Current projects and past projects

FNR ATTRACT Surface and interface science on photovoltaic materials (Sunspot)

PhD Students: Thibaut Gallet and Christian Kameni Boumenou

Duration: 03.2017 - 03.2022

Solar cells play an important role in the future low carbon energy production of the earth. Highly efficient and cheap devices are necessary. The aim of the project SUNSPOT (SUrface and iNterface Science on PhOtovoltaic maTerials) is to improve the understanding of the surfaces and interfaces of polycrystalline materials that are relevant for photovoltaics. The project focusses on Cu(In,Ga)Se2 and hybrid organic inorganic perovskites (HOIPs) which show exceptionally high power conversion efficiencies despite their polycrystalline nature. Scanning tunneling microscopy and spectroscopy in conjunction with Kelvin probe force microscopy in ultra-high vacuum will be used to analyse these materials on the nanometer scale. The goal is to develop models that predict how the nature of polycrystalline surfaces and grain boundaries need to be in order to achieve high quality solar cells. The influence of foreign atoms on grain boundaries and the effect of the polycrystalline surfaces on junction formation will be analysed in detail. Absorber surfaces and cross sections of solar cell devices will be studied. Phase transitions in the HOIPs will be investigated by temperature dependent measurements and the effect of H2O and O2 will be analysed in detail in order to study the stability of the absorbers on the nanometer scale.

It is the aim to understand how the modifications of the surfaces and grain boundaries contributed to the mostly empirical improvements of these high performance solar cell technologies in the last years. Only then further improvements can be made to close the gap between polycrystalline thin film solar cells and single crystalline devices. Moreover the models developed within this project can be translated to other new emerging thin film solar cell technologies. The CIGSe absorbers and devices will be prepared in already existing groups of the host institution where polycrystalline and epitaxial samples can be fabricated. The perovskite absorbers will be grown via vacuum evaporation within the SUNSPOT group. Special care will be taken to analyse clean surfaces which is a prerequisite for a development of consistent models. The proposal is in line with the focus of the host institution, covers one of the priorities of the University of Luxembourg and of the FNR and strong collaborations within the university and with the Luxembourg Institute of Science and Technology are foreseen.

FNR CORE Grain boundaries in solar cells (GRISC)

PhD Student: Evandro Martin Lanzoni

Duration: 06.2018-06.2021

Within the GRISC project (GRain boundaries In Solar Cells) scanning probe microscopy (SPM), Hall effect and Atom Probe tomography (APT) measurements will be used to study the grain boundaries (GBs) in Cu(In,Ga)Se2 (CIGSe).This material is used to manufacture thin film solar cells. In industry, the growth of the absorber layer is carried out on low cost glass substrates. This results in polycrystalline films with a lot of grain boundaries which need to be passivated and understood as good as possible in order to further increase the power conversion efficiency of the solar cells.In contrast to most other reports in literature, we will grow CIGSe absorbers on GaAs wafers with one defined GB (i.e. on a bicrystal). This allows combining the results from the Hall measurements (which is a macroscopic technique) with SPM and APT measurements, which are local techniques. We will use scanning tunneling microscopy and spectroscopy to study the local density of states at the grain boundaries and we will use Kelvin Probe force microscopy to study work function changes at GBs. Moreover conductive atomic force microscopy will be applied to study current transport along the GBs. All measurements will be carried out in ultra-high vacuum and the samples will be transferred into the SPM chamber without prior air exposure. Hall effect measurements will be carried out to measure the temperature dependence of the conductivity and of the mobility across the GBs.The CIGSe samples will be grown on GaAs wafers in a metal organic vapor phase epitaxy system.

We will grow pure CuInSe2 and Cu(In,Ga)Se2 absorbers and the growth will be carried out on random high angle GBs. We will study the GB properties prior and after a post deposition treatment (PDT) with alkali metals such as Na and F. The PDT treatment will be carried out in a molecular beam epitaxy system where NaF or KF will be thermally evaporated. In addition metallic Na or F will be evaporated in the SPM machine for which alkali metal dispensers will be used. The PDT treatments will be done without air exposure to increase reproducibility and to assure that SPM measurements are not falsified by surface contaminations. Finally, we will supplement the measurements with APT measurements carried out at the RWTH. This allows us to link the GB properties as measured with SPM and Hall to the grain boundary composition.The GRISC project will substantially improve the understanding of the GBs in CIGSe since we combine local measurements carried with SPM, transport measurements via Hall and compositional measurements via APT on a well defined GB. This combination makes the project unique and the outcomes will be of high importance for CIGSe solar cells and beyond.

