The Fabrication and Characterisation of Semiconductor Sensitized Photoanodes
Author(s)
King, Lauren
Type
Thesis
Abstract
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.
(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.
Date Issued
2012-12
Date Awarded
2013-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Riley, Jason
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)