10 April 2018

Luxembourg researchers show that sodium enhances indium-gallium interdiffusion in CIGS solar cell production

© Semiconductor Today Magazine / Juno PublishiPicture: Disco’s DAL7440 KABRA laser saw.

Research led by the University of Luxembourg has investigated solar cell manufacturing processes and proved that assumptions about chemical processes that have been commonplace for the past 20 years are, in fact, inaccurate (Colombara et al, Nature Communications vol9 (2018) article no. 826; DOI: 10.1038/s41467-018-03115-0).

In the past, scientists discovered by accident that the efficiency of copper indium gallium diselenide (CIGS) solar cell technology improves vastly if sodium is added to the light-absorbing layer. At the same time, they observed that the sodium impacts growth of this layer and the interaction of the other chemical elements, namely it inhibits the mixing of gallium and indium. This leads to less homogenous layers and thus impairs the results. Therefore, in the past, scientists and manufacturers believed that the ideal way to produce a solar cell was to only add the sodium after the growth process was concluded.

By using a different approach, researchers at the University of Luxembourg’s Physics and Materials Science Research Unit, along with four internationals partners, have shown that the truth is more nuanced. While conventionally the light-absorbing layer is made up of thousands of individual grains, the research group chose a more demanding fabrication strategy and grew the layer as a single grain. “If the absorber is made of only one grain, adding a small amount of sodium helps to homogenize the distribution of the elements,” says principal investigator Diego Colombara, now Marie Curie Research Fellow at the International Iberian Nanotechnology Laboratory. “More than 20 years of previous research have consistently shown the opposite effect on absorbers made of many grains,” he adds.

Picture: Microscope image of solar cell absorber made of one grain (black and white) and corresponding chemical analysis showing the concentration of gallium (orange) and indium (purple).

The conclusion is that sodium has a dual effect: it homogenizes the elements inside each grain but it slows down homogenization in the interplay between grains. “This gives us the opportunity to rethink how we produce solar cells,” concludes Dr Phillip Dale, head of the research group at the University of Luxembourg’s Laboratory for Energy Materials (LEM) and an Attract fellow of the Luxembourg National Research Fund (FNR). “In the future, these insights might lead to improvements in the manufacturing process.”

Tags: PV

Visit: www.nature.com/articles/s41467-018-03115-0

Visit: https://wwwen.uni.lu/research/fstc/physics