DFG Perovskite semiconductors: From fundamental properties to devices (SPP 2196)

PhD Student: Joana Ferreira Machado

Collaboration with Michael Saliba (Darmstadt) and Robin Ohmann (Siegen)

This project explores epitaxially grown tin (Sn) perovskites as alternatives to lead (Pb) based materials. Importantly, Sn perovskites have a narrow bandgap that is in the ideal range for a single-junction solar cell, which will also enable all-perovskite multijunction solar cells. In addition, they are more environmentally friendly. However, so far power conversion efficiencies of Sn perovskites are falling short to their lead counterparts. One of the most critical obstacles to overcome is the tendency of Sn2+ to oxidize to Sn4+. Here, we will employ epitaxial growth methods to avoid solvents that promote chemical reactions leading to oxidation of Sn and we will characterize these Sn perovskites down to the atomic level. Specifically, we will experimentally evidence the atomic structure of Sn perovskite surfaces such as CH3NH3SnI3 to localize Sn4+ defects and understand interface phenomena that readily occur in perovskite solar cells. Furthermore, we will add dopants and adsorbates, such as environmental gas molecules to the surfaces, to fundamentally study the specific interactions that occur on the atomic scale with respect to performance enhancements, degradation, and oxidation.

To bridge the size gap to the application, we will use surface techniques on the micrometer scale probing large-scale inhomogeneities, grain boundaries, workfunctions, contact potential differences and surface photovoltages. A full understanding on a device level necessitates the fabrication of Sn-based perovskite solar cells using state of the art solution-processing as a benchmark. Then we will apply the optimized architectures and use the knowledge from the nanoscopic and microscopic characterizations as well as epitaxially grown Sn perovskite absorbers to fabricate novel Sn based solar cells that are highly efficient and long-term stable. We believe that the correlation between atomic, micro- and macroscale on the same type of samples will be particularly fruitful to gain a thorough understanding of Sn perovskites.

FNR AFR CALL - Functionalization of MoS2 via ion beams and alkali metal

PhD Student: Jonathan Rommelfangen

Duration: September 2019 - September 2023

Two dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) exhibit fascinating properties, which are distinct from their corresponding 3D crystals. Examples include a transition from an indirect to a direct bandgap semiconductor in few layer and single layer MoS2, single photon emission from edge states in WSe2 and photoluminescence quantum yields close to unity. Potential applications in transistors and solar cells have already been demonstrated for some TMDs. Especially, the edges of some 2D materials exhibit novel physical properties that need to be understood better. Recently, single photon emission has been observed at the edges of WSe2 flakes. The effects are linked to defects at the edges.

In this PhD project, we plan to develop an atomic scale model of the defects at step edges and their influence on optical and vibrational properties. Furthermore, we will study the impact of locally varying strain on the properties of MoS2 flakes and we will perform model experiments to understand the effect of extrinsic and substrate-induced doping on the optical and vibrational properties of MoS2 flakes. We will use scanning probe techniques such as scanning tunneling microscopy and Kelvin Probe force microscopy to study defects and strain in MoS2 on the nanometer scale. We will supplement these measurements with Photoluminescence and Raman measurements. Defects will be introduced via ion beam erosion at grazing where the sputtering yield at the step edges are much higher than on flat terraces. Ion implantation with subsequently annealing will be used change the strain in MoS2 and extrinsic doping will be carried out via alkali metals intercalation. We plan to have secondments in Aachen and Oldenburg, and throughout the project we will have support from a theory group at the UL who specialized on density functional theory of 2D materials.

FNR PRIDE Photovoltaics Advanced Concepts for high Efficiency (PACE)

PhD Student: Himanshu Phirke

Collaboration with Philip Dale, Susanne Siebentritt, Ludger Wirtz (University of Luxembourg), Tom Wirtz, Torsten Granzow, Emmanuel Defay, Sebastjan Glinsek and Renaud Leturcq (LIST)

The demand for renewably generated electricity will only increase. Common photovoltaic modules consist of single p-n junction solar cells made of silicon or the more cost effective CdTe or Cu(In,Ga)Se2. All have an upper theoretical light to electric power conversion efficiency of 33 %, known as the Shockley-Queisser limit (SQL). This is breakable when the sunlight is used more effectively than is done in single junction devices. Multi-junction devices made of III-V semiconductors do surpass the SQL with efficiencies of around 39 % under normal conditions and around 46 % under concentrated light, showing the potential of multi-junction and concentrator solar cells, albeit at very high cost. Breaking the SQL using cost-effective materials still needs to be demonstrated and devices will require new materials, more components and greater structural complexity.

