triplet-fusion-upconversion-nanocapsule-synthesis

Triplet Fusion Upconversion Nanocapsule Synthesis

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has ...

Harvard University

Stanford University

Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications, with stereolithography a particularly successful approach. However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion. Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures, but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.

Triplet fusion upconversion nanocapsules for volumetric 3D printing.

Light-emitting diodes utilizing perovskite nanocrystals have generated strong interest in the past several years, with green and red devices showing high efficiencies. Blue devices, however, have lagged significantly behind. Here, it is shown that the device architecture plays a key role in this lag and that NiO , a transport layer in one of the highest efficiency devices to date, causes a significant reduction in perovskite luminescence lifetime. An alternate transport layer structure which maintains robust nanocrystal emission is proposed. Devices with this architecture show external quantum efficiencies of 0.50% at 469 nm, seven times higher than state-of-the-art devices at that wavelength. Finally, it is demonstrated that this architecture enables efficient devices across the entire blue-green portion of the spectrum. The improvements demonstrated here open the door to efficient blue perovskite light-emitting diodes.

Reducing Architecture Limitations for Efficient Blue Perovskite Light-Emitting Diodes.

Recent advances in photoredox catalysis have made it possible to achieve various challenging synthetic transformations, polymerizations and surface modifications. All of these reactions require ultraviolet- or visible-light stimuli; however, the use of visible-light irradiation has intrinsic challenges. For example, the penetration of visible light through most reaction media is very low, leading to problems in large-scale reactions. Moreover, reactants can compete with photocatalysts for the absorption of incident light, limiting the scope of the reactions. These problems can be overcome by the use of near-infrared light, which has a much higher penetration depth through various media, notably biological tissue. Here we demonstrate various photoredox transformations under infrared radiation by utilizing the photophysical process of triplet fusion upconversion, a mechanism by which two low-energy photons are converted into a higher-energy photon. We show that this is a general strategy applicable to a wide range of photoredox reactions. We tune the upconversion components to adjust the output light, accessing both orange light and blue light from low-energy infrared light, by pairwise manipulation of the sensitizer and annihilator. We further demonstrate that the annihilator itself can be used as a photocatalyst, thus simplifying the reaction. This approach enables catalysis of high-energy transformations through several opaque barriers using low-energy infrared light.

Photoredox catalysis using infrared light via triplet fusion upconversion.

Optical upconversion based on triplet fusion (TF), also known as triplet-triplet annihilation, is a process by which two or more low-energy photons are converted to one higher energy photon. This process requires two components, a sensitizer which absorbs the incident low-energy photons and an annihilator which emits the higher energy photons. While much attention has been given to the investigation of new types of sensitizers, very little work has been done on the exploration of new annihilators. In this work, we show that the singlet energy of diketopyrrolopyrroles (DPPs) can be altered by modifying the pendant aryl substituents to the core. This allows us to meet the energetic requirements necessary for TF upconversion and demonstrates DPPs as a new class of annihilator molecules. Using this new DPP platform, the output wavelength from upconversion can easily be tuned, which will greatly diversify the number of applications of DPPs in upconversion technologies.

Tunable Emission from Triplet Fusion Upconversion in Diketopyrrolopyrroles.

Optical upconversion is a net process by which two low energy photons are converted into one higher energy photon. There is vast potential to exploit upconversion in applications ranging from solar energy and biological imaging to data storage and photocatalysis. Here, we link two upconverting chromophores together to synthesize a series of novel tetracene dimers for use as annihilators. When compared with the monomer annihilator, TIPS-tetracene, the dimers yield a strong enhancement in the triplet fusion process, also known as triplet-triplet annihilation, as demonstrated  a large increase in upconversion efficiency and an order of magnitude reduction of the threshold power for maximum yield. Along with the ongoing rapid improvements to sensitizer materials, the dimerization improvements demonstrated here open the way to a wide variety of emerging upconversion applications.

Annihilator dimers enhance triplet fusion upconversion.

In Fig. 1c of this Letter, the orange axis label of the graph should have read 'FDPP upconversion photoluminescence (AU)' instead of 'TTBP upconversion photoluminescence (AU)'. This has been corrected online.

Author Correction: Photoredox catalysis using infrared light via triplet fusion upconversion.

Silicon dominates contemporary solar cell technologies. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton). Singlet exciton fission in the molecular semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap. When the triplet excitons are transferred to silicon they create additional electron-hole pairs, promising to increase cell efficiencies from the single-junction limit of 29 per cent to as high as 35 per cent. Here we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using electric-field-effect passivation to enable the efficient energy transfer of the triplet excitons formed in the tetracene. The maximum combined yield of the fission in tetracene and the energy transfer to silicon is around 133 per cent, establishing the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate.

