Colorado Renewable Energy Collaboratory Partners for Clean Energy Center for Revolutionary Solar Photoconversion updateSummer 2010 CRSP Research Profile Plasma Sheds Light on Mysteries of PV Efficiency Stars are made of plasma, an ionized gas comprising a complex mixture of gas-phase species. So it’s remarkable that CRSP researchers are using plasmas to create photovoltaic (PV) devices that can better convert energy from our own star, the sun, into power we can use here on Earth. A CRSP research team, made up of re- searchers from CSU and NREL, has been using plasmas to modify PV materials and improve the interfaces between the layers of materials in thin-film solar cells. The goal is to increase efficiency in PV devices. Ellen Fisher is an analytical/materials The CRSP plasma processing project team Ina Martin (left) and Ellen Fisher are shown chemist and the project’s principle investiga- includes Ina Martin, an analytical chemist at in the laboratory with a low-pressure rf plasma reactor. The two chemists work to- tor at CSU. “We know we can use plasmas NREL. Her role is to extend the character- gether on a CRSP project that uses plasmas to change materials and get different device ization of the modified materials and evalu- results, but we need to know exactly how it to modify PV materials and improve the ate the resulting devices under real-world interfaces between the layers of materials in works,” she says. conditions. The rest of the team includes thin-film solar cells. Credit: Jeff Shearer. The team is developing new materials for Michael Elliott, a CSU electrochemist with solar cells by taking known materials, such a background in PV device testing; Patrick McCurdy, a CSU staff scientist who special- chemistry, applying these results to as titanium dioxide (TiO2), and improv- ing their properties. Such improvements izes in surface science and who helps ana- PV devices, then using the device perfor- include applying plasmas to modify material lyze data; and CSU graduate student Dan mance to determine what additional modifica- surfaces to reduce “electron traps.” So Pulsipher, who performs research in the lab. tions need to be made to the materials.” when photons strike and free up electrons CRSP has funded the plasma processing To learn more about the project, visit to make electricity, more electrons can flow project through mid-2010, but the project will www.chm.colostate.edu/erf/photovoltaics.htm through the material. The more free-flowing need additional funding to continue. Fisher electrons there are, the more electricity is seeking additional sources of funding, in- is produced relative to the amount of sun cluding from the U.S. Department of Energy The Center for Revolutionary exposure. (DOE), and hopes any further work could Photoconversion (CRSP) For example, a promising PV technology is be a large component of a new graduate student’s thesis. is dedicated to the basic and applied the TiO2-based, dye-sensitized solar cell. research necessary to create revolutionary Such cells can now be produced on a vari- “The entire world needs to replace fossil fuel new solar energy technologies as well as ety of inexpensive substrates, making them energy sources with clean renewable en- educational and training opportunities. ideal for building-integrated PV applications. ergy,” she says. “This project is so interesting The research underpins renewable energy Each dye cell consists of a photoanode, because ultimately, it’s a part of a worldwide technologies, commonly called third- such as TiO2, a dye sensitizer, an electro- effort and everyone working on PV has dif- generation solar photon conversion, for lyte, and a photocathode. These cells cur- ferent approaches. Collectively, we’re bound the highly efficient and cost-competitive rently have an efficiency of 11%; solid-state to find solutions to this very real problem.” production of both electricity and fuels dye cells have an efficiency of about 5%. via direct solar processes. The CRSP Martin concurs, adding, “So much of PV re- The CRSP team has learned that by using participating research institutions are the search is a materials science problem. As a a plasma process on the TiO surface, they National Renewable Energy Laboratory 2 scientist, I also find it satisfying to participate may be able to modify the dye/TiO interface (NREL), University of Colorado at Boulder 2 in the process of improving materials prop- to make it amenable to new dyes, and to (CU), Colorado State University (CSU), boost the overall cell performance. erties through understanding the gas-phase and Colorado School of Mines (CSM). 