In a study investigating the manufacturing process of solar cells, a team led by the group of FNR ATTRACT Fellow Dr Phillip Dale at the University of Luxembourg has discovered that 20-year old assumptions on chemical processes in the production of solar cells are in fact, inaccurate. The findings have just been published in the prestigious journal Nature Communications.
Photovoltaic solar panels convert sunlight into electrical power. The panels absorb the incoming light, which excites electrons sending them off in a predefined direction in order to generate an electric current that can drive motors or light a bulb.
This works through the interaction of several layers of semiconductors and metals in the solar panel. The cells are manufactured in a complex process where several chemical elements are deposited on a glass substrate, typically by evaporation. Thereby, a solar cell “grows”, layer by layer.
A different approach
In the past, scientists discovered by accident that the efficiency of one type of solar cell technology improves vastly if they add sodium to the light absorbing layer. At the same time, they observed that the sodium impacts the 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 from the Physics and Materials Science Research Unit at the University of Luxembourg, along with four internationals partners, now were able to show 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. They found that when the absorber was made of just one grain adding a small amount of sodium helps to homogenize the distribution of the elements.
Possible improvements to manufacturing process
“This is very surprising, because more than 20 years of previous research have consistently shown the opposite effect on absorbers made of many grains,” said Diego Colombara, now Marie Curie Research Fellow at the International Iberian Nanotechnology Laboratory and principal investigator of the study.
The conclusion of the researchers is that sodium has a dual effect: it homogenises the elements inside each grain but it slows down homogenisation in the interplay between grains.
“This gives us the opportunity to rethink how we produce solar cells. In the future, these insights might lead to improvements in the manufacturing process,” concluded FNR ATTRACT Fellow Dr Phillip Dale, the Head of the research group at the Laboratory for Energy Materials (LEM) at the University of Luxembourg.
Publication
‘Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers’, Nature Communications doi:10.1038/s41467-018-03115-0
PRIDE Doctoral Training Unit
Dr Phillip Dale is the coordinator of a new Doctoral Training Unit (DTU) that received funding in the 2017 FNR PRIDE Call. The unit is entitled Photovoltaics: Advanced Concepts for High Efficiency and has been awarded 7 PhD grants by the FNR.
