Optical upconversion via sensitized triplet-triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities. Here, we report a ...solid-state thin film for infrared-to-visible upconversion that employs lead sulphide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm, two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equivalent to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing molecular sensitizers, given their small exchange splitting, wide wavelength tunability, broadband infrared absorption, and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.
Hybrid interfaces combining inorganic and organic materials underpin the operation of many optoelectronic and photocatalytic systems and allow for innovative approaches to photon up- and ...down-conversion. However, the mechanism of exchange-mediated energy transfer of spin-triplet excitons across these interfaces remains obscure, particularly when both the macroscopic donor and acceptor are composed of many separately interacting nanoscopic moieties. Here, we study the transfer of excitons from colloidal lead sulfide (PbS) nanocrystals to the spin-triplet state of rubrene molecules. By reducing the length of the carboxylic acid ligands on the nanocrystal surface from 18 to 4 carbon atoms, thinning the effective ligand shell from 13 to 6 Å, we are able to increase the characteristic transfer rate by an order of magnitude. However, we observe that the energy transfer rate asymptotes for shorter separation distances (≤10 Å) which we attribute to the reduced Dexter coupling brought on by the increased effective dielectric constant of these solid-state devices when the aliphatic ligands are short. This implies that the shortest ligands, which hinder long-term colloidal stability, offer little advantage for energy transfer. Indeed, we find that hexanoic acid ligands are already sufficient for near-unity transfer efficiency. Using nanocrystals with these optimal-length ligands in an improved solid-state device structure, we obtain an upconversion efficiency of (7 ± 1)% with excitation at λ = 808 nm.
Semiconductor nanocrystals are a promising class of materials for a variety of novel optoelectronic devices, since many of their properties, such as the electronic gap and conductivity, can be ...controlled. Much of this control is achieved via the organic ligand shell, through control of the size of the nanocrystal and the distance to other objects. We here simulate ligand-coated CdSe nanocrystals using atomistic molecular dynamics, allowing for the resolution of novel structural details about the ligand shell. We show that the ligands on the surface can lie flat to form a highly anisotropic “wet hair” layer as opposed to the “spiky ball” appearance typically considered. We discuss how this can give rise to a dot-to-dot packing distance of one ligand length since the thickness of the ligand shell is reduced to approximately one-half of the ligand length for the system sizes considered here; these distances imply that energy and charge transfer rates between dots and nearby objects will be enhanced due to the thinner-than-expected ligand shell. Our model predicts a non-linear scaling of ligand shell thickness as the ligands transition from “spiky” to “wet hair”. We verify this scaling using transmission electron microscopy on a PbS nanoarray, confirming that this theory gives a qualitatively correct picture of the ligand shell thickness of colloidal quantum dots.
Triplet excitons are ubiquitous in organic optoelectronics, but they are often an undesirable energy sink because they are spin-forbidden from emitting light and their high binding energy hinders the ...generation of free electron-hole pairs. Harvesting their energy is consequently an important technological challenge. Here, we demonstrate direct excitonic energy transfer from 'dark' triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90 ± 13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near-infrared light-emitting applications, where effective molecular phosphors are lacking at present. In particular, this route to 'brighten' low-energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells.
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•A new method for solving chemical reaction kinetics with disorder in the rates (unimolecular or bimolecular) is presented.•The method uses a mean-field approximation to quickly ...compute self-consistent steady state populations.•We illustrate the proposed method with a working example of H2 production on a heterogeneous surface.
We propose a method to quickly compute steady state populations of species undergoing a set of chemical reactions whose rate constants are heterogeneous. Using an average environment in place of an explicit nearest neighbor configuration, we obtain a set of equations describing a single fluctuating active site in the presence of an averaged bath. We apply this Mean Field Steady State (MFSS) method to a model of H2 production on a disordered surface for which the activation energy for the reaction varies from site to site. The MFSS populations quantitatively reproduce the KMC results across the range of rate parameters considered.
Density functional theory-based high-throughput materials and drug discovery has achieved tremendous success in recent decades, but its power on organic semiconducting molecules suffered ...catastrophically from the self-interaction error until the nonempirical but expensive optimally tuned range-separated hybrid (OT-RSH) functionals were developed. An OT-RSH transitions from a short-range (semi)local functional to a long-range Hartree–Fock exchange at a distance characterized by a molecule-specific range-separation parameter (ω). Herein, we propose a stacked ensemble machine learning model that provides an accelerated alternative of OT-RSH based on system-dependent structural and electronic configurations. We trained ML-ωPBE, the first functional in our series, using a database of 1970 molecules with sufficient structural and functional diversity, and assessed its accuracy and efficiency using another 1956 molecules. Compared with nonempirical OT-ωPBE, ML-ωPBE reaches a mean absolute error of 0.00504a 0 –1 for optimal ω’s, reduces the computational cost by 2.66 orders of magnitude, and achieves comparable predictive power in optical properties.
High internal quantum efficiency semiconductor nanocrystal (NC)-based photon upconversion devices are currently based on a single monolayer of active NCs. Devices are therefore limited in their ...external quantum efficiency based on the low number of photons absorbed. Increasing the number of photons absorbed is expected to increase the upconversion efficiency, yet experimentally increasing the number of layers does not appreciably increase the upconverted light output. We unravel this mystery by combining kinetic modeling and transient photoluminescence spectroscopy. The inherent energetic disorder stemming from the polydispersity of the NCs means that the kinetics are governed by a stochastic transfer matrix. By drawing the rates from a probabilistic distribution and constructing a reaction network with realistic connectivity, we are able to fit complex photoluminescence traces with a very simple model. We use this model to explain the thickness-dependent performance of the upconversion devices and can attribute the reduced efficiencies to the low excitonic diffusivity of the exciton within the NC layers and increased back transfer of the created singlets from the organic annihilator rubrene. We suggest some avenues for overcoming these limitations in future devices.
Organic light-emitting diodes (OLEDs) are promising candidates for solid-state lighting but suffer from decreased efficiency at the high current density required for lighting applications. This ...decreased efficiency is called roll-off, and its cause remains the subject of much debate. Here, a combination of targeted experiments and novel modeling is used to quantify the importance of different microscopic processes in OLED roll-off. This study utilizes a new formalismmean-field steady-state kineticsthat can rapidly simulate macroscopic device characteristics based on molecular-scale rates. Specifically, the model can disentangle exciton–charge annihilation (ECA) from exciton–exciton annihilation (EEA)two processes that dominate roll-off. Next, two OLEDs are fabricated that primarily differ in the radiative lifetime of the emitters. This change is found to have a critical effect on the roll-off mechanism; the faster-emitting device is dominated by ECA and the slower device by EEA. One set of molecular rate constants is able to reproduce both the external quantum efficiency and photoluminescence quantum yield roll-off in each device, suggesting that the resulting mechanistic insights are robust. These results suggest that tuning the emitting layer can change the roll-off behavior in an OLED, and the speed and simplicity of the kinetic model opens the door for rapid design of more efficient devices.
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