Sensitization of silicon by singlet exciton fission in tetracene.

Singlet exciton fission is a mechanism that could potentially enable solar cells to surpass the Shockley-Queisser efficiency limit by converting single high-energy photons into two lower-energy triplet excitons with minimal thermalization loss. The ability to make use of singlet exciton fission to enhance solar cell efficiencies has been limited, however, by the sparsity of singlet fission materials with triplet energies above the bandgaps of common semiconductors such as Si and GaAs. Here, we employ a high-throughput virtual screening procedure to discover new organic singlet exciton fission candidate materials with high-energy (>1.4 eV) triplet excitons. After exploring a search space of 4482 molecules and screening them using time-dependent density functional theory, we identify 88 novel singlet exciton fission candidate materials based on anthracene derivatives. Subsequent purification and characterization of several of these candidates yield two new singlet exciton fission materials: 9,10-dicyanoanthracene (DCA) and 9,10-dichlorooctafluoroanthracene (DCOFA), with triplet energies of 1.54 eV and 1.51 eV, respectively. These materials are readily available and low-cost, making them interesting candidates for exothermic singlet exciton fission sensitization of solar cells. However, formation of triplet excitons in DCA and DCOFA is found to occur via hot singlet exciton fission with excitation energies above ∼3.64 eV, and prominent excimer formation in the solid state will need to be overcome in order to make DCA and DCOFA viable candidates for use in a practical device.

Discovery of blue singlet exciton fission molecules via a high-throughput virtual screening and experimental approach.

Triplet-triplet annihilation upconversion (TTA-UC) is an unconventional photophysical process that yields high-energy photons from low-energy incident light and offers pathways for innovation across many technologies, including solar energy harvesting, photochemistry, and optogenetics. Within aromatic organic chromophores, TTA-UC is achieved through several consecutive energy conversion events that ultimately fuse two triplet excitons into a singlet exciton. In chromophores where the singlet exciton is roughly isoergic with two triplet excitons, the limiting step is the triplet-triplet annihilation pathway, where the kinetics and yield depend sensitively on the energies of the lowest singlet and triplet excited states. Herein we report up to 40-fold improvements in upconversion quantum yields using molecular engineering to selectively tailor the relative energies of the lowest singlet and triplet excited states, enhancing the yield of triplet-triplet annihilation and promoting radiative decay of the resulting singlet exciton. Using this general and effective strategy, we obtain upconversion yields with red emission that are among the highest reported, with remarkable chemical stability under ambient conditions.

Molecular Engineering of Chromophores to Enable Triplet-Triplet Annihilation Upconversion.

Nanocrystals based on halide perovskites offer a promising material platform for highly efficient lighting. Using transient optical spectroscopy, we study excitation recombination dynamics in manganese-doped CsPb(Cl,Br) perovskite nanocrystals. We find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by  calculations, we can attribute the enhanced emission rates to increased charge carrier localization through lattice periodicity breaking from Mn dopants, which increases the overlap of electron and hole wave functions locally and thus the oscillator strength of excitons in their vicinity. Our report of a fundamental strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing efficiencies in light-emitting applications.

Charge Carrier Localization in Doped Perovskite Nanocrystals Enhances Radiative Recombination.

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA)Pb(I Br  ). Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites.

Halide Mixing Inhibits Exciton Transport in Two-dimensional Perovskites Despite Phase Purity.

Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M) can be used to tailor and enhance their light emission. Although crucial to maximize performance, an understanding of the universal working mechanism for such doping is still missing. Here, we directly compare the optical properties of nanocrystals containing the most commonly employed dopants, fabricated under identical synthesis conditions. We show for the first time unambiguously, and supported by first-principles calculations and molecular orbital theory, that element-unspecific symmetry-breaking rather than element-specific electronic effects dominate these properties under device-relevant conditions. The impact of most dopants on the perovskite electronic structure is predominantly based on local lattice periodicity breaking and resulting charge carrier localization, leading to enhanced radiative recombination, while dopant-specific hybridization effects play a secondary role. Our results suggest specific guidelines for selecting a dopant to maximize the performance of perovskite emitters in the desired optoelectronic devices.

Daniel N. Congreve

Rowland Institute

Harvard University

Triplet Fusion Upconversion Nanocapsule Synthesis