University of Groningen
Transfer of Triplet Excitons in Singlet Fission-Silicon Solar Cells
Daiber, Benjamin
DOI:
10.33612/diss.163964740
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Publication date: 2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Daiber, B. (2021). Transfer of Triplet Excitons in Singlet Fission-Silicon Solar Cells: Experiment and Theory Towards Breaking the Detailed-Balance Efficiency Limit. University of Groningen.
https://doi.org/10.33612/diss.163964740
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A B ST R A C T
This thesis explores the theory and experimental design of singlet fission-silicon solar cells. Singlet fission is a process that can convert one high-energy photon into two excitons of roughly half the high-energy. When com-bined with a lower-bandgap material like silicon, singlet fission materials can increase the efficiency of solar cells by using the energy of blue and green part of the incoming light more efficiently. To enable this dream we have to then disassociate or transfer these triplet excitons so we can extract the additional energy in the singlet fission process and make it us-able as a real-life electricity source. In this thesis we demonstrate several theoretical and experimental insights that can help with the development of useful singlet fission-solar cells.
chapter 1 introduces the singlet fission process and its application in solar cells. We discuss the difference between inorganic and organic semiconductors and how that difference presents special challenges when combining the two.
chapter 2 describes how a thin layer of quantum dots can help with transfer from a singlet fission material into silicon. We calculate the trans-fer efficiency for the Förster Resonant Energy Transtrans-fer (FRET) mechanism and find that, since silicon is an indirect bandgap semiconductor, the transfer can only be efficient if the quantum dot layer is very close to the silicon surface. We modify the standard FRET model to describe the transfer from a dipole donor (the quantum dot) into a bulk acceptor (the silicon) and find that the distance dependence weaker, predicting a higher transfer efficiency than expected from the standard model. chapter 3 contains solar cell efficiency calculations for three different transfer mechanisms. One mechanism is FRET transfer for which we use
160 bibliography
the FRET model from Chapter II to calculate a realistic but optimistic solar cell efficiency that is much higher than of just the silicon solar cell alone. Transfer can also happen by directly transferring the triplet exciton via Dexter transfer, for which we find an even higher efficiency, if the energy levels of the singlet fission material and silicon match well. The last transfer mechanism we discuss is via charge transfer, dissociating the triplet exciton at the silicon interface. This transfer mechanism has the highest efficiency gains of the three and puts the least constraints in the singlet exciton energies, but also adds experimental complexity.
chapter 4 discusses a new method of detecting evidence for triplet exciton transfer by quenching of the delayed photoluminescence of tetracene, a singlet fission material, on a silicon surface. Detecting quench-ing is necessary to determine if transfer occurs and we combined height maps and photoluminescence lifetime data of hundreds of small tetracene islands to correlate height and lifetime. We model photoluminescence in the islands with a diffusion model and find that we expect shorter lifetimes for thinner islands. We then apply this method to different silicon surface treatments and find that there is no quenching in these specific surface treatments.
chapter 5 demonstrates a singlet fission silicon solar cell with energy transfer of triplet excitons from tetracene into silicon. We detect the characteristic behavior of the solar cell current under a magnetic field and find evidence for triplet energy transfer if the protective layers of the silicon solar cell have been removed and the cell with tetracene has been exposed to air. We then use photoluminescence decay data and fit a differential equation describing the different species in tetracene that allows us to quantify the transfer efficiency. This solar cell is only the second demonstration of a singlet fission-silicon solar cell and works with a surprisingly simple geometry once the crystal packing of the singlet fission material is favorable for energy transfer.
S A M E N VAT T I N G
van de proefschrift:Overdracht van Triplet Excitons in Singlet Splitsing-Siliciumzonnecellen
Experimenten en Theorie Omtrent het Doorbreken van de Efficiëntielimiet van de Gedetailleerde Balans
Deze thesis onderzoekt de theorie en het ontwerp van singlet splitsing-silicium zonnecellen. Singlet splitsing is een proces waarbij één hooge-nergetisch foton kan worden omgezet in twee excitonen met een lagere energie, elk met ruwweg de helft van de oorspronkelijke fotonenergie. Wanneer ze gecombineerd worden met een materiaal met lage band-kloof, zoals silicium, kunnen singlet splitsing materialen de efficiëntie van zonnecellen verhogen door de energie uit het blauwe en groene deel van inkomende licht efficiënter te gebruiken. Om deze droom te kunnen verwezenlijken moeten de triplet excitonen gedissocieerd danwel overgedragen worden zodat de toegevoegde energie van het singlet split-singsproces kan worden geëxtraheerd en kan worden gebruikt als echte bron van elektriciteit. In deze thesis onderzoeken wij enkele theoretische en experimentele inzichten die kunnen helpen om de ontwikkeling van singlet splitsing zonnecellen realiteit te maken.
hoofdstuk 1 introduceert het singlet splitsingsproces en zijn toepas-sing voor zonnecellen. We bediscussiëren het verschil tussen anorganische en organische halfgeleiders en hoe dit verschil uitdagingen vormt wan-neer beiden gecombiwan-neerd worden.
