Vladimir Bulović

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (250)1710.71 Total impact

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    ABSTRACT: Charge-transfer (CT) states, bound combinations of an electron and a hole on separate molecules, play a crucial role in organic optoelectronic devices. We report direct nanoscale imaging of the transport of long-lived CT states in molecular organic donor-acceptor blends, which demonstrates that the bound electron-hole pairs that form the CT states move geminately over distances of 5-10 nm, driven by energetic disorder and diffusion to lower energy sites. Magnetic field dependence reveals a fluctuating exchange splitting, indicative of a variation in electron-hole spacing during diffusion. The results suggest that the electron-hole pair of the CT state undergoes a stretching transport mechanism analogous to an 'inchworm' motion, in contrast to conventional transport of Frenkel excitons. Given the short exciton lifetimes characteristic of bulk heterojunction organic solar cells, this work confirms the potential importance of CT state transport, suggesting that CT states are likely to diffuse farther than Frenkel excitons in many donor-acceptor blends.
    Nature Material 09/2015; DOI:10.1038/nmat4424 · 36.50 Impact Factor
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    ABSTRACT: Plexcitons are polaritonic modes that result from the strong coupling between excitons and plasmons. We consider plexcitons emerging from the interaction of excitons in an organic molecular layer with surface plasmons in a metallic film. We predict the emergence of Dirac cones in the two-dimensional bandstructure of plexcitons due to the inherent alignment of the excitonic transitions in the organic layer. These Dirac cones may open up in energy by simultaneously interfacing the metal with a magneto-optical layer and subjecting the whole system to a perpendicular magnetic field. The resulting energy gap becomes populated with topologically protected one-way modes which travel at the interface of this plexcitonic system. Our theoretical proposal suggests that plexcitons are a convenient and simple platform for the exploration of exotic phases of matter as well as of novel ways to direct energy flow at the nanoscale.
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    ABSTRACT: Abrupt switching behavior and near-zero leakage current of nanoelectromechanical (NEM) switches are advantageous properties through which NEMs can outperform conventional semiconductor electrical switches. To date, however, typical NEMs structures require high actuation voltages and can prematurely fail through permanent adhesion (defined as stiction) of device components. To overcome these challenges, in the present work we propose a NEM switch, termed a "squitch," which is designed to electromechanically modulate the tunneling current through a nanometer-scale gap defined by an organic molecular film sandwiched between two electrodes. When voltage is applied across the electrodes, the generated electrostatic force compresses the sandwiched molecular layer, thereby reducing the tunneling gap and causing an exponential increase in the current through the device. The presence of the molecular layer avoids direct contact of the electrodes during the switching process. Furthermore, as the layer is compressed, the increasing surface adhesion forces are balanced by the elastic restoring force of the deformed molecules which can promote zero net stiction and recoverable switching. Through numerical analysis, we demonstrate the potential of optimizing squitch design to enable large on-off ratios beyond 6 orders of magnitude with operation in the sub-1 V regime and with nanoseconds switching times. Our preliminary experimental results based on metal-molecule-graphene devices suggest the feasibility of the proposed tunneling switching mechanism. With optimization of device design and material engineering, squitches can give rise to a broad range of low-power electronic applications.
    ACS Nano 08/2015; 9(8). DOI:10.1021/acsnano.5b02476 · 12.88 Impact Factor
  • W.J. Jo · D.C. Borrelli · V. Bulović · K.K. Gleason
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    ABSTRACT: Abstract Polyselenophene (PSe) donor layers are successfully integrated into organic photovoltaic devices (OPV) for the first time. Thin, patterned films of this insoluble semiconductor were fabricated using a vacuum-based vapor-printing technique, oxidative chemical vapor deposition (oCVD) combined with in-situ shadow masking. The vapor-printed PSe exhibits a reduced optical bandgap of 1.76 eV and enhanced photo-responsivity in the red compared to its sulfur containing analog, polythiophene. These relative advantages are most likely explained by selenium's enhanced electron-donating character compared to sulfur. The HOMO level of PSe was determined to be at -4.85 eV. The maximum power conversion efficiency achieved was 0.4% using a bilayer heterojunction device architecture with C60 as the donor.
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    ABSTRACT: We report an ultra-bright, highly efficient, low roll-off, inverted quantum dot-based red light emitting device (QLED) using solution-processed zinc oxide nanoparticles and cesium carbonate films as the electron injection and hole blocking layers, respectively. Record luminance of 165,000 Cd/m2 has been obtained at a current density of 1000 mA/cm2 with a low driving voltage of 5.8 V for deep red device with CIE coordinates of (0.69, 0.31).
    06/2015; 46(1). DOI:10.1002/sdtp.10462
