Quantum Dot Sensitized Solar Cells Thesis

Quantum Dot Sensitized Solar Cells Thesis-53
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.• advancing the understanding of the interfacial charge transfer kinetics in a Cd S QDSSC based on aqueous Fe(CN)6^4-/Fe(CN)6^3- electrolyte.

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.• advancing the understanding of the interfacial charge transfer kinetics in a Cd S QDSSC based on aqueous Fe(CN)6^4-/Fe(CN)6^3- electrolyte.

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Luminescent and conductive carbon dots or C-dots were also included in this cell to improve electron transfer and transport.

Incident photon to current conversion efficiency or IPCE measurements revealed an optimal coverage of the visible spectrum due to FRET.

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.

• 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.• What are the optimal reduced and oxidised species concentrations in a ferrocyanide/ferricyanide electrolyte to maximise the performance of a Cd S QDSSC?• Which parameters in the diode model of the Cd S QDSSC cell have the most influence on cell performance with this redox electrolyte and which among these parameters are sensitive to tolerance changes?• 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.Colloidal nanocrystals or quantum dots (QDs) have attained immense scientific interest as light sensitive materials due to band gap tunability by size control, ease of preparation, multiple exciton generation and low cost.In the last decade, although reasonably high power conversion efficiencies (PCEs) of 5% have been realized in quantum dot sensitized solar cells (QDSSCs), the efficiencies of QDSSCs continue to lag behind that of dye sensitized solar cells or DSSCs.There is a plethora of renewable energy sources that wait for us to be harnessed - wind, geothermal, wave, solar energies, to name a few - which are more than enough to supply our energy demand.The sun, with its enormous amount of free energy at 3 x 10^24 Joules/year, is estimated to be capable of covering 10,000 times the world's energy requirement at the beginning of the 21st century.

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