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

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.

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.