Aqueous Cadmium Chalcogenide Quantum Dots: Synthesis, Characterization, Spectroscopic Investigation of Electron Transfer Properties and Photovoltaic Device Performance
Coughlin, Kathleen M.
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The research presented in this thesis will focus on (1) synthesis and characterization of aqueous CdSe quantum dots (QDs) as alternatives to organic QDs; (2) the roles that linkers and capping groups play on the physical and optical properties of aqueous QDs; (3) performance of QD-sensitized solar cells (QDSSC) incorporating magic-sized clusters (MSCs) vs regular QDs (RQDs) and as a function of the molecular linkers between QDs and TiO 2 ; and (4) the effect of the polysulfide electrolyte on QD-functionalized TiO 2 films and their performance in QDSSCs. The use of QDs as light harvesters has grown over the last few decades due to their unique properties. Water-dispersible QDs are of increasing interest because their syntheses are straightforward, environmentally-benign and more cost-effective. CdSe and CdS QDs were synthesized at room temperature under ambient conditions, by combining a Cd precursor and either Na 2 SeSO 3 (for CdSe) or Na 2 S 2 O 3 (for CdS), in basic aqueous reaction mixtures. Three different ligands were utilized as capping groups (cysteinate (Cys), mercaptopropionate (MP), and mercaptosuccinate (MS)). Changing the capping-agent, the reagent concentrations and the temperature changed the photophysical properties of the QDs. When Cys was used as the capping-agent, MSCs were formed. When high concentrations of Cys were used or when the reaction mixture was heated, RQDs were formed. When MP and MS were used in the synthesis of CdSe, RQDs were formed. In the synthesis of CdS, MS caused the formation of RQDs, and MP caused formation of a mixture of RQDs and MSCs. Transient absorption spectroscopy and photoelectrochemical experiments were performed to understand the influence of capping-agent and electronic properties (MSCs vs. RQDs) on the efficiency of electron transfer from photoexcited QDs to TiO 2 . The Cys-CdSe-functionalized TiO 2 exhibited more efficient electron injection and/or slower recombination, leading to improved efficiency of QDSSCs. Devices made with Cys-CdSe RQDs exhibited higher efficiencies than QDSSCs with Cys-CdSe MSCs, indicating that the presence of Cys induces greater efficiency in QDSSCs. For QDSSCs, a polysulfide electrolyte is commonly used instead of I - /I 3 - . Effects of the polysulfide electrolyte and Na 2 S on CdSe-functionalized TiO 2 films were explored to elucidate the chemistry that occurs when CdSe is exposed to the electrolyte in the QDSSCs. Photophysical changes to the CdSe-functionalized films occurred when exposed to the polysulfide electrolyte. Upon immersion of CdSe-functionalized TiO¬ 2 films into solutions of Na 2 S, an initial red shift in the absorption spectrum was observed, followed by a decrease and blue shift of the band. Based on XPS analysis of the films, it was determined that the S within the electrolyte (1) replaced Se or (2) bound to the CdSe-functionalized film without replacing Se. Photoelectrochemical measurements were acquired to analyze device performance with varying [S] within the electrolyte. Upon increase of S, the efficiency of the device increased and gave rise to a better fill factor in the photocurrent-photovoltage data collected.