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Reducing the life cycle environmental impacts of kesterite solar photovoltaics: comparing carbon and molybdenum back contact options

  • LCA FOR MANUFACTURING AND NANOTECHNOLOGY
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Abstract

Purpose

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.

Methods

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.

Conclusions

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.

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Notes

  1. Such price fluctuations adversely impacted the silicon photovoltaics market—providing a lesson for device researchers about the importance of considering materials abundance, market expansion, and price impacts. With the emergence of silicon solar panels in the 2000s, there followed a boom in demand for polysilicon, which caused a rise in spot prices. As the market responded and more producers came online, the spot prices plummeted from more than $475/kg in 2007 to less than $20/kg by 2014—the result of temporary oversupply of highly pure silicon to the market (Ciszek 2014). Such price volatility, though temporary, served as a warning to many producers about the long-run sustainability of developing solar photovoltaics that utilize materials subject to large price fluctuations—and especially use of materials for which increased levels of production are not sustainable.

  2. This is based on a simple calculation, where a 100 cm*100 cm panel 0.0001 cm thick would weigh approximately 10 g given a molybdenum density of 10.2 g/cm. Cost per 100 g based on current prices as of December 2014 available from alibaba.com for molybdenum sputtering targets (Alibaba 2014).

  3. Though by volume more graphite is used in a device, the density of molybdenum is over 4.6 times that of graphite. Thus, for a 5-μm-thick graphite layer as is modeled in this paper compared to a 1-μm-thick molybdenum layer, the mass of material used is very similar.

  4. This primarily refers to the production of molybdenum as the dataset was accessed in an aggregated form with any co-product already allocated by mass. For the primary production of graphene or graphite back contacts, we observe no co-products. We allocated impacts to copper based on mass.

  5. At the beginning of the project, the record device achieved 10.1 % efficiency, and during progress 11.1 % efficiency was achieved (Todorov et al. 2013). Furthermore, 10.5 % was deemed to represent a top-level device.

  6. Though graphene thickness is an assumption, it is also a key driver as to why a graphene device may be more optimal than a graphite device. While a graphene device could be much thicker than what is modeled (7 nm), it is unlikely a graphene device would be used unless it is feasible at such thicknesses due to additional costs and reduced benefits. Thicknesses are based on personal communication and review of relevant scholarly articles, primarily Li et al. 2009; Guo et al. 2010.

  7. We do not run this model for graphite and molybdenum because although improvements to all of the processes are likely, the graphene process is in the earliest stage of development and thus more significant refinements may be attainable during the transition to industrial scale up, as the lab-scale production methods are themselves likely to still improve.

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Acknowledgments

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.

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  1. Daniel J Evans School of Public Policy and Governance, University of Washington, Seattle, WA, 98195, USA

    Ryan P. Scott & Alison C. Cullen

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  1. Ryan P. Scott
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  2. Alison C. Cullen
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Correspondence to Ryan P. Scott.

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Scott, R.P., Cullen, A.C. Reducing the life cycle environmental impacts of kesterite solar photovoltaics: comparing carbon and molybdenum back contact options. Int J Life Cycle Assess 21, 29–43 (2016). https://doi.org/10.1007/s11367-015-0978-4

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