1 CRSP Research Profile cells and carefully verified that the SHG measurement was truly an interface signal. Postdoc Research a Critical Lasers Reveal These results will be the basis of new exper- Success Factor for CRSP iments that will probe how interfaces change The three research profiles featured Nature of Interface under illumination by sunlight, which, in turn, in this issue of CRSP Update have a could lead to breakthroughs in efficiency and common thread: each one benefits from Surfaces, Processes lower the costs of PV devices. research contributions by postdoctoral This interface research, like the plasma candidates. Postdocs are able to As PV materials become thinner and rely process research, has been funded through dedicate themselves to specific research on complex material systems, a critical chal- mid-2010. In the future, Teplin hopes to assignments, making them an important lenge is understanding how the interfaces obtain funding to apply SHG techniques to part of scientific research in general and between semiconductors work. While one organic solar cells, which are made of very for CRSP in particular. CRSP team is using plasma to explore inexpensive polymers and show promise This makes for a two-way street of the these interfaces (see page 1), another is us- to dramatically reduce the cost of photo- best kind. The research laboratories ing an advanced optical technique to unlock voltaics. These devices are almost entirely benefit from a tailored, focused research the secrets of interface physics. controlled by interfaces that separate power- effort, and the postdocs—who earn their Charles “Chaz” Teplin of NREL and Charles generating charges that are excited by coveted positions because they are the Rogers of CU are both physicists leading a sunlight. This separation frees the electrons most talented among their peers—have research team that is using lasers to study to produce electricity, but the process is an opportunity to work with the world’s how electrons and electron holes move from poorly understood and SHG experiments leading scientists while advancing their one material to another in semiconducting may provide crucial new insights that would own careers. interfaces called heterojunctions. be helpful to device engineers. Postdocs have strengthened the linkages When light is shined on a surface, a small Teplin and Rogers work with Howard Branz, among CRSP research members. Ina portion is reflected at twice the original an NREL principal scientist. Long He, a CU Martin completed her degree at CSU, joining NREL as a postdoc working in frequency. With the help of modern, pulsed graduate student, conducts the actual labo- thin crystalline-silicon solar cells. She lasers, it is possible to collect this “second ratory experiments. Matt Page, a scientist has now moved on to Case Western as harmonic generation” or SHG light. The with NREL’s heterojunction research team, an associate professor and is setting up intensity of the SHG light arises only as provides the samples for study. Together, the a renewable energy laboratory there. a result of the interface properties of the team hopes to develop a tool that is useful Andrew Herring was an NREL postdoc materials under illumination. Thus, SHG for every PV technology. in electrochemical reduction of carbon experiments can monitor interesting inter- “The combination of fundamental and ap- dioxide before joining the faculty at CSM. face processes. This interface sensitivity is plied work really appeals to me,” says Teplin. Charles Teplin completed his degree unusual; most PV measurement techniques “As a scientist, I want to understand how with Chuck Rogers at CU and joined sense mainly bulk properties and yield little the devices work, but it’s wonderful to be NREL as a postdoc looking at cladding information about interfaces. working on something that will help new PV layers for glass. Today, he is a senior During the course of the research, the team technologies make it to market.” scientist at NREL and collaborator in a performed SHG experiments on silicon solar 2009 CRSP project with Rogers. CRSP Member Companies Abengoa Solar Applied Materials Ascent Solar Technologies DuPont Evident Technologies G24 Innovations General Motors Konarka Lockheed Martin Motech Industries Sharp Tokyo Electron Total Toyota To inquire about CRSP membership, send e-mail to: CRSPinfo@ Long He, a graduate student at CU, uses lasers and measures their second harmonic generation coloradocollaboratory.org light to study how electrons and electron holes move from one material to another in semicon- ducting interfaces called heterojunctions. Credit: Hsiangsheng Ku. 2 CRSP Research Profile plines to bear including theoretical modeling, computational analysis, and surface char- CRSP News & Events Harvesting acterization. The principal investigators are John Turner of NREL and Andrew Herring, New CRSP Leadership Team—Early Visible Light for Mark Lusk, and Thomas Furtak of CSM. in 2010, CRSP welcomed some A key member of the research team is S. new members to its leadership Solar Hydrogen Kishore Pilli, a postdoc at CSM, who may team. Craig Taylor of CSM replaced have had the toughest assignment. “His job Arthur Nozik of NREL as the CRSP Production was to figure out if the POM absorbs vis- scientific director. John Benner and ible light, and if not, work within the design Kaitlyn VanSant, both of NREL, came Using hydrogen as a fuel confers a host of space to make it absorb light,” says Andrew onboard as CRSP managing director benefits.