Organic Ferroelectric Photovoltaics

Jinsong Huang, Stephen Ducharme, and Alexei Gruverman
Nebraska MRSEC

Photovoltaics is a method of converting solar radiation into electricity.  Some semiconducting materials exhibit a property known as the photoelectric effect that causes them to absorb light and release electrons. In addition to the semiconductors, ferroelectric materials have been employed to create ferroelectric-photovoltaic devices. In these devices, a ferroelectric thin film is used as a light absorbing layer and the electric field created by ferroelectric polarization is the driving force for the photocurrent. However the efficiency of the photovoltaic effect in these devices is low due to the low solar light absorption efficiency, low electric conductivity, and short lifetime of photoinduced charge carriers. MRSEC researchers at the University of Nebraska come up with a method for overriding these constraints by combining the strong polarization electric field from a ferroelectric material with the strong absorption and conduction capabilities of polymer semiconductors (Nature Materials, 10, 296 (2011)). With rationale of their device design, the efficiency of organic ferroelectric photovoltaic can be higher than that of both organic photovoltaic and ferroelectric photovoltaic. The proposed device also shows unique functions such as switchable polarity and efficiency. This discovery may lead to a novel approach to photovoltaics resulting in the enhanced efficiency of converting solar radiation into electricity.

The figure illustrates how the introduced electric field by the ferroelectric polymer layer facilitates the charge separation and extraction in polymer solar cell.

Theoretical and Experimental Characterization of Structures of MnAu Nanoclusters

J. M. Xiao Cheng Zeng, Jeffrey E. Shield, David J. Sellmyer
Nebraska MRSEC

He Kai
University of Maryland

Highly-symmetrized MnAu nanoalloys may possess high magnetic moments for potential application. The magnetic properties of MnAu nanoclusters exhibit strong dependence on the cluster sizes and morphologies. Determining the most stable morphologies as well as their spin-polarization patterns is important for their further application.  Researchers at University of Nebraska MRSEC performed a joint theoretical/experimental study to investigate relative stabilities of various highly-symmetrized morphologies of MnAu clusters with different sizes (ACS NANO 5, 9966 9 (2011)). Based on an extensive search, they found that the antiferromagnetic spin configurations are the most stable for all the morphologies investigated. As the size increases, the most stable morphology of MnAu nanocluster evolves from a core-shell structure to L1₀ structure, and starting from 1.7 nm size the L1₀ structures become more stable, confirmed by the high-resolution transmission electron microscopy experiment. Both the antiferromagnetic and ferromagnetic states for the L1₀ MnAu nanoclusters larger than 2 nm are likely to be energetically stable at room temperature. Because their closeness in cohesive energies, there is possible existence of both antiferromagnetic and ferromagnetic states for L1₀ MnAu nanoclusters in different sizes, and this mixed system can be useful for applications such as exchange bias.

Organic Molecular Layers for Efficient Charge Injection

Peter A. Dowben, Axel Enders
Nebraska MRSEC

Luis Rosa
University of Puerto Rico - Humacao

Julian Velev
University of Puerto Rico - Río Piedras

High conductivity and efficient charge injection into organic layers could lead to the design of more efficient organic solar cells and molecular electronics, especially light emitting diodes. Most organic materials are however insulators and only few exhibit high conducting properties. Nebraska MRSEC researchers in collaboration with their colleagues at University of Puerto Rico have discovered that zwitterion molecules of the p-benzoquinonemonoimine type are different from being a standard insulator. Using state of the art electron spectroscopy techniques and sophisticated first-principles calculations the researchers explored the electronic band structure of these organic molecular layers. They found a compelling evidence of the electronic pockets at the Fermi energy in these molecules thus indicating a possibility of charge injection. This discovery broadens the class of organic compounds that may be used for efficient charge injection and hence for the design of novel organic solar cells and light emitting diodes.

