31 results on '"Vladimir Bulovic"'
Search Results
2. Silver Nanowire Back Electrode Stabilized with Graphene Oxide Encapsulation for Inverted Semitransparent Organic Solar Cells with Longer Lifetime
- Author
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Vladimir Bulovic, Jeremiah Mwaura, Jeffrey C. Grossman, Thomas Sannicolo, and Woo Hyun Chae
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Materials science ,Organic solar cell ,Graphene ,Oxide ,Energy Engineering and Power Technology ,Nanotechnology ,Silver nanowires ,Encapsulation (networking) ,law.invention ,chemistry.chemical_compound ,chemistry ,law ,Electrode ,Materials Chemistry ,Electrochemistry ,Chemical Engineering (miscellaneous) ,Electrical and Electronic Engineering ,Building-integrated photovoltaics - Abstract
Semitransparent organic solar cells (ST-OSCs) have garnered strong interest for building integrated photovoltaics (PV) and wearable electronics. Although seen as an appealing low-cost PV technology...
- Published
- 2021
3. Nanocrystal-Sensitized Infrared-to-Visible Upconversion in a Microcavity under Subsolar Flux
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Marc A. Baldo, Ting-An Lin, Mengfei Wu, Vladimir Bulovic, and Jan Tiepelt
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Materials science ,Photon ,business.industry ,Infrared ,Mechanical Engineering ,Physics::Optics ,Bioengineering ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Photon upconversion ,Orders of magnitude (time) ,Nanocrystal ,Optoelectronics ,General Materials Science ,Quantum efficiency ,Photonics ,0210 nano-technology ,business ,Absorption (electromagnetic radiation) - Abstract
Infrared-to-visible photon upconversion could benefit applications such as photovoltaics, infrared sensing, and bioimaging. Solid-state upconversion based on triplet exciton annihilation sensitized by nanocrystals is one of the most promising approaches, albeit limited by relatively weak optical absorption. Here, we integrate the upconverting layers into a Fabry-Perot microcavity with quality factor Q = 75. At the resonant wavelength λ = 980 nm, absorption increases 74-fold and we observe a 227-fold increase in the intensity of upconverted emission. The threshold excitation intensity is reduced by 2 orders of magnitude to a subsolar flux of 13 mW/cm2. We measure an external quantum efficiency of 0.06 ± 0.01% and a 2.2-fold increase in the generation yield of upconverted photons. Our work highlights the potential of triplet-triplet annihilation-based upconversion in low-intensity sensing applications and demonstrates the importance of photonic designs in addition to materials engineering to improve the efficiency of solid-state upconversion.
- Published
- 2021
4. Charge-Carrier Recombination in Halide Perovskites
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Kyle Frohna, David Emin, Samuel D. Stranks, David S. Ginger, Vladimir Bulovic, Dane W. deQuilettes, and Thomas Kirchartz
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Photoluminescence ,010405 organic chemistry ,Chemistry ,Band gap ,business.industry ,General Chemistry ,010402 general chemistry ,Polaron ,01 natural sciences ,0104 chemical sciences ,Semiconductor ,Chemical physics ,Charge carrier ,business ,Rashba effect ,Recombination ,Perovskite (structure) - Abstract
The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.
- Published
- 2019
5. Predicting Low Toxicity and Scalable Solvent Systems for High Speed Roll-to-Roll Perovskite Manufacturing
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Richard Swartwout, Rahul Patidar, Emma Belliveau, Benjia Dou, David Beynon, Peter Greenwood, Nicole Moody, Dane W. deQuilettes, Moungi Bawendi, Trystan M. Watson, and Vladimir Bulovic
- Abstract
This manuscript introduces solvent toxicity in solar perovskite ink chemistries as a major technoeconomic limitation for the growth of the technology. More specifically, the capital and operational cost of handling such toxic chemicals to maintain a safe working environment can lead to significant added costs. As all record power conversion efficiency devices to date have been solution processed, this represents a major challenge for the perovskite optoelectronic field and of printed electronics as a whole. Knowing this limitation, we propose that solvent selections for ink chemistries should be more quantitative and focus on lowering toxicity. To this end, we show that a Hansen solubility model is effective in predicting ink systems using lower toxicity solvents. We also show that inks formed from this method are applicable for high-speed slot-die coating, limiting the need for long anneal times. These methods and results demonstrate a useful framework for quantitatively engineering solvent systems with reduced toxicity while simultaneously maintaining and surpassing performance. It therefore provides a pathway and major step forward towards the commercialization of solution coated perovskite technologies.
