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Title: The fabrication and characterisation of semiconductor sensitized photoanodes
Author: King, Lauren
ISNI:       0000 0004 2741 9178
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Cadmium selenide (CdSe) quantum dots (QDs) and molybdenum disulfide (MoS2) are considered as effective light harvesting assemblies for application in semiconductor sensitized solar cells (SSSCs). CdSe QDs were synthesized following a trioctylphosphine oxide (TOPO) stabilized hot injection method. Prior to sensitization, QDs are subject to a common purification; cycles of alternate precipitation/re‐dispersion in a nonsolvent/ solvent. Our study reveals the critical role of purification. With enhanced purification, the QD concentration at a functionalised surface has been shown to increase 5‐fold. Imaging reveals that QD agglomerates on the surface decrease in size and increase in population. Photocurrent measurements demonstrate the importance of the morphology and QD population on QD photoinjection and thus the necessity to control purification. Polysulfide electrolyte is the most commonly utilized electrolyte in quantum dot sensitized solar cells (QDSSCs). To date, there have been relatively few investigations into the stability of CdSe QDs in polysulfide solution. Bulk CdSe crystals have long been known to undergo sulfur substitution reactions resulting in CdS layers of a few nanometres thickness at the surface of CdSe crystals. Here, post exposure to polysufide a red‐shift in the absorbance, and photocurrent onset of QDs is observed. Through structural, chemical and optical studies of QD only, and QD sensitized TiO2 samples, the shift in onset is attributed to a combination of change in both QD structure and film morphologies. Due to their photocatalytic stability and appropriate band gaps, group 6 transition metal dichalcogenides (TMD) such as MoS2 have long been considered candidates for photoelectrochemical cells (PEC). Low dimensional materials have recently attracted significant attention, and in particular monolayer MoS2 has been highlighted for its unique optical properties. Upon decreasing thickness, the indirect band gap of bulk MoS2 shifts to a direct gap material for monolayer crystals. Here we investigate the photoelectrochemical properties of ultra‐thin films of chemically exfoliated MoS2 and its composites with TiO2 nanoparticles. MoS2 monolayer films are shown to exhibit effective PEC properties similar to bulk materials, generating photocurrent at excitation wavelengths above the direct band gap edge at ~660 nm. We also demonstrate that MoS2 monolayers sensitized to TiO2 behave as effective photosensitizers. We find that in PEC cells with TiO2‐MoS2 composite photoanodes, excited electrons in MoS2 are able to inject into TiO2 while holes are removed by the electrolyte so as to generate electrical current from incident light. Our results demonstrate the potential of solution‐processed MoS2 monolayers for PEC applications.
Supervisor: Riley, Jason Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral