Towards uncovering triplet-pair decoherence mechanisms in singlet fission Eman Bu Ali The University of Sheffield, UK Singlet fission (SF) is a carrier multiplication phenomenon that transpires in organic semiconductors. The absorption of a photon in conjugated organic substances results in the creation of a singlet exciton (S N ). The transformation of a spin-0 singlet exciton into double spin-1 triplet excitons is referred to as SF. Triplet-triplet annihilation (TTA) is a photophysical opposite process in which a pair of spin-1 low-energy triplet excitons merge to form a single spin-0 high-energy exciton 1 . This multiexciton generation process has been studied over the past decade primarily because of its promise to improve solar cell efficiency 2 , as one high-energy photon creates two low-energy excited states without losses due to thermalization. Controlling the population of singlet and triplet states is one critical approach for increasing the efficiency of organic photovoltaics. The kinetic model represents the process of SF is: S 1 ↔ 1 (T T) ↔ (T...T ) ↔T 1 +T 1 In which the bounded triplet pair state 1 (T T) can be created using a spin-allowed mechanism after initial excitation. This intermediate triplet pair decorrelates to form a weakly bounded triplet pair state (T...T) resulting in the separation of the triplet pair into two independent triplet excitons 1,3 . Despite the process being studied since the late 60s, the mechanism of SF is still not fully understood. The goal of this work is to answer the questions “What are the mechanisms of triplet pair decoherence?” and “How does triplet-pair behaviour depend on molecular morphology/microstructure?”. The sample used in this work, (diF-TES-ADT), is an organic semiconductor material. In addition to the air- and photo-stability of this material, the basic brickwork crystalline structure with no evident phase transition documented between 100 K and room temperature makes it a preferred material for these studies 4 . In this work, we will investigate how sample morphology impacts the dynamics of SF and TTA, as well as the triplet pair coherence time, using a variety of transient magneto-optical spectroscopy and microscopy measurement techniques. References 1. D. G. Bossanyi, M. Matthiesen, S. Wang, J. A. Smith, R. C. Kilbride, J. D. Shipp, D. Chekulaev, E. Holland, J. E. Anthony, J. Zaumseil, et al. , “Emissive spin-0 triplet-pairs are a direct product of triplet–triplet annihilation in pentacene single crystals and anthradithiophene films,” Nature Chemistry , vol. 13, no. 2, pp. 163–171, 2021. 2. M. Hanna and A. Nozik, “Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers,” Journal of Applied Physics , vol. 100, no. 7, p. 074510, 2006. 3. X. Qiao and D. Ma, “Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission,” Materials Science and Engineering: R: Reports , vol. 139, p. 100519, 2020. 4. C. K. Yong, A. J. Musser, S. L. Bayliss, S. Lukman, H. Tamura, O. Bubnova, R. K. Hallani, A. Meneau, R. Resel, M. Maruyama, et al. , “The entangled triplet pair state in acene and heteroacene materials,” Nature communications , vol. 8, no. 1, pp. 1–12, 2017.
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