Electric Field Control of Magnetization

Abhijit Mardana, Stephen Ducharme, and Shireen Adenwalla
Nebraska MRSEC

To change the magnetization of a ferromagnet usually requires a magnetic field.  So, for example, if we put a compass needle into the high field of an MRI machine, we can no longer trust it to swivel to the North.  Similarly, the magnetic stripes on credit cards and key cards can be destroyed in high magnetic fields.  Electric fields don’t have the same effect on magnetic materials, which is just as well for everyday applications. However there are specific applications in which the ability to use an electric rather than a magnetic field to produce changes in the magnetization offer distinct advantages. This is because producing tightly focused electric fields is much easier than producing a tightly focused magnetic field. In high density magnetic memories, when we want to switch only one bit (and not all its neighbors), making sure the magnetic field doesn’t spill over requires very careful design.   
How can we design materials in which electric fields can change the magnetization? Nebraska MRSEC researchers have demonstrated that this may be achieved by combining a ferroelectric, a material with a permanent electric polarization that produces a large electric field, with a ferromagnet.  Using a very thin cobalt film (only a few atoms thick) overlaid with a soft, plastic ferroelectric the researchers altered the direction of the magnetization by switching the polarization of the ferroelectric. Unlike magnetization changes that are achieved using magnetic field, this change in direction is irreversible – switching the electric field back doesn’t change the magnetization. This discovery may lead to electrically controlled magnetic data storage promising higher densities than those available today. 

Enhanced Ferroelectric Stability by Interface Engineering

A. Gruverman and E. Y. Tsymbal
Nebraska MRSEC

C.-B. Eom
University of Wisconsin

 X. Pan
University of Michigan

Ferroelectric materials are characterized by a spontaneous polarization that can be switched by external electric field. This property is important for various technological applications such ferroelectric random access memories.  However, when ferroelectric film thickness is reduced down to a nanoscale the ferroelectric polarization may become unstable due to strong depolarization fields and interface effects. Nebraska MRSEC researchers in collaboration with their colleagues at Universities of Wisconsin and Michigan have predicted and demonstrated that interface engineering may be efficiently used to stabilize ferroelectric polarization at the nanoscale. Using sophisticated first-principles calculations and advanced fabrication and characterization techniques the researchers showed that deposition of a two-unit cell thick non-ferroelectric strontium titanate layer at the interface enhances the polarization of ferroelectric barium titanate layer, making it switchable by an applied electric filed. This discovery provides a new insight into the switching and retention behavior of thin-film ferroelectric materials and paves the way to atomic scale property engineering in ferroelectric-based electronic devices. This work was partly supported by the Nanoelectronics Research Initiative of Semiconductor Research Corporation.   

Picture: Experimental observation of enhanced polarization of a barium titanate thin film. Left images have the same contrast indicating that polarization of barium titanate is not switched by electric field. Right images have different contrast indicating that the interface engineered film can be switched by an electric field.  .

Nebraska MSREC WoPHY11 Conference

Verona Skomski and Jeffrey Shield
Nebraska MRSEC

The University of Nebraska held its third Women in Physics Conference October 20-22, 2011, organized by MRSEC faculty member Axel Enders. The theme of this year’s conference was “Materials Girls.” The conference highlighted progress in Materials Science done by undergraduate women in physics, chemistry, and engineering at colleges and universities. Over 75 undergraduate students from 24 states participated in this year’s conference and most of the students presented either invited oral talks or posters.

Highlights of the WoPHY11 Conference included:

• plenary talks by accomplished women scientists from around the country
• invited presentations by undergraduate students upon nomination by their faculty mentor
• career workshop where women faculty shared their experiences with the participants
• poster session, conference banquets, and social events for the students to discuss science and connect with each other
• lab tours at the Physics Department of the University of Nebraska-Lincoln

Nebraska MRSEC Professor/Student Pairs Program

Jeffrey Shield
Nebraska MRSEC

The Nebraska MRSEC Professor/Student Pairs Program brings in professor/student pairs from non-research intensive four-year institutions to conduct research with Nebraska MRSEC scientists.  The goal is to provide a research experience which benefits both the participants and the MRSEC projects.  For the professor, this program provides an opportunity to conduct new research, access to facilities typically unavailable at their home institution, and make strong and lasting connections with MRSEC researchers.  For the student, this program provides an opportunity to conduct world-class research and exposure to new and different expertise and capabilities. For the MRSEC scientist, this program provides collaborative and mentoring opportunities.  Ideally, the professor/student pairs continue to expand on MRSEC-related research at their home institutions, thus further building their relationship.
In summer 2011, within this program, Professor Khalid Eid and undergraduate Taylor Reid from Miami University (Oxford, OH) worked with Nebraska MRSEC researcher, Andrei Sokolov, to characterize the interface between a ferromagnetic semiconductor and different ferromagnetic metals. This research may shed light into the understanding of the magnetic and electronic properties of these interfaces which are important for creating novel electronic devices with enhanced functionalities

Picture: Prof. Khalid Eid (back left) and Student Taylor Reid (right) from Miami University (Oxford, OH).