abstract: Semiconductor nanostructures are promising building blocks for light management in thin silicon solar cells and silicon-based tandems due their tunable optical properties. The present dissertation is organized along three main research areas: (1) characterization and modeling of III-V nanowires as active elements of solar cell tandems, (2) modeling of silicon nanopillars for reduced optical losses in ultra-thin silicon solar cells, and (3) characterization and modeling of nanoparticle-based optical coatings for light management. First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells. Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths. Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

Le photovoltaïque (PV) s’est imposé comme un acteur majeur de l’énergie. L’innovation dans ce domaine passera sans doute par le PV à haut rendement sur des couches minces flexibles et légères permettant son déploiement dans les applications mobiles. Cette thèse étudie le développement de cellules solaires III-V à structures quantiques visant des concepts PV hauts rendements tels les cellules solaires à bande intermédiaire (IBSC). Ces IBSC se sont montrés limités du fait de l’échappement thermique des porteurs à température ambiante ainsi que la faible absorption optique sous le gap. Nous avons évalué la topologie, le mécanisme d’échappement thermique, la structure quantique ainsi que l’absorption de boites quantiques en In(Ga)As dans un matériau hôte en Al0.2GaAs à grand gap. Nous avons aussi caractérisé de manière quantitative comment opère ce système et avons amélioré son design optique. Sous une forte irradiation, nous avons mis en évidence l’apparition d’une population de porteurs chauds dans les boites quantiques. Par ailleurs, l’effet d’absorption sequentielle à deux photons (S-TPA) a été démontré. Nous avons observé une augmentation de ce S-TPA d’un facteur x5-10 grâce à du management de la lumière réalisé notamment avec des cavités de Fabry-Pérot. Des nanostructures périodiques ont aussi été fabriquées dans le cas de cellules solaires à multi-puits quantiques par l’utilisation de lithographie en nanoimpression. Dans l’ensemble cette étude vise à discuter la possibilité de réaliser des cellules solaires à porteurs chauds assistés d’une bande intermédiaire et améliorées par un management optique afin d’ouvrir la voie pour des cellules à hauts rendements. / In the past decade, photovoltaics (PV) has become a key player for the future of worldwide energy generation. Innovation in PV is likely to rely on high efficiency PV with flexible and lightweight thin films to enable PV deployement for mobile applications. In the framework of the Japanese-French laboratory “NextPV”, this thesis investigates the development of III-V quantum structured solar cells to explore high-efficiency photovoltaic concepts especially intermediate band solar cells (IBSC). Quantum structured IBSC have proven to be limited by thermal escape at room temperature and by low subbandgap light absorption. Following a consistent approach, we evaluate the topology, thermal escape mechanism, quantum structure and optical absorption of In(Ga)As quantum dots in a wide gap Al0.2GaAs host material. We also characterize quantitatively the device operation and improve the optical design. For a high irradiation, we evidence a hot carrier population in the quantum dots. At the same time, sequential two-photon absorption (S-TPA) is demonstrated both optically and electrically. We also show that S-TPA for both subbandgap transitions can be enhanced by a factor x5-10 with light management techniques, for example by implementation of Fabry-Perot cavities with the different epitaxial transfer methods that we developed. More advanced periodical nanostructures were also fabricated in the case of multi-quantum well solar cells using nanoimprint lithography techniques. Overall we discuss the possibility of realizing intermediate-band-assisted hotcarrier solar cells with light management to open the path for high-efficiency quantum structured IBSC.