Langmuir Blodgett films from colloidal dispersions of silicon quantum dots and their potential use in third generation photovoltaics

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.