Tuneable emission in single molecule dyads mediated by a charge transfer state.

The demand for fluorescent organic dyes across a broad range of applications has led to investigation into tuneable emission dyes. The tuneable nature of these dyes makes them desirable for applications in a variety of fields, including organic light-emitting diodes (OLEDs), optical sensing devices, and fluorescence imaging. In recent investigations, there have only been a handful of mechanisms used to tune emission. Herein, we present four novel perylene-acene dyads that undergo solvent tuneable emission, and propose a novel mechanism for this tuneability based on the presence of a charge transfer state. These dyes were shown to reach photoluminescence quantum efficiencies (PLQEs) as high as 45%, depending on the solvent, showing the ability for this mechanism to be used to access higher PLQE tuneable emission.

Micellular fluorescence resonance energy transfer based fluorescent ratiometric response to hydrocarbon analytes.

Fluorescence resonance energy transfer (FRET) has been utilised to develop numerous selective and sensitive fluorescent ratiometric sensors. Typically, FRET-based fluorescent ratiometric sensors rely on chemical interactions between the sensor and analyte to illicit a response, thus unreactive hydrocarbons are a neglected analyte and a source for new sensors. By containing an unbound donor-acceptor system within micelles, energy transfer is enabled by spatial confinement. This offers the potential of a ratiometric response as a hydrocarbon analyte is added. Introducing a hydrocarbon analyte to this system causes micelles to swell, increasing the donor-acceptor distance and thus reducing the amount of observed energy transfer. We present InP/ZnS quantum dot donors interacting with a Nile Red acceptor, confined by cetyltrimethylammonium bromide (CTAB)-based micelles. We alleviated spatial confinement of the pair within micelles using common laboratory solvents to represent hydrocarbons, (toluene, hexane and octadecene). We constructed calibration curves for each solvent and found effective sensing ranges of 0.009-0.21, 0.008-0.27 and 0.003-0.06 M for toluene, hexane and octadecene, respectively. This study contributes towards the development of new hydrocarbon sensors utilising this new mechanism.

Luminescent light diffuser for diffuse lighting applications.

The lighting industry currently accounts for a significant proportion of all energy demand. Luminescent white lighting is often impure, inefficient, expensive, and detrimentally emits as a point source, meaning the light is emitted from a focused point. A luminescent light diffuser offers the potential to create a spatially broad lighting fixture. We developed a luminescent light diffuser consisting of three commercially available luminescent dye species (rhodamine 6G, fluorescein, 7-diethylamino-4-methylcoumarin) dispersed within a polymer matrix (polyvinyl alcohol), or commercial paint, and coated on a planar waveguide. A Light-emitting diode (LED) (385 nm) is directed into the waveguide which excites the luminescent species, coating the panel, creating a device that emits spatially broad pure white light. As the emission depends on escape cone emission from the waveguide, the device's emission was found to depend highly on the coating film quality and components. We present two systems: a small 40 mm × 40 mm prototype, made using standard water-soluble polymer (polyvinyl alcohol), to study the underlying operational principles, and a 100 mm × 100 mm device with optimized efficiency fabricated with a clear commercial paint. By doping the polymer matrix with scattering silica microparticles we achieved a maximum photon outcoupling efficiency of 78%, whilst maintaining colour purity with an increased device size of more than 300 times (compared with the input LED). This work shows that it is possible to construct an inexpensive and spatially broad lighting source, whilst maintaining colour purity at a low cost.

Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission.

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 ± 5%) before quantitative harvesting of the triplet excitons (95 ± 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.

Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites.

We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.

Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120%.

Multiple exciton generation (MEG) in semiconducting quantum dots is a process that produces multiple charge-carrier pairs from a single excitation. MEG is a possible route to bypass the Shockley-Queisser limit in single-junction solar cells but it remains challenging to harvest charge-carrier pairs generated by MEG in working photovoltaic devices. Initial yields of additional carrier pairs may be reduced due to ultrafast intraband relaxation processes that compete with MEG at early times. Quantum dots of materials that display reduced carrier cooling rates (e.g., PbTe) are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we demonstrate PbTe quantum dot-based solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultrafast time scale with pump-probe transient absorption and pump-push-photocurrent measurements, we identify a delayed cooling effect above the threshold energy for MEG.

Heterostructured Nanotetrapod Luminophores for Reabsorption Elimination within Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) concentrate light via luminescence within a planar-waveguide and have potential use for building-integrated photovoltaics. However, their commercialization and potential applications are currently hindered greatly by photon reabsorption, where emitted waveguided light is parasitically reabsorbed by a luminophore. Nanotetrapod semiconductor materials have been theorized to be excellent luminophores for LSCs owing to their inherently large Stokes shifts. Here we present the first nanotetrapod-based LSCs (5 × 5 × 0.3 cm3) reported in the literature. External quantum efficiencies as high as 4.9 ± 0.5% were achieved under AM1.5G conditions. We also perform an in-depth investigation by optical characterization of the different operational metrics of our nanotetrapod-based LSCs and show reabsorption to be eliminated (mean number of average reabsorption events per photon equal to 0.00) in our most extended nanotetrapod devices.

