Research Highlights

OVERVIEW

About the EFRC

The Center for Nano- and Molecular Science and Technology (CNM) at The University of Texas at Austin is the site of an Energy Frontier Research Center (EFRC) funded by U.S. Department of Energy (DOE) Office of Basic Energy Sciences.

According to the DOE, the EFRC program represents “a major effort to accelerate the scientific breakthroughs needed to build a 21st-century energy economy.” Our EFRC is one of 46 DOE-funded Centers in the United States. More information from the DOE regarding $777M EFRC program can be found at the DOE EFRC website.

The DOE plans to support our EFRC for an initial period of five years (beginning August 1, 2009) with approximately $15M in funds allocated from the 2009 American Recovery and Reinvestment Act (stimulus package).

Research Focus

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Our EFRC, titled “Understanding Charge Separation and Transfer at Interfaces in Energy Materials and Devices (EFRC:CST),” is focused on advancing the understanding and design of nanostructured molecular materials for organic photovoltaic (OPV) and electrical energy storage (EES) applications. OPV materials, including solar cells, and EES materials, including batteries for all-electric vehicles, are highly promising for future energy needs. However, very little is understood about the interfacial charge separation and transfer processes that contribute to these materials’ energy conversion efficiencies and overall performance.

We will use newly developed sub-ensemble techniques (including single-molecule spectroscopy and imaging) coupled with theoretical methods to answer key outstanding questions on charge separation and transfer for solar cell and battery materials. More information about research conducted in the EFRC:CST can be found under the “Research” tab.

The expected outcomes of this program include:

  • new R&D tools
  • new high-performance materials
  • the education of a new generation of energy researchers

The EFRC:CST Team

The EFRC:CST is directed by Professor Paul F. Barbara, who is also Director of the CNM. Our EFRC team is composed of 18 CNM-affiliated faculty members from UT’s College of Natural Sciences and Cockrell School of Engineering, as well as partnering researchers from Sandia National Laboratories. Approximately 30 graduate students and postdoctoral fellows are supported by the EFRC:CST each year. A full listing of EFRC investigators can be found under the “People” tab.

We are advised by an External Advisory Board consisting of world-renowned scientists whose research interests are highly aligned with those of the EFRC:CST. In addition, a Technology Advisory Board consisting of affiliates in the solar and battery industries also advises the EFRC:CST.

EFRC:CST Activities

The EFRC:CST hosts an annual meeting each year that is open to all EFRC investigators and affiliates. We also host symposia throughout the year that feature invited speakers and discussions of research topics that are of broad appeal to the EFRC team.

We also aim to educate students at all levels about energy research through a host of educational activities as described under the “Education” tab.

MESSAGE FROM THE DIRECTOR

barbara-paul

Dear Colleagues and Visitors:

Welcome to the website of the Department of Energy Energy Frontier Research Center on Charge Separation and Transfer at Interfaces in Energy Materials and Devices!

We hope this website will serve to help you learn about how understanding charge separation and transfer is critical to advancing the modeling and design of new materials for energy needs. My co-investigators, including faculty, staff scientists and graduate students, are committed to developing breakthroughs in the molecular tools and concepts for this rapidly advancing field.

We are interested in developing relationships with small and large industrial organizations. If you would like to pursue this, please click on the “Industry” tab to learn more.

For prospective graduate students, please browse through our “Research” and “Education” tabs, and feel free to contact us if you’d like to learn more.

We look forward to communicating with you all as we continue our aggressive pursuit of energy solutions for the 21st century.

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Paul F. Barbara
Richard J.V. Johnson Welch Regents Chair in Chemistry and
Director, EFRC:CST

OPVs AND SOLAR CELLS: THE BASICS

Harvesting solar energy is a key endeavor for this century as we face ever-decreasing fossil fuel world reserves and ever-increasing environmental crises emanating from greenhouse gas production. Current solar cell technologies are largely silicon-based. Devices used for human energy consumption yield solar power conversion efficiencies of ~10-20%, which is sufficient to meet a significant fraction of energy demands in the industrialized world, but manufacturing such devices costs energy and produces greenhouse gases. In order to recover such energetic costs, up to four years of continuous operation are required for each silicon-based device. This is a significant fraction of the device lifetime, and given the processing costs of silicon solar cells, this requirement represents a serious obstacle in the widespread implementation of solar cells in homes and businesses. For solar energy to become a more substantial and usable energy source, we need to find alternative solar technologies that reduce device processing costs.

