Silicon quantum dots embedded in SiO2/Si3N4 hybrid matrix for tandem photovoltaic cells

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Copyright: Di, Dawei
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Abstract
Silicon is the core element for microelectronics and conventional photovoltaics. The absorption and emission of photons in bulk Si are limited by its indirect electronic band gap that requires phonons for momentum conservation. In quantum confined low dimensional systems such as Si quantum dots (Si QDs), this momentum conservation becomes less stringent due to Heisenberg uncertainty principle. Si QD based materials provide the additional advantage of a tunable electronic band gap that is particularly important in the realisation of an all-Si tandem solar cell, a third generation photovoltaic device that aims to exceed the Shockley-Queisser Limit at low cost. The thesis investigation begins with the fabrication and analysis of single junction Si QD solar cells on quartz substrates, using Si QDs embedded in SiO2 matrix. The impacts of post-metallisation treatments such as phosphoric acid (H3PO4) etching, nitrogen (N2) gas anneal and forming gas (Ar: H2) anneal on the cells electrical and photovoltaic properties have been studied. The Si QD solar cells investigated in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from electrical measurements suggest that the performance of the solar cell is strongly limited by poor carrier transport. This limiting factor can be partly eliminated by forming gas annealing. To enhance current transport in the cell, we propose a new nanostructure of Si QDs in SiO2/Si3N4 hybrid matrix as a possible improvement to the conventional Si nanostructure - Si QDs in SiO2 matrix . Initial work on the synthesis of undoped Si QDs in SiO2/Si3N4 shows the size and crystallization of the Si QDs can be controlled by factors including the annealing method and the thicknesses of the nanoscale layers, as evidenced in X-ray diffraction and Raman studies. Photoluminescence measurements indicate the apparent band gap of the material increases with reducing nanocrystal (NC) size, exhibiting the effect of quantum confinement. It is also shown that the conventional Si quantum dots in SiO2 matrix approach can lead to the formation of over-sized Si nanocrystals especially when doped with phosphorous, making the size-dependent quantum confinement less effective. By replacing the SiO2 tunnel barriers by the Si3N4 layers, the new material manages to constrain the growth of doped Si quantum dots effectively and enhances the apparent band gap. Besides, electrical characterization on Si QD/c-Si hetero-interface test structures indicates the new material possesses improved vertical carrier transport properties. To gain further understanding of the photon absorption and emission mechanisms of the new material, doped and undoped Si QDs in SiO2/Si3N4 matrix are characterised optically via spectroscopic and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption spectra using the Hishikawa s method and Tauc s analysis. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Lastly, we demonstrate all-Si light-emitting diodes (LEDs) based on boron-doped Si QD/c-Si p-n heterojunction structures, which show electroluminescence in the visible/infrared regions. The EL peaks at ~ 1.7 eV are a further evidence of enhanced electronic band gap of Si QDs due to quantum confinement. These LEDs are fabricated using the same materials and techniques developed in this thesis for Si QD cells. The electroluminescence spectra of these diodes can be modified by changing the quantum confining barriers from SiO2 to Si3N4. The results are the first demonstration of electroluminescence from boron-doped Si nanocrystals. The nanostructure material proposed in this thesis Si QDs in SiO2/Si3N4 hybrid matrix has exhibited improvements in a number of material properties, including nanocrystal size control, quantum confinement and current transport, over the conventional Si QDs in SiO2 matrix. Despite the necessity for further optimisation, this novel material is evidently a promising building block for the realisation of all-Si tandem solar cells.
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Author(s)
Di, Dawei
Supervisor(s)
Conibeer, Gavin
Green, Martin
Perez-Wurfl, Ivan
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Publication Year
2012
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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