Prof. Dr. Caterina Cocchi

Prof. Cocchi erforscht die elektronischen und optischen Eigenschaften von Halbleiter-Nanomaterialien durch computergestützte Modellierung auf atomarer Ebene. Ihr aktueller Fokus liegt auf Metall-Halogenid-Perowskiten und zweidimensionalen Materialien (wie MoS₂), wo sie untersucht, wie Ladungsträger unter Lichteinfluss ultraschnell transportiert werden und wie Grenzflächen zwischen verschiedenen Materialien diese Prozesse beeinflussen. Diese Erkenntnisse ermöglichen es, Halbleiter-Bauelemente wie Solarzellen, Photodetektoren und Photokathodenmaterialien gezielt zu verbessern. Die Methoden sind relevant für Photovoltaik, Optoelektronik und Beschleunigertechnik.

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• Grundlagen des molkularen Dotierens organischer Halbleiter

  Quelle ↗

  Unsere heutige Informationsgesellschaft basiert auf der Fähigkeit, anorganische Halbleiter wie etwa Silizium kontrolliert zu dotieren und damit deren elektrische Eigenschaften gezielt an anwendungsspezifische Anforderungen anzupassen. Für viele opto-elektronische Anwendungen haben sich allerdings organische Halbleiter als bessere Alternative herauskristallisiert, insbesondere weil deren optische Bandlücke leicht durch chemische Modifikation angepasst werden kann und weil sich diese kostengünstig und großflächig auf flexible Trägermaterialien aufbringen lassen. Auch für diese Materialklasse wurden bereits Verbesserungen etwa von organischen Leuchtdioden oder Solarzellen durch Dotierung mit geeigneten molekularen Dotanden demonstriert. Allerdings stellen sich die dabei erreichbaren Dotiereffizienzen als unerwartet gering heraus, was die Tatsache unterstreicht, dass die zugrundeliegenden Prozesse weit weniger verstanden sind. Dies zeigt sich insbesondere darin, dass zwei wichtige Klassen organischer Halbleiter - kleine, konjugierte Moleküle und konjugierte Polymere - bei Dotierung grundlegend unterschiedliches Verhalten aufweisen, wofür bis heute keine zufriedenstellende Erklärung vorgeschlagen werden konnte. Im Rahmen des gegenwärtigen Projektes soll dieses zentrale Problem gelöst werden, um eine wissensbasierte Verbesserung bestehender organischer opto-elektronischer Bauteile ebenso zu ermöglichen wie die Realisierung neuartiger Funktionalität.

• Ab-initio-Modellierung der Photoemissionseigenschaften von Cs2Te als Photokathodenmaterial für Teilchenbeschleuniger

  Quelle ↗

  The overarching goal of this doctoral project, with planned duration of 36 months, is to understand, predict, and control the photoemission properties of the photocathode material Cs2Te. To accomplish this task, state-of-the-art methods of solid-state theory and computational materials science will be employed. The knowledge of the electronic structure of the material is the key to disclose the fundamental physical mechanisms ruling the creation of photo-excited charge-carriers upon light absorption, their scattering mechanisms, and their final emission from the cathode. Due to the little existing information about the nanoscale electronic properties of Cs2Te, the proposed study will begin with a detailed characterization of the bulk material in the framework of density-functional theory (DFT) and many-body perturbation theory (MBPT) following the path proposed in the recent publication C. Cocchi et al., J. Phys.: Condens. Matter 31, 014002 (2018). A quantitative comparison with experimental results is enabled by the adopted parameter-free theoretical scheme. The information about the electronic structure of bulk Cs2Te can be used as an input to describe photoemission by adopting and extending the Spicer’s three step model. The response of the material to photo-absorption (step 1) is given by the dielectric tensor computed from MBPT. Scattering mechanisms of the photo-excited electrons (step 2) that are relevant in semiconductors involve the electron-phonon interaction, which is accessible within the framework of DFT. Emission rates (step 3) can be inferred consequently. The extension of Spicer’s model includes the revision of the relevant physical quantities (absorption, scattering, and emission rates) including quantum-mechanical many-particle effects, which are included in the aforementioned first-principles scheme. The application of the extended photoemission model on bulk Cs2Te is the first step to validate it. Refinements will be needed to enhance the comparison with experiments and operational characteristics, taking into account defects and surface effects. To identify relevant structures within such a huge configurational space, we will perform a computational high-throughput screening. Surfaces will be automatically created and optimized using DFT. After assessing and ranking their stability, the electronic structure of the most stable surfaces will be investigated in view of modeling their emission properties. Due to the significantly larger amount of atoms that is needed to describe a surface compared to a bulk, MBPT calculations will be performed only in selected critical cases where DFT qualitatively fails. The prior assessment of the photoemission properties of bulk Cs2Te will serve as a benchmark for the surfaces. Defected structures will be investigated adopting the same approach.

• Electronic structure of cesium-based photocathode materials from density functional theory: performance of PBE, SCAN, and HSE06 functionals

  2021

  Electronic Structure · DOI

  Abstract The development of novel materials for vacuum electron sources in particle accelerators is an active field of research that can greatly benefit from the results of ab initio calculations for the characterization of the electronic structure of target systems. As state-of-the-art many-body perturbation theory calculations are too expensive for large-scale material screening, density functional theory offers the best compromise between accuracy and computational feasibility. The quality of the obtained results, however, crucially depends on the choice of the exchange–correlation potential, v xc . To address this essential point, we systematically analyze the performance of three popular approximations of v xc [PBE, strongly constrained and appropriately normed (SCAN), and HSE06] on the structural and electronic properties of bulk Cs 3 Sb and Cs 2 Te as representative materials of Cs-based semiconductors employed in photocathode applications. Among the adopted approximations, PBE shows expectedly the largest discrepancies from the target: the unit cell volume is overestimated compared to the experimental value, while the band gap is severely underestimated. On the other hand, both SCAN and HSE06 perform remarkably well in reproducing both structural and electronic properties. Spin–orbit coupling, which mainly impacts the valence region of both materials inducing a band splitting and, consequently, a band-gap reduction of the order of 0.2 eV, is equally captured by all functionals. Our results indicate SCAN as the best trade-off between accuracy and computational costs, outperforming the considerably more expensive HSE06.

• Quantitative Analysis of Doping-Induced Polarons and Charge-Transfer Complexes of Poly(3-hexylthiophene) in Solution

  2020