Mechanistic Understanding and Rational Design of Quantum Dot/Mediator Interfaces for Efficient Photon Upconversion

Abstract

Conspectus The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet–triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet–triplet energy losses compared to metal–ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step’s efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems. This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 ...

Shallow distance-dependent triplet energy migration mediated by endothermic charge-transfer

“…These consistencies further support that the transfer rates measured here are dominated by a hole-transfer-like process. We notice that, in addition to the inorganic shells, phenylene bridges are also frequently used to control the distance between QD donors and molecular acceptors [42][43][44][45] . The β values obtained in those studies, however, are not quantitatively comparable with each other.…”

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…2,3 However, because of their o-negligible electric dipole, 4,5 molecular triplets are commonly generated using a sensitizer 6 via sequential photon absorption, spin-dephasing, and triplet energy transfer (TET) to a molecular acceptor. 7 Examples of sensitizers include coordination complexes, due to their strong absorption bands and high triplet yield via intersystem crossing, 6,[8][9][10][11][12][13][14][15] as well as colloidal quantum dots (QDs), [16][17][18][19][20][21][22][23] metal-halide perovskite lms and nanocrystals, [24][25][26] and materials containing lanthanide atoms. 5,27 Triplet fusion (TF) occurs when the energy from two or more triplet-excited acceptors are combined to yield a higher-energy excited state, which may generate a spin-singlet excitation capable of emitting a photon.…”

“…6,12,23,[33][34][35] Triplet sensitization using QDs is an increasingly common strategy to achieve triplet-fusion upconversion (TUC) due to latent advantages such as their size-tunable optical gap, large molar absorption cross-section, rapid/near-isoergic spindephasing, and modiable surface chemistry. [16][17][18][19][20][21][22][23]34,[36][37][38][39][40] By combining these properties with the long intrinsic lifetimes of triplet excitons on many molecular chromophores, TUC performance has been able to access new performance regimes of incident wavelength, anti-Stokes shi, quantum yield, and low-threshold operation. 18,20,23,40,41 Specically, lead sulde (PbS) QDs possess advantageously long excited-state lifetimes compared to other commonly employed semiconductor nanocrystals.…”

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…2,3 However, because of their oft-negligible electric dipole, 4,5 molecular triplets are commonly generated using a sensitizer6 via sequential photon absorption, spin-dephasing, and triplet energy transfer (TET) to a molecular acceptor. 7 Examples of sensitizers include coordination complexes, due to their strong absorption bands and high triplet yield via intersystem crossing, 6,[8][9][10][11][12][13][14][15] as well as colloidal quantum dots (QDs), [16][17][18][19][20][21][22][23] metal-halide perovskite films and nanocrystals, [24][25][26] and materials containing lanthanide atoms. 5,27 Triplet fusion (TF) occurs when the energy from two or more triplet-excited acceptors are combined to yield a higher-energy excited state, which may generate a spin-singlet excitation capable of emitting a photon.…”

“…6,23,[32][33][34] Triplet sensitization using QDs is an increasingly common strategy to achieve triplet-fusion upconversion (TUC) due to latent advantages such as their size-tunable optical gap, large molar absorption cross-section, rapid/near-isoergic spin-dephasing, and modifiable surface chemistry. 16,17,[36][37][38][39][18][19][20][21][22][23]33,35 By combining these properties with the long intrinsic lifetimes of triplet excitons on many molecular chromophores, TUC performance has been able to access new performance regimes of incident wavelength, anti-Stokes shift, quantum yield, and low-threshold operation. 18,20,23,35, 40 Specifically, lead sulfide (PbS) QDs possess advantageously long excited-state lifetimes compared to other commonly employed semiconductor nanocrystals.…”