles (UCLA), and Prof. Lin Chen and co-workers at Northwestern University. The objective of this multidisciplinary collaborative project is to develop organic solar cells with power conversion efficiency competitive with amorphous inorganic solar cells. Novel materials (both donor and acceptor materials) with the best power conversion efficiencies and controlled absorption windows will be synthesized. Detailed structural and spectroscopic studies will be performed to probe and understand exciton generation, charge separation, and transport mechanism in these newly developed materials, which will form the foundation for device fabrication. Novel geometryies of tandem solar cells will be developed to maximize solar energy conversion. Close collaborations with active feedback among the team will lead to rapid realization of the research goal.

The overall viability of organic photovoltaics (OPV) as a sustainable energy technology will be assessed based on their environmental, health, and economic impacts via collaboration with a Dr. Peter Sinsheimer on urban planning, Prof. Zhu Yifang on public health, and Mr. Timothy Malloy on societal impact. Issues associated with technology diffusion and possible commercial, institutional, legal, policy, technological, and cultural barriers associated with the design, development, and adoption of new sustainable technologies will be evaluated experimentally and/or based on viable models. This collaborative effort will be used as a platform to attract underrepresented undergraduate students to the research area of sustainable energy pathways. The project will provide great opportunity to educate and train graduate students and postdoctoral associates in system-based research in the area of sustainable energy pathways.

If this project succeeds, it will result in alternative solar energy harvest materials and devices that can be manufactured in an environmentally friendly approach, with flexibility, and at a low cost. It will change the landscape of renewable energy and exert significant impact on sustainable economic development and environmental protection. The resulting technology may penetrate into our daily life, as it may be used in a wide range of electronic devices like portable battery chargers, energy saving computer displays, and house decorations. Students working on this project will be prepared to become leaders in renewable energy research and development.

TECHNICAL SUMMARY:
This Materials World Network project will investigate the design rules and compatibility of n-type fullerene molecules with photoactive polymers, and apply the obtained understanding to achieve breakthrough performance in photovoltaic devices. The centerpiece of the technical approach is the rational design and synthesis of novel fullerene materials to address three key issues: a) raising LUMO Energy level of fullerene for higher photo-voltage; b) polarity fine-tuning of the fullerene by introducing substitutes to achieve optimized morphology; and c) absorption enhancement through introducing chromophores to the fullerene compounds. The relationship between the structures of these novel acceptors and the energy levels, charge generation/recombination, absorption, solubility, electron mobility, surface energy, and morphology of the donor/acceptor blend films will be comprehensively investigated. This project will be in collaboration with Prof. Kung-Hwa Wei of NCTU in Taiwan. The combined study will lead to understanding on the design rules of n-type fullerenes with emphasis on not just the LUMO-HOMO position, but also the relation between morphology, charge carrier lifetime, and charge transport in the materials system. This understanding is important for achieving organic photovoltaic performance milestones.

NON-TECHNICAL SUMMARY:
Organic photovoltaics (OPVs) are an area of growing importance in exploring sustainable energy sources. This Materials World Network project seeks to provide a deep understanding of the core issues in OPV materials for energy device technology through a complementary international collaboration. This project will provide a platform for undergraduate training and showcase the importance of renewable energy to society. The project will provide education and awareness to K-12 students on clean-energy issues, science and careers in association with the outreach programs in UCLA's NSF Clean-Green IGERT program. A summer camp opportunity for high and middle school students will be provided as part of the project. In addition, a course on renewable energy and solar cells, as well as an OPV solar cell teaching module for UCLA Clean & Green IGERT are planned.

This project is supported by the Polymers Program and the Office of Special Programs in the Division of Materials Research.

