Quantum Dot Sensitized Solar Cells Thesis

• Studying the interfacial charge transfer kinetics and transport of a Cd S QDSSC via controlling the reduced and oxidised species of redox electrolyte.• Writing an algorithm in Matlab using a single diode equation for solar cell simulation and another algorithm to simulate the sensitivity of the fitted parameters. Size engineering studies should be extended to much larger QD sizes and temperature and molar ratios being the parameters to focus on still.The aim of this thesis is thus to understand how the concentration of the redox electrolyte affects kinetics and dynamics of electrons at the interfaces of the Cd S QDSSC which was achieved by: • reviewing previous and current works on the enhancement of QDSSC conversion efficiency and studies on QDs.

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The main research questions addressed in this thesis are: • What are the alternative ways of controlling and handling QDs and how these handling conditions affect QD's ageing?

• How will ferrocyanide/ferricyanide redox electrolyte affect the interfacial charge transfer kinetics in a Cd S QDSSC?

• Designing an optimal reduced and oxidised species concentration combination and observe its effect on the cell's conversion efficiency. Pb S QD size engineering can be done by keeping the precursor ratio constant while the injection temperature variable. Pb S QDs can be stored in air/dark without effect on its optical properties after one bubbling in nitrogen. Pb S QDs remain optically stable after 60 days in air/dark environment. Pb S QDs can be dried when needed to be transported and re-dispersed without adverse effect on the absorption. 0.2 M reduced species concentration is the optimal reduced species concentration in this study. 0.01 M oxidised species concentration results in relatively slower charge recombination at the Ti O2 surface hence high FF results in longer lifetime thus higher open circuit voltage (VOC). At fixed oxidised species concentration (0.01 M) in the electrolyte, a sufficiently low (2 where the ideal value is 1) as the irradiation intensity was increased. The extracted parameters that were sensitive to slight changes (± 1%) were identified as the ideality factor, n, and shunt resistance, Rh. The extracted ideality factors result showed that the interfacial recombination increased once the irradiation is more than 100% i.e. Emission and excitation spectral measurements on QDs should also be conducted. Further studies on the QD ageing beyond 180 days in order to establish QD's practical lifetime. Further studies on this QDSSC model should be focused on other components such as the sensitiser, counter-electrode, and passivating agent since the maximum theoretical VOC has already been achieved in this study. Reasons why the ideality factor increases (moving away from ideal) as the irradiation intensity is increased need to be further investigated together with ways to improve the charge recombination kinetics. Since the maximum theoretical VOC is almost achieved in this thesis, it is recommended that the next study be focused on how to improve the short circuit current (JSC) and fill factor (FF).

In summary, the optimised ferrocyanide/ferricyanide concentration ratio of the redox electrolyte in the QDSSC examined in this thesis has been found to be 0.2/0.01 M resulting in a VOC of 0.8 V, a FF of 0.66, and JSC of 3.8 m A/cm^2, corresponding to an IPCE of 57% at 410 nm and overall conversion efficiency of 2%.

• To what extent does a ferrocyanide/ferricyanide electrolyte with optimised concentrations improve the overall QDSSC performance?

These questions were answered by: • Synthesising and characterising quantum dots (using Pb S as model) by using established and modified parameters.

Apart from the use of carbon nanostructures, another concept, FRET, was also exploited in this work to realize improved efficiencies in QDSSCs.

An electrode tethered QD assembly of Zn S/Cd S/Zn S was used as the donor and copper phthalocyanine (Cu Pc) molecules dissolved in the electrolyte were used as the acceptors.

The most common method of harvesting solar energy is through photovoltaic (PV) technology in which next-generation PV technologies are vastly becoming popular due to limitations in the mainstream solar PVs i.e. One of these next-generation PVs is the quantum dot sensitised solar cell (QDSSC), the focus in this thesis.

Quantum dots (QD) which are semiconductor nanomaterials used as sensitiser in QDSSCs, are physically very small in size, usually below 10 nm.


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