The Rise and Future of Discrete Organic-Inorganic Hybrid Nanomaterials.

Hybrid nanomaterials (HNs), the combination of organic semiconductor ligands attached to nanocrystal semiconductor quantum dots, have applications that span a range of practical fields, including biology, chemistry, medical imaging, and optoelectronics. Specifically, HNs operate as discrete, tunable systems that can perform prompt fluorescence, energy transfer, singlet fission, upconversion, and/or thermally activated delayed fluorescence. Interest in HNs has naturally grown over the years due to their tunability and broad spectrum of applications. This Review presents a brief introduction to the components of HNs, before expanding on the characterization and applications of HNs. Finally, the future of HN applications is discussed.

Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor.

Organic photovoltaics (OPVs) promise cheap and flexible solar energy. Whereas light generates free charges in silicon photovoltaics, excitons are normally formed in organic semiconductors due to their low dielectric constants, and require molecular heterojunctions to split into charges. Recent record efficiency OPVs utilise the small molecule, Y6, and its analogues, which - unlike previous organic semiconductors - have low band-gaps and high dielectric constants. We show that, in Y6 films, these factors lead to intrinsic free charge generation without a heterojunction. Intensity-dependent spectroscopy reveals that 60-90% of excitons form free charges at AM1.5 light intensity. Bimolecular recombination, and hole traps constrain single component Y6 photovoltaics to low efficiencies, but recombination is reduced by small quantities of donor. Quantum-chemical calculations reveal strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge how current OPVs operate, and renew the possibility of efficient single-component OPVs.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions.

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Spectroscopy and thermochemistry of a jet-cooled open-shell polyene: 1,4-pentadienyl radical.

The 1,4-pentadienyl (vinylallyl) radical has been observed for the first time by optical spectroscopy. An excitation spectrum is recorded on m/z 67 by resonant two-color two-photon ionization spectroscopy. Several bands are observed with the origin transition identified at 19 449 cm(-1). The spectrum is assigned by a comparison with ab initio frequencies calculated at the CASPT2/cc-pVTZ level of theory, with an accompanying Franck-Condon calculation of the excitation spectrum, including Dushinsky mixing. The b(1) and a(2) outer C-C bond torsional modes are calculated to halve in frequency upon electronic excitation, bringing about their appearance in the excitation spectrum. This can be readily understood by considering the torsional sensitivity of the frontier molecular orbital energies. High-level quantum chemical calculations of the radical stabilization energy, resulting in a value of nearly 120 kJ mol(-1), provide quantitative confirmation that this radical is highly stabilized.

The photophysical properties of naphthalene bridged disilanes.

Structural isomers of naphthalene-bridged disilanes were prepared via catalytic intramolecular dehydrocoupling of disilyl precursors using Wilkinson's catalyst. Interestingly, it was observed that interchanging the side groups on the silicon atoms altered the photophysical properties of the bridged disilanes. Herein, we report the first example of naphthalene bridged disilanes forming excimers in non-polar solvents. Cyclic voltammetry experiments and DFT calculations were performed to analyse the band gaps of the compounds and σ-π mixing in the bridged disilanes.

Whispering-Gallery Mode Lasing in Perovskite Nanocrystals Chemically Bound to Silicon Dioxide Microspheres.

Cesium lead halide perovskite nanocrystals exhibit high photoluminescence quantum efficiencies and tunability across the visible spectrum. This makes these crystals ideal candidates for solar panels, light-emitting diodes, lasers, and especially nanolasers. Due to the versatility of cation substitution in perovskite nanocrystals, they can be grown on amine-functionalized silicon dioxide nanoparticles, where the amine linker replaces the standard cation structure. Selectively growing luminescent nanocrystals on spherical silicon dioxide microspheres results in the opportunity to populate whispering-gallery modes in these spherical silica microspheres. In this case, the nanocrystal halide composition can be used to selectively tune the emission wavelength mode, and microsphere radius to tune the mode spacing. This silicon dioxide attachment also adds to the overall stability of the system. Through photoluminescence microscopy measurements, we show whispering gallery modes in individual perovskite-coated microspheres for CsPbBr3 and CsPbI3 nanocrystals on 9.2 µm diameter silica spheres and compare these to theoretically predicted optical modes. In CsPbBr3, we provide evidence that these modes will lase under optical excitation, with a threshold of 750 µJ/cm2. This study presents a novel system that, through optimization, could be a promising pathway to achieve facile and stable perovskite nanolasers.

Inter-ligand energy transfer in dye chromophores attached to high bandgap SiO2 nanoparticles.

