Modelling and characterization of carrier transport through nanostructures

Photovoltaics is considered to be one of the best alternatives to fossil fuels to meet global energy demand. The improvement of solar cell efficiency beyond the Shockley-Queisser limit and towards the thermodynamic limit may be realized through “third generation photovoltaic” concepts. Two major strands of research in third generation photovoltaics are hot carrier solar cells and tandem solar cells. In the former, carriers are extracted before they cool down to the band edges, while the latter employs superlattices for more effective conversion of sunlight. High efficiencies in these cells are achieved by using materials engineered at the nanoscale. A hot carrier solar cell has two parts: a hot carrier absorber and a selective energy contact for energetically filtering electrons. Quantum dot double barrier structures are the most suitable candidates for making energy selective contacts. In this work, I have developed a 3 dimensional model using effective mass approximation and scatter matrix methods to study resonant tunnelling transport through energy selective contacts. The impacts of disorder in the position and size of the quantum dots on the conductivity of the structure are investigated. A novel design using alternating layered dielectrics is analysed using this model and found to have significant advantages over double barrier structures lacking this feature. The confined energy in quantum dots depends on their size and shape. Recent characterization techniques have shown that only a few dots in a self assembled layer represent an ordered array of spherically shaped dots. An efficient, self-consistent 3 dimensional, model for calculating energy confinement and wavefunctions in quantum dots of arbitrary shapes is developed using Multi Grid algorithms. Silicon quantum dots and their interaction in different dielectric materials are analysed for the best performance in tandem solar cells. Optically assisted I-V is carried out on structures with silicon quantum dots in silicon dioxide. Probing the hot carrier population electrically in steady state as a function of spectrally selective optical excitation shows negative differential resistance, indicating resonant tunnelling behaviour. This modelling and characterization work has produced results that are relevant in understanding the transport mechanisms in nanostructures and optimizing the design of energy selective contacts in hot carrier solar cells as well as quantum dot structures in tandem solar cells.