The field of solar cells has and continues to develop extremely fast. The industry has moved from being niche to a major energy source. The efficiencies of industrial modules, as well as the records in the lab improve every year.

“17 years ago, tandem cells were considered too expensive and not industrially viable, now they are predicted the next big step in industry.”

Over the last 25 years, Susanne Siebentritt, Professor in Physics at the University of Luxembourg since 2007, has used optical methods to study solar cells.

“I have moved from the basic studies of material properties to the analysis of complete devices. Our methods now help predict the efficiency of solar cells before we finish all production steps. Our goal is to contribute to making thin film solar cells better, because we are convinced that they have to play a crucial role in mitigating the climate crisis. ”

Susanne Siebentritt Full Professor in Physics & Head of Laboratory for Photovoltaics at the University of Luxembourg

Benefits of tandem approach to solar cells

Siebentritt’s research interest is twofold: the development of new thin film solar cells and the semiconductor physics of the materials used in these cells.

With her team, Siebentritt works on tandem solar cells. A tandem solar cell consists of two layers of semiconductors that each absorb different wavelengths of light and convert this energy into electricity.

In a typical thin-film tandem solar cell, these semiconductors are stacked on top of each other. Layering them in this way yields two solar cells that, together, produce a tandem effect. Much like having two cyclists on a tandem bike combine their power, layering solar cells increases their efficiency for converting sunlight to electricity.

These combined solar cells can namely exploit a broader spectrum of light wavelengths.

Among the team’s highlights are making a detailed analysis of the electronic defects in chalcopyrites, the material they use in their solar cells. The team also explained how certain types of disorder reduce the voltage in solar cells and how to reduce this disorder.  Additionally, a new passivated back contact that solves a major problem in these solar cells has been developed, as well as the first transparent wide gap chalcopyrite solar cells that can be used in tandem solar cells.

From collaborations to informing photovoltaics policies

Since Luxembourg has very little photovoltaic industry, Siebentritt is very involved with industry across Europe, but also had one project with Luxembourgish industry on detectors, which are in a way similar to solar cells. Siebentritt also gets occasionally questions from members of Luxembourg parliament on photovoltaics in general.

Siebentritt collaborates closely with researchers at the Luxembourg Institute of Science and Technology (LIST), who provide special analysis for her solar cells.

“My FNR projects always include some subcontracting with LIST – which has established a very fruitful cooperation. And all of our PRIDE projects were together with LIST – and I enjoy all of them, in particular for the cooperation. The main part of the actual research in the lab is done by my PhD students and postdoc. They enable what I do. I learn from the good ones. While in Luxembourg I have been training as the main supervisor 23 PhD students and 20 postdocs. Some of my PhD students became my postdoc afterwards. Many of them work now in photovoltaics or semiconductor industry. Some have positions in research institutions, some have become professors. ”

Susanne Siebentritt Full Professor in Physics & Head of Laboratory for Photovoltaics at the University of Luxembourg

Siebentritt was also recognised with an FNR Award for Outstanding Mentor, as well as an FNR Award for Outstanding Scientific Publication (together with co-authors). She has also been heavily involved in several PRIDE programmes – FNR-supported Doctoral Training Units.

Siebentritt for example coordinated the PRIDE Doctoral Training Unit (DTU) MASSENA, together with Emmanuel Defay from the Luxembourg Institute of Science and Technology (LIST), which involved training 22 PhD candidates on the topic of materials for sensing and energy harvesting.

Key questions for the future

“There are currently two options for the top cell in tandem cells: perovskites or chalcopyrites. Perovskites have very good efficiencies but issues with stability, whereas chalcopyrites are proven stable, but so far lack the efficiency. The race is open, I decided to work on improving the efficiency of chalcopyrites.”

“The technology will clearly move towards tandem cells. My dream is to finish an idea, that we first realised in 2002: a tandem solar cell, where both cells are chalcopyrite. And the dream is to show efficiencies better than the best single junction cells. This would represent a cell that’s higher in efficiency than state of the art and at the same time lower in carbon emissions. ”

Susanne Siebentritt Full Professor in Physics & Head of Laboratory for Photovoltaics at the University of Luxembourg

Susanne Siebentritt’s FNR projects (main applicant- PRIDE not included)