V. Bulovic

ETH Zurich, Zürich, ZH, Switzerland

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Publications (94)301.76 Total impact

  • [show abstract] [hide abstract]
    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: 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
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    ABSTRACT: Growth of semiconducting nanostructures on graphene would open up opportunities for the development of flexible optoelectronic devices, but challenges remain in preserving the structural and electrical properties of graphene during this process. We demonstrate growth of highly uniform and well-aligned ZnO nanowire arrays on graphene by modifying the graphene surface with conductive polymer interlayers. Based on this structure, we then demonstrate graphene cathode-based hybrid solar cells using two different photoactive materials - PbS quantum dots and the conjugated polymer P3HT - with AM 1.5G power conversion efficiencies of 4.2% and 0.5%, respectively, approaching the performance of ITO-based devices with similar architectures. Our method preserves beneficial properties of graphene and demonstrates that it can serve as a viable replacement for ITO in various photovoltaic device configurations.
    Nano Letters 12/2012; · 13.03 Impact Factor
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    ABSTRACT: We present two-dimensional Fourier transform optical spectroscopy measurements of two types of molecular J-aggregate thin films and show that temperature-dependent dynamical effects govern exciton delocalization at all temperatures, even in the presence of significant inhomogeneity. Our results indicate that in the tested molecular aggregates, even when the static structure disorder dominates exciton dephasing dynamics, the extent of exciton delocalization may be limited by dynamical fluctuations, mainly exciton-phonon coupling. Thus inhomogeneous dephasing may mediate the exciton coherence time while dynamical fluctuations mediate the exciton coherence length.
    The Journal of Physical Chemistry B 11/2012; · 3.61 Impact Factor
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    ABSTRACT: We demonstrate series-integrated multijunction organic photovoltaics fabricated monolithically by vapor-deposition in a transposed subcell order with the near-infrared-absorbing subcell in front of the green-absorbing subcell. This transposed subcell order is enabled by the highly complementary absorption spectra of a near-infrared-absorbing visibly-transparent subcell and a visible-absorbing subcell and motivated by the non-spatially-uniform optical intensity in nanoscale photovoltaics. The subcell order and thicknesses are optimized via transfer-matrix formalism and short-circuit current simulations. An efficient charge recombination zone consisting of layers of BCP/Ag/MoOx leads to negligible voltage and series-resistance losses. Under 1-sun illumination the multijunction solar cells exhibit a power conversion efficiency of 5.5 ± 0.2% with an FF of 0.685 ± 0.002 and a V(OC) of 1.65 ± 0.02 V, corresponding to the sum of the V(OC) of the component subcells. These devices exhibit a broad spectral response (in the wavelength range of 350 nm to 850 nm) but are limited by subcell external quantum efficiencies between 20% and 30% over the photoactive spectrum.
    Physical Chemistry Chemical Physics 09/2012; 14(42):14548-53. · 3.83 Impact Factor
  • Richard R. Lunt, Vladimir Bulovic
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    ABSTRACT: The practical efficiency limits for nanostructured photovoltaics including organic small molecule, dye-sensitized, polymer, and colloidal-quantum-dot architectures are assessed a posterori. Five decades since Shockley and Queisser derived the theoretical power conversion efficiency limit of single-junction photovoltaic cells, researchers have still not demonstrated such high performance for any photovoltaic device system. Hence, in evaluating the achievable performance of a comparatively new photovoltaic technologies, such as nanostructured PVs, it is prudent to estimate the upper limit of achievable efficiencies based on trends of the best technical demonstrations across the nanostructured platforms. This analysis is utilized to give a clear perspective on the potential market viability of these technologies in the near future and outline the challenges necessary to overcome this threshold. These technologies are compared and contrasted to provide an overview for the potential of each for reducing thermal losses with ``Third Generation'' concepts accessible to nanostructured PVs that can subsequently impact cost structures.
    02/2012;
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    ABSTRACT: For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(ethylenedioxythiophene) with poly(styrenesulfone) (PSS) as the solid state dopant. However, graphene's low surface free energy renders uniform coverage of PEDOT:PSS challenging. Here, we introduce a novel, yet simple, vapor printing method via oxidative chemical vapor deposition (oCVD) for creating patterned PEDOT layers directly onto the graphene surface. Graphene electrodes were synthesized under both LPCVD (˜300 φ/sq at ˜92%T) and APCVD (˜450 φ/sq at ˜92%T). We demonstrate that using the donor material tetraphenyldibenzoperiflanthene (DBP) with highly efficient graphene electrodes (without any doping) yields organic solar cells with performances comparable to those of the ITO reference devices (ηp, LPCVD = 3.01%, ηp, APCVD = 2.49%, and ηp, ITO = 3.20%).
