PhD's SoPPoM

Nee
 
 

Armin ESMAEIL ZAGHI

SIM Flanders

* Department:
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44 – bus 2450, B-3001 Heverlee

* Title of PhD dissertation:
Nanopowder suspension processinf of chalcogenide semiconductors for thin film solar cell applications

* Promoter:
Prof.dr.ir. Jef Vleugels

* Co-promoter:
Prof. dr. ir. Jef Poortmans

* Summary:
The objective of the PhD was the development of a solution based fabrication technique for chalcogenide CuIn(S,Se)2 (CIS) and CuIn1-xGax(S,Se)2 (CIGS) semiconductors as light absorber layers for solar cell application, via printing of a nanopowder precursor dispersion followed by an annealing/selenization step. To reach the main objectives of this research, a systematic study was conducted to provide an in-depth understanding of the material science aspects of chalcogenide semiconductor processing. The following topics were addressed: synthesis of high purity chalcogenide precursor materials by high energy milling, development of precursor ink and coating technique for precursor layer deposition, understanding the recrystallization and grain growth of precursor coatings during the annealing process, and optoelectronic characterization and photovoltaic performance evaluation of the nanopowder based chalcogenide semiconductor absorber layers.

* Date of Defense:
december 2014
 

Hamed Ravash

* Department:
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44 - bus 2450, B-3001 Heverlee

* Title of PhD dissertation:
3D phase-field simulations of sintering and coarsening in polycrystalline multi-phase materials 

* Promoter:
Prof. dr. ir. Nele Moelans

* Co-promoter:
Prof. dr. ir. Jef Vleugels

* Summary:
As the functionality, performance and safety demands on engineering materials continue to rise, the use of multi-component, multi-phase alloys processed under carefully controlled conditions becomes ever more common. The microstructure of a multi-phase material contains boundaries between grains of the same phase and grains of different phases. Microstructural evolution in multi-phase materials is controlled by two competing phenomena, Ostwald ripening and grain growth. Although both phenomena are diffusion driven processes, they exhibit two different coarsening regimes in time, t, with an average domain size increase proportional to t1/2 for grain growth and t1/3 for Ostwald ripening. Assuming long-range bulk diffusion controlled growth, the interfacial energy, volume fraction and diffusional mobility in each phase are the essential parameters which determine the morphological evolution.

During this PhD work, the microstructural evolution in two and three phase materials was studied using Phase-field simulation. This meso-scale modeling method is a powerful tool used for studying the evolution of interfaces without explicit tracking them. In the phase-field method, microstructures are represented by a set of continuous field variables that describe the size, shape and spatial arrangement of the grains and the spatial composition variations. The temporal evolution of the phase-field variables, and thus the microstructure, is obtained by numerically solving a set of coupled partial differential equations derived from a thermodynamic free energy functional.

In order to produce a result with a statistical relevance, 3-D simulations for a large number of grains are required. However, realistic three-dimensional phase-field coarsening simulations demand significant amounts of computation power. Therefore, an efficient implementation of the phase-field simulation is required. In this work, the bounding-box implementation software to model grain growth in anisotropic systems was developed to account for the grain coarsening in multi-phase multi-component material systems. The developed software can be used for an arbitrary number of phases and elements with a desired system size.

In order to analyze the effect of the interfacial energy and diffusivity of elements in each phase on the evolving morphology, a series of 3-D phase-field simulations was performed for systems containing three solid phases. In order to study the effect of the interfacial energy and phase volume fraction, another series of 3-D phase-field simulations was performed for systems containing one solid and one liquid phase.

The obtained morphological and topological information were comparable with formerly performed simulations and experimental results for single phase and two phase materials. Furthermore, the microstructural features of a liquid-phase sintered NbC-Co material were measured in 2-D and 3-D and compared with the simulation results. The results from the microscopy and the simulations agreed well and verified that the selected physical model in our simulations can be successfully used to model the microstructural evolution in multi-phase material systems.

* Date of Defense:
december 2014

Jeroen Drijkoningen

* Department:
UHasselt - IMO-IMOMEC, Material physics, Wetenschapspark 1, 3590 Diepenbeek

* Title of phD dissertation:
Advanced Scanning Probe Microscopy of Novel Generation Solar Cells

* Promoter:
Prof. dr. ir. Jean Manca

* Co-promoter:
Prof. dr. ir. Jan D'Haen

* Summary:
To understand the macroscopic behavior of novel generation solar cells a more thorough understanding of the nanoscale behaviour is needed, which in turn can be studied using several advanced techniques e.g. electron beam or scanning probe based techniques. In this thesis novel generation solar cells and absorber layers were studied using standard and advanced scanning probe based techniques. More specifically morphology and local electrical properties were analysed by Kelvin Probe Force Microscopy and Conductive AFM to elucidate the effect of grain boundaries in inorganic absorber layers, assess the effect of interlayers on the photovoltaic properties of organic solar cells and to try to understand the initial contact degradation of metallic contacts in organic devices.

Serena Jacovo

* Department:
Semiconductor Physics Laboratory, Department of Physics, KULeuven, Celestijnenlaan 200 D, 3001 Leuven

* Title of PhD dissertation:
Comparative study by means of ESR of point defects at interfaces and interlayers of Si with various insulators

* Promoter:
Prof. dr. ir. Andre Stesmans

* Summary:
Using electron spin resonance (ESR) as major experimental technique, the work carried out deals with a fundamental study of some specific Si/insulator interfaces and newly-conceived functional layers and thin films as encountered in innovative device architectures, of actual interest because of rising challenges in future device progress. The inherent quality of the interfaces and thin layers is assessed through probing the atomic properties of occurring paramagnetic point defects, where a main focus is on atomic identification and quantification.

Targeted subjects include the thermal (211)Si/SiO2 structure as encountered in emerging Si nanowire entities and the c-Si3N4/(111)Si interface as well as interfaces and dielectric layers of advanced solar cell structures and low- oxide/Si entities. In addition, a study is intended of the GaAs/GaAs-oxide interface, where the current understanding of the detrimental interface aspects of this highly wanted high-mobility entity is almost exclusively based on theoretical exploration.