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The efficiency of solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplet–triplet annihilation (TTA) in organic chromophores. The quantum yield of TTA is often low due to competing processes. The singlet pathway, where a high energy photon is emitted, is one of three possible outcomes of an encounter between two triplet excitons. The quintet pathway is often too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using many-body perturbation theory in the GW approximation and the Bethe–Salpeter equation, we calculate the energy release in both the singlet and triplet pathways for 59 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and why the quantum yield of TTA is typically low. That said, our results also indicate that the performance of emitters from known chemical families may be improved by chemical modifications, such as functionalization with side groups, and that new chemical families could be explored to discover more TTA emitters.
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Delayed photoacidity produced through the triplet–triplet annihilation of a neutral pyranine derivative
A novel pyranine derivative, Et HPTA-OH, was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of Et HPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of Et HPTA-OH was found to be highly photoacidic (p K a * = 8.74 in acetonitrile). A series of aniline and pyridine bases were used to investigate excited-state proton transfer (ESPT) from singlet Et HPTA-OH, and rate constants for singlet quenching via ESPT were determined ( k q = 5.18 × 10 9 to 1.05 × 10 10 M −1 s −1 ). Et HPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet–triplet annihilation (TTA) process to reform singlet Et HPTA-OH on timescales of up to 80 μs. Detection of ESPT photoproducts on timescales comparable to that of TTA singlet regeneration provides strong evidence for photoacidic behavior stemming from the regenerated singlet Et HPTA-OH.
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- Award ID(s):
- 1726291
- PAR ID:
- 10200936
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 21
- Issue:
- 29
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 16353 to 16358
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9cp02929j
