• Søgaard, Nicolaj Brink

Abstract

Production, storage, and transport of green energy is one of the most urgent challenges for the entire world today, making any research progress within this field valuable. One of the technologies used today is solar cells. However, the efficiency of current commercial solar cells could be higher. Multi-junction solar cells can increase this efficiency by improving the energy gained from high-energy photons and absorbing a larger part of the spectrum. Semiconductors with different sizes of bandgaps are needed to build such devices, and one way to achieve this is through quantum dots, where the bandgap increases as the size decreases.<br/>A good candidate for such multi-junction solar cells is Ge_1−xSn_x nanoparticles investigated in this thesis. A typical inert synthesis of Ge_1−xSn_x nanoparticles that produces a GeO₂ contamination in the resulting powder is investigated to locate the origin of the oxygen contamination. The contamination source is not directly found, but indications suggest that the most possible sources of contamination are the solvents, the glove box, or the Schlenk line used in the reactions. A new autoclave synthesis of Ge_1−xSn_x nanoparticles is presented to circumvent the oxidation of the Ge precursor. The autoclave is a non-inert synthesis prepared under ambient conditions, in contrast to typical syntheses of Ge_1−xSn_x nanoparticles. The synthesis succeeded in producing phase pure Ge_1−xSn_x nanoparticles with Sn concentrations up to about 4% and sizes of 11-25 nm. When more than 4% Sn was used in the synthesis, a side-product of β-Sn appeared in the resulting particles while keeping the Sn concentration of the Ge_1−xSn_x particles constant but increasing their size up to about 55 nm. The results from <i>in situ</i> powder X-ray diffraction (PXRD) measurements on the synthesis indicate that the Sn precursor is catalyzing the reaction and that crystals are only formed after becoming about 10 nm in size.<br/>The optical properties of the particles from the non-inert syntheses were investigated with different techniques. The particles show indications of semiconductor properties from the terahertz time-domain spectroscopy (THz-TDS) and ultraviolet-visible spectroscopy (UV-Vis) measurements. In THz-TDS, the sheet conductance of one set of the Ge particles increases as a function of pump laser power on the sample, indicating increased sheet conductance at higher excitation powers. In the UV-Vis measurements, one set of the Ge_1−xSn_x particles shows an energy-dependent transmission as expected in a semiconductor material. This indicates that these particles still hold potential for use in solar cells when combining the results above with the crude estimate of the diffusion length of 12.7 µm. The particles must be characterized much further before their use in solar cell-like devices is considered. However, the main challenge for this use of the particles is to produce films where the transport of free carriers between particles is greatly improved while making thin films of high quality with the particles.<br/>Finally, chemical vapor deposition (CVD) growth of Ge_1−xSn_x films was also studied during the change of research environment at Forschungszentrum Jülich. This resulted in the discovery of a temperature-independent change of Sn concentrations in the growth of Ge_1−xSn_x films. The change in Sn concentration is instead achieved by changing the N2 carrier gas flow, which makes it possible to increase and decrease the Sn concentration of consecutive layers interchangeably.<br/>

Topics

• nanoparticle
• impedance spectroscopy
• phase
• thin film
• Oxygen
• semiconductor
• powder X-ray diffraction
• quantum dot
• chemical vapor deposition
• terahertz time-domain spectroscopy