Photon Upconversion Systems Research Articles

Red-to-blue photon up-conversion with high efficiency based on a TADF fluorescein derivative

A new photon up-conversion system with a TADF fluorescein derivative as a photosensitizer was developed to achieve a quite large anti-Stokes shift from red to blue with a fairly high up-conversion emission quantum yield. This TADF photosensitizer has a very small ΔEST to provide a 207 nm anti-Stokes shift photon up-conversion. Meanwhile, it has a quite long triplet state lifetime (22.11 μs) and a large molar extinction coefficient to assure a high up-conversion efficiency of 11.2%. These results fully demonstrated the potential of the TADF fluorescein derivative as the photosensitizer used in photon up-conversion for the first time.

Recent advances in the application triplet–triplet annihilation-based photon upconversion systems to solar technologies

Upconversion (UC) of low-energy photons into high-energy light can in principle increase the efficiency of solar devices by converting photons with energies below the energy absorption threshold into radiation that can be utilized. Among UC mechanisms, the sensitized triplet–triplet annihilation-based upconversion (sTTA-UC) is the most recent that would be applied in solar technologies. sTTA-UC was demonstrated using sunlight in 2006, and due to its high efficiency at low excitation intensities with noncoherent light, it is considered a promising strategy for the recovering of subbandgap photons. Here, we describe briefly the working principle of the sTTA-UC, and we review the most recent advances of its use in solar applications.

Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules.

Molecular control of energy transfer is an attractive proposition because it allows chemists to synthetically tweak various kinetic and thermodynamic factors. In this Perspective, we examine energy transfer between semiconductor nanocrystals (NCs) and π-conjugated molecules, focusing on the transmitter ligand at the organic-inorganic interface. Efficient transfer of triplet excitons across this interface allows photons to be directed for effective use of the entire solar spectrum. For example, a photon upconversion system composed of semiconductor NCs as sensitizers, bound organic ligands as transmitters, and molecular annihilators has the advantage of large, tunable absorption cross sections across the visible and near-infrared wavelengths. This may allow the near-infrared photons to be harnessed for photovoltaics and photocatalysis. Here we summarize the progress in this recently reported hybrid upconversion platform and point out the challenges. Since triplet energy transfer (TET) from NC donors to molecular transmitters is one of the bottlenecks, emphasis is on the design of transmitters in terms of molecular energetics, photophysics, binding affinity, stability, and energy offsets with respect to the NC donor. Increasing the efficiency of TET in this hybrid platform will increase both the up- and down-conversion quantum yields, potentially exceeding the Shockley-Queisser limit for photovoltaics and photocatalysis.

Metallophthalocyanines as triplet sensitizers for highly efficient photon upconversion based on sensitized triplet-triplet annihilation.

Soluble palladium and platinum phthalocyanines with coumarin moieties were synthesized with Q bands in the red and near-IR regions, in which the molar extinction coefficients were up to 1.01 × 10(5) cm(-1) mol(-1). These metallophthalocyanines were coupled with rubrene and applied in photon upconversion systems based on triplet-triplet annihilation. The highest upconversion efficiency of the palladium phthalocyanine was 5.6%, which is higher than that of the platinum phthalocyanine-rubrene system. The larger molar extinction coefficient resulted in high upconversion capability (>10(5) cm(-1) mol(-1)) and low saturation incident power (<20 mW cm(-2)).

Triplet-triplet annihilation photon-upconversion: towards solar energy applications.

Solar power production and solar energy storage are important research areas for development of technologies that can facilitate a transition to a future society independent of fossil fuel based energy sources. Devices for direct conversion of solar photons suffer from poor efficiencies due to spectrum losses, which are caused by energy mismatch between the optical absorption of the devices and the broadband irradiation provided by the sun. In this context, photon-upconversion technologies are becoming increasingly interesting since they might offer an efficient way of converting low energy solar energy photons into higher energy photons, ideal for solar power production and solar energy storage. This perspective discusses recent progress in triplet-triplet annihilation (TTA) photon-upconversion systems and devices for solar energy applications. Furthermore, challenges with evaluation of the efficiency of TTA-photon-upconversion systems are discussed and a general approach for evaluation and comparison of existing systems is suggested.

Photon Upconverting Liquids: Matrix-Free Molecular Upconversion Systems Functioning in Air

A nonvolatile, in-air functioning liquid photon upconverting system is developed. A rationally designed triplet sensitizer (branched alkyl chain-modified Pt(II) porphyrin) is homogeneously doped in energy-harvesting liquid acceptors with a 9,10-diphenylanthracene unit. A significantly high upconversion quantum yield of ∼28% is achieved in the solvent-free liquid state, even under aerated conditions. The liquid upconversion system shows a sequence of efficient triplet energy transfer and migration of two itinerant excited states which eventually collide with each other to produce a singlet excited state of the acceptor. The observed insusceptibility of upconversion luminescence to oxygen indicates the sealing ability of molten alkyl chains introduced to liquefy chromophores. The involvement of the energy migration process in triplet-triplet annihilation (TTA) provides a new perspective in designing advanced photon upconversion systems.

Efficient S 2 state production in ZnTPP–PMMA thin films by triplet–triplet annihilation: Evidence of solute aggregation in photon upconversion systems

Non-coherent photon upconversion by triplet–triplet annihilation (TTA) of metalloporphyrins in degassed solid thin PMMA films has been investigated. Dual S 1 and S 2 fluorescence of the model chromophore ZnTPP is used to monitor TTA directly. Evidence that aggregation is required for TTA to occur with the measured efficiencies is presented.