Grain boundaries and potassium post-deposition treatments in chalcopyrite solar cells

Doctoral thesis (2022)

Over the last years, alkali post-deposition treatments (PDT) have been attributed as the main driver for the continuous improvements in the power conversion efficiency (PCE) of Cu(In,Ga)Se2 (CIGSe) solar ... [more ▼]

Over the last years, alkali post-deposition treatments (PDT) have been attributed as the main driver for the continuous improvements in the power conversion efficiency (PCE) of Cu(In,Ga)Se2 (CIGSe) solar cells. All the alkali elements have shown beneficial optoelectronic effects, ranging from sodium to cesium, with many reports linking the improvements to grain boundary (GB) passivation. The most common process for alkali incorporation into the CIGS absorber is based on the thermal evaporation of alkali fluorides in a selenium atmosphere. Besides the demonstrated improvements in performance, disentangling the individual contributions of the PDTs on the GB, surface, and bulk is very challenging because of the many concurring chemical reactions and diffusion processes. This thesis aims to investigate how pure metallic potassium interacts with CIGSe epitaxially grown on GaAs (100) and multi-crystalline GaAs. Surface sensitive Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS) measurements are used to, in-situ, analyze changes in workfunction and compositional changes before and after each deposition step. Inert gas transfer systems and ultrahigh vacuum (UHV) are used to keep the pristine surface properties of the CIGSe. An in-depth understanding of how different KPFM operation modes and environments influence the measured workfunction is discussed in detail in this thesis. It is shown that AM-KPFM, the most common KPFM operation mode, leads to misinterpretations of the measured workfunction at GBs on rough samples. Frequency modulation KPFM (FM-KPFM), on the other hand, turns out to be the most suitable KPFM mode to investigate GB band bending. Pure metallic potassium evaporation on CIGSe epitaxially grown on GaAs (100) leads to diffusion of K from the surface down to the CIGS/GaAs interface even in the absence of GBs. Evaporation of metallic K is performed using a metallic dispenser, in which the evaporation rate can be controlled to deposit a few monolayers of K. The deposition is done in UHV, and an annealing step is used to diffuse K from the surface to the bulk. Pure metallic potassium is also evaporated on CIGSe epitaxially grown on multicrystalline GaAs substrate, where well-defined GBs are present. Negligible workfunction changes at the GB were observed. XPS shows a strong Cu depletion after K deposition followed by annealing. Interestingly, the amount of K on the absorber surface after the K-deposition and subsequent annealing is almost equal to the amount of Cu that diffused into the bulk, suggesting a 1:1 exchange mechanism and no KInSe2 secondary phase. [less ▲]

Forschungsbericht 1: Ergebnisse der Studie "Social Work Labour Market in Luxembourg" - Gründe der ehemaligen BSSE-Studierenden für die Wahl einer akademischen Ausbildung im Bereich der Sozial- und Erziehungswissenschaften an der Universität Luxemburg.

Report (2022)

Trainees in an EU institution: Getting ready for a multilingual workplace

Scientific Conference (2022, June 24)

Institutions of the European Union welcome over 2,000 trainees every year (EPSO 2022). What can the young graduates learn about the specifics of language use in a multilingual workplace? To find out, the ... [more ▼]

Institutions of the European Union welcome over 2,000 trainees every year (EPSO 2022). What can the young graduates learn about the specifics of language use in a multilingual workplace? To find out, the present sociolinguistic study interviewed 8 juniors (trainees) and 4 seniors (permanent employees) in a terminology and communication unit in an EU institution in Luxembourg. Qualitative data were collected from September 2020 to May 2021 in a participatory manner (McIntyre 2008), joining the recent interest in folklinguistic empowerment (Rymes 2021). The researcher took part in discussions with participants and encouraged their in-depth reflection on communication in the multilingual workplace. Iterative interviews, viewed as social interaction with critical reflective positioning of the researcher (Deppermann 2013), were accompanied by observation of meetings (online, due to Covid-19 pandemics), reflective drawing of metaphors by participants (Molinié 2009), and written reflections. Participants were free to define what language and multilingualism meant to them, as the study also aimed to avoid implicit methodological nationalism (Kraus 2018). Informed by the results of a pilot study in the same workplace (Lovrits and de Bres 2021), the collected data were subjected to a discursive analysis of stances (Jaffe 2009). Results showed that trainees’ understanding of language competence at work changed over the time. Aligning with stances of their supervisors towards the end of the traineeship stay, the juniors no longer focused on a linguistic standard in everyday communication. Instead, they began to pay more attention to the socio-emotional and relational aspects of their language use. The trainees reconstructed their understanding of (flexible) multilingualism as a basis for personal well-being at work and stopped fixating on formal linguistic "mistakes". Moreover, since research has emphasized socio-pragmatic diversity in the process of meaning making (Verschueren 2000), the seniors recognized the trainees' need to make the pragmatic aspects of everyday multilingualism explicit. This revelation has led to changes in language management in the workplace and has the potential to further inspire lifelong learning and professional development in the changing conditions of the global labour market (Budach and de Saint-Georges 2017). The findings are particularly relevant for professionals who facilitate transitions from monolingual to multilingual environments. The study reveals the importance of metalinguistic awareness in people coming from linguistically homogeneous settings into the so called "post-monolingual conditions" (Yildiz 2012) of an international workplace. [less ▲]

