Lead sulfide quantum dots and their application for solar cells

Abstract

Quantum dot sensitized solar cells (QDSSCs) are interesting third generation solar cells that have potential to address the current energy related issues due to their low manufacturing cost, ease of fabrication as well as good performance. Quantum dots (QDs) offer several advantages such as size tunable band gaps across a wide range of energy levels, high molar extinction coefficients and enhanced stability. Among them, colloidal near infrared (NIR) QDs of lead sulfide (PbS) are attractive due to their narrow bulk bandgap, large exciton Bohr radii and the possibility of multiple exciton generation. Utilizing these QDs in solar cells with extendable IR absorption is promising. However, the progress of PbS QDSSCs is lacking due to the limited understanding regarding the synthesis and surface chemistry of QDs. The development of QDSSCs is also hindered by lack of proper counter electrode materials for the reduction of electrolytes. Hence, further developments in the synthesis and application of new materials for QDSSCs are necessary. This PhD project focuses on the materials development for PbS QDSSCs such as PbS QD synthesis, surface ligand exchange of PbS QDs, and the development of new counter electrode materials. The following researches are included in this thesis: 1) A robust method to synthesize monodisperse lead sulfide (PbS) QDs is presented. PbS QDs with different sizes is produced by stepwise heating of the preformed seed QDs in the presence of excess oleic acid. A combination of "living" monomer addition and Ostwald ripening is identified as the mechanism for such QD growth processes. 2) The detailed synthesis mechanism of PbS QDs is investigated. Here, the various synthesis parameters influencing the nucleation and growth of PbS QDs are elucidated. In addition, the detailed understanding of the synthesis mechanism is used to guide the synthesis of PbS QDs at ultra-small regime. 3) A versatile solution phase ligand exchange of PbS QDs in the presence of Pb-thiolate as the exchanging ligands is presented. The ligand exchange procedure better preserves the optical properties of PbS QDs and is applicable to a number of ligand/solvent systems. 4) The implementation of PbS QDs in QDSSCs is presented. The treatment of PbS QD photoelectrodes with cadmium salts is necessary to maintain the stability of PbS QDs in polysulfide based electrolytes. In addition, the number of cycles of CdS and ZnS treatment is optimized to achieve a photoconversion efficiency of 1.77 %. 5) Finally, N-doped CNₓ/CNT hetero-electrocatalyst materials using polydopamine is synthesized, which are explored as counter electrode materials for dye-sensitized solar cell (DSSC). These CNₓ/CNTs material show excellent electrocatalytic activities towards the reduction of tri-iodide electrolytes with the optimized solar devices using CNₓ/CNTs showing comparable performance (7.3 %) to reference Pt based devices (7.1 %).