Reducing the life cycle environmental impacts of kesterite solar photovoltaics: comparing carbon and molybdenum back contact options
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When fully developed, kesterite photovoltaics will require large quantities of earth minerals including copper, zinc, tin, and sulfur to generate electricity. This leads to questions about which material options can maximize the environmental sustainability of devices. Molybdenum is used as the back contact in kesterite photovoltaic devices, but can cause a detrimental reaction with the absorber layer limiting conversion efficiency. As a result, numerous substitutes or solutions are suggested including carbon-based back contacts. While molybdenum back contacts have been characterized in past environmental assessments, the impacts of graphene and graphite in comparison were unknown. Of paramount interest is the fact that graphene is an emerging nanomaterial with the potential to provide game-changing benefits in a variety of fields; however, the potential for human and environmental health risks to be introduced by new applications remains uncertain.
We apply life cycle assessment (LCA) to the selection of photovoltaic back contacts for emergent solar devices. Specifically, we use TRACI 2.0 to analyze impacts associated with molybdenum, graphite, and graphene back contact alternatives. For data sources, we provide calculated unit processes for graphene and graphite back contacts and utilize open source life cycle databases including the United States Life Cycle Inventory. We explore the sensitivity of the model to assumptions regarding processes and inputs using sensitivity analysis and simulation.
Results and discussion
The results demonstrate that engineering factors, such as the amount of methane used in graphene production, as well as design factors, such as the thickness of potential graphite devices, can determine whether materials substitutions will result in environmental and health gains. Without improvements to graphene production methods, we find that graphene back contacts are associated with more significant health impacts. Graphite back contacts on the other hand are associated with increases in environmental indicators—though these increases are at levels that should not prove problematic in terms of overall impacts of solar photovoltaics.
In conclusion, both graphite and graphene back contacts would provide potential technological improvements, but present additional risks that may need to be considered. Specific attention to graphene chemical vapor deposition improvements as well as efforts to reduce the thickness of graphite back contacts to below 5 μm are necessary to ensure that improved technical efficiency does not jeopardize the social and environmental goals of solar photovoltaics.
KeywordsGraphene Life cycle assessment Nanotechnology Photovoltaics
In this completion of this paper, we acknowledge the work of Cate Fox - Lent and Igor Linkov of the Risk and Decision Science Center of the US Army Corps of Engineers. We also express our appreciation to Dr. Hugh Hillhouse (PI) and Wes Williamson of the University of Washington. This research was supported in part by the National Science Foundation under Grant Number CHE-1230615. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Partial support for this research came from a Eunice Kennedy Shriver National Institute of Child Health and Human Development research infrastructure grant, R24 HD042828, to the Center for Studies in Demography & Ecology at the University of Washington.
- Al-Hosiny NM, Moussa MA (2010) Characterization of carbon nanotubes (CNTs) Polymethyl methacrylate (PMMA) composite films. In: Enabling Science and Nanotechnology (ESciNano), 2010 International Conference, pp 1–2. IEEE, 2010Google Scholar
- Alibaba (2014) Molybdenum sputter target-molybdenum sputter target manufacturers, suppliers and exporters on Alibaba.com Aluminum Sheets. In: Alibaba. http://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=molybdenum+sputter+target
- Arvidsson R, Molander S, Sandén BA (2013) Review of potential environmental and health risks of the nanomaterial graphene. Hum Ecol Risk Assess 19(4):873–887Google Scholar
