nn9b09339_si_001.pdf (1.23 MB)
Direct vs Delayed Triplet Energy Transfer from Organic Semiconductors to Quantum Dots and Implications for Luminescent Harvesting of Triplet Excitons
journal contribution
posted on 2020-03-18, 19:13 authored by Victor Gray, Jesse R. Allardice, Zhilong Zhang, Simon Dowland, James Xiao, Anthony J. Petty, John E. Anthony, Neil C. Greenham, Akshay RaoHybrid inorganic–organic materials
such as quantum dots
(QDs) coupled with organic semiconductors have a wide range of optoelectronic
applications, taking advantage of the respective materials’
strengths. A key area of investigation in such systems is the transfer
of triplet exciton states to and from QDs, which has potential applications
in the luminescent harvesting of triplet excitons generated by singlet
fission, in photocatalysis and photochemical upconversion. While the
transfer of energy from QDs to the triplet state of organic semiconductors
has been intensely studied in recent years, the mechanism and materials
parameters controlling the reverse process, triplet transfer to QDs,
have not been well investigated. Here, through a combination of steady-state
and time-resolved optical spectroscopy we study the mechanism and
energetic dependence of triplet energy transfer from an organic ligand
(TIPS-tetracene carboxylic acid) to PbS QDs. Over an energetic range
spanning from exothermic (−0.3 eV) to endothermic (+0.1 eV)
triplet energy transfer we find that the triplet energy transfer to
the QD occurs through a single step process with a clear energy dependence
that is consistent with an electron exchange mechanism as described
by Marcus–Hush theory. In contrast, the reverse process, energy
transfer from the QD to the triplet state of the ligand, does not
show any energy dependence in the studied energy range; interestingly,
a delayed formation of the triplet state occurs relative to the quantum
dots’ decay. Based on the energetic dependence of triplet energy
transfer we also suggest design criteria for future materials systems
where triplet excitons from organic semiconductors are harvested via QDs, for instance in light emitting structures or the
harvesting of triplet excitons generated via singlet
fission.