Flexible Solar Cells Could Release Toxic Metals After Disposal

From backpacks that recharge cellphones to window glazing that generates solar power, there is a growing market for flexible thin-film photovoltaic cells that convert sunlight into electricity. However, a new study shows that certain thin-film solar cells currently on the market could leach hazardous amounts of cadmium after disposal (Environ. Sci. Technol. 2013, DOI: 10.1021/es402969c). In contrast, an emerging technology, organic thin-film solar cells, would not release unsafe levels of metals.

Because they are cheaper, lighter, and more versatile than conventional silicon-wafer solar panels, flexible thin-film solar cells are expected to grow in use, reaching an estimated total power output of 1.6 gigawatts worldwide by 2020. They are made by depositing thin layers of semiconducting materials on a flexible substrate such as plastic or metal foil. Despite recycling efforts, many consumer products made with the cells are likely to be dumped in landfills or waterways, says Markus Lenz of the University of Applied Sciences & Arts Northwestern Switzerland. And solar cells integrated into buildings could be broken up and dumped when those buildings are later demolished.

To assess potential environmental contamination from disposal of the cells, Lenz and his colleagues tested metal leaching from two types of plastic-encapsulated thin-film solar cells: copper indium gallium selenide (CIGS) cells, a technology currently used in consumer products, and organic photovoltaic (OPV) cells, an emerging technology that contains metals including zinc and aluminum.

They first cut the cells into 1.5- by 3-cm pieces to mimic their fragmentation after landfilling or demolition of the material. Then they exposed the fragments to a variety of fluids that the materials might encounter in the environment—including acidic rainwater, seawater, and lake water— for up to four months. They measured metals leaching from the two types of thin-film cells and then applied an environmental model to predict the concentrations of metals that the materials would release. For example, they modeled scenarios such as how much metal 1 kg of material would leach into a cubic meter of sea or lake water.

The CIGS cells leached several metals, including molybdenum, zinc, aluminum, selenium, and cadmium. In particular, the team predicts that damaged CIGS cells would release amounts of cadmium leading to environmental concentrations that exceed World Health Organization safe drinking water limits in all the scenarios they tested, with concentrations more than 50 times higher than the limit set for acidic rainwater in arid climates. In contrast, predicted environmental metal concentrations for OPV cells were well below WHO drinking water limits for all the metals tested.

Vasilis Fthenakis, the founding director of the Center for Life Cycle Analysis at Columbia University, says the study highlights the importance of legislating safe disposal and recycling of photovoltaic cells. However, he says the testing conditions the group applied, including the small size of the cell fragments and the low pH of the test rainwater, may unrealistically overestimate metal leaching from the materials.

Lenz and his colleagues, who are members of SUNFLOWER, a European consortium of academic and industry members developing OPV cells, conclude that the cells offer a less polluting alternative to CIGS cells.

Fthenakis notes, however, that it is also important to consider efficiency when assessing environmental impact. CIGS cells, he says, have the potential to become efficient enough to compete in price with fossil fuels such as coal, which emits hundreds of times more cadmium than the solar cells. In contrast, according to the National Renewable Energy Laboratory, the best OPV cells are about half as efficient.