Technical Description: Solution-processed oligomer-based organic photovoltaic materials are promising candidates for next-generation solar cells due to their low-cost potential, ease of fabrication, and tunable characteristics. The relationship between molecular structure and materials morphology critically affects performance characteristics such as charge mobility. This collaborative project, sponsored by the Designing Materials to Revolutionize and Engineer Our Future (DMREF) program, explores the structure-morphology relationship, and then uses knowledge gained to employ a screening and iterative design approach. The project unites computational, synthetic, and device construction and characterization research labs at UCLA to accelerate development of these materials. A tiered computational approach for multiscale morphology modeling of oligomeric donor material is being developed, using a combination of rapid sampling and accurate ranking techniques using integrated electronic structure calculations, crystal structure prediction, molecular dynamics (MD) simulations, and charge transport calculations. Crystal structure prediction methods sample possible packing arrangements and provide initial geometries for MD simulations of each arrangement, upon which high-accuracy accelerated MD with a polarizable force field can be employed to simulate local chemical properties and disorder. As part of this approach, known first-principles and charge transport analysis methods predict donor morphology-influenced properties such as hole mobility and bulk electronic structure. From combinations of large libraries of donor and acceptor subunits, screened oligomers with promising electronic structures and morphologies are synthesized via site-selective cross-coupling reactions. Microscopy and x-ray diffraction methods are used to analyze morphologies, including morphology changes due to annealing, and to compare to theoretical predictions. Photoluminescence quenching, transient absorption spectroscopy, and charge extraction by linearly increasing voltage are used to assess device performance. This tightly knit collaborative effort enables feedback from experimental results to be used for iterative systematic tuning of candidate molecules and improvements of computational methods.

Non-technical Description: The need for clean and affordable energy demands the development and improvement of alternative energy sources. The sun supplies the Earth with 9000 times the world's current energy consumption, making solar power an attractive option. Oligomer (small molecule)-based organic solar cells are low cost relative to the current solar technology and have experienced significant increases in power efficiency in recent years. But major improvements are needed to enhance its commercialization potential. In a collaborative project, computational, synthetic, and device characterization research labs at UCLA are developing new methods and models to improve prediction of materials morphology - the way in which many molecules fit together - in these devices. The goal is to predict the performance of oligomer-based organic solar cell performance and to accelerate the discovery of new materials. From vast libraries of candidate molecules, the performance of new high-performance devices is screened computationally to predict promising molecules to be used to create and test devices and then improve performance of these devices. This project involves graduate students in diverse research teams to learn and develop interdisciplinary skills. Students involved gain experience in each aspect of the project. Encouragement of the next generation of scientists to engage in STEM careers is being fostered through mentorship of undergraduate students and through K-12 outreach.

Abstract

Non-Technical: The sun provides clean and abundant energy source for human being, but solar energy has not been widely used due to the high cost. The key to achieving affordable solar photovoltaic (PV) technologies is to develop techniques that offer both high performance and low material and processing costs. This research will combine the strength of two types of high performance thin film solar cell technologies - copper indium gallium diselenide (CIGS) solar cells and organic-inorganic hybrid perovskite solar cells to realize tandem solar cells and address key issue. The research will significantly enhance the thin film solar cell efficiency from 20% to 30% and still keep low cost. The proposed research represents a new front in solar cell research society. It will impact national and global efforts in developing new and sustainable energy strategies and technologies, thus impacting the nation's economy and improving environmental quality by reducing the use of fossil fuels. The education component of in this project offers students (from high school to graduate-level) the opportunity to experience all of these trainings and learn the skills to face future scientific challenges. This project also provides a platform for undergraduate training and showcases the importance of renewable energy to society.

Technical: The objective of this research is to advance solution processed perovskite/CIGS tandem solar cell technology by reducing thermalization losses of hot carriers generated by photons with larger energies than the bandgap, toward high performance (a target efficiency of 30%). The approach is to design and synthesize both perovskite and CIGS subcells with high performances and spectrum-matched absorption, and a tunnel junction with minimal optical and electric losses, based on full consideration of charge generation, charge separation and transport/collection in a monolithic configuration. The centerpiece of technical approach includes: (1) perovskite solar cell with high PCEs and spectrum-matched absorption will be designed and synthesized; detailed structural and spectroscopic characterizations will be performed, which form the foundation for device fabrication; (2) high performance inorganic CIGS solar cell will be applied to cover the Near-IR (from 800nm to 1200nm) portion; (3) single junction devices based on the materials in the two spectral ranges will be fabricated to achieve optimized performance; charge generation, charge separation and transport mechanism in these devices will be studied; (4) novel nano-functional interlayer and novel geometry of hybrid tandem solar cells will be developed to maximize solar energy conversion; (5) solution processed transparent electrodes will be will be developed to realize all solution process hybrid tandem solar cells. Upon success, this will be the first solution processed solar cell reaching 30% efficiency mark, which is an important milestone for low-cost manufacturing of high efficiency solar cell.

This project is jointly funded by the Electronics, Photonics, and Magnetic Devices (EPMD) Program in the Division of Electrical, Communications and Cyber Systems (ECCS) and the Electronic and Photonic Materials (EPM) Program in the Division of Materials Research (DMR).