The aim of this Doctoral Training Unit is to create a cohort of scientists with expertise in advanced photovoltaic concepts necessary to overcome the SQL. This will be achieved by a stimulating and ambitious research and training programme. The research will focus on multi-junction and concentrator solar cells, and on understanding the necessary materials and interface properties. The final objectives are to synthesize beyond state of the art tandem and micro-concentrator solar cells using cost-effective materials. The research will be supported by training to place it in the broader context of other high efficiency ideas such as intermediate band, and hot carrier solar cells. It will consist of workshops by international experts, placements, a retreat, and organization of an international workshop. All of these objectives will be met by combining the expertise of the photovoltaic scientists of the University of Luxembourg, the materials and characterization scientists of the Luxembourg Institute of Science and Technology, and the external international scientists and companies involved.

FNR Core 2020 Hybrid Perovskite Surfaces (HYPS)

PI: Alex Redinger
Post-doctoral researcher: Jeremy Hieulle

Solar cells are very important for the production of environmentally friendly electricity on large scales. In order to produce better solar cells, which can convert light more efficiently into electrical power, new materials need to be developed and improved. Hybrid perovskites are an emerging class of materials that exhibit excellent optical and electrical properties that makes them very suitable for solar cells. A solar cell device consists of many different materials that are stacked on top of each other. The final device performance will be determined by a complex interplay between all these layers.

Consequently, the surface and interfaces of the materials play a crucial role in the development of high efficiency devices and need to be understood as good as possible. Within this project the fundamental surface properties of several types of perovskites will be investigated with surface sensitive techniques such as Photoelectron Spectroscopy (PES) and Scanning Probe Microscopy (SPM). We will combine both techniques to develop a consistent model of the electronic structure of the surface. We will carry out measurements at temperatures between 100K and 350K in order to better understand the effect of mobile ions in the material. Furthermore, we will develop novel synthesis routines based on vacuum evaporation where we will develop a dedicated alkali doping strategy to reduce the ionic mobility in the perovskite thin films.

FINAL REPORT:

Within the HYPS project, we studied halide perovskite surfaces under various external stimuli using a combination of scanning probe techniques and photoelectron spectroscopy. In particular, we used Kelvin Probe Force Microscopy (KPFM) to measure work function changes, aiming to identify secondary phases and observe degradation on the nanometer scale. We complemented this with scanning tunneling spectroscopy to assess how degradation affects the local density of states. The electronic properties revealed by scanning probe techniques were supplemented by photoelectron spectroscopy to monitor changes in surface composition and track shifts in the oxidation states of the elements present in the films. Additionally, we evaluated the influence of the measurements themselves -especially the effect of X-ray flux during XPS - to ensure that observed surface properties were not artifacts caused by beam-induced degradation.

We developed a model that accurately describes how state-of-the-art perovskites (power conversion efficiency >20%) degrade under white light exposure. The degradation mechanism involved both light and defects. Nanometer-scale analysis revealed that degradation seeds were located in regions where PbI2 precipitations formed. These findings were supported by detailed density functional theory calculations conducted in collaboration and published in [P3].

In collaboration with EPFL, we also studied the impact of organic passivation layers. The synthesis and photoluminescence analysis were published in [P2], and their influence during white light exposure is discussed in [P3]. Furthermore, we extended our rate equation model to halide perovskites with varying bromine contents to study ion motion during light exposure .

The results generated many new ideas and follow-up work. Notably, we developed a tool to track degradation at the nanoscale via KPFM and discovered that degradation strongly depends on light wavelength. We speculate that identifying the specific degradation-triggering wavelengths could help design more resilient perovskite materials. The postdoctoral researcher hired in this project successfully applied for an FNR CORE Junior grant, which began in mid-2024. The HYPS project was crucial for identifying key degradation mechanisms.

HYPS also played a key role in developing advanced experimental capabilities to study semiconductors. The resulting system, now unique in Luxembourg, enables XPS and UPS measurements as a function of temperature and includes a UHV-compatible chamber for thin-film growth. It is fully compatible with other STM and AFM systems in the physics department. This setup has since been used to study materials such as Sn-based perovskites, Cu(In,Ga)Se2, 2D materials, and layered perovskites. HYPS was essential in building the required expertise and infrastructure for these studies.