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- 2021
6. Radiative Efficiency Limit with Band Tailing Exceeds 30% for Quantum Dot Solar Cells
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Moungi G. Bawendi, Joel Jean, Thomas S. Mahony, Deniz Bozyigit, Jakub Holovský, Melany Sponseller, and Vladimir Bulovic
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Materials science ,Renewable Energy, Sustainability and the Environment ,business.industry ,Photovoltaic system ,Energy conversion efficiency ,Energy Engineering and Power Technology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Copper indium gallium selenide solar cells ,Cadmium telluride photovoltaics ,0104 chemical sciences ,Fuel Technology ,Absorption edge ,Chemistry (miscellaneous) ,Quantum dot ,Materials Chemistry ,Optoelectronics ,Spontaneous emission ,Thin film ,0210 nano-technology ,business - Abstract
Thin films of colloidal quantum dots (QDs) are promising solar photovoltaic (PV) absorbers in spite of their disordered nature. Disordered PV materials face a power conversion efficiency limit lower than the ideal Shockley–Queisser bound because of increased radiative recombination through band-tail states. However, investigations of band tailing in QD solar cells have been largely restricted to indirect measurements, leaving their ultimate efficiency in question. Here we use photothermal deflection spectroscopy (PDS) to robustly characterize the absorption edge of lead sulfide (PbS) QD films for different bandgaps, ligands, and processing conditions used in leading devices. We also present a comprehensive overview of band tailing in many commercial and emerging PV technologies—including c-Si, GaAs, a-Si:H, CdTe, CIGS, and perovskites—then calculate detailed-balance efficiency limits incorporating Urbach band tailing for each technology. Our PDS measurements on PbS QDs show sharp exponential band tails, w...
- Published
- 2017
7. High-Speed Vapor Transport Deposition of Perovskite Thin Films
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Nicole Moody, Ella Louise Wassweiler, Anurag Panda, Michel Nasilowski, Klavs F. Jensen, Anna Osherov, Vladimir Bulovic, Richard Swartwout, Maximilian T. Hoerantner, Aidan E. Driscoll, Haomiao Zhang, Moungi G. Bawendi, Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology. Department of Chemical Engineering, Massachusetts Institute of Technology. Department of Chemistry, Massachusetts Institute of Technology. Research Laboratory of Electronics, and Massachusetts Institute of Technology. Department of Physics
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010302 applied physics ,Materials science ,business.industry ,Energy conversion efficiency ,vapor deposition ,fluid dynamics ,02 engineering and technology ,Chemical vapor deposition ,021001 nanoscience & nanotechnology ,7. Clean energy ,01 natural sciences ,manufacturing ,thin-film ,Chemical engineering ,Photovoltaics ,solar cells ,0103 physical sciences ,Deposition (phase transition) ,General Materials Science ,Thin film ,0210 nano-technology ,business ,perovskite ,Research Article ,Perovskite (structure) - Abstract
Intensive research of hybrid metal-halide perovskite materials for use as photoactive materials has resulted in an unmatched increase in the power conversion efficiency of perovskite photovoltaics (PVs) over the last couple of years. Now that lab-fabricated perovskite devices rival the efficiency of silicon PVs, the next challenge of scalable mass manufacturing of large perovskite PV panels remains to be solved. For that purpose, it is still unclear which manufacturing method will provide the lowest processing cost and highest quality solar cells. Vapor deposition has been proven to work well for perovskites as a controllable and repeatable thin-film deposition technique but with processing speeds currently too slow to adequately lower the production costs. Addressing this challenge, in the present work, we demonstrate a high-speed vapor transport processing technique in a custom-built reactor that produces high-quality perovskite films with unprecedented deposition speed exceeding 1 nm/s, over 10× faster than previous vapor deposition demonstrations. We show that the semiconducting perovskite films produced with this method have excellent crystallinity and optoelectronic properties with 10 ns charge carrier lifetime, enabling us to fabricate the first photovoltaic devices made by perovskite vapor transport deposition. Our experiments are guided by computational fluid dynamics simulations that also predict that this technique could lead to deposition rates on the order of micrometers per second. This, in turn, could enable cost-effective scalable manufacturing of the perovskite-based solar technologies. Keywords: solar cells; perovskite; thin-film; vapor deposition; manufacturing; fluid dynamics, National Science Foundation (U.S.) (Award 1541959), National Science Foundation (U.S.) (Grant 1605406)
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- 2019
8. Epitaxial Dimers and Auger-Assisted De-Trapping in PbS Quantum Dot Solids
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William Tisdale, Jeffrey C. Grossman, Adam Willard, Vladimir Bulovic, Joel Jean, Huashan Li, Mark Weidman, Elizabeth Lee, Nabeel Dahod, Wenbi Shcherbakov-Wu, Yun Liu, and Rachel H. Gilmore
- Abstract
Electronic trap states limit the overall power conversion efficiency of quantum dot (QD) solar cells by inhibiting charge carrier transport and reducing the open-circuit voltage. Here, we explore the dynamic interaction of charge carriers between band edge states and sub-band trap states using broadband transient absorption spectroscopy. In monodisperse arrays of 4-5 nm diameter PbS QDs, we observe an optically active trap state ~100-200 meV below the band edge that occurs at a frequency of 1 in ~2500 QDs. Uncoupled QD solids with oleic acid ligands show trap-to-ground-state recombination that resembles Auger recombination. In electronically coupled QD solids, we observe entropically-driven uphill thermalization of trapped charge carriers from the trap state to the band edge via two distinct mechanisms: Auger-assisted charge transfer (~35 ps) and thermally activated hopping (~500 ps). Photophysical characterization combined with atomistic simulations and high-resolution transmission electron microscopy suggest that these states arise from epitaxially fused pairs of QDs – rather than electron or hole traps at the QD surface – offering new strategies for improving the efficiency of QD solar cells.
