Vladimir Bulović

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (237)1532.72 Total impact

  • 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; DOI:10.1021/acsnano.5b00938 · 12.03 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 · 10.74 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; DOI:10.1039/C4EE04073B · 15.49 Impact Factor
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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 · 15.41 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.
    Nano Letters 12/2014; 15(1). DOI:10.1021/nl5024244 · 12.94 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 · 36.43 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; DOI:10.1021/nn504444g · 12.03 Impact Factor
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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 · 15.41 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; DOI:10.1038/nmat3984 · 36.43 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 · 12.03 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 · 12.94 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 · 10.74 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
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    ABSTRACT: We demonstrate a method for fabricating organic optical microcavities which can be electrostatically actuated to dynamically tune their optical output spectra. Fabrication of an integrated organic micro-opto-electro-mechanical system (MOEMS) cavity is enabled by the solvent-free additive transfer of a composite membrane. Electrical actuation and optical characterization of a completed cavity show resonance tuning greater than 20 nm for membrane deflections of over 200 nm at 50 V.
    2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS); 01/2014
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    ABSTRACT: The mainstream commercialization of colloidal quantum dots (QDs) for light-emitting applications has begun: Sony televisions emitting QD-enhanced colors are now on sale. The bright and uniquely size-tunable colors of solution-processable semiconducting QDs highlight the potential of electroluminescent QD light-emitting devices (QLEDs) for use in energy-efficient, high-color-quality thin-film display and solid-state lighting applications. Indeed, this year’s report of record-efficiency electrically driven QLEDs rivaling the most efficient molecular organic LEDs, together with the emergence of full-color QLED displays, foreshadow QD technologies that will transcend the optically excited QD-enhanced products already available. In this article, we discuss the key advantages of using QDs as luminophores in LEDs and outline the 19-year evolution of four types of QLEDs that have seen efficiencies rise from less than 0.01% to 18%. With an emphasis on the latest advances, we identify the key scientific and technological challenges facing the commercialization of QLEDs. A quantitative analysis, based on published small-scale synthetic procedures, allows us to estimate the material costs of QDs typical in light-emitting applications when produced in large quantities and to assess their commercial viability.
    09/2013; 38(09). DOI:10.1557/mrs.2013.181
  • Organic Electronics 09/2013; 14(9):2257-2268. DOI:10.1016/j.orgel.2013.05.004 · 3.68 Impact Factor
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    ABSTRACT: New tetraalkylcyclobutadiene–C60 adducts are developed via Diels–Alder cycloaddition of C60 with in situ generated cyclobutadienes. The cofacial π-orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C60 significantly (ca. 100 and 300 meV for mono- and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open-circuit voltages (VOC) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene–C60 monoadduct displays an open-circuit voltage (0.61 V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3-hexylthiophene/([6,6]-phenyl-C61-butyric acid methyl ester) composite (0.58 V and 2.57%, respectively). The role of the cofacial π-orbital interactions between C60 and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance.
    Advanced Functional Materials 06/2013; 23(24). DOI:10.1002/adfm.201203251 · 10.44 Impact Factor
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    ABSTRACT: Colloidal quantum dot photovoltaics show great promise for future solar energy conversion applications but remain limited by inefficient charge carrier extraction. Ordered arrays of ZnO nanowires, shown here in blue, can decouple light absorption and carrier collection, yielding a significant relative enhancement in the photocurrent and efficiency of quantum dot solar cells. Further details can be found in the article by Vladimir Bulović and co-workers on page 2790.
    Advanced Materials 05/2013; 25(20):2789. DOI:10.1002/adma.201370134 · 15.41 Impact Factor
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    ABSTRACT: Vertical arrays of ZnO nanowires can decouple light absorption from carrier collection in PbS quantum dot solar cells and increase power conversion efficiencies by 35%. The resulting ordered bulk heterojunction devices achieve short-circuit current densities in excess of 20 mA cm(-2) and efficiencies of up to 4.9%.
    Advanced Materials 05/2013; 25(20). DOI:10.1002/adma.201204192 · 15.41 Impact Factor

Publication Stats

13k Citations
1,532.72 Total Impact Points

Institutions

  • 2002–2015
    • Massachusetts Institute of Technology
      • • Department of Electrical Engineering and Computer Science
      • • Department of Materials Science and Engineering
      Cambridge, Massachusetts, United States
  • 2010
    • Tufts University
      • Department of Chemical and Biological Engineering
      Medford, MA, 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