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    ABSTRACT: Quantum dot photovoltaics (QDPV) offer the potential for low-cost solar cells. To develop strategies for continued improvement in QDPVs, a better understanding of the factors that limit their performance is essential. Here, we study carrier recombination processes that limit the power conversion efficiency of PbS QDPVs. We demonstrate the presence of radiative sub-bandgap states and sub-bandgap state filling in operating devices by using photoluminescence (PL) and electroluminescence (EL) spectroscopy. These sub-bandgap states are most likely the origin of the high open-circuit-voltage (VOC) deficit and relatively limited carrier collection that have thus far been observed in QDPVs. Combining these results with our perspectives on recent progress in QDPV, we conclude that eliminating sub-bandgap states in PbS QD films has the potential to show a greater gain than may be attainable by optimization of interfaces between QDs and other materials. We suggest possible future directions that could guide the design of high-performance QDPVs.
    Nano Letters 04/2015; 15(5). DOI:10.1021/acs.nanolett.5b00513 · 13.59 Impact Factor
  • Wendi Chang · Gleb M Akselrod · Vladimir Bulovic
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    ABSTRACT: Direct modification of exciton energy has been previously used to optimize the operation of organic optoelectronic devices. One demonstrated method for exciton energy modification is through the use of the solvent dielectric effects in doped molecular films. To gain a deeper appreciation of the underlying physical mechanisms, in this work we test the solid-state solvation effect in molecular thin films under applied external pressure. We observe that external mechanical pressure increases dipole-dipole interactions, leading to shifts in the Frenkel exciton energy and enhancement of the time-resolved spectral red shift associated with the energy-transfer-mediated exciton diffusion. Measurements are performed on host:dopant molecular thin films, which show bathochromic shifts in photoluminescence (PL) under increasing pressure. This is in agreement with a simple solvation theory model of exciton energetics with a fitting parameter based on the mechanical properties of the host matrix material. We measure no significant change in exciton lifetime with increasing pressure, consistent with unchanged aggregation in molecular films under compression. However, we do observe an increase in exciton spectral thermalization rate for compressed molecular films, indicating enhanced exciton diffusion for increased dipole-dipole interactions under pressure. The results highlight the contrast between molecular energy landscapes obtained when dipole-dipole interactions are increased by the pressure technique vs. the conventional dopant concentration variation methods, which can lead to extraneous effects such as aggregation at higher doping concentrations. The present work demonstrates the use of pressure-probing techniques in studying energy disorder and exciton dynamics in amorphous molecular thin films.
    ACS Nano 04/2015; 9(4). DOI:10.1021/acsnano.5b00938 · 12.88 Impact Factor
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    ABSTRACT: Charge transfer states play a crucial role in organic photovoltaics, mediating both photocurrent generation and recombination losses. In this work, we examine recombination losses as a function of the electron-hole spacing in fluorescent charge transfer states, including direct monitoring of both singlet and triplet charge transfer state dynamics. Here we demonstrate that large donor-acceptor separations minimize back transfer from the charge transfer state to a low-lying triplet exciton 'drain' or the ground state by utilizing external pressure to modulate molecular spacing. The triplet drain quenches triplet charge transfer states that would otherwise be spin protected against recombination, and switches the most efficient origin of the photocurrent from triplet to singlet charge transfer states. Future organic solar cell designs should focus on raising the energy of triplet excitons to better utilize triplet charge transfer mediated photocurrent generation or increasing the donor-acceptor spacing to minimize recombination losses.
    Nature Communications 03/2015; 6:6415. DOI:10.1038/ncomms7415 · 11.47 Impact Factor
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    ABSTRACT: Solar energy is one of the few renewable, low-carbon resources with both the scalability and the technological maturity to meet ever-growing global demand for electricity. Among solar power technologies, solar photovoltaics (PV) are the most widely deployed, providing 0.87% of the world's electricity in 2013 and sustaining a compound annual growth rate in cumulative installed capacity of 43% since 2000. Given the massive scale of deployment needed, this article examines potential limits to PV deployment at the terawatt scale, emphasizing constraints on the use of commodity and PV-critical materials. We propose material complexity as a guiding framework for classifying PV technologies, and we analyze three core themes that focus future research and development: efficiency, materials use, and manufacturing complexity and cost.
    Energy & Environmental Science 02/2015; 8(4). DOI:10.1039/C4EE04073B · 20.52 Impact Factor
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    ABSTRACT: Utilizing stiction, a common failure mode in micro/nano electromechanical systems (M/NEMS), we propose a method for the controlled fabrication of nanometer-thin gaps between electrodes. In this approach, a single lithography step is used to pattern cantilevers that undergo lateral motion towards opposing stationary electrodes separated by a defined gap. Upon wet developing of the pattern, capillary forces induce cantilever deflection and collapse leading to permanent adhesion between the tip and an opposing support structure. The deflection consequently reduces the separation gap between the cantilever and the electrodes neighboring the point of stiction to dimensions smaller than originally patterned. Through nanoscale force control achieved by altering device design, we demonstrate the fabrication of nanogaps having controlled widths smaller than 15 nm. We further discuss optimization of these nanoscale gaps for applications in NEM and molecular devices.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 02/2015; 2015:85-88. DOI:10.1109/MEMSYS.2015.7050892
