Morphology of passivating organic ligands around a nanocrystal

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 TEM on a PbS nanoarray, confirming that this theory gives a qualitatively correct picture of the ligand shell thickness of colloidal quantum dots.Comment: 17 Pages, 9 Figure

Triplet-sensitization by lead halide perovskite thin films for near-infrared-to-visible upconversion

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination

Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications.Comment: updated author metadat

Energy Transfer In A Synthetic Dendron-based Light Harvesting System

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.5]{ismsfigure.eps} \end{wrapfigure} Single molecule experiments based on Förster resonance energy transfer (FRET) are now capable of detecting energy funneling in branched molecules. Here we present the synthesis, as well as the optical characterization of a dendron coupled to two donor dyes (Cy3) and one acceptor dye (Cy5). Characterization of the dendron by ensemble absorption and emission spectroscopy shows that the molecule is capable of light harvesting; yielding a FRET signal from the acceptor that is greater than expected for a single donor. Additionally, we investigated an energy transfer cascade upon UV excitation of the conjugated backbone, resulting in several competing energy transfer pathways with the same total energy transfer as direct FRET. The first pathway is FRET from the backbone to Cy3 and resulting FRET to Cy5, with the competing pathway that allows direct energy transfer to Cy5 from the backbone via superexchange. Structural simulations in solution, as well as direct imaging by scanning tunneling microscopy show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for imaging single molecule studies of light harvesting

Single molecule optical absorption at room temperature detected by scanning tunneling microscopy

The high spatial resolution of the scanning tunneling microscope makes it a powerful tool to investigate single molecules deposited on a variety of conductive or semi-conductive surfaces. By adding laser absorption, we are able to simultaneously examine single molecules with high spatial and high energy resolution. Our method of single molecule absorption detected by scanning tunneling microscopy (SMA-STM) relies on backside illumination to cut down on tip heating effects. The evanescent wave of a laser undergoing total internal reflection nearly saturates excitation of molecules on the surface, thus changing the net local density of states enough for STM detection. The excitation laser is amplitude modulated, allowing for simultaneous detection of the STM current (image) and its derivative (absorption signal) by a lock-in amplifier. Although this approach for the most part overcomes the junction heating effects, a new problem arises. It is no longer possible to use arbitrary substrates - apart from being atomically flat and conductive they must also be optically transparent at the wavelength of excitation. Previous studies involved E11 absorption spectroscopy of carbon nanotubes (CNT) on silicon substrates. For further studies of SMA we have chosen molecules with a more defined absorption band in the visible region: organic fluorophores and quantum dots, as well as higher excited states of CNTs. 15 nm thick platinum gold hybrid films deposited by electron beam evaporation onto c-plane sapphire substrates serve as substrates. Low resistance, sufficient light transmission and atomically flat island surfaces make these strong candidates for optical experiments. As expected, SMA-STM performed on quantum dots and carbon nanotubes deposited by dry contact transfer onto a Pt-Au film, resulted in a strong, phase dependent absorption signal. Results from a collaboration with a theoretical group at the University of Washington aid in the explanation of the observed shapes of excited electron density in PbS quantum dots Stark-shifted so that different electronic states contribute to the absorption signal. Semiconducting-to-metallic transitions in CNTs have been imaged and identified directly by SMA-STM, and also characterized by I-V curves. Finally the synthesis, optical and surface characterization of a dendron functionalized with two green donor dyes (Cy3) and one red acceptor dye (Cy5) through flexible linkers, that will be used in the next generation of SMA-STM experiments on metal films, is presented. Single-molecule experiments based on Förster resonant energy transfer (FRET) or on single molecule absorption spectroscopy (SMA) are now capable of studying energy funneling, exciton blockade, singlet fission, and a variety of other processes that involve multiple photoactive groups interacting on a single molecular backbone. Characterization of the dendron and of control molecules with fewer donors or no acceptor by ensemble absorption and emission spectroscopy shows that the system is capable of light harvesting, producing an intramolecular FRET signal from the acceptor greater than expected from a single donor. Additionally, intramolecular energy transfer upon UV excitation of the conjugated backbone is investigated. The photophysical behavior of this light harvesting dendron can be rationalized by a simple Förster/superexchange model. Simulations and scanning tunneling microscopy of single dendron molecules show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for single molecule studies of light harvesting

Correction: Engineering 3D perovskites for photon interconversion applications.

[This corrects the article DOI: 10.1371/journal.pone.0230299.]

Turning On TTA: Aggregation-Induced Energy Landscape Modification

The development of efficient solid-state photon upconversion (UC) devices remains paramount for practical applications of the technology, and in recent years, incorporating perovskite thin films as triplet sensitizers for triplet-triplet annihilation (TTA) based UC has provided a promising solution. In the pursuit of finding an ‘ideal annihilator’ in order to maximize the apparent anti-Stokes shift, we investigate naphtho[2,3-a]pyrene (NaPy) as an annihilator in both solution-based and perovskite-sensitized TTA-UC systems. Surprisingly, we observe different emissive behaviors of NaPy in the solid state based on the excitation wavelength. Under direct excitation, a high energy transition S1’ dominates the emission spectrum, while upconversion results in predominate emission from a lower lying state S1”. We propose that this is the result of aggregation-induced lowering of the singlet excited state thus changing the fundamental energic landscape. Aggregation decreases the singlet energy below the energy level of the triplet pair state ¬1(TT), yielding energetically favorable emission from the aggregated state S1” and weak emission from the higher lying singlet state S1’

Engineering 3D perovskites for photon interconversion applications.

In this review, we highlight the current advancements in the field of triplet sensitization by three-dimensional (3D) perovskite nanocrystals and bulk films. First introduced in 2017, 3D perovskite sensitized upconversion (UC) is a young fast-evolving field due to the tunability of the underlying perovskite material. By tuning the perovskite bandgap, visible-to-ultraviolet, near-infrared-to-visible or green-to-blue UC has been realized, which depicts the broad applicability of this material. As this research field is still in its infancy, we hope to stimulate the field by highlighting the advantages of these perovskite nanocrystals and bulk films, as well as shedding light onto the current drawbacks. In particular, the keywords toxicity, reproducibility and stability must be addressed prior to commercialization of the technology. If successful, photon interconversion is a means to increase the achievable efficiency of photovoltaic cells beyond its current limits by increasing the window of useable wavelengths