Langmuir Blodgett films of colloidal silicon quantum dots for hot carrier photovoltaics

Abstract

Colloidal quantum dots (QDs) materials have been found on a promising path to achieve practical low-cost and high-efficiency third generation solar cells. The solution processability of colloidal QDs enables low-cost fabrication of various kinds of colloidal QDs solar cells. Moreover, favourable carrier dynamics are revealed in colloidal QDs, such as multiple exciton generation and slowed carrier cooling, which potentiate the demonstration of third generation solar cells. This thesis develops methods to assembly solution-form colloidal Si QDs into uniform close packed films and investigates carrier dynamics in such films, and subsequently proves that Si QDs can be employed as qualified hot carrier absorber materials. Uniform close packed single and multiple-layer films of colloidal Si QDs are fabricated using Langmuir Blodgett (LB) technique, by reasonably tuning the conventional LB processes. Unique hot carrier related effects are revealed via a series of optical spectroscopies. Using Raman scattering, size-dependent phonon confinements and LB film enhanced acoustic phonon folding are inspected. Employing transient photoluminescence spectra, a coherent lifetime evolution is established. Finally utilizing excitation power dependent continuous wave photoluminescence spectroscopy, carrier temperatures are estimated. Si QDs are proved to be potential as hot carrier absorbers from the aspects of effective phonon manipulation, prolonged carrier decay lifetimes and elevated carrier temperatures. Since these studies are systematically carried out on Si QDs of three different sizes, some plausible mechanisms are able to be put forward. In addition, based on the comparative studies on the drop cast and LB films, the hot carrier related effects are shown to be strengthened in ordered arrays, and thus the optimum film thickness (13 LB layer) is proposed for hot carrier absorber.