Increasingly, the world is facing an energy shortage, even though the amount of sunlight that Earth receives in just one hour is enough to meet the electricity demands of every human being for a year. Clearly, solar panels are vital to harnessing as much of this renewable energy as possible. However, the lifetime of the cells used in solar panels is limited: after 20-25 years, their efficiency drastically wanes. This causes two key challenges: disposing of old solar cells creates electrical waste, and we need solar cells to be as efficient as possible during their working lifespan.
A team of researchers from the University of Coimbra in Portugal, led by Rui Vilão, have been investigating some new candidate materials for use in solar cells. In particular, they used muon spectroscopy to study the use of Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) as possible absorber materials. These are intended for thin film technology, typically just a few nanometres to a few micrometres thick. This is much thinner than the wafers used in conventional silicon-based solar cells, which can be up to 200 times thicker.
By using much less material, and using elements which are much more abundant in the Earth’s crust, it becomes cheaper to obtain the materials and then manufacture the solar cells. Reductions in volume also mean there’s less material to recycle afterwards, further decreasing the environmental impact of solar cell production.
During the production of these solar cells, the components produced need to be kept very clean. However, even the cleanest labs have some trace elements in the air. For example, the most abundant element in the universe is hydrogen, which is so small that it can get into materials, even in labs that would normally be considered completely clean. As these materials require exactly the right chemical composition, this hydrogen can lead to defects that change the material properties.
These defects can negatively impact the cells’ performance and lifespan. Luckily, scientists have a helpful tool they can use: muons.
Using the EMU instrument at ISIS, Rui and the team combined positive muons with electrons to create muonium, which has very similar properties to hydrogen. Crucially, using muonium is a controllable technique for the research team to probe how hydrogen-like atoms affect their solar cells. This shows how hydrogen affects their solar cells, much more effectively than previous methods.
Additionally, Rui explains, muonium is highly sensitive to defects in the materials. He says, “we’re only now becoming aware in the muon community that this sensitivity to defects can be used as a ‘defectometer’.”
This study also acts as preparation to working out how muonium affects materials. Not only did the team successfully characterise hydrogen impurities, which was a completely new results in CZTS, they obtained data to progress understanding of muonium interactions.
This study only looked at the absorber layer of solar cells, but this is just one of many components. There are several other layers in the cell such as the buffer layer, windows, electric contacts, and substrates, and future research will expand to include these.
