Research Highlights / Full Story

to Advance Renewable Energy

Every hour the sun radiates more energy onto the Earth's surface than is consumed globally in one year. To harness the power of solar energy, improvements in the efficiency of photovoltaics and electrical storage are required to reduce variability and intermittency of solar power. RIT's NanoPower Research Laboratories (NPRL) are making significant advances in the development of new materials and devices utilizing nanomaterials and nanotechnology for energy conversion, energy storage, and power systems development.

Dr. Ryne Raffaelle founded the NPRL in 2001 and has developed the laboratory into a center that spans over 7,000 square feet across the IT Collaboratory, College of Science, and Center for Integrated Manufacturing Studies buildings with 15 faculty and staff members, along with 23 students. The labs are equipped with a wet chemistry synthesis facility; a photovoltaic characterization facility; a thermal, spectroscopic, and microscopic characterization facility; battery testing lab; and a laser lab. The NPRL is internationally recognized for its capabilities in synthesis and characterization of nanomaterials. Recently, the U.S. Department of Energy named Raffaelle director of the National Center for Photovoltaics.

"NPRL's expertise is unique in that not only do they focus on fundamental material research, but also advanced device research. From developing advanced photovoltaics to enhancing battery technology that better support these renewable energies and synthesizing new materials that will enable the advancement of these new technologies—this is the breadth of the expertise of the NPRL," says Raffaelle.

Improving Photovoltaic Efficiency

With the national emphasis toward renewable energy, chances are you have a solar-powered device in your home—whether it is a light, calculator, or solar panels to help power your home. However, in order to support large-scale energy demands with solar power, significant advancements in photovoltaic (or solar cell) technologies are required. The first generation of solar cells found in most devices available today uses silicon. While silicon is highly abundant and these types of solar cells can be easily manufactured, the best power conversion efficiency is only 24%. The NPRL is focused on developing semiconducting photovoltaics using both III-V materials and polymers, with the bottom line goal to increase efficiency while decreasing the manufacturing costs associated with cell fabrication. Increased efficiency and reduced cost lead directly to cheaper electricity (cost per watt).

III-V materials are naturally better suited for photovoltaic applications because they are better absorbers of light than silicon. Another advantage is the versatility and availability of different III-V materials, leading to the ability to stack multiple absorber layers. This approach provides a better match between the III-V absorbers and the solar spectrum, reducing the amount of sunlight lost to heat, which in turn increases the efficiency of the cell. Standard commercial silicon solar cells available today are approximately 15-20% efficient—that is to say, only 15-20% of the sunlight is actually converted into electricity and the other 80-85% is lost to heat.

"With III-V materials we will be able to harness different parts of the solar spectrum, increasing the overall efficiency of the solar cell," says Dr. Seth Hubbard, assistant professor of physics and microsystems engineering.

Additionally, the incorporation of nanostructures into III-V materials seeks to develop new paradigms for photovoltaic conversion. Nanostructured photovoltaics make use of electrical and optical properties of nanomaterials that can be controlled by changing the particle size at the atomic level. The main challenge of using nanostructures with III-V photovoltaic cells is the actual incorporation of the nanostructures into the solar cells without disrupting the fundamental structure of the cell.

Many questions need to be answered—What's the best way to insert the nanostructures? What size is needed? What is the best location? What type of cell designs are necessary? While research is underway to address these long-term considerations to III-V nanostructured photovoltaics, the lab has shown shortterm progress through the use of quantum dots. Quantum dots can be used to tune a solar cell and allow the absorption of the cell to be controlled. The NPRL is one of the few research groups in the world that have been able to show enhanced efficiency in solar cells through the use of quantum dots. By incorporating InAs and GaSb quantum dots into a standard GaAs solar cell, the research team has been able to show improvements in the current density.

"Our research predicts with the integration of quantum dots efficiency could reach levels as high as 47%," says Hubbard.

Another approach to increase efficiency and reduce cost per watt is through the use of high concentration photovoltaic solar cells. Instead of covering a large area—like the roof of a home—with a traditional flat-plate system, sunlight is focused down using parabolic mirrors or lenses. Similar to what happens when you put a magnifying glass over a piece of paper in the sun and the paper burns, the sunlight is intensified from 500 or 1,000 watts per meter2 at normal intensity to tens or hundreds of thousands watts per meter2. However, in concentrator photovoltaics, this intense light is converted to electricity. This approach allows engineers to replace III-V materials with less expensive materials, thus making the system more cost effective. In addition, while an increase in temperature reduces efficiency, the added efficiency from concentration far outweighs the decreased efficiency caused from the heat. Researchers at the NPRL are focused on improving both the efficiency and reliability of these concentrator cells and the integration of these cells into system-level designs.

