Decoherence and excitation-transfer mechanisms in semiconductor quantum dots

The aim of this thesis is to supply a theoretical study of the interaction mechanisms between quantum dots beyond the simple picture of macroatoms. Coulomb interaction between excitons, exciton-phonon interaction as well as radiative interaction are, in particular, considered. These mechanisms can be exploited to coherently couple quantum dots, thus being the physical tool enabling quantum information processing using quantum-dot-based logic gates. On the other hand, the same mechanisms are responsible for the decoherence of the quantum-state that prevent the storing of the quantum information. Already if considered as simple two-level systems, quantum dots are subject to mutual interaction. Quantum dots in the excited state can be considered as dipoles, and are thus coupled with each other via the dipole-dipole electrostatic interaction. This results in excitation transfer between dots over distances of a few tens of nanometers. In Chapter 2 we show that taking into account the retarded nature of the electromagnetic field results in a correction to this effect, that become a leading contribution at large distances, effectively coupling quantum dots over distances of a few hundreds of nanometers. Strong exciton-phonon-coupling in quantum dots results in a very efficient decoherence mechanism. The strongly localized polarization in an excited quantum dot can induce virtual phonon emission and reabsorption processes which act as a phase-destroying mechanism. In quantum dot molecules the decay rate of the interband polarization is almost one order of magnitude larger than in the single quantum dot case, and depends on the interdot distance. The description of this coupling mechanism is possible only beyond the marcoatom picture. In Chapter 3 we develop a model that describes the phonon-mediated interaction between quantum dots in a dot molecule, explaining the strong distance dependence of the exciton dephasing rates in terms of a matching condition between the phonon wavelength and the interdot distance, which enhances the phonon-assisted scattering from bright to dark states. The heterodyne spectral interferometry is a novel implementation of transient nonlinear spectroscopy that enables to study the transient nonlinear polarization emitted from individual localized electronic transitions, in both intensity and phase. Two-dimensional spectra obtained by means of this technique display signals that can be associated to the coherent coupling between different resonances of the system under study. This technique is theoretically modeled for the first time in Chapter 4, where a very satisfactory description of the measured spectra is provided, showing that coherent coupling between different optical transitions of a quantum system result in off-diagonal peaks of the two-dimensional excitation spectrum. Furthermore, we show that in the low intensity regime, each spectral signal can be associated to a specific pair of coupled resonances as long as the level structure of the system under study is known

Microcrystalline and micromorph device improvements through combined plasma and material characterization techniques

Hydrogenated microcrystalline silicon (μc-Si:H) growth by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) is studied in an industrial-type parallel plate KAI reactor. Combined plasma and material characterization techniques allow to assess critical deposition parameters for the fabrication of high quality material. A relation between low intrinsic stress of the deposited i-layer and better performing solar cell devices is identified. Significant solar cell device improvements were achieved based on these findings: high open circuit voltages above 520 mV and fill factors above 74% were obtained for 1 μm thick μc-Si:H single junction cells and a 1.2 cm2 micromorph device with 12.3% initial (Voc=1.33 V, FF=72.4%, Jsc=12.8 mA cm−2) and above 10.0% stabilized efficiencies

A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

To further lower production costs and increase conversion efficiency of thin-film silicon solar modules, challenges are the deposition of high-quality microcrystalline silicon (μc-Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc-Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post-deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub-oxide doped layers, for reaching high efficiency stable thin film silicon solar cells

Optimization of the Asymmetric Intermediate Reflector Morphology for High Stabilized Efficiency Thin n-i-p Micromorph Solar Cells

This paper focuses on our latest progress in n-i-p thinmicromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by lowpressure chemical vapor deposition of zinc oxide.We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-μm-thick μc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm−2 , resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-μm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells

On the Interplay Between Microstructure and Interfaces in High-Efficiency Microcrystalline Silicon Solar Cells

This paper gives new insights into the role of both the microstructure and the interfaces in microcrystalline silicon (μc- Si) single-junction solar cells. A 3-D tomographic reconstruction of a μc-Si solar cell reveals the 2-D nature of the porous zones, which can be present within the absorber layer. Tomography thus appears as a valuable technique to provide insights into the μc- Si microstructure. Variable illumination measurements enable to study the negative impact of such porous zones on solar cells performance. The influence of such defectivematerial can bemitigated by suitable cell design, as discussed here. Finally, a hydrogen plasma cell post-deposition treatment is demonstrated to improve solar cells performance, especially on rough superstrates, enabling us to reach an outstanding 10.9% efficiency microcrystalline singlejunction solar cell

New progress in the fabrication of n–i–p micromorph solar cells for opaque substrates

In thispaper,weinvestigatetandemamorphous/microcrystallinesiliconsolarcellswithasymmetric intermediatereflectorsgrowninthen–i–psubstrateconfiguration.Wecomparedifferenttypesof substrateswithrespecttotheirlight-trappingpropertiesaswellastheirinfluenceonthegrowthof single-junctionmicrocrystallinecells.Ourmostpromisingbackreflectorcombinesatexturedzinc oxide filmgrownbylow-pressurechemicalvapordeposition,asilverfilmforreflection,andazinc oxide bufferlayer.Grownonthissubstrate,microcrystallinecellsexhibitexcellentresponseinthe infrared whilekeepinghighopen-circuitvoltageandfillfactor,leadingtoefficienciesofupto10.0%. After optimizingthemorphologyoftheasymmetricintermediatereflector,weachieveann–i–p micromorphsolarcellstabilizedefficiencyof11.6%,using270nmand1.7 mm ofsiliconfortheabsorber layer oftheamorphoustopcellandthemicrocrystallinebottomcell,respectively.Usingthisoriginal devicearchitecture,wereachefficienciesclosetothoseofstate-of-the-artn–i–pandp–i–nmicro- morph devices,demonstratingapromisingroutetodeposithigh-efficiencythin-filmsiliconsolarcells on opaquesubstrates

Light Harvesting Schemes for High Efficiency Thin Film Silicon Solar Cells

In Thin Film Silicon (TF-Si) solar cells light harvesting schemes must guarantee an efficient light trapping in the thin absorber layers without decreasing the silicon layers quality and consecutively the p-i-n diodes electrical performance. TF-Si solar cells resilience to the substrate roughness is reported to be possibly improved through optimizations of the cell design and of the silicon deposition processes. By further tailoring the superstrate texture, amorphous silicon / microcrystalline silicon (a-Si:H/mu c-Si:H) tandem solar cells with an initial efficiency up to 13.7 % and a stabilized efficiency up to 11.8 % are demonstrated on single-scale textured superstrates. An alternative approach combining large and smooth features nanoimprinted onto a transparent lacquer with small and sharp textures from as-grown LPCVD ZnO is then shown to have a high potential for further increasing TF-Si devices efficiency. First results demonstrate up to 14.1 % initial efficiency for a TF-Si tandem solar cell

Advanced nanostructured materials for pushing light trapping towards the Yablonovitch limit

We give an overview on recent progress in the synthesis, fabrication and integration of advanced nanostructured materials for efficient light trapping in high-efficiency thin-film silicon solar cells