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.
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.
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.
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.
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.