Excited triplets of organic molecules have the interesting features of long excitation lifetime and spin polarization. Among various functions of excited triplets, our recent works on the development of chromophore assemblies exhibiting photon upconversion and singlet fission will be presented.

Triplet-triplet annihilation based photon upconversion (TTA-UC) from visible light to UV light is useful for various photocatalysts including artificial photosynthesis, but its low efficiency (<10%) has been a problem for several years. We have developed a chromophore pair that exhibits a high TTA-UC efficiency of over 20%, which is two times higher than the previous record for Vis-to-UV TTA-UC. The Ir coumarin complex shows strong absorption in the visible range and relatively weak absorption in the UV range and does not quench the upconverted UV emission. By combining the Ir coumarin complex with TIPS-naphthalene, we have achieved a high TTA-UC efficiency of over 20% and a very low threshold excitation intensity I_th below solar irradiance.^1,2 Furthermore, we have recently developed bicontinuous porous monoliths showing visible to UV TTA-UC and will introduce the details in the talk.

We are also interested in the spin degree of freedom of singlet fission in the context of quantum information science (QIS).³ We developed chromophore assemblies and dimers exhibiting singlet fission, and successfully observed quantum coherence of quintet triplet dimer state at room temperature for the first time. We found that the suppressed chromophore dynamics is crucial for achieving both of quintet formation and quantum coherence.

Photon upconversion offers the prospect of combining low energy near-infrared radiation to make high energy photons in the visible wavelengths. The goal is to improve photovoltaics and introduce new bioimaging techniques. In order to harness the intrinsic ability of colloidal semiconductor nanocrystals to couple strongly with light, it is important to efficiently outcouple energy from photoexcited quantum dots (QDs), much like how nature uses molecular antennas to direct light during photosynthesis. This talk focuses on the organic-inorganic interface for triplet-fusion based photon upconversion, where orbital overlap between the QD donor and molecular acceptor is critical for efficient energy transduction. In the past 8 years, we have learnt a great deal about how the electronic interactions between chalcogenide nanocrystals and acene conjugated molecules affect the photon upconversion quantum yield. We apply the lessons learnt to extending photon upconversion to silicon nanocrystal light absorbers, addressing synthetic challenges with invaluable insight from time-resolved spectroscopy.

Quantum dot-organic hybrids have become popular as triplet-photon converting systems, for example as triplet sensitizers for triplet-triplet annihilation photon upconversion and photon multiplication based on singlet fission.[1] The organic ligand in such hybrid structures plays a crucial role in mediating triplet exciton transfer between the organic molecules and inorganic quantum dots in solution, and many studies have studied the intricate details of triplet mediation via the ligand.[2] Here I will higlight another aspect of the ligand, namely as a directing agent in bulk organic-semiconductor/quantum-dot blend films. These blends, comprising organic semiconductors and inorganic quantum dots  are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase separate during processing to form films, compromising both their structural and electronic properties. Here, I will explain how a suitable organic ligand can achieve well-dispersed inorganic-organic blend films, as characterised by X-ray and neutron scattering and electron microscopy. As proof-of-concept I will show two examples of these films applied to singlet fission based photon multiplication[3] and triplet-triplet annihilation based photon upconversion. Due to the optimal blend morphology triplet excitons can be transferred with near unity efficiently across the organic-inorganic interface, while the TIPS-tetracene films maintain efficient SF (190% yield) in the organic phase, resulting in 95% of the triplet excitons generated via SF in the organic phase being harvested by the QDs. By changing the relative energy between organic and inorganic components yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, this talk will exemplify a highly versatile approach to overcome long-standing challenges in the blending of organic semiconductors with QDs with relevance for many optical and optoelectronic applications.

The conversion of low-energy light into photons of higher energy based on sensitized triplet–triplet annihilation (sTTA) upconversion in bicomponent systems is emerging as the most promising wavelength-shifting methodology to recover sub-bandgap solar photons, because it operates efficiently at excitation powers as low as the solar irradiance. Excellent efficiencies have been obtained in liquid environments as well as in prototype upconversion enhanced solar cells, but the research is still focused on the realization of affordable solid states upconverters suitable to be implemented in current solar technologies. We show here that controlled confinement of the upconverting materials in nanostructured or nanosized materials can improve the material performance at low powers. [1] [2] [3] [4] The result presented will show demonstrate how this strategy can represent a crucial guideline for the future development of upconverting photonic devices operating at subsolar irradiances suitable for technological implementation.

[1]  Mattiello et al. Diffusion-free intramolecular triplet–triplet annihilation in engineered conjugated chromophores for sensitized photon upconversion. ACS Energy Letters 7 (8), 2435-2442 (2022)

[2] A.Ronchi & A. Monguzzi; Developing solid-state photon upconverters based on sensitized triplet–triplet annihilation. J. Appl. Phys. 129 (5), 050901 (2021)

[3] Saenz et al. Nanostructured Polymers Enable Stable and Efficient Low‐Power Photon Upconversion, Adv. Funct. Mater. 31, 1, 2004495 (2021) 

[4] Meinardi et al. Quasi-thresholdless photon upconversion in metal–organic framework nanocrystals, Nano Letters 19 (3), 2169-2177 (2019)

The upconversion of light has numerous potential applications ranging from photocatalysis, photovoltaics, imaging, and advanced manufacturing. The strategies being investigated to achieve this include energy transfer upconversion in lanthanide nanoparticles, and sensitized triplet fusion, also known as photochemical upconversion or triplet-triplet annihilation upconversion (TTA-UC). The latter involves the generation of triplet excited states by a sensitizer and their subsequent transfer to an emitter. Emitter triplets annihilate (fuse) to generate an emissive singlet state from which a higher energy photon is emitted. This strategy is spectrally adaptable, and high efficiencies are routinely achieved in solution, approaching the quantum yield ceiling of 50%.

Recently, there has been progress in translating triplet fusion upconversion into the solid state, which is desirable from a device engineering standpoint.[1] There are many possible solid-state device architectures. Broadly these can be categorized into materials wherein the sensitizer and emitter are evenly distributed, and heterogeneous devices where triplet generation is spatially separated from triplet fusion. The latter strategy is attractive, as the upconverted singlet state is protected to an extent from Förster resonance energy transfer back to the sensitizer. A simple concept would be to lay down a monolayer of sensitizer overlaid with a thin film of emitter. However, such devices suffer from poor ligh absorption.

In dye-sensitized solar cells (DSSCs), the problem of poor light absorption by a monolayer of chromophores is addressed with a nanostructured metal oxide support, such that incoming light passes through a multitude of chromophore-bearing interfaces. Indeed, DSSCs incorporating TTA have been demonstrated.[2]  As there are many steps in photochemical upconversion, it is desirable to investigate certain aspects in isolation. To concentrate on the solid-state sensitization process, here we present a strategy where the sensitizer is chemisorbed on a nanoporous Al₂O₃ support, but the emitter remains in solution.[3]

Using time-resolved spectroscopies, we show that the sensitizer triplets are long-lived, do not suffer significant aggregation-induced quenching and can be efficiently transferred to the emitter chromophores. We demonstrate devices with internal upconversion quantum yields as high as 9.4%, which is 19% of the maximum achievable value and 38% of what can be achieved with the diphenylanthracene emitter. This shows that using dyes bound to nanoporous Al₂O₃ films is a viable strategy to sensitize upconversion within a solid state material, and bodes well for future development with solid state emitters. Initial work on liquid-chromophore and fully solid state systems based on the same Al₂O₃ films will also be presented.

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.