Semiconductor Nanocrystals Based on Group IV Materials: Synthesis and Characterization towards Applications in Solar Cells

<p>Quantum dots have attracted a lot of interest in the past decade due to their physical and chemical properties. Quantum dots offer exciting possibilities for third generation photovoltaic devices. Light emitting quantum dots are stronger emitters than conventional organic dyes and are more resistant to degradation. This thesis focuses on the solution phase synthesis of semiconducting nanoparticles containing only easily available and relatively non-toxic materials, unlike cadmium containing nanoparticles. As an example, CdSe has been heavily studied for its outstanding optical properties. But the toxicity of cadmium encourage towards the use of other materials combining low toxicity with efficient emitting properties, such as silicon or germanium. We concentrate our research to silicon, germanium, tin, tin/germanium and Cu2ZnSnS4 (CZTS) nanoparticles. Tin based nanocrystals are poor emitters but have great potential as light harvesters in solar cells due to great semiconducting properties. The potential applications, crystal structures and properties of the target materials are described in Chapter 1. Chapter 2 details the characterization techniques used to define the nanoparticles synthesized in this research. Size and shape of the nanocrystals was evaluated using Transmission Electron Microscopy (TEM). The crystals structure was determined by X-ray diffraction (XRD) or Selected Area Electron Diffraction (SAED). The surface termination of quantum dots was assessed via Fourier Transform Infrared Spectroscopy (FTIR). Finally, the optical properties were determined using UV-Visible and photoluminescence spectroscopies. Silicon quantum dots (SiQDs) exhibit strong blue photoluminescence. The emission phenomenon of silicon nanostructures is still heavily debated in the literature. Chapter 3 looks into the origin of this fluorescence. The quantum dots were synthesized following a chemical reduction method in the presence of a surfactant. We evaluate the influence of the nanoparticle size variation on the optical properties. Then we explore the role of the passivation molecule on the surface of the silicon quantum dots on the light absorption and emission phenomena. The synthesis of CZTS nanoparticles via a solution phase process is described in Chapter 4. The aim of this research was the production of small monodisperse particles. We investigate the influence of the solvent environment in high temperature decomposition syntheses, followed by the study of a novel chemical reduction method for CZTS nanocrystals. Chapter 5 regroups the research conducted on germanium and tin quantum dots, as well as the study on germanium/tin alloy. Germanium quantum dots, strong light emitters, were characterized optically in this study. The semiconducting phase of tin has great physical properties but is unstable in an ambient environment. So far reported tin nanoparticles synthesized via a solution process display only the metallic structure of tin. Presenting similar structural properties, germanium is expected to stabilize the quantum dot configuration when alloyed to tin. In Chapter 6 are described three different collaborative projects towards the application of silicon quantum dots in solar cells. First silicon quantum dots were anchored to zinc oxide nanowires arrays. Then we investigated the optical properties of SiQDs blended in a matrix of block copolymers. The third project looks into the effect of SiQDs spread over the surface of a working silicon solar cell. Finally, the last chapter presents an overall conclusion and summarizes the main findings of this study. It also introduces perspectives for future work with concepts on how to overcome the problems encountered in this research and ideas towards concrete industrial application of quantum dots.</p>

Semiconductor Nanocrystals Based on Group IV Materials: Synthesis and Characterization towards Applications in Solar Cells

