Projet de thèse en Electronique et Optoélectronique, Nano- et Microtechnologies
Sous la direction de Maria Tchernycheva.
Thèses en préparation à Paris Saclay , dans le cadre de École doctorale Electrical, optical, bio-physics and engineering (Orsay, Essonne ; 2015-....) , en partenariat avec Centre de Nanosciences et de Nanotechnologies (laboratoire) , Photonique (equipe de recherche) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 01-03-2017 .
Cf la description en anglais
Nanoscale analyses of semiconductor nanowire properties for optimization of third generation photovoltaic converters
The dependency on fossil fuels is today a major problem. In order to ensure the energetic independence, the use of renewable energies has become indispensable. Among the alternative sustainable energy resources, the most promising is the solar energy conversion, which can deliver a power of tens of milliWatts per cm2. Today, a record conversion efficiency above 40% is achieved for a 3-junction tandem cell based on III-V semiconductors. However, the cost per kilowatt-hour of electricity generated with this photovoltaic (PV) technology is far too high for widespread applications. The problem of creating cost-effective high-performance solar cells is still open. One alternative is to exploit the advantages of III-V semiconductor nanowires (NWs) to develop novel PV devices with high conversion efficiency and moderate production cost. Thanks to the strain accommodation by the free lateral surface and the small footprint, NWs can be grown on low-cost misfitting substrates and highly mismatched materials can be stacked within the NW without formation of dislocations. Moreover, NW arrays have very attractive optical properties such as a small optical reflectance and strong capability for efficient light trapping leading to an increased absorption in comparison to thin films. To enhance the performance of NW PV devices, bottlenecks related to the design, material growth and processing have to be removed. In particular, for NW solar cells standard macroscopic PV characterization averaged over millions of nano-objects does not provide all the information necessary to understand the device physics and to optimize the performance. It is essential to push the comprehensive analyses down to the nanometric scale and to probe individual nanowire p-n junctions in order to analyze the material quality, to determine the surface recombination rate, to assess the wire-to-wire homogeneity and to detect eventual failures. The objective of this PhD project is to achieve an in-depth understanding of fundamental physical phenomena governing the carrier generation, extraction and collection in NWs at different scales. To this end, the PhD candidate will make use of our recently developed multi-scale technique combining electrical (Electron Beam Induced Current) and optical (Laser Beam Induced Current and cathodoluminescence) characterization tools (illustrated in Fig. 1) to tackle the conversion mechanism from the single NW level up to the whole device level. These advanced characterization tools will provide access to the key parameters governing the PV conversion (minority carrier diffusion length, carrier density, surface recombination velocity, etc). Combining the advanced modeling with the experimental investigation, the candidate will establish novel device architectures for high-efficiency PV converters. The recruited PhD fellow will also participate in the nanowire device fabrication using electron beam lithography in the dedicated clean-room facilities of CTU-IEF-Minerve. In addition, the PhD candidate will have an opportunity to collaborate with several of the best European research groups via the on-going European projects of the group.