Vladimir Bulović

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (224)1310.43 Total impact

  • [Show abstract] [Hide abstract]
    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 materials. 10/2014;
  • 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; · 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 05/2014; · 14.83 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 materials. 05/2014;
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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; · 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; · 13.03 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 01/2014; 5:3646. · 10.74 Impact Factor
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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.
    MRS Bulletin. 09/2013; 38(09).
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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). · 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. · 14.83 Impact Factor
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    ABSTRACT: We study the origin of efficiency roll-off (also called "efficiency droop") in colloidal quantum-dot light-emitting diodes through the comparison of quantum-dot (QD) electroluminescence and photoluminescence. We find that an electric-field-induced decrease in QD luminescence efficiency-and not charge leakage or QD charging (Auger recombination)-is responsible for the roll-off behavior, and use the quantum confined Stark effect to accurately predict the external quantum efficiency roll-off of QD light-emitting diodes.
    Physical Review Letters 05/2013; 110(21):217403. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate room temperature lasing through the polaritonic mode of a J-aggregate microcavity in which losses from exciton-exciton annihilation and slow polariton relaxation typical of direct J-aggregate excitation are circumvented via intra-cavity pumping. The pumping scheme utilizes an organic dye layer (DCM) within the cavity with an emission band overlapping the entire lower J-aggregate polariton branch spectrum, hence forcing DCM lasing to occur through the strongly-coupled mode. This cavity architecture, which separates strong coupling and gain into two materials, presents a general and flexible design for polariton devices and allows for the use of a wide range of materials, organic and inorganic, to be integrated into the cavity.
    Optics Express 05/2013; 21(10):12122-8. · 3.55 Impact Factor
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    ABSTRACT: We report a colour-saturated, red quantum-dot light-emitting device (QLED) using an inverted organic-inorganic hybrid device structure and colloidal CdSe-CdS (core-shell) quantum-dot emitters. The strong electronic coupling of quantum dots to an adjacent layer of ZnO nanocrystals (which form the electron transport layer) facilitates charge transfer, which is responsible for both injecting electrons and maintaining an optimal charge balance for the quantum dot emitters. We show that QLED performance can be modified by controlling the distance of the electroluminescence recombination zone within the quantum dot film from the quantum dot-ZnO interface. Devices are reported with a luminous efficiency of 19 cd A-1, corresponding to an external quantum efficiency of 18% (which is close to the theoretical maximum of 20%) and an internal quantum efficiency of 90%. The corresponding luminous power efficiency exceeds 25 lm W-1 due to the low operating voltage of the device.
    Nature Photonics 05/2013; 7(5):407-412. · 27.25 Impact Factor
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    ABSTRACT: Integration of water splitting catalysts with visible light absorbing semiconductors would enable direct solar-energy-to-fuels conversion schemes such as those based on water splitting. A disadvantage of some common semiconductors that possess desirable optical bandgaps is their chemical instability under the conditions needed for oxygen evolution reaction (OER). In this study, we demonstrate the dual benefits gained from using a cobalt metal thin-film as the precursor for the preparation of cobalt-phosphate (CoPi) OER catalyst on cadmium chalcogenide photoanodes. The cobalt layer protects the underlying semiconductor from oxidation and degradation while forming the catalyst and simultaneously facilitates the advantageous incorporation of the cadmium chalcogenide layer into the CoPi layer during continued processing of the electrode. The resulting hybrid material forms a stable photoactive anode for light-assisted water splitting.
    ACS Applied Materials & Interfaces 03/2013; · 5.90 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 02/2013; · 14.83 Impact Factor
  • Timothy P. Osedach, Trisha L. Andrew, Vladimir Bulović
