About this Research Topic
Exciton spin management in organic semiconductors promises to unveil new aspects of wavelength-shifting technologies that could positively affect many different research fields, including those of energy, photocatalysis, photodetection, photonics and biophotonics. The exploitation of organic composites as spin-converting functional materials paves the way for the development of numerous technologically important applications in which light-management and restructuring of the solar spectrum is envisioned as a crucial step to surpassing state-of-the-art device performances. Utilization of zero-spin singlet excitons in organic semiconductors promises the development of photovoltaic devices with solar radiation-to-electrical energy efficiencies higher than the thermodynamic limit set by Shockley – Queisser. Similar to the process of quantum cutting observed in lanthanide-based inorganic phosphors, some organic materials facilitate down-conversion of a high-energy singlet exciton to two low-energy triplet excitons. This singlet-fission photochemical reaction corresponds to an exciton multiplication process that can be potentially utilized by well-designed down-converting overlayers attached on single-junction solar cell devices to reduce the efficiency losses due to hot exciton thermalization. Prior to reaching the photoactive layer of the solar cell device, the incident solar radiation can travel through the down-converting layer where a pair of low-energy triplet excitons can be generated for each high-energy singlet exciton formed upon absorption of light. Another way of restructuring the solar spectrum before it enters a light-driven optoelectronic device is the attachment of an up-converting layer that facilitates the fusion of two low-energy triplet excitons, which generate one high-energy singlet exciton. In organic materials, photon energy up-conversion operates through the annihilation of triplet excitons; the formation of a high-energy singlet exciton manifests in the generation of anti-Stokes luminescence, even when moderate photoexcitation intensities are used comparably to those of the solar spectrum. Photon energy up-converting layers promise to harvest those low photon energies that otherwise would not be absorbed by the photoactive layer of an optoelectronic device and to use them in the generation of high-energy emissive excitons that can sensitize photocurrent generation, chemical reactions and charge transfer.
This Research Topic aims to pinpoint the latest findings in the fields of singlet-exciton fission and triplet-exciton fusion, and to highlight the significance of solid-state microstructure on the quantum efficiency of both processes. It is very likely that in the mechanism of singlet-exciton fission and triplet-exciton fusion processes the participation of an intermediate entangled spin-zero state is involved, the formation of which depends strongly on the specific structural motifs of the composite under study. Upon sharing the knowledge produced by numerous research groups that are active in either experimental or theoretical study of singlet-fission and triplet-fusion organic systems in the solid state, this Research Topic aims to establish a roadmap for the systematic development of structure-property-function correlations in organic composites for spin-exciton management, and to facilitate the crossover between the two seemingly independent fields. By highlighting the common origin of the two processes and by underlining their strong dependence on microstructure, deep insight can be gained into the factors that impede high quantum efficiencies in the solid state.
Keywords: Triplet-triplet annihilation, triplet-exciton fusion, singlet-exciton fission, up-conversion, down-conversion, delayed fluorescence, organic photonics
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