Abstract
Third generation photovoltaics (PV) presents exciting possibilities for reaching power
conversion efficiencies of up to 66 % through the use of nano‐regime materials, exploiting
both the quantum confinement and phonon confinement effects. Quantum dot (QD)
structures enable property tunability and manipulation over the physical characteristics
providing building blocks for device engineering of third generation PV applications such as
tandem solar cells and hot carrier solar cells.
This thesis reports the fabrication of single and multilayer films, up to ten layers, of silicon
quantum dots (Si QDs) from colloidal dispersions in chloroform using Langmuir‐Blodgett
apparatus. The physical, optical and phononic properties of both solution and Si QD films
are reported. This research is the first to examine the film property evolution as the films
form multilayer structures.
Si QDs surface functionalized with dodecene and octadecene were found to have an
average diameter of 2.7 ± 0.6 nm and 2.7 ± 0.8 nm, respectively, as determined by
transmission electron microscopy. Si QD films were observed to show room temperature
photoluminescence (PL) of 1.66 eV and 1.64 eV and the optical bandgap of the Si QD films
were determined through absorption spectroscopy as 2.0 eV and 1.75 eV, for the
dodecene and octadecene‐Si QDs, respectively. The optical bandgap and PL emission peak
are blue‐shifted from the indirect bandgap of bulk silicon, confirming quantum
confinement. Quantum confinement effects do not increase with increasing QD packing
(surface pressure) or the number of layers; however, a small (~ 20 meV) red‐shift in the
peak emission and optical bandgap were recorded when the colloidal QDs initially
assemble into single layer films. Raman spectroscopy was used to confirm phonon
confinement in the Si QD films, observed as a Stokes‐shift of the Si Raman peak at 514 cm‐1
and 513 cm‐1 for the dodecene and octadecene‐Si QD films, respectively. Phonon
confinement is observed; however, further investigation into the low‐wavenumber phonon
modes is recommended.
Together the presence of phonon and quantum confinement effects make colloidal silicon
QD films an attractive material for use in third generation PV, in both the tandem solar cell
and hot carrier solar cell devices.