This thesis presents advancements on the modeling of quantum dots using Fourier-space k.p theory and on the use of InGaN quantum dots for ratchet band solar cells. Fourier-space based methods have generally assumed sharp material interfaces for electronic structure, strain and piezoelectric potential calculations in quantum dot systems. Additionally, standard Fourier-space methods have often assumed uniform elastic and dielectric constants for the strain and piezoelectric potential calculations. We present generalized methods to include smoothly varying alloy profiles for the quantum dots, including spatially varying elastic and dielectric constants for the strain and piezoelectric potential calculations. For the case of InGaN/GaN quantum dots, we show that the elastic and dielectric constants corrections are important for accurate strain, piezoelectric potentials, and electronic structure. The smooth alloy profiles are constructed by convolving sharp alloy profiles with a Gaussian, and we show that the electronic structure strongly depends on the smoothing kernel, indicating the need for precise alloy profiles for accurate electronic structures. We also present a new method that facilitates the coupling of strain into the k.p Hamiltonian when considering isolated dots, greatly reducing the computational costs of calculating the Hamiltonian matrix elements. Using the methods, we investigate the use of InGaN/GaN quantum dot superlattices as ratchet band solar cells, where we propose to use the piezoelectric potential to generate a ratchet. The piezoelectric potential can spatially separate confined electron and hole states, creating a spatial ratchet in order to reduce recombination. From our quantum dot k.p model, we calculate optical light absorption cross sections and present an improved method to calculate bound-to-continuum absorption, where electrons are excited out of the dots. In this method, we approximate the continuum states as bulk k.p states for GaN. By coupling our k.p model absorptions into a detailed balance model, we predict power conversion efficiencies. We find that a large number of QD layers is necessary to achieve sufficiently strong absorption as to reach high efficiencies, highlighting one of the key issues of QD-based solar cells. We consider systems which consists of up to 131000 layers of quantum dots and an ideal Lambertian back reflector with two different QD geometries. The first geometry possesses a strong spatial ratchet and can reach a maximum efficiency of 36% under a 1-sun 6000 K black-body spectrum. We verify the existence of the spatial ratchet through the optical properties of the system, showing that it truly has the potential to block recombination. We also present an efficiency optimized system that reaches a 42% detailed balance efficiency but does not have a spatial ratchet.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/43330 |
| Date | 28 February 2022 |
| Creators | Robichaud, Luc-Eugène |
| Contributors | Krich, Jacob |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Attribution-ShareAlike 4.0 International, http://creativecommons.org/licenses/by-sa/4.0/ |
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