The results were presented at major conferences such as MRS, IEEE, IMC20, and ECOSS, as well as in invited talks by the PI and postdoc. We also participated in outreach events, including the University of Luxembourg’s open-door days and the 2021 Science Festival, to share our research with the public.

Outcomes:

Publications:

[P6] Jeremy Hieulle, Anurag Krishna, Hazem Adel Musallam, Tom Aernouts and Alex Redinger, Modeling the FA and I losses in Mixed-halide Perovskite through Chemical Rate Equations: Insights into Light induced Degradation, EES Solar, accepted (2025)

[P5] J. Ferreira Machado, J. Hieulle, A Vanderhaegen, A. Redinger, Light-induced Degradation of Methylammonium Tin iodide Absorber layers, Journal of Materials Chemistry A, 13, 1, 517-525 (2025).

[P4] M. Abbasli, J. Hieulle, J. Schrage, D. Wilks, A. Samad, U. Schwingenschlögl, A. Redinger, C. Busse, and, R. Ohmann, Tin Halide Perovskite Epitaxial Films on Gold Surfaces: Atomic Structure and Stability, Advanced Functional Materials, 2403680 (2024).

[P3] J. Hieulle, A. Krishna, A. Boziki, J.-N. Audinot, M. U. Farooq, J. Ferreira Machado, M. Mladenovic, H. Phirke, A. Singh, T. Wirtz, A. Tkatchenko, M. Grätzel, A. Hagfeldt and A. Redinger, Understanding and decoupling the role of wavelength and defects in light-induced degradation of metal-halide perovskites, Energy & Environmental Science, 17, 284–295 (2024). {13 citations}

[P2] A. Krishna, V. Škorjanc, M. Dankl, J. Hieulle, H. Phirke, A. Singh, E.A. Alharbi, H. Zhang, F. Eickemeyer, S.M. Zakeeruddin, G.N. M. Reddy, A. Redinger, U. Rothlisberger, M. Grätzel, A. Hagfeldt, Mitigating the Heterointerface Driven Instability in Perovskite Photovoltaics, ACS Energy Letters, 8, 3604-3613 (2023).

[P1] Ajay Singh, J. Hieulle, Joana Ferreira Machado, Sevan Gharabeiki, Weiwei Zuo, Muhammad Uzair Farooq, Himanshu Phirke, Michael Saliba, and Alex Redinger, Coevaporation Stabilizes Tin-Based Perovskites in a Single Sn-Oxidation State, Nano Letters, 22, 17, 7112–7118 (2022).

Conferences:

[C5] "Understanding Photo-Induced Degradation in Perovskites: A Kinetic Model Approach", IEEE PVSEC 2025, Montreal

[C4] "Comprendre la dégradation photo-induite dans les pérovskites : une approche par modèle cinétique", Forum Sondes Locales, SPA, 2025

[C3] “Probing Light-Induced Degradation in Metal Halide Perovskites at the Nanoscale”, (invited talk), 20th International Microscopy Congress (IMC20), Busan, Korea, September 2023.

[C2] “Intrinsic Light Instabilities in Metal Halide Perovskites”, (Oral presentation), Materials Research Society, MRS fall meeting, Boston, USA, November 2022.

[C1] “Unraveling the Intrinsic Light Instability in Metal Halide Perovskites”, (Oral presentation), ECOSS35, European Conference on Surface Science, Belval, Luxembourg, August 2022

Seminars and colloquium talks :

[S2] "Halide perovskite photovoltaics", A. Redinger, University of Siegen, Germany, May 2025

[S1] “Metal Halide Perovskites: From Surface Properties to Material Stability and Device Stability” (invited seminar), J. Hieulle, Institute of Physics and Chemistry of Materials of Strasbourg (IPCMS), Strasbourg, France, May 2022.

Outreach:

2023 Participation in the Open-door days of the University of Luxembourg. It is an event where scientists are doing a lab tour to the general public and explains the research that is performed at the University of Luxembourg.

2022 Participation in the Open-door days of the University of Luxembourg.

2021 Participation in the 13th Science Festival, organized by the city of Luxembourg. It is an event organized every 2 years where scientists describe their work to the general public through small experiments, games, and leisure activities.

