DOI: https://doi.org/10.15368/theses.2008.23
Available at: https://digitalcommons.calpoly.edu/theses/8
Date of Award
6-2008
Degree Name
MS in Engineering - Materials Engineering
Department/Program
Materials Engineering
Advisor
Richard Savage
Abstract
Photovoltaics harness energy emitted from the sun. When the sun's energy is absorbed by a photovoltaic cell array, photons of light are converted into current. The amount of current produced by a photovoltaic cell is calculated by the difference between the energy of the incoming photon and the energy required for an electron to travel across the band gap of the photovoltaic cell. Traditional solar cells are commonly manufactured from silicon, which have a bandgap energy of 1.1 eV. If the photon's energy is equal to or greater than the band gap of silicon, electrons are excited from the valence band to the conduction band. Electron excitation between these respective bands enables current flow within the photovoltaic cell. Increasing the number of photons energies equal to the bandgap of the photovoltaic material will increase the amount of current produced.
The objective of this research was to explore the utilization of quantum dots to increase the amount of light collected by a silicon-based photovoltaic cell. Although the electromagnetic spectrum of the sun is broadband, only a finite portion of the spectrum can be harnessed by current solar cell technology. For example, the excess energy of ultraviolet light, when compared to the bandgap of silicon, is generally lost to thermalization; which prevents current production. Therefore the range of the electromagnetic spectrum that is available to a solar cell for electric current is limited. One mechanism to increase the efficiency of solar cells is to increase the electromagnetic spectrum collected.
Quantum dots are known to down convert high-energy photons to lower energy photons; thereby expanding the useable electromagnetic spectrum. This study investigated the changes associated with dispersing quantum dots above the surface of a photovoltaic cell, as well as, measuring how the electric current of the device is affected. The quantum dots were purchased from Evident Technologies and were made from CdSe/ZnS. Once acquired, the quantum dots were suspended in microfluidic channels fabricated from polydimethylsiloxane (PDMS). Toluene and water were respectively chosen to disperse the quantum dots. The compatibility of these solvents with PDMS was explored. The change in current was investigated when the microfluidic channels filled with quantum dots were applied to the surface of the photovoltaic cell.