Circumventing Spectrum Mismatch - Studies of Triplet-Triplet Annihilation Upconversion, Singlet Fission and Two-Photon Absorption in Photoactive Materials
Doctoral thesis, 2023
Solar energy stands out for its potential to supply the global energy demand by itself. Therefore, it is of great value to expand the use of processes that involve light such as conversion to electricity or fuels, or to drive high-energy reactions. However, the limited availability of photons with the desired energy required to induce a certain photophysical process poses a challenge. To circumvent this spectral mismatch, processes that up- and down-convert photon energy can be used. In this work the focus lies on photon upconversion through triplet-triplet annihilation (TTA-UC) and two-photon absorption (2PA) and downconversion through singlet fission (SF).
One key challenge for up- and downconversion processes is that for them to be useful they must be incorporated into practical devices. Therefore, one of the overall objectives of this work is to evaluate photoactive materials that both change the photon energy and have potential to be incorporated with a working device. This means moving away from diffusion control in liquid solution.
Self-assembling organogels, with chromophores covalently attached to the gelator backbone, were tested as platforms for TTA-UC and SF. The results show that chromophore-chromophore interactions can be tuned by choice of substitution position, and that it is possible to obtain photon energy conversion in the studied self-assembling gels, although efficiencies need to be improved for practical applications. The results indicate that there is potential for future development of gel-based self-assembled photoactive materials.
To demonstrate how TTA-UC can be applied to circumvent spectral mismatches in devices, it was used to sensitize the well-known catalyst titanium dioxide (TiO2). A TTA-UC solution was used to absorb visible light and the upconverted emission was in turn used to sensitize a TiO2 thin film. Despite needs for optimizing the setup, visible light photoexcitation could be confirmed and the number of holes in valence band could be quantified to mM concentrations, using a redox couple, which at the same time confirmed the reactivity of the sensitized TiO2.
One key challenge for up- and downconversion processes is that for them to be useful they must be incorporated into practical devices. Therefore, one of the overall objectives of this work is to evaluate photoactive materials that both change the photon energy and have potential to be incorporated with a working device. This means moving away from diffusion control in liquid solution.
Self-assembling organogels, with chromophores covalently attached to the gelator backbone, were tested as platforms for TTA-UC and SF. The results show that chromophore-chromophore interactions can be tuned by choice of substitution position, and that it is possible to obtain photon energy conversion in the studied self-assembling gels, although efficiencies need to be improved for practical applications. The results indicate that there is potential for future development of gel-based self-assembled photoactive materials.
To demonstrate how TTA-UC can be applied to circumvent spectral mismatches in devices, it was used to sensitize the well-known catalyst titanium dioxide (TiO2). A TTA-UC solution was used to absorb visible light and the upconverted emission was in turn used to sensitize a TiO2 thin film. Despite needs for optimizing the setup, visible light photoexcitation could be confirmed and the number of holes in valence band could be quantified to mM concentrations, using a redox couple, which at the same time confirmed the reactivity of the sensitized TiO2.
singlet fission
solar energy conversion
two-photon absorption
energy transfer
triplet-triplet annihilation upconversion
spectroscopy.
carbon nitride quantum dots
self-assembly gels
photochemistry
10:an, Chemistry building, Kemigården 4 (Chalmers, Johanneberg campus)
Opponent: Malcolm D. E. Forbes, Bowling Green State University, United States
Author
Deise Fernanda Barbosa de Mattos
Chalmers, Chemistry and Chemical Engineering, Chemistry and Biochemistry
Covalent incorporation of diphenylanthracene in oxotriphenylhexanoate organogels as a quasi-solid photon upconversion matrix
Journal of Chemical Physics,;Vol. 153(2020)
Journal article
Structure-Property Relationships in Self-Assembled TIPS-Pentacene Organogels, Johnstone, M. D., de Mattos, D. F. B., Ringström, R., Ruiu, A., Mencaroni, L., Albinsson, B., Mårtensson, J., Abrahamsson, M., Sundén, H.
Towards TiO2 Photochemistry with Visible-to-UV Triplet-Triplet Annihilation Upconversion, de Mattos, D. F. B., Abrahamsson, M.
