Showing papers by "Moungi G. Bawendi published in 2014"

Improved performance and stability in quantum dot solar cells through band alignment engineering

Abstract: Fabricating low-temperature solution-processed solar cells with good power-conversion efficiency and stability in ambient conditions has proved challenging. The use of ligands that protect colloidal quantum dots from degradation in air and tune their energy levels is now shown to be a viable approach for the realization of spin-coated solar cells with very high efficiency. Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing1, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Improved performance and stability in quantum dot solar cells through band alignment engineering

Abstract: Fabricating low-temperature solution-processed solar cells with good power-conversion efficiency and stability in ambient conditions has proved challenging. The use of ligands that protect colloidal quantum dots from degradation in air and tune their energy levels is now shown to be a viable approach for the realization of spin-coated solar cells with very high efficiency. Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing1, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Energy level modification in lead sulfide quantum dot thin films through ligand exchange.

Abstract: The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the perfo...

A transferable model for singlet-fission kinetics

Abstract: Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 A. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

Core/Shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency

Abstract: CdSe/CdS core/shell quantum dots (QDs) have been optimized toward luminescent solar concentration (LSC) applications. Systematically increasing the shell thickness continuously reduced reabsorption up to a factor of 45 for the thickest QDs studied (with ca. 14 monolayers of CdS) compared to the initial CdSe cores. Moreover, an improved synthetic method was developed that retains a high-fluorescence quantum yield, even for particles with the thickest shell volume, for which a quantum yield of 86% was measured in solution. These high quantum yield thick shell quantum dots were embedded in a polymer matrix, yielding highly transparent composites to serve as prototype LSCs, which exhibited an optical efficiency as high as 48%. A Monte Carlo simulation was developed to model LSC performance and to identify the major loss channels for LSCs incorporating the materials developed. The results of the simulation are in excellent agreement with the experimental data.

Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals

Abstract: Lead sulphide colloidal nanocrystals are now used to harvest non-emissive triplet excitons generated in a tetracene layer. Depending on the length of the ligands capping the nanocrystals, exciton transfer efficiency as high as 90% can be achieved.

Magneto-fluorescent core-shell supernanoparticles.

Abstract: Magneto-fluorescent particles have been recognized as an emerging class of materials that exhibit great potential in advanced applications. However, synthesizing such magneto-fluorescent nanomaterials that simultaneously exhibit uniform and tunable sizes, high magnetic content loading, maximized fluorophore coverage at the surface and a versatile surface functionality has proven challenging. Here we report a simple approach for co-assembling magnetic nanoparticles with fluorescent quantum dots to form colloidal magneto-fluorescent supernanoparticles. Importantly, these supernanoparticles exhibit a superstructure consisting of a close-packed magnetic nanoparticle 'core', which is fully surrounded by a 'shell' of fluorescent quantum dots. A thin layer of silica coating provides high colloidal stability and biocompatibility, and a versatile surface functionality. We demonstrate that after surface pegylation, these silica-coated magneto-fluorescent supernanoparticles can be magnetically manipulated inside living cells while being optically tracked. Moreover, our silica-coated magneto-fluorescent supernanoparticles can also serve as an in vivo multi-photon and magnetic resonance dual-modal imaging probe.

Robust excitons inhabit soft supramolecular nanotubes

Abstract: Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions--prerequisites for efficient energy transport--are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials.

Deconstructing the photon stream from single nanocrystals: from binning to correlation

Abstract: Prior to the advent of single-molecule fluorescence spectroscopy, many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals, ranging from intensity “binning” techniques to more sophisticated methods based on single-photon counting. In particular, we highlight photon correlation – a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics, uncovering the basic processes governing spectral linewidths and spectral diffusion, and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

p‐i‐n Heterojunction Solar Cells with a Colloidal Quantum‐Dot Absorber Layer

Abstract: A quantum-dot (QD) p-i-n heterojunction solar cell with an increased depletion region is demonstrated by depleting the QD layer from both the front and back junctions. Due to a combination of improved charged extraction and increased light absorption, a 120% increase in the short-circuit current is achieved compared with that of conventional ZnO/QD devices.

Coherent exciton dynamics in supramolecular light-harvesting nanotubes revealed by ultrafast quantum process tomography.

Abstract: Long-lived exciton coherences have been recently observed in photosynthetic complexes via ultrafast spectroscopy, opening exciting possibilities for the study and design of coherent exciton transport Yet, ambiguity in the spectroscopic signals has led to arguments against interpreting them in terms of exciton dynamics, demanding more stringent tests We propose a novel strategy, quantum process tomography (QPT), for ultrafast spectroscopy and apply it to reconstruct the evolving quantum state of excitons in double-walled supramolecular light-harvesting nanotubes at room temperature from eight narrowband transient grating experiments Our analysis reveals the absence of nonsecular processes, unidirectional energy transfer from the outer to the inner wall exciton states, and coherence between those states lasting about 150 fs, indicating weak electronic coupling between the walls Our work constitutes the first experimental QPT in a “warm” and complex system and provides an elegant scheme to maximize infor

Metabolic Tumor Profiling with pH, Oxygen, and Glucose Chemosensors on a Quantum Dot Scaffold

Abstract: Acidity, hypoxia, and glucose levels characterize the tumor microenvironment rendering pH, pO2, and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These noninvasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.

Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals

Abstract: The understanding of the photophysics of visible-emitting colloidal nanocrystals (NCs) has long been aided by single-molecule studies of their emission. Until recently, no suitable detection technologies have existed for corresponding studies of shortwave-infrared (SWIR) emitters. Now, the use of superconducting nanowire single-photon detectors (SNSPDs) enables the detailed study of SWIR NC emission dynamics at the single-emitter level. Here, we report a detailed analysis of the emission dynamics of individual InAs/CdZnS NCs emitting in the SWIR region. We observe blinking akin to the type A and type B blinking previously observed in visible-emitting CdSe NCs. We determine the intrinsic radiative lifetime of several InAs/CdZnS NCs and find examples ranging from 50–200 ns, indicative of a quasi-type-II electronic structure. We also measure g0(2) for several of these NCs and find that their biexciton emission quantum yields vary from <1% up to 43%.

Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation

Abstract: The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.

Enhanced photovoltaic performance with co-sensitization of quantum dots and an organic dye in dye-sensitized solar cells

Abstract: CdSe quantum dots of two different sizes exhibiting a maximum emission at 495 nm (CdSe495) and 545 nm (CdSe545) were combined with di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II) (N719) or 2-cyano-3-{5-[7-(4-diphenylamino-phenyl)benzo[1,2,5]thiadiazol-4-yl]-thiophen-2-yl}-acrylic acid (TBTCA) resulting in four novel hybrid organic–inorganic sensitizers, which were used in the fabrication of dye-sensitized solar cells. The results showed that with N719, both the CdSe dots decreased the power conversion efficiencies when compared to a standard device consisting only of N719 as the sensitizer. With the organic dye TBTCA, CdSe545 showed no significant effect, while CdSe495 interacted favorably, leading to a 25% increase in power conversion efficiency compared to a device sensitized solely by TBTCA. Studies on excited-state lifetimes of N719 in the presence of CdSe did not distinguish between energy and/or charge transfer mechanisms. On the other hand, time correlated single photon counting experiments on the photoelectrodes suggest that the advantages due to the CdSe495–TBTCA combination could be ascribed to FRET from quantum dots to the organic dye and to a further contribution, as suggested by IPCE spectra, consisting of electron transfer via cascade from the LUMO level of TBTCA to CdSe495 to TiO2, which produces a higher flux of electrons in the external circuit.

Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation

Abstract: The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.

Quantum dot solar cells with band alignment engineering

Abstract: A solar cell and method of making are disclosed. The solar cell includes an acceptor layer a donor layer treated with a first quantum dot (QD) ligand and a blocking layer treated with a second, different, QD ligand. The acceptor layer has an acceptor layer valence band and an acceptor layer conduction band. The donor layer has a donor layer valence band and a donor layer conduction band, the donor layer valence band is higher than the acceptor layer valence band, the donor layer conduction band is higher than the acceptor layer conduction band. The blocking layer least partially blocks electron flow in at least one direction, the blocking layer having a blocking layer valence band and a blocking layer conduction band, the blocking layer valence band is higher than the donor layer valence band, the blocking layer conduction band is higher than the donor layer conduction band.

Surface binding of nanoparticle based drug delivery to tissue

Abstract: Microparticles and nanoparticles and compositions thereof are provided. The microparticles and nanoparticles and compositions may be used for the treatment of musculoskeletal disease, such as osteoarthritis and injury such as trauma.

A phase I study of the safety and activation of a cathepsin-activatable fluorescent cancer-specific probe LUM015.

Abstract: TPS11135 Background: Local recurrence is a common mode of failure for patients with cancer. Intra-operative detection of microscopic residual disease in the tumor bed could be used to decrease the ...

Corrigendum: A transferable model for singlet-fission kinetics

Abstract: Nature Chemistry 6, 492–497 (2014); published online 4 May 2014; corrected after print 21 May 2014.10.1038/nchem.1945 In the version of this Article originally published, the author name Moungi G. Bawendi was missing the middle initial. This has now been corrected in the online versions of the Article.

Short-wavelength infrared (swir) fluorescence in vivo and intravital imaging with semiconductor nanocrystals

Abstract: InAs based core-shell particles which leads to tunable, narrow emitting semiconductor nanocrystals with a very high quantum yield which can be preserved in physiological buffers with long stability can used for short wavelength infrared (SWIR) imaging. Increased resolution with reduced read time and increased imaging frequency can provide advantages in in vivo applications.

Quantum dot dye-sensitized solar cell

Abstract: Quantum dot dye-sensitized solar cell (QDDSSC) comprising an anode, a cathode, an electrolyte between the anode and the cathode, wherein the anode comprises: -a semiconductor electrode layer absorbed with at least one organic dye, said organic dye comprising at least one triaryl-amine group and at least one benzo-heterodiazole group; -at least one quantum dot (QD) distributed within the semiconductor electrode layer, said quantum dot (QD) having an average diameter ranging from 1.5 nm to 3.6 nm, preferably ranging from 1.6 nm to 3.2 nm.

Bowtie plasmonic aperture for single quantum emitter absorption measurement

Abstract: Bowtie apertures with gap sizes of less than 30nm are fabricated successfully by lift-off process. Simulations show that they have mode area as small as 0.011(l/n)2, which is two orders smaller than a conventional tightly focused laser spot.

Liaison de surface de médicaments à base de nanoparticules administrés à des tissus

Abstract: L'invention concerne des microparticules, des nanoparticules et des compositions comprenant celles-ci. Ces microparticules, nanoparticules et compositions peuvent etre utilisees pour le traitement des maladies musculo-squelettiques, telles que l'arthrose et les lesions de type trauma.