Performance enhancement of quantum dot sensitised solar cells through enhanced interfacial charge transfer kinetics

Evangelista, R 2019, Performance enhancement of quantum dot sensitised solar cells through enhanced interfacial charge transfer kinetics, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Performance enhancement of quantum dot sensitised solar cells through enhanced interfacial charge transfer kinetics
Author(s) Evangelista, R
Year 2019
Abstract 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.

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. Silicon-based solar PVs. 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. Because of this minuteness the QD's optical and electronic properties differ from those of its bulk material's properties, such that it will absorb/emit light usually from the visible to infrared wavelength in the solar spectrum. In addition, these properties can be controlled by tuning the parameters during synthesis, opening up a number of applications in biotechnology, electronics, photovoltaics, and quantum computing. This thesis focuses on the photovoltaic application of QDs specifically investigating the liquid junction QDSSC.

There have been previous studies focusing on the components such as electrode, sensitiser, counter-electrode, and limited studies on 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 CdS 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 CdS QDSSC based on aqueous Fe(CN)6^4-/Fe(CN)6^3- electrolyte. • analysing the effects of varying concentrations of the reduced (Fe(CN)6^4-) and oxidised (Fe(CN)6^3-) species in a ferrocyanide/ferricyanide electrolyte on the performance of a CdS QDSSC. • identifying the most influential factors on the output of a CdS QDSSC using the Matlab software for optimised fitting of a theoretical vs. experimental voltage-current curve based on the diode model. • disseminating the results of the investigation conducted via publication in peer-reviewed journals.

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 CdS QDSSC? • What are the optimal reduced and oxidised species concentrations in a ferrocyanide/ferricyanide electrolyte to maximise the performance of a CdS QDSSC? • Which parameters in the diode model of the CdS QDSSC cell have the most influence on cell performance with this redox electrolyte and which among these parameters are sensitive to tolerance changes? • 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 PbS as model) by using established and modified parameters. • Studying the interfacial charge transfer kinetics and transport of a CdS 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. • Designing an optimal reduced and oxidised species concentration combination and observe its effect on the cell's conversion efficiency.

Summarising the findings from this thesis: 1. PbS QD size engineering can be done by keeping the precursor ratio constant while the injection temperature variable. 2. PbS QDs can be stored in air/dark without effect on its optical properties after one bubbling in nitrogen. 3. PbS QDs remain optically stable after 60 days in air/dark environment. 4. PbS QDs can be dried when needed to be transported and re-dispersed without adverse effect on the absorption. 5. 0.2 M reduced species concentration is the optimal reduced species concentration in this study. 6. 0.01 M oxidised species concentration results in relatively slower charge recombination at the TiO2 surface hence high FF results in longer lifetime thus higher open circuit voltage (VOC). 7. At fixed oxidised species concentration (0.01 M) in the electrolyte, a sufficiently low (<0.02 M) reduced species (ferrocyanide) concentration controls the anodic limiting current. 8. A Matlab algorithm found that the ideality factor deteriorated (n>2 where the ideal value is 1) as the irradiation intensity was increased. 9.The extracted parameters that were sensitive to slight changes (± 1%) were identified as the ideality factor, n, and shunt resistance, Rh. 10. The extracted ideality factors result showed that the interfacial recombination increased once the irradiation is more than 100% i.e. 120%, 130% via solar simulation.

Recommendations from this study are: 1. Size engineering studies should be extended to much larger QD sizes and temperature and molar ratios being the parameters to focus on still. Emission and excitation spectral measurements on QDs should also be conducted. 2. Further studies on the QD ageing beyond 180 days in order to establish QD's practical lifetime. 3. 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. 4. 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. 5. 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 mA/cm^2, corresponding to an IPCE of 57% at 410 nm and overall conversion efficiency of 2%.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Energy Generation, Conversion and Storage Engineering
Keyword(s) Semiconductor quantum dot sensitised solar cells
Ferrocyanide/ferricyanide redox couple
Photovoltage decay
Transient photocurrent
SILAR deposition
Single diode equation
Matlab simulation
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Created: Thu, 30 May 2019, 16:53:32 EST by Pinipa Sugandi
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