Two-Photon Absorption Characteristics of EDTA Functionalized CNQDs in Water, Mistry, L., de Mattos, D. F. B., Larsson, H., Martin, C. B., Abrahamsson, M.
The theoretical maximum efficiency of a present-day solar cell is around 32%. The reason for this number being so low is a mismatch between the solar spectrum and the so-called bandgap of the solar cell, which leads to a lot of sunlight not being used. Light that does not have enough energy to overcome this bandgap is not converted into electricity and light with more energy than necessary is not used as efficiently as it could be. In this work, materials with the potential to circumvent this mismatch issue have been studied.
Such materials must be able to either upconvert or downconvert the light energy to match the bandgap of the solar cell. Three such up- and downconversion processes are described in this work, namely singlet fission, upconversion through triplet-triplet annihilation and two photon absorption.
In singlet fission, one high energy light particle, a photon, can be transformed into two low energy charge carriers through the interaction between molecules with certain properties and light. Photon upconversion through triplet-triplet annihilation can be thought of as combining two low energy light particles into one with higher energy. Both processes require that the molecules are organized in a favorable way, for specific interactions between two molecules, in a number of energy transfer steps. Two-photon absorption is another upconverting process, where a molecule or material absorbs two low energy photons simultaneously and reaches a high energy state. In all cases, it may sound easy but in reality, only a few molecules and materials are able to do this. In addition, the small fractions of time in which these processes occur are difficult to measure and are a key for understanding and controlling them.
In this work, the conversion of light energy through singlet fission, triplet-triplet annihilation upconversion and two-photon absorption was studied in self-assembled gels and quantum dots. It also includes a case study showing how triplet-triplet annihilation upconversion can be used to circumvent the mismatch between the light spectrum and the bandgap of a semiconductor.
The understanding of the detailed sequence of energy transfer mechanism in the light conversion processes and the structure of the photoactive material may help to increase the efficiency of solar cells. Moreover, this provides information suitable to be applied in other areas such as bioimaging and digital displays.
Such materials must be able to either upconvert or downconvert the light energy to match the bandgap of the solar cell. Three such up- and downconversion processes are described in this work, namely singlet fission, upconversion through triplet-triplet annihilation and two photon absorption.
In singlet fission, one high energy light particle, a photon, can be transformed into two low energy charge carriers through the interaction between molecules with certain properties and light. Photon upconversion through triplet-triplet annihilation can be thought of as combining two low energy light particles into one with higher energy. Both processes require that the molecules are organized in a favorable way, for specific interactions between two molecules, in a number of energy transfer steps. Two-photon absorption is another upconverting process, where a molecule or material absorbs two low energy photons simultaneously and reaches a high energy state. In all cases, it may sound easy but in reality, only a few molecules and materials are able to do this. In addition, the small fractions of time in which these processes occur are difficult to measure and are a key for understanding and controlling them.
In this work, the conversion of light energy through singlet fission, triplet-triplet annihilation upconversion and two-photon absorption was studied in self-assembled gels and quantum dots. It also includes a case study showing how triplet-triplet annihilation upconversion can be used to circumvent the mismatch between the light spectrum and the bandgap of a semiconductor.
The understanding of the detailed sequence of energy transfer mechanism in the light conversion processes and the structure of the photoactive material may help to increase the efficiency of solar cells. Moreover, this provides information suitable to be applied in other areas such as bioimaging and digital displays.
Vattenoxidation på titandioxidytor genom uppkonvertering av synligt ljus
Swedish Energy Agency (2017-006789), 2018-01-01 -- 2021-12-31.
Driving Forces
Sustainable development
Areas of Advance
Nanoscience and Nanotechnology
Energy
Materials Science
Subject Categories
Physical Chemistry
Atom and Molecular Physics and Optics
Materials Chemistry
Chemical Sciences
ISBN
978-91-7905-939-2
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5405
Publisher
Chalmers
10:an, Chemistry building, Kemigården 4 (Chalmers, Johanneberg campus)
Opponent: Malcolm D. E. Forbes, Bowling Green State University, United States