<p>Quantum dots have attracted a lot of interest in the past decade due to their physical and chemical properties. Quantum dots offer exciting possibilities for third generation photovoltaic devices. Light emitting quantum dots are stronger emitters than conventional organic dyes and are more resistant to degradation. This thesis focuses on the solution phase synthesis of semiconducting nanoparticles containing only easily available and relatively non-toxic materials, unlike cadmium containing nanoparticles. As an example, CdSe has been heavily studied for its outstanding optical properties. But the toxicity of cadmium encourage towards the use of other materials combining low toxicity with efficient emitting properties, such as silicon or germanium. We concentrate our research to silicon, germanium, tin, tin/germanium and Cu2ZnSnS4 (CZTS) nanoparticles. Tin based nanocrystals are poor emitters but have great potential as light harvesters in solar cells due to great semiconducting properties. The potential applications, crystal structures and properties of the target materials are described in Chapter 1. Chapter 2 details the characterization techniques used to define the nanoparticles synthesized in this research. Size and shape of the nanocrystals was evaluated using Transmission Electron Microscopy (TEM). The crystals structure was determined by X-ray diffraction (XRD) or Selected Area Electron Diffraction (SAED). The surface termination of quantum dots was assessed via Fourier Transform Infrared Spectroscopy (FTIR). Finally, the optical properties were determined using UV-Visible and photoluminescence spectroscopies. Silicon quantum dots (SiQDs) exhibit strong blue photoluminescence. The emission phenomenon of silicon nanostructures is still heavily debated in the literature. Chapter 3 looks into the origin of this fluorescence. The quantum dots were synthesized following a chemical reduction method in the presence of a surfactant. We evaluate the influence of the nanoparticle size variation on the optical properties. Then we explore the role of the passivation molecule on the surface of the silicon quantum dots on the light absorption and emission phenomena. The synthesis of CZTS nanoparticles via a solution phase process is described in Chapter 4. The aim of this research was the production of small monodisperse particles. We investigate the influence of the solvent environment in high temperature decomposition syntheses, followed by the study of a novel chemical reduction method for CZTS nanocrystals. Chapter 5 regroups the research conducted on germanium and tin quantum dots, as well as the study on germanium/tin alloy. Germanium quantum dots, strong light emitters, were characterized optically in this study. The semiconducting phase of tin has great physical properties but is unstable in an ambient environment. So far reported tin nanoparticles synthesized via a solution process display only the metallic structure of tin. Presenting similar structural properties, germanium is expected to stabilize the quantum dot configuration when alloyed to tin. In Chapter 6 are described three different collaborative projects towards the application of silicon quantum dots in solar cells. First silicon quantum dots were anchored to zinc oxide nanowires arrays. Then we investigated the optical properties of SiQDs blended in a matrix of block copolymers. The third project looks into the effect of SiQDs spread over the surface of a working silicon solar cell. Finally, the last chapter presents an overall conclusion and summarizes the main findings of this study. It also introduces perspectives for future work with concepts on how to overcome the problems encountered in this research and ideas towards concrete industrial application of quantum dots.</p>

Molecular Dynamics Simulations of the Thermal Conductivity of Silicon-Germanium and Silicon-Germanium-Tin Alloys

In this work, we investigate the thermal conductivity properties of Si 1 − x Ge x and Si 0.8 Ge 0 Sn 2 y alloys. The equilibrium molecular dynamics (EMD) is employed to calculate the thermal conductivities of Si 1 − x Ge x alloys when x is different at temperatures ranging from 100 K to 1100 K. Then nonequilibrium molecular dynamics (NEMD) is used to study the relationships between y and the thermal conductivities of Si 0.8 Ge 0.2 Sn 2 y alloys. In this paper, Ge atoms are randomly doped, and tin atoms are doped in three distributing ways: random doping, complete doping, and bridge doping. The results show that the thermal conductivities of Si 1 − x Ge x alloys decrease first, then increase with the rise of x , and reach the lowest value when x changes from 0.4 to 0.5. No matter what the value of x is, the thermal conductivities of Si 1 − x Ge x alloys decrease with the increase of temperature. Thermal conductivities of Si 0.8 Ge 0.2 alloys can be significantly inhibited by doping an appropriate number of Sn atoms. For the random doping model, thermal conductivities of Si 0.8 Ge 0.2 Sn y alloys approach the lowest level when y is 0.10. Whether it is complete doping or bridge doping, thermal conductivities decrease with the increase of the number of doped layers. In addition, in the bridge doping model, both the number of Sn atoms in the [001] direction and the penetration distance of Sn atoms strongly influence thermal conductivities. The thermal conductivities of Si 0.8 Ge 0.2 Sn y alloys are positively associated with the number of Sn atoms in the [001] direction and the penetration distance of Sn atoms.