A perspective on triplet fusion upconversion: triplet sensitizers beyond quantum dots
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Prospective Article
A perspective on triplet fusion upconversion: triplet sensitizers beyond quantum dots Zachary A. VanOrman†, Alexander S. Bieber†, Sarah Wieghold, and Lea Nienhaus Florida State University, 95 Chieftan Way, Tallahassee, FL 32306, USA
, Department of Chemistry and Biochemistry,
Address all correspondence to Lea Nienhaus at [email protected] (Received 3 July 2019; accepted 7 August 2019)
Abstract The processes of singlet fission and triplet fusion could allow state-of-the-art photovoltaic devices to surpass the Shockley–Queisser limit by optimizing the utilized solar spectrum by reducing thermal relaxation and inaccessible sub-bandgap photons, respectively. Triplet fusion demands precise control of the spin-triplet state population, and requires a sensitizer to efficiently populate the triplet state of an acceptor molecule. In this perspective, we highlight the established field of sensitized upconversion and further examine alternative triplet sensitization routes, including the possibility of bulk solid-state semiconductors as triplet sensitizers, which provide a new avenue for charge transfer-based triplet sensitization rather than excitonic triplet energy transfer.
Introduction Efficient control and generation of spin-triplet states are essential for the understanding and advancement of current technologies, especially in regard to applications in photovoltaic (PV) devices. The processes of singlet fission (SF) and triplet fusion, or triplet– triplet annihilation upconversion (TTA-UC) depend on spintriplet states to interconvert the wavelengths of the incident light and the resulting emission. The SF process involves the splitting of a higher-lying singlet state into two triplet states that can be harvested as photocurrent in PV devices,[1–5] whereas the opposite process of TTA-UC combines two triplet states to a higher-lying singlet state in a spin-allowed process that results in upconverted emission upon relaxation of the resultant singlet state.[6–8] TTA is commonly observed in heavily conjugated organic molecules, such as polyacenes. Here, the electronic interactions between two triplet states on adjacent molecules allow for “annihilation” to a single higher lying singlet state. In general, UC as a result of TTA has wide-ranging applications which include overcoming the Shockley–Queisser limit in single-junction solar cells,[9,10] photocatalysis,[11–14] threedimensional displays,[15] bioimaging,[16] and sub-bandgap sensitization of silicon-based devices for infrared (IR) imaging.[17] However, as transitions from the singlet ground state to excited spin-triplet states are “spin-forbidden,” the triplet state must be indirectly populated, or sensitized, in order to efficiently harness their energy. This sensitization process can occur through a spin-allowed Dexter triplet energy transfer (TET) mechanism,[18] where the triplet state of the acceptor molecule can
† These authors contributed equally to this work.
be populated by a direct excitonic energy transfer from the sensitizer or through cha
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