    02/2012;
  • F. M. Yaul, V. Bulovic, J. H. Lang
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    ABSTRACT: This paper presents a flexible underwater pressure sensor array which achieves a 1.5 pascal pressure resolution using a 16-bit analog/digital converter. Each sensor consists of a polydimethysiloxane (PDMS) diaphragm and a resistive strain gauge made of a conductive carbon black-PDMS composite. A functional linear array of four sensors with a 15 mm center-to-center spacing is demonstrated, and the dynamic response of the sensors is characterized and modeled.
    Journal of Microelectromechanical Systems 01/2012; 21(4):500-503. · 2.13 Impact Factor
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    ABSTRACT: Semiconductor nanocrystals, or quantum dots (QDs), promise to drive advances in electronic light generation. It was recently shown that long range transport of charge, which is typically required for electric excitation and which is inherently limited in nanosized materials, can be avoided by developing devices that operate through local, field-assisted generation of charge. We investigate such devices that consist of a thin film of CdSe/ZnS core-shell QD placed between two dielectric layers and that exhibit electroluminescence under pulsed, high field excitation. Using electrical and spectroscopic measurements, we are able to elucidate the dynamics of charge within the QD layer and determine that charge trapping and field-induced luminescence quenching are the main limitations of device performance.
    Journal of Applied Physics. 01/2012; 111(11):113701.
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    ABSTRACT: We report a new method for additive fabrication of thin (125±15 nm thick) gold membranes on patterned silicon dioxide (SiO2) substrates for acoustic MEMS. The deflection of these membranes, suspended over cavities in a SiO2 dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. This process uses a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto SiO2 substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactiveion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm2 in area are fabricated and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28 μm diameter cavities. Young's modulus of these films is shown to be 74±17 GPa, and their potential sound pressure generation at 15 V is calculated.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2012;
  • M. Scott Bradley, Vladimir Bulovic
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    ABSTRACT: We demonstrate arrays of molecular organic thin film microcavities with 5 $\mu{\rm m}$ lateral features fabricated by thin-film contact-patterning using polydimethylsiloxane stamps. The contact-patterning method enables engineering of shifts in the microcavity resonance energy across the microcavity array. Small resonance shifts are imparted by patterning thickness of organic films in multilayer structures, and large resonance shifts are formed by patterning thick films. The contact-patterning method is scalable, extendable to patterning sub-micronmeter organic photonic device features.
    IEEE Photonics Technology Letters 01/2012; 24(2):104-106. · 2.04 Impact Factor
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    ABSTRACT: This paper presents the design, fabrication, testing and evaluation of a MEMS switch that employs a metal-polymer nanocomposite as its active material. The nanocomposite is formed by doping a polymer with conducting nanoparticles. The conductivity of the nanocomposite changes 10,000-fold as it is mechanically compressed. In this demonstration the compressive squeeze is applied with electric actuation. Since squeezing initiates the switching behavior, the device is referred to as a “squitch”.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2012;
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    ABSTRACT: We demonstrate single-walled carbon nanotube (SWCNT)/P3HT polymer bulk heterojunction solar cells with an AM1.5 efficiency of 0.72%, significantly higher than previously reported (0.05%). A key step in achieving high efficiency is the utilization of semiconducting SWCNTs coated with an ordered P3HT layer to enhance the charge separation and transport in the device active layer. Electrical characteristics of devices with SWCNT concentrations up to 40 wt % were measured and are shown to be strongly dependent on the SWCNT loading. A maximum open circuit voltage was measured for SWCNT concentration of 3 wt % with a value of 1.04 V, higher than expected based on the interface band alignment. Modeling of the open-circuit voltage suggests that despite the large carrier mobility in SWCNTs device power conversion efficiency is governed by carrier recombination. Optical characterization shows that only SWCNT with diameter of 1.3-1.4 nm can contribute to the photocurrent with internal quantum efficiency up to 26%. Our results advance the fundamental understanding and improve the design of efficient polymer/SWCNTs solar cells.
    Nano Letters 12/2011; 11(12):5316-21. · 13.03 Impact Factor
