Building Towards Singlet Fission Solar Cells - from photophysics to application

Autor: Hosseinabadi, Parisa
Jazyk: angličtina
Rok vydání: 2023
Předmět:
DOI: 10.26190/unsworks/24813
Popis: Singlet fission (SF) is a photochemical process whereby high-energy photons can be absorbed by a molecular material to produce two triplet lower-energy excitons. The process could help improve the efficiency of silicon solar cells where an excessive energy loss through the thermalisation of photogenerated carriers occurs. This Ph.D. dissertation is dedicated to understanding SF from photophysics of the process to the application in photovoltaic (PV) solar cells and contains four original research projects (chapters 4-7) to fulfil this aim. The photophysical stages of the SF process was investigated using ultrafast laser techniques whereby time-resolved spectroscopy (absorption and emission) was used to determine the evolution of the singlet and triplet populations. Singlet populations are emissive at char- acteristic energies and can therefore be probed using photoluminescence spectroscopy. Probing the population of non-emissive triplet states is best performed using absorption spectroscopy. Using these techniques, the role and behaviour of intermediate states in the singlet fission process could be evaluated. The work begins with the identification of a self-absorption effect when performing steady-state absorption and emission spectroscopy that takes place in concentrated solutions of singlet fission within nodips-tetracene in Toluene solvent. This led to a series of experiments whereby the behaviour of intermediate states could be probed in nodips-tetracene using transient absorption spectroscopy, confirming the existence of an intermediate state in the SF process. The detection and behaviour of an intermediate state was confirmed in nodips tetracene by employing time-resolved photoluminescence (TRPL) using a set of samples where the nodips-tetracene was diluted using Toluene. Importantly this result adds to a body of evidence surrounding the role of an intermediate excimer state, which although present, does not appear to be necessary for the singlet fission process. Spatial-dependent photoluminescence (PL) emission shows that in the organised and rigid area in the pure sample (meniscus), the emission spectra contain more excimer-character, while in the bulk area where molecules can diffuse more freely, the emission spectra have more singlet-character. To augment the efficiency of a silicon solar cell the energy must be transferred from an overlying molecular layer into the silicon solar cell. To achieve this some preliminary work was performed to determine the extent by which energy can be transferred from the non-emissive triplet-excitons to an emissive species. Using TRPL technique energy transfer from the triplets state of 8-hydroxyquinoline (Q) to the Ytterbium (Yb+3) ion was observed. To integrate singlet fission into a silicon solar cell, it will be necessary to add a molecular layer next to the silicon solar cell, which will change the optical dielectric environment for light entering the device. As a result, optimisation of the solar cell anti-reflection coating (ARC) for the tetracene-Si structure was performed. The optical properties of tetracene thin film on Si wafer was acquired using ellipsometer and a python-based package using the transfer matrix method was employed for ARC optimisation. A double layer coating (DLARC) containing MgF2 and TiO2 with a thickness of 127 and 22 nm, respectively, was confirmed to gain less than 2.5% reflection of incident light. Finally, the efficiency and economics of SF-enhanced PV was determined. For this purpose, tetracene-Si PV cell was simulated under realistic conditions using the PVsyst computer software based on a Si-mono 50 Wp cell from Generic company. Comparing the efficiency of primary conventional cell and SF-enhanced solar cell has shown 19% increase. Furthermore, the operating temperature of SF- and conventional PV cells was compared within two different climate environments confirming that SF-enhanced PV cell operates in 1-2 ◦C lower temperature, especially in higher irradiance. This can increase the life span of solar cells. The levelized cost of efficiency (LCOE) was calculated for SF-enhanced PV cells to determine the maximum allowable cost to fabricate these cells and the cost-effectiveness of area-related expenses for the extra efficiency achieved by SF-enhanced PV cells.
Databáze: OpenAIRE
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