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Abstract
Si based quantum dot (QD) tandem cells have been considered as promising structures to obtain high energy conversion efficiencies through reducing thermalisation losses while still maintaining relatively low production costs by implementation of thin-film fabrication techniques. This type of solar cell is based on the idea that the bandgap of the Si based QD material can be engineered and applied for better utilization of the spectrum by stacking multiple cells on top of each other. A single-junction Si QD solar cell showing an open circuit voltage of 493 mV has been realized through sputtering and annealing alternating Si rich oxides (SRO) and silicon dioxide (SiO2) bilayers onto fused silica substrates, forming a mesa-type p-i-n structure. However, a current-crowding effect arises due to the thinness and
highly resistive nature of this mesa-type structure, limiting the current from the device to a negligible level.
In order to minimize the current-crowding effect, this thesis proposes a vertical structure for Si QD solar cells. This vertical structure is based on a conductive substrate, Molybdenum (Mo), to circumvent lateral carrier transport to contacts. A proof-of-concept study is demonstrated to discuss the feasibility of incorporating Mo into the proposed vertically structured device. Experimental work has been conducted to address two aspects: the suitability of Mo as a conductive substrate material; and the compatibility between Mo and Si QD materials subject to high temperature annealing processes.
In addition, Raman spectroscopy is employed as a major diagnostic tool to examine the properties of Si QDs fabricated via the solid-phase method. A simulation approach for the Raman spectra based on the one-phonon confinement model (PCM) is applied for the deconvolution of different Raman patterns. The objective of this model is to extract Si QDs size distribution information and estimate the Si crystalline fractions. For the first time, the reliability of this Raman model is compared and verified through simulation of photoluminescent (PL) spectra of the investigated sample.
The most important contributions and findings of this thesis are:
• First demonstration of extraction of Si QDs size distribution information from a combination of Raman and PL simulations.
• First study of annealing effects on Mo properties up to 1100 ℃.
• Demonstration of the compatibility between Si QDs and Mo, as well as between Mo processed at high temperatures.
• Demonstration of the compatibility of Mo thin films with various substrate types.
• Development of effective approaches to protect Mo from oxidation at elevated temperatures.
• Fabrication of the first vertically structured single-junction Si QD solar cell on a Mo conductive substrate, with 𝑉oc=200 mV and 𝐽sc=0.12 mA/cm2.