A cornerstone of the NPRL is the lab's unmatched solar cell testing capabilities. The laboratory follows a strict standard for solar simulation and specification so results can be compared universally. A standard is especially critical when qualifying devices for space-based applications. The NPRL has built a Class A Close Match Solar Simulator that allows researchers to test how cells will perform under both terrestrial conditions at different locations (by adjusting the spectrum based on the location) and also extraterrestrial conditions, similar to the conditions that solar panels on our communications satellites encounter.

The fundamental theory of nanostrucuted III-V photovoltaics suggests a significant increase in solar cell efficiency. "However, many questions remain to be answered and a lot of it comes down to the materials," says Hubbard. "We are looking wherever we can to find a way to break the current limits of photovoltaics using nanotechnology." Generating electricity is half the challenge; then it becomes a question of creating storage solutions that can support mass power generation.

The Power in the Black Powder

Carbon nanotubes have shown tremendous potential for power generation and storage devices due to their remarkable properties. Said to be one of the most conductive and strongest materials yet discovered, the synthesis, characterization, and application of carbon nanotubes have been one of the core research focuses of the NPRL since its founding.

"Like diamonds, single-walled carbon nanotubes (SWCNTs) are another allotrope of carbon. In the laboratory we are modifying the conditions to increase the quantity of SWCNTs we produce, along with the purity of the material," says Dr. Brian Landi, assistant professor of chemical engineering.

One of the main challenges of SWCNTs is the ability to verify quality and consistency of the material. Recent characterization studies conducted at NPRL have provided a better understanding of the fundamental properties, helping researchers to better control material properties.

Using optical spectroscopy, the lab has developed a patent-pending technique that measures the purity of the material. The material is dispersed in a solvent and using spectroscopy, absorption peaks are measured to determine the purity of the sample. Using a simple calibration curve from the peak height intensity, NPRL is able to standardize the purity of its SWCNT materials and assess the purity of the materials provided by commercial vendors.

Extensive work in the lab is being done to use carbon nanotube (CNT) wires to replace ordinary copper wiring in spacecraft applications, such as communication systems. CNT wires are much stronger, more flexible and durable, and lighter, which provides significant weight savings advantages for space-based applications. As the accessibility of copper diminishes, CNT wires have the ability to replace traditional wiring in a wide variety of everyday applications. Currently, conductivity of CNT wires is about equal to copper. The lab is working to improve conductivity by treating the CNT wires through exposure to acids, bases, and ionic solutions.

It is clear carbon nanotubes can provide a powerful solution for renewable energy technologies; however, continued characterization studies are required to be able to develop the material with the consistency and dependability carbon nanotube-based devices demand.

An Energy Storage Solution

Because of their unique electrochemical and mechanical properties, one of the devices carbon nanotubes are ideal for are lithium ion batteries. Lithium ion batteries, like the one found in your cellphone, provide superior energy density over any other battery available to date. By nature, lithium is the lightest metal and therefore batteries using lithium have the highest specific capacity. "This is the very reason consumer electronics have been able to make significant advances in the last decade," explains Landi.

With the demand for better energy storage, the NPRL is working to enhance the lithium ion battery by replacing the graphite materials used as the anode with carbon nanotube papers. Carbon nanotubes offer many distinct advantages over the graphite material. Carbon nanotubes are not only conductive electrically but absorb lithium and because it is lighter in nature, the capacity is immediately increased threefold. Their strength and flexibility also make it attractive to prevent cracking during operation or in vibration environments.

"We are seeing that higher purity materials have higher capacity. In addition, when we change the electrolytes in the battery, it affects how it interacts with the nanotubes and that also changes the capacity," says Landi.

These results demonstrate not only the importance of understanding how the material properties affect the device performance, but also how all the constraints of the device—assembly, electrolytes, etc.—affect the material. For this reason, NPRL is focused on not only being an expert on materials, but also the devices and the testing of these devices.

A Research and Development Center for Industry and Government

The laboratory provides a research and development backbone for industrial collaborators, including BP Solar, Ohmcraft, Lockheed Martin, Northrop Grumman, Boeing Spectrolab, Greatbatch, ITT, Emcore Photovoltaics, Reflexsite, and Alpha V (an RIT startup company), along with governmental agencies such as NASA and the Air Force Research Laboratory. The support of local, state, and federal governments has also made the laboratory's research efforts possible, helping to advance renewable energy technologies that are critical to addressing the global energy crisis.

"Renewable energy technologies, like the ones under development at NPRL, present great promise to addressing the global energy crisis. However, legislation is key. For people like us to want to have renewable energy sources powering our homes, legislation has to pave the way," says Raffaelle. "The support of our congressmen Chris Lee and Steve Israel, as well as former congressmen Jim Walsh, Randy Kuhl, and Tom Reynolds, has been integral to the success of our lab. Efforts like this are what will help to make our research become reality for you and me."