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    ABSTRACT: Core-shell PbS-CdS quantum dots enhance the peak external quantum efficiency of shortwave-infrared light-emitting devices by up to 50-100-fold (compared with core-only PbS devices). This is more than double the efficiency of previous quantum-dot light-emitting devices operating at wavelengths beyond 1 μm and results from the passivation of the PbS cores by the CdS shells against in situ photoluminescence quenching. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 01/2015; 27(8). DOI:10.1002/adma.201404636 · 17.49 Impact Factor
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    ABSTRACT: We study the dielectric constant of lead sulfide quantum dot (QD) films as a function of the volume fraction of QDs by varying the QD size and keeping the ligand constant. We create a reliable QD sizing curve using small-angle X-ray scattering (SAXS), thin-film SAXS to extract a pair-distribution function for QD spacing, and a stacked-capacitor geometry to measure the capacitance of the thin film. Our data support a reduced dielectric constant in nanoparticles.Keywords: Dielectric constant; quantum dot; PbS; SAXS; capacitance
    Nano Letters 12/2014; 15(1). DOI:10.1021/nl5024244 · 13.59 Impact Factor
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    ABSTRACT: 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.
    Nature Material 10/2014; 13(11). DOI:10.1038/nmat4097 · 36.50 Impact Factor
  • Parag B Deotare · Thomas S Mahony · Vladimir Bulović
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    ABSTRACT: We report an ultracompact low-threshold laser with an Alq3:DCM host:guest molecular organic thin film gain layer. The device uses a photonic crystal nanobeam cavity which provides a high quality factor to mode volume (Q/V) ratio and increased spontaneous emission factor along with a small footprint. Lasing is observed with a threshold of 4.2 μJ/cm(2) when pumped by femtosecond pulses of λ = 400 nm wavelength light. We also model the dynamics of the laser and show good agreement with the experimental data. The inherent waveguide geometry of the structure enables easy on-chip integration with potential applications in biochemical sensing, inertial sensors, and data communication.
    ACS Nano 09/2014; 8(11). DOI:10.1021/nn504444g · 12.88 Impact Factor
  • Dong-Kyun Ko · Patrick R Brown · Moungi G Bawendi · Vladimir Bulović
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    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.
    Advanced Materials 07/2014; 26(28). DOI:10.1002/adma.201401250 · 17.49 Impact Factor
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    ABSTRACT: 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 processing, 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.
    Nature Material 05/2014; 13(8). DOI:10.1038/nmat3984 · 36.50 Impact Factor
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    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 performance optimization of QD optoelectronic devices.
    ACS Nano 05/2014; 8(6). DOI:10.1021/nn500897c · 12.88 Impact Factor
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    ABSTRACT: Colloidal quantum dots (QDs) are promising materials for use in solar cells, light emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length - which exceeds 30 nm in some cases - can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.
    Nano Letters 05/2014; 14(6). DOI:10.1021/nl501190s · 13.59 Impact Factor
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    ABSTRACT: Transport of nanoscale energy in the form of excitons is at the core of photosynthesis and the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal and spectral visualization of exciton transport in molecular crystals and disordered thin films. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of the thin film, we show that this transition to subdiffusive transport occurs at earlier times as disorder is increased. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology, which has wide implications for the design of excitonic materials and devices.
    Nature Communications 04/2014; 5:3646. DOI:10.1038/ncomms4646 · 11.47 Impact Factor
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    ABSTRACT: We propose nanoelectromechanical (NEM) switches that operate via electromechanical modulation of tunneling current through several-nanometer-thick switching gaps. In such a device, direct contact between electrodes is avoided by utilizing self-assembled molecular layers to define the switching gap. Electrostatic compression of the molecular layer reduces the tunneling gap leading to an exponential increase in the tunneling current, turning on the switch. With removal of an applied voltage, the compressed layer provides the elastic restoring force necessary to overcome the surface adhesive forces, turning off the switch. Thus, the proposed tunneling NEM switch may enable low-voltage operation while simultaneously mitigating device failure due to stiction. This principle is experimentally investigated using a prototype two-terminal tunneling NEM switch with a switching gap formed by a fluorinated decanethiol layer. In this device, the presence of the molecular film promotes repeatable switching. A comparison of the switch operation with a theoretical model indicates electrostatic compression of the molecular switching gap.
    2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS); 01/2014

Publication Stats

14k Citations
1,710.71 Total Impact Points


  • 2003–2015
    • Massachusetts Institute of Technology
      • Department of Electrical Engineering and Computer Science
      Cambridge, Massachusetts, United States
  • 2004
    • Brown University
      • School of Engineering
      Providence, RI, United States
  • 1994–2001
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States
  • 1996
    • University of California, Los Angeles
      • Department of Chemistry and Biochemistry
      Los Ángeles, California, United States