New laboratory infrastructure:

Within the project HYPS we purchases a mu-metal chamber in order to carry out Photoelectron spectroscopy measurements. The existing analyser was transferred to that system. The final system is shown in the figure below:

FNR CORE projet TAILS (2021-2024)
How Tail states in the Absorber Influence and Limit Solar cell efficiency

PI: Prof. Susanne Siebentritt
co-PI: Prof. Alex Redinger
PhD Students: Sevan GHARABEIKI; Muhammad Uzair FAROOQ

Thin film solar cells will have to play a major role for the future low-carbon energy supply. Their carbon emissions, averaged over their lifetime, including production, can be lower than any other electricity source. They are based on polycrystalline absorbers, which contain grain boundaries. Any semiconductor contains tail states which decay from the bands into the band gap. It has been observed that polycrystalline materials have more tail states. Indirect indications exist that grain boundary properties and tail states are linked. Furthermore, a correlation has been observed, covering a wide range of solar cell technologies, that links the amount of tail states with the open-circuit voltage of the solar cells. This correlation is not quantitatively explained on the basis of existing models.
The aim of the proposed research is to establish and explain a correlation between grain boundary properties and tail states, as well as to quantitatively explain the dependence of the open-circuit voltage and the tail states. We will investigate two very different thin film technologies, that both have reached efficiencies above 23%: perovskites and chalcopyrites. We will use photoluminescence to measure tail states and other electronic defect states and the open-circuit voltage capabilities of the absorbers. Different methods of scanning probe microscopy will be used to investigate the properties of grain boundaries. The efficiency and in particular the open circuit voltage of complete solar cells will be measured.

FNR CORE projet EPICHALK (2024-2027)
How Tail states in the Absorber Influence and Limit Solar cell efficiency

PI: Prof. Susanne Siebentritt
co-PI: Prof. Alex Redinger
PhD Students: Penda FALL; Elham BAGHESTANI
Postdoc: Adriana ROETTGER

Chalcopyrite (Cu(InGa)Se2) is used as the absorber in thin film solar cells, that are stable, show high efficiency and low carbon emission. Postdeposition treatments with heavy alkalis has been shown to be a key factor for high efficiency. From a wide range of investigations it is clear that alkali treatments modify the surface, the bulk and the grain boundaries of the absorber. However, the interplay between the observed changes, the electronic structure and the efficiency of solar cells is under discussion. In our lab we have recently been able to grow thick, high electronic quality epitaxial chalcopyrite films. These films open new possibilities to study the diffusion of alkalis and their localized influence on the electronic structure of the bulk and of grain boundaries. This new growth method allows to prepare solar quality films with and without grain boundaries, to make solar cells from them and study directly the interplay between alkali diffusion and accumulation, electronic quality and solar cell efficiency.

FNR PRIDE: Materials for a Sun-Powered Energy Transition (SPETRA) (2025-2030)

PhD Student: Yannick BUSCHHARDT

Collaboration with Philip Dale, Susanne Siebentritt, Ludger Wirtz (University of Luxembourg), Emmanuel Defay (University of Luxembourg and LIST), Sebatjan Glinsek (LIST), Jorge Iniguez-Gonzalez (University of Luxembourg and LIST), Jean-Sébastien Thomann (LIST)

It is widely recognized that a rapid transition from fossil fuels to renewable energies is critical for reducing greenhouse gas emissions and ensuring a habitable planet. It is also widely accepted that the increased use of solar energy is the most effective option.

We know that photovoltaics must play a central role in the energy production, and that renewable “green hydrogen” is urgently needed as a fuel in sectors where electrification is not viable (e.g., heavy transportation). We also know that our ability to tackle the climate challenge will depend on the discovery or optimization of materials that enable an efficient sunlight-to-energy conversion and production of green hydrogen. Ultimately, it all relies on basic physical and chemical processes by which sunlight is captured by suitable materials that allow to either transform it into electricity directly or use it to power clean processes, such as the production of green hydrogen from water.

SPETRA will run 8 cutting-edge research projects aiming exactly at that: the discovery and optimization of materials and physical/chemical processes that allow us to make the most of solar energy, thus enabling a sun-powered energy transition. To do this, we have assembled a team of experts from the University of Luxembourg and the Luxembourg Institute of Science and Technology, who possess the complementary equipment and skills needed to tackle this challenge. We will explore the most advanced photovoltaic materials for a direct sun-to-electricity conversion (4 projects), as well as alternative strategies that may allow us to create even more efficient solar panels by also making use of the heat generated by sunlight (2 projects). Additionally, we will study approaches to obtain hydrogen from water via chemical reactions (and novel catalytic materials) that we will optimize so sunlight boosts their efficiency (2 projects).