- Classen M, Althaus H, Blaser S, Scharnhorst W, Tuchschmid M, Jungbluth N, Emmenegger MF (2007) Life cycle inventories of metals. Final report ecoinvent data v2.0:1945Google Scholar
- Cui H, Liu X, Liu F, Hao X, Song N, Yan C (2014) Boosting Cu2ZnSnS4 solar cells efficiency by a thin Ag intermediate layer between absorber and back contact. Appl Phys Lett 104:041115Google Scholar
- Department of Defense (2013) Fiscal year 2012 annual energy management reportGoogle Scholar
- Environment Canada (2013) Assessment of the environmental performance of solar photovoltaic technologiesGoogle Scholar
- Green Delta TC (2013) Openlca 1.3Google Scholar
- Heijungs R, Suh S (2002) The computational structure of life cycle assessment. Vol. 11 SpringerGoogle Scholar
- Hillhouse H (2014) Personal communication, March 10. University of Washington CampusGoogle Scholar
- Johnson AL, Entley WR, Maroulis PJ (2000) Reducing PFC gas emissions from CVD chamber cleaning. Solid State Technol 43(12):103–114Google Scholar
- Kidd R (2011) Army power and energy. http://www.greengov2011.org/presentations/CleanEnergy/GreenGov-2011-CleanEnergy-S5-RichardKidd.pdf
- Landfield Greig A (2008) Life cycle inventory of metallurgical molybdenum products. Four Elements Consulting Inc for the International Molybdenum Association, BrusselsGoogle Scholar
- Meillaud F, Boccard M, Bugnon G, Despeisse M, Hänni S, Haug F-J, Persoz J, Schüttauf J-W, Stuckelberger M, Ballif C (2015) Recent advances and remaining challenges in thin-film silicon photovoltaic technology. Mater Today 18:378–384Google Scholar
- NSF Award Search (2015) Award#1230615—SEP: a sustainable pathway to terawatt-scale solution-processed solar cells from earth abundant elements (2013). In: The National Science Foundation. http://nsf.gov/awardsearch/showAward?AWD_ID=1230615. Accessed 9 April 2015
- Olson DW (2015) Graphite: USGS mineral commodity summaries, January 2015. U.S. Geological SurveyGoogle Scholar
- Pehnt M (2002) Life-cycle Assessment of Fuel Cells in Mobile and Stationary Applications, Ph.D. Dissertation, VDI–Verlag Fortschrittsberichte, Vol. 6 No. 476, DusseldorfGoogle Scholar
- PE International (2015) Process data set: ferro molybdenum; primary production; production mix, at plant; (en). In: Gabi Documentation. http://gabi-documentation-2014.gabi-software.com/xml-data/processes/7fc8d435-42ba-4418-8bd5-8ea4d3b01b42.xml
- Polyak D (2008) Molybdenum. United States Geological Survey (ed) Vol. 2008 minerals yearbookGoogle Scholar
- Polyak D (2014) Molybdenum: USGS Mineral Commodity Summaries, January 2015. U.S. Geological SurveyGoogle Scholar
- Polyak D (2015) Molybdenum: USGS Mineral Commodity Summaries, January 2015. U.S. Geological SurveyGoogle Scholar
- R Core Team (2012) R: a language and environment for statistical computingGoogle Scholar
- Scheer R, Schock H-W (2011) Chalcogenide photovoltaics: physics, technologies, and thin film devices. John Wiley & SonsGoogle Scholar
- Scragg JJ, Kubart T, Wätjen JT, Ericson T, Linnarsson MK, Platzer-Björkman C (2013) Effects of back contact instability on Cu2ZnSnS4 devices and processes. Chem Mater 25:3162–3171Google Scholar
- Shin B, Zhu Y, Bojarczuk N, Chey SJ, Guha S (2012) [High efficiency Cu2ZnSnSe4 solar cells with a TiN diffusion barrier on the molybdenum bottom contact. In: Photovoltaic Specialists Conference (PVSC), 38th IEEE. IEEE, Austin, p 000671–000673Google Scholar
- Todorov TK, Tang J, Bag S, Gunawan O, Gokmen T, Zhu Y, Mitzi DB (2013) Beyond 11% efficiency: characteristics of state-of-the-art Cu2ZnSn(S,Se)4 solar cells. Adv Energy MaterGoogle Scholar
- US Census Bureau (2012) TIGER/Line shapefiles [machine-readable data files]/ prepared by the U.S. Census Bureau (2012)Google Scholar
- US Geological Survey (2013) Mineral commodity summaries. GraphiteGoogle Scholar
- US Life Cycle Inventory Database (2013) National renewable energy laboratory, 2012. Accessed 19 November 2013: https://www.lcacommons.gov/nrel/search
- Zhivov A, Liesen R, Richter S, Jank R, Underwood D, Neth D, Woody A, Bjork C, Duncan S (2012) Net zero building cluster energy systems analysis for US army installations. ASHRAE Trans 118:751–766Google Scholar