Presenter: Return Migration, Welfare, and the French-Luxembourg Borderlands after the First World War

Presentation (2022, June 23)

HTR fine tuning for medieval manuscripts models: strategies and evaluation.

Speeches/Talks (2022)

Young People and COVID-19: Social, Economic, and Health Consequences of Infection Prevention and Control Measures among Young People in Luxembourg – Technical Report 2021 Longitudinal Survey

Report (2022)

1 Energy-Efficient Hybrid Beamforming for Multi-Layer RIS-Assisted Secure Integrated Terrestrial-Aerial Network

in IEEE Transactions on Communications (2022), 70(6), 4189-4210

The integration of aerial platforms to provide ubiq- uitous coverage and connectivity for densely deployed terrestrial networks is expected to be a reality in emerging sixth-generation networks. Energy ... [more ▼]

The integration of aerial platforms to provide ubiq- uitous coverage and connectivity for densely deployed terrestrial networks is expected to be a reality in emerging sixth-generation networks. Energy-effificient design and secure transmission are two crucial issues for integrated terrestrial-aerial networks. With this focus, due to the potential of RIS in substantially saving power consumption and boosting the security of private information by enabling a smart radio environment, this paper investigates the energy-efficient hybrid beamforming for multi- layer reconfigurable intelligent surface (RIS)-assisted secure in- tegrated terrestrial-aerial network for defending against simul- taneous jamming and eavesdropping attacks. Specifically, with the available of angular information based imperfect channel state information (CSI), we propose a framework for the joint optimization of user’s received precoder, terrestrial BS’s and HAP’s digital precoder, and multi-layer RIS analog precoder to maximize the system energy efficiency (EE) performance. For the design of received precoder, a heuristic beamforming scheme is proposed to convert the worst-case problem into a min-max one such that a closed-form solution is derived. For the design of digital precoder, we propose an iterative sequential convex approximation approach via capitalizing the auxiliary variables and first-order Taylor series expansion. Finally, a monotonic vertex-update algorithm with penalty convex concave procedure is proposed to obtain analog precoder with low computational complexity. Numerical results show the superiority and effective- ness of proposed optimization framework and architecture [less ▲]

The COVID-19 Pandemic as Enhancer of Existing Challenges in the Educational System

Presentation (2022, June 22)

Young People and COVID-19: Social, Economic, and Health Consequences of Infection Prevention and Control Measures among Young People in Luxembourg – Technical Report 2021 Cross-Sectional Survey

Report (2022)

Complex Dance of Light-Seeking Algae in Light Gradients

Article for general public (2022)

A population of photosynthetic algae has been shown to exhibit a highly nonlinear response to light, forming dynamic structures in light-intensity gradients. Two longstanding questions about this light ... [more ▼]

A population of photosynthetic algae has been shown to exhibit a highly nonlinear response to light, forming dynamic structures in light-intensity gradients. Two longstanding questions about this light response regard how light-seeking cells move in a light-intensity gradient and whether this motion depends on cell concentration. Aina Ramamonjy and colleagues investigated the dynamics of dilute and semi dilute suspensions of these algae in a light-intensity gradient (varying from darkness to bright green light). The results could improve our understanding of how groups of photosynthetic organisms arrange themselves into dynamic patterns to control the amount of light that they receive. [less ▲]