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    ABSTRACT: We report a 20-fold enhancement in the fluorescence of the organic dye DCM when resonantly coupled to a strongly optically absorbing structure of a thin film of spin-deposited molecular J-aggregates in a critically coupled resonator (JCCR) geometry. A submonolayer equivalent of DCM molecules is shown to absorb and re-emit 2.2% of the incident resonant photons when coupled to the JCCR enhancement structure, compared to 0.1% for the bare film of same thickness on quartz. Such a JCCR structure is a general energy focusing platform that localizes over 90% of incident light energy within a 15 nm thin film layer in the form of excitons that can subsequently be transferred to colocated lumophores. Applications of the exciton-mediated concentration of optical energy are discussed in the context of solid-state lighting, photodetection, and single photon optics.
    ACS Nano 12/2011; 6(1):467-71. · 12.06 Impact Factor
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    Richard R. Lunt, Vladimir Bulovic
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    ABSTRACT: We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.
    Applied Physics Letters 03/2011; 98(11):113305-113305-3. · 3.79 Impact Factor
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    ABSTRACT: We demonstrate resonant-cavity colloidal quantum dot light-emitting diodes that exhibit electroluminescence with enhanced spectral purity and directionality and show that the enhancement is due to modification of the optical modes of the system. OCIS codes: (160.4236) Nanomaterials, (230.3670) Light-emitting diodes
    01/2011;
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    ABSTRACT: We report a new method for fabricating thin (140 nm thick) suspended metal films in MEMS. Our MEMS fabrication process employs micro-contact printing. It avoids the use of solvents and etchants, obviating the need for deep reactive-ion etching and other harsh chemical treatments. Solvent absence during fabrication also avoids the deleterious effects of MEMS stiction that can result during wet processing. Elevated temperature processing is also avoided to enable MEMS fabrication on flexible polymeric substrates. Thin films up to 0.78 mm2 in area are fabricated and the deflection of 25 μm diameter films is demonstrated. These films can be utilized in pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of MEMS sensors.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2011;
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    ABSTRACT: High storage capacity memory cells were fabricated by integrating organic molecules, with dimension ~1 nm, into flash memory devices. The stored charge within the molecules was directly probed using Kelvin force microscopy, revealing strong confinement of stored charge over time. Molecular memories with increased storage capacity were achieved via addition of dopant molecules.
    Electron Devices Meeting (IEDM), 2011 IEEE International; 01/2011
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    ABSTRACT: Nanocrystal (NC) films have been proposed as an alternative to bulk semiconductors for electronic applications such as solar cells and photodetectors. One outstanding challenge in NC electronics is to robustly control the carrier type to create stable p-n homojunction-based devices. We demonstrate that the postsynthetic addition of Cd to InAs nanocrystals switches the resulting InAs:Cd NC films from n-type to p-type when operating in a field effect transistor. This method presents a stable, facile way to control the carrier type of InAs nanocrystals prior to deposition. We present two mechanisms to explain the observed switch in carrier type. In mechanism 1, Cd atoms are incorporated at In sites in the lattice and act as acceptor defects, forming a partially compensated p-type semiconductor. In mechanism 2, Cd atoms passivate donor-type InAs surface states and create acceptor-type surface states. This work represents a critical step toward the creation of p-n homojunction-based NC electronics.
    ACS Nano 12/2010; 4(12):7373-8. · 12.06 Impact Factor
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    ABSTRACT: A fully photolithographic dual threshold voltage (V<sub>T</sub>) organic thin-film transistor (OTFT) process suitable for flexible large-area integrated circuits is presented. The nearroom-temperature (<; 95 °C) process produces integrated dual V<sub>T</sub> pentacene-based p-channel transistors. The two V<sub>T</sub> 's are enabled by using two gate metals of low (aluminum) and high (platinum) work function. The Al and Pt gate OTFTs exhibit nominally identical current-voltage transfer curves shifted by an amount ΔV<sub>T</sub>. The availability of a high-V<sub>T</sub> device enables area-efficient zero-Vos high-output-resistance current sources, enabling high-gain inverters. We present positive noise margin inverters and rail-to-rail ring oscillators powered by a 3-V supply-one of the lowest supply voltages reported for OTFT circuits. These results show that integrating nand p-channel organic devices is not mandatory to achieve functional area-efficient low-power organic integrated circuits.
    IEEE Transactions on Electron Devices 12/2010; · 2.06 Impact Factor

Publication Stats

1k Citations
301.76 Total Impact Points

Institutions

  • 2012
    • ETH Zurich
      • Integrated Systems Laboratory
      Zürich, ZH, Switzerland
  • 2004–2012
    • Massachusetts Institute of Technology
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering and Computer Science
      Cambridge, MA, United States
    • Brown University
      • School of Engineering
      Providence, RI, United States
  • 1995–2001
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States