Artificial light harvesters require ordered arrangement of chromophores. We covalently attach three organic chromophore ligands to silicon dioxide nanoparticles. This allows us to study inter-ligand energy transfer when attached to SiO2 nanoparticles, creating a simple system with a large ratio of donors to acceptors. Using steady-state and transient spectroscopy measurements we quantify this energy transfer between ligands. We show a maximum transfer efficiency of 30% and measure the 2D diffusion length of anthracene carboxylic acid on SiO2 to be between 0.6 and 2.2 nm.

Excimer Formation in Carboxylic Acid-Functionalized Perylene Diimides Attached to Silicon Dioxide Nanoparticles.

The creation of artificial light-harvesting complexes involves the ordered arrangement of chromophores in space. To guarantee efficient energy-transfer processes, organic dyes must be brought into close proximity, often leading to aggregation and the formation of excimer states. In recent years, the attachment of ligand-based chromophores to nanoparticles has also generated interest in relation to improved solar harvesting and spin-dependent electronic interactions such as singlet fission and upconversion. We explore the covalent attachment of two novel perylene-diimide (PDI) carboxylic acid ligands to silicon dioxide nanoparticles. This allows us to study electronic interactions between the ligands when attached to nanoparticles because these cannot couple to the wide band gap silicon dioxide. One of the synthesized PDI ligands has sterically hindering phenols in the bay position and undergoes minimal optical changes upon attachment, but the other forms an excimer state with a red-shifted and long-lived florescence. As such, molecular structure changes offer a method to tune weak and strong interactions between ligand layers on nanocrystal surfaces.

Singlet Fission and Triplet Transfer to PbS Quantum Dots in TIPS-Tetracene Carboxylic Acid Ligands.

Singlet exciton fission allows for the generation of two triplet excitons for each photon absorbed within an organic semiconductor. Efficient harvesting of these triplets could allow for the Shockley-Queisser limit on the power conversion efficiency of single-junction photovoltaics to be broken. Here, we show that singlet fission molecules bound directly to PbS quantum dots as ligands can undergo singlet fission with near unity efficiency and can transfer triplets sequentially into the PbS with near unity efficiency. Within the PbS, the excitations recombine, giving rise of the emission of photons. This allows for the doubling of the quantum dot photoluminescence quantum efficiency when photons are absorbed by the singlet fission ligand, as compared to when directly absorbed in the quantum dot. Our approach demonstrates that it is possible to convert the exciton multiplication process of singlet fission into a photon multiplication process and provides a new path to harness singlet fission with photovoltaics.

Long-lived polarization memory in the electronic states of lead-halide perovskites from local structural dynamics.

Anharmonic crystal lattice dynamics have been observed in lead halide perovskites on picosecond timescales. Here, we report that the soft nature of the perovskite crystal lattice gives rise to dynamic fluctuations in the electronic properties of excited states. We use linear polarization selective transient absorption spectroscopy to study the charge carrier relaxation dynamics in lead-halide perovskite films and nanocrystals. We find that photo-excited charge carriers maintain an initial polarization anisotropy for several picoseconds, independent of crystallite size and composition, and well beyond the reported timescales of carrier scattering. First-principles calculations find intrinsic anisotropies in the transition dipole moment, which depend on the orientation of light polarization and the polar distortion of the local crystal lattice. Lattice dynamics are imprinted in the optical transitions and anisotropies arise on the time-scales of structural motion. The strong coupling between electronic states and structural dynamics requires a unique interpretation of recombination and transport mechanisms.

Photon Reabsorption in Mixed CsPbCl3:CsPbI3 Perovskite Nanocrystal Films for Light-Emitting Diodes.

Cesium lead halide nanocrystals, CsPbX3 (X = Cl, Br, I), exhibit photoluminescence quantum efficiencies approaching 100% without the core-shell structures usually used in conventional semiconductor nanocrystals. These high photoluminescence efficiencies make these crystals ideal candidates for light-emitting diodes (LEDs). However, because of the large surface area to volume ratio, halogen exchange between perovskite nanocrystals of different compositions occurs rapidly, which is one of the limiting factors for white-light applications requiring a mixture of different crystal compositions to achieve a broad emission spectrum. Here, we use mixtures of chloride and iodide CsPbX3 (X = Cl, I) perovskite nanocrystals where anion exchange is significantly reduced. We investigate samples containing mixtures of perovskite nanocrystals with different compositions and study the resulting optical and electrical interactions. We report excitation transfer from CsPbCl3 to CsPbI3 in solution and within a poly(methyl methacrylate) matrix via photon reabsorption, which also occurs in electrically excited crystals in bulk heterojunction LEDs.

Highly Efficient Perovskite Nanocrystal Light-Emitting Diodes Enabled by a Universal Crosslinking Method.

The preparation of highly efficient perovskite nanocrystal light-emitting diodes is shown. A new trimethylaluminum vapor-based crosslinking method to render the nanocrystal films insoluble is applied. The resulting near-complete nanocrystal film coverage, coupled with the natural confinement of injected charges within the perovskite crystals, facilitates electron-hole capture and give rise to a remarkable electroluminescence yield of 5.7%.