The EFRC:CST focuses on conjugated polymers, which are a highly promising class of materials that are used to make organic photovoltaics (OPVs). OPVs currently receive considerable interest owing to their exciting possibilities to fabricate low-cost, large-area solar panels. The most common system design of polymer-based OPV materials consists of a heterogeneous polymer layer sandwiched between two electrodes, as shown in the schematic here. The polymer layer’s heterogeneity comes from a blend of “donor” and “acceptor” chemical components, which are selected to promote the motion of electrons and “holes” toward opposite electrodes. (A hole is an imaginary particle that represents the lack of charge that remains when an electron moves from its original position.)

ICT
Upon exposure to sunlight, the system absorbs photons (hv), thus generating electron-hole pairs, also known as excitons, at and near the many donor/acceptor interfaces in the polymer layer. Under the right conditions, the excitons dissociate into individual electrons (e-) and holes (h+). This process is referred to as interfacial charge separation. Separated electrons and holes can migrate through the polymer layer toward their respective electrodes (referred to as interfacial charge transfer), and this process produces electrical current.

OPV materials are evaluated both in terms of their light to electric power conversion efficiency and in terms of their external quantum efficiency (the percentage of photons converted to electrical current). However, the light-to-power conversion efficiency of the best OPV materials developed over the last decade is currently limited to ~5%. This is over a factor of two too low to be competitive as a serious technology. The lack of efficiency in these materials is largely attributed to the fact that their molecular-level mechanisms of interfacial charge separation and transfer (CST) processes remain unknown, without a broadly accepted theoretical description. Therefore, the EFRC:CST seeks to unravel the complex interfacial processes that govern efficiency loss mechanisms through integrative theoretical and experimental studies, thus enabling major breakthroughs in OPV development.

EES AND BATTERIES: THE BASICS

Virtually all portable electronic devices, including cell phones, PDAs and laptop computers, rely on chemical energy stored in batteries. Batteries are an example of an electrical energy storage (EES) technology, and the U.S. Department of Energy has stated that major advances in EES technology are vital for developing sustainable energy for the 21st century. In particular, high energy density batteries are needed to fully utilize renewable energy sources (such as solar and wind energy) because they allow for long-term storage of collected energy for use at a later time (such as at night, or when it’s not windy outside).

Lithium ion (Li+) batteries in particular have revolutionized the portable electronics market as they offer high energy density, long shelf life, and a wider temperature of operation over conventional aqueous electrolyte-based battery systems. Li+ batteries are also being intensively pursued for hybrid electric vehicle, plug-in hybrid electric vehicle, and electric vehicle applications.

Li

The EFRC:CST focuses on Li+ battery materials based on nanostructured phospho-olivine (LixMPO4) and lithium metal alloys (LixSi, LixGe). To construct a battery, these materials are assembled in configurations similar to that shown here. Energy is stored in these batteries through the movement of Li+ between the cathode and the anode. Both the anode and cathode are actually complex composites containing not only active materials for Li+ storage, but also other components, namely a polymeric binding agent and conductive carbon filler, which help facilitate Li+ ion and electron transfer processes. We refer to these processes collectively as Li+-coupled charge transfer.

Batteries constructed from these advanced materials offer exceptional promise for higher energy density (Wh/Kg) and power density (W/Kg) applications compared to conventionally used Li+/carbon anodes and layered cobalt oxide cathode materials. Depending on the specific choice of materials for the battery, the voltage, capacity, life, and safety can vary dramatically. Unfortunately, the practical energy density of commercial batteries is approximately 25-50% of their theoretical values because of inefficiencies related to the mass/volume of the inert components (binders and conductive fillers), irreversible energy losses associated with phase and volume changes, and incomplete utilization of the active material in the battery due to poor ionic and electronic communication.

A key requirement for advancing this technology is to disentangle the factors that govern Li+-coupled charge transfer at these complex interfaces and to elucidate degradation processes in the batteries. Therefore, the EFRC:CST seeks to obtain a fundamental understanding of Li+-coupled charge transfer, lithium insertion/extraction mechanisms, and phase formation processes in these emerging EES materials through highly coordinated experimental and theoretical investigations to provide breakthroughs for enhancing the energy density, power density, and long-term stability of advanced EES devices.