SEMINAR: "Nanostructures for Photovoltaics: A Case Study in Silicon Nanowire Solar Cells, Loucas Tsa
March 9 @ 10:30 AM - 11:30 AM - BRK 1001
ABSTRACT: In the last decade there has been a significant, resurgent interest in renewable energy systems. Solar energy conversion is of particular interest owing to the abundance of the source. Approximately 85% of today’s commercial solar cells are based on crystalline Si (mean module efficiencies of 14–16%), yet they are relatively expensive (~$1.6/W). On the other hand, ~15% of the PV market is based on CdTe thin films, which provide lower cost (<$1/W) though with lower module efficiencies (9–12%). Concurrently, the intersection of solar energy and nanotechnology research has flourished in the last 5 or so years. The above discussion highlights three key questions facing the solar energy field: 1) how can the efficiency of solar cells be increased to competitive levels with other energy sources? 2) how can the cost of solar cells be decreased to a level suitable first for secondary and ultimately for primary power generation? 3) how can both of these goals be achieved in a single solar cell device and related manufacturing process? These questions lead to yet another question that is the central theme of this talk: can nanotechnology be used to address either of the above three questions from an industrial perspective, and if so, how?
The presentation will explore the recent literature in the application of various classes of nanostructures to photovoltaics. These are classified as: (a) nanocomposites & nanostructured materials, (b) quantum wells, (c) nanowires & nanotubes, (d) nanoparticles & quantum dots. Both the potential advantages of each nanostructure approach, as well as the disadvantages will be discussed from an industrial perspective, with an emphasis on possible future areas of research interest.
A specific example from our own work on Si nanowire solar cells will also be described [1, 2]. This includes efforts in engineering such structures by control of key CVD process parameters, and their impact on critical array features for PV device performance. Array features such as mean diameter, length, dopant density, and alignment, and the impact of process temperature, precursor gas chemistry, and flow rates on these features will be discussed. Of particular interest is the growth of Si NWs on metal substrates, which enable back contacts to the nanowire arrays and the potential for flexible solar cells. Nucleation and growth of such nanostructures using barrier layers on stainless steel will be provided. The presentation will also highlight research in depositing conformal amorphous silicon and silicon oxide layers on Si NW arrays using plasma-enhanced chemical vapor deposition (PECVD), showing the impact of pressure, temperature, and array density on the thin film thickness uniformity and the mechanisms associated with the observed trends.
Various generic scientific challenges facing the use of nanostructures in PV, e.g. charge transport phenomena, surface recombination, etc. will also be discussed. The talk will conclude with a summary of the future prospects of using nanostructures in PV.
 L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, “Silicon nanowire solar cells,” Applied Physics Letters 91, 233117 (2007). [also published in Virtual Journal of Nanoscale Science & Technology]
 L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, “Silicon nanowire solar cells: device physics, fabrication, and optoelectronic properties,” Proc. 23rd European Photovoltaic Solar Energy Conversion Conference, 1AP.1.3 (2008).
BIO: Dr. Loucas Tsakalakos is a Senior Scientist and Project Leader at the General Electric – Global Research Center in Niskayuna, New York. He received his BS degree (1995) from Rutgers University, and his MS (1998) and PhD (2000) degrees in Materials Science & Engineering (with minors in Solid State Physics and Microelectromechanical Systems) from the University of California, Berkeley. His expertise is in the integration of heterogeneous thin film and nanostructured materials systems for micro and nano device applications, and also has extensive experience in the characterization of materials. Since joining GE Global Research in 2000, Dr. Tsakalakos has designed and implemented integrated electronic and sensor systems for defense applications, studied cathode materials for lighting applications, and is a founding team member of GE’s Nanotechnology Program. He has more than 9 years experience in program management, including as a Principal Investigator for NIST ATP, DARPA, and DOE-funded programs. His current research is focused on development of nanostructured materials and devices, primarily using nanowires/tubes, working with multi-disciplinary teams both within GE and in collaboration with external partners. He is currently leading the advanced/next-generation PV efforts within GE Global Research’s Solar Energy Platform, which includes research to apply nanostructures to photovoltaics. Dr. Tsakalakos is a member of Tau Beta Pi (the National Engineering Honor Society), is author or co-author of over thirty journals, conference proceedings, and book chapter publications, and holds five US patents. He sits on the program committees of several international conferences in the areas of nanotechnology and clean/solar technologies.
- Deborah Starewich