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    ABSTRACT: For organic photovoltaics (OPVs) to become a viable source of renewable energy, the synthesis of organic active-layer materials will need to be scaled to thousands of kilograms. Additionally, the ultimate cost of these materials will need to be low enough to constitute only a small fraction of the cost of the solar cell module. In this study, we present a quantitative analysis, based on published small-scale synthetic procedures, to estimate the materials costs for several promising OPV materials when produced in large quantities. The cost in dollars-per-gram ($ per g) is found to increase linearly with the number of synthetic steps required to produce each organic photoactive compound. We estimate the cost-per-Watt ($ per Wp) as a function of power conversion efficiency (PCE) for an archetypal OPV structure and find that a relatively simple molecule requiring only 3 synthetic steps will contribute a cost of 0.001 to 0.01 $ per Wp, given a solar module PCE of 10%. In contrast, a relatively complicated molecule requiring 14 synthetic steps will contribute costs in the range of 0.075 to 0.48 $ per Wp. Our findings suggest that the commercial viability of an OPV technology may depend on the synthetic accessibility of its constituent active layer materials. Additionally, this work stresses the importance of optimizing synthetic routes to minimize solvent and reagent usage as well as to minimize the number of required workup procedures in the scaled production of OPV materials.
    Energy & Environmental Science 02/2013; 6(3):711-718. · 11.65 Impact Factor
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    ABSTRACT: PbS colloidal quantum dot heterojunction solar cells have shown significant improvements in performance, mostly based on devices that use high-temperature annealed transition metal oxides to create rectifying junctions with quantum dot thin films. Here, we demonstrate a solar cell based on the heterojunction formed between PbS colloidal quantum dot layers and CdS thin films that are deposited via a solution process at 80 °C. The resultant device, employing a 1,2-ethanedithiol ligand exchange scheme, exhibits an average power conversion efficiency of 3.5%. Through a combination of thickness-dependent current density-voltage characteristics, optical modeling, and capacitance measurements, the combined diffusion length and depletion width in the PbS quantum dot layer is found to be approximately 170 nm.
    Nano Letters 02/2013; · 13.03 Impact Factor
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    ABSTRACT: Since their inception 18 years ago, electrically driven colloidal quantum-dot light-emitting devices (QD-LEDs) have increased in external quantum efficiency from less than 0.01% to around 18%. The high luminescence efficiency and uniquely size-tunable colour of solution-processable semiconducting colloidal QDs highlight the potential of QD-LEDs for use in energy-efficient, high-colour-quality thin-film display and solid-state lighting applications. Indeed, last year saw the first demonstrations of electrically driven full-colour QD-LED displays, which foreshadow QD technologies that will transcend the optically excited QD-enhanced lighting products already available today. We here discuss the key advantages of using QDs as luminophores in LEDs and outline the operating mechanisms of four types of QD-LED. State-of-the-art visible-wavelength LEDs and the promise of near-infrared and heavy-metal-free devices are also highlighted. As QD-LED efficiencies approach those of molecular organic LEDs, we identify the key scientific and technological challenges facing QD-LED commercialization and offer our outlook for on-going strategies to overcome these challenges.
    Nature Photonics 01/2013; 7. · 27.25 Impact Factor
  • Katherine W Song, Ronny Costi, Vladimir Bulović
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    ABSTRACT: The electrophoretic deposition of thin films of colloidal quantum dots is an alternative to spin-casting and printing for large-area, high-throughput processing of quantum-dot (QD) optoelectronics. In this study, QD light-emitting diodes (QD-LEDs) fabricated with electrophoretically deposited films of CdSe/ZnS core/shell QDs are demonstrated for the first time.
    Advanced Materials 12/2012; · 14.83 Impact Factor

Publication Stats

6k Citations
1,310.43 Total Impact Points

Institutions

  • 2002–2014
    • Massachusetts Institute of Technology
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering and Computer Science
      • • Department of Chemistry
      Cambridge, Massachusetts, United States
  • 2011–2012
    • ETH Zurich
      • • Integrated Systems Laboratory
      • • Department Information Technology and Electrical Engineering
      Zürich, ZH, Switzerland
    • Michigan State University
      • Department of Chemical Engineering and Materials Science
      East Lansing, MI, United States
  • 2010–2012
    • Tufts University
      • Department of Chemical and Biological Engineering
      Medford, MA, United States
  • 2007
    • Nishi kyushu University
      Hukuoka, Fukuoka, Japan
  • 2004
    • Brown University
      • School of Engineering
      Providence, RI, United States
  • 1995–2001
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States