- Published
- 2019
9. Photovoltaic Performance of PbS Quantum Dots Treated with Metal Salts
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Dong Kyun Ko, Moungi G. Bawendi, Andrea Maurano, Vladimir Bulovic, Su Kyung Suh, Donghun Kim, Gyu Weon Hwang, and Jeffrey C. Grossman
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Materials science ,Passivation ,Band gap ,business.industry ,Photovoltaic system ,General Engineering ,General Physics and Astronomy ,Halide ,Charge (physics) ,Nanotechnology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Solar cell efficiency ,Quantum dot ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business ,Dark current - Abstract
Recent advances in quantum dot surface passivation have led to a rapid development of high-efficiency solar cells. Another critical element for achieving efficient power conversion is the charge neutrality of quantum dots, as charge imbalances induce electronic states inside the energy gap. Here we investigate how the simultaneous introduction of metal cations and halide anions modifies the charge balance and enhances the solar cell efficiency. The addition of metal salts between QD deposition and ligand exchange with 1,3-BDT results in an increase in the short-circuit current and fill factor, accompanied by a distinct reduction in a crossover between light and dark current density-voltage characteristics.
- Published
- 2016
10. The Role of Electron–Hole Separation in Thermally Activated Delayed Fluorescence in Donor–Acceptor Blends
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Daniel N. Congreve, Marc A. Baldo, Troy Van Voorhis, Wendi Chang, Vladimir Bulovic, and Eric Hontz
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Photoluminescence ,Chemistry ,Analytical chemistry ,Electron ,Electron hole ,Fluorescence ,Acceptor ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,Intersystem crossing ,Chemical physics ,OLED ,Singlet state ,Physical and Theoretical Chemistry - Abstract
Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from nonemissive triplet states via reverse intersystem crossing (RISC) to the bright singlet state. Here we present the rather surprising finding that in TADF materials that contain a mixture of donor and acceptor molecules the electron–hole separation fluctuates as a function of time. By performing time-resolved photoluminescence experiments, both with and without a magnetic field, we observe that at short times the TADF dynamics are insensitive to magnetic field, but a large magnetic field effect (MFE) occurs at longer times. We explain these observations by constructing a quantum mechanical rate model in which the electron and hole cycle between a near-neighbor exciplex state that shows no MFE and a separated polaron-pair state that is not emissive but does show magnetic field dependent dynamics. Interestingly, the model suggests that only a portion of TADF in these blends comes fr...
- Published
- 2015
11. Quantum-Dot Size and Thin-Film Dielectric Constant: Precision Measurement and Disparity with Simple Models
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Patrick O. Brown, Vladimir Bulovic, Moungi G. Bawendi, Daniel K. Harris, and Darcy D. W. Grinolds
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Materials science ,business.industry ,Scattering ,Mechanical Engineering ,Bioengineering ,General Chemistry ,Dielectric ,Condensed Matter Physics ,Capacitance ,Condensed Matter::Soft Condensed Matter ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,Optics ,chemistry ,Quantum dot ,Volume fraction ,Optoelectronics ,General Materials Science ,Lead sulfide ,Thin film ,business ,Constant (mathematics) - 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.
- Published
- 2014
12. Ultracompact Low-Threshold Organic Laser
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Vladimir Bulovic, Parag B. Deotare, and Thomas S. Mahony
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Materials science ,Physics::Optics ,General Physics and Astronomy ,02 engineering and technology ,7. Clean energy ,01 natural sciences ,Waveguide (optics) ,law.invention ,Optics ,law ,0103 physical sciences ,Nanotechnology ,General Materials Science ,Spontaneous emission ,Photonic crystal ,010302 applied physics ,Photons ,Mode volume ,Organic laser ,business.industry ,Lasers ,General Engineering ,021001 nanoscience & nanotechnology ,Laser ,Femtosecond ,Microscopy, Electron, Scanning ,Optoelectronics ,Crystallization ,0210 nano-technology ,business ,Lasing threshold - 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.
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- 2014
13. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange
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Moungi G. Bawendi, Ni Zhao, Jeffrey C. Grossman, Patrick O. Brown, Donghun Kim, Vladimir Bulovic, and Richard R. Lunt
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Chemistry ,Band gap ,General Engineering ,General Physics and Astronomy ,Nanotechnology ,Dipole ,chemistry.chemical_compound ,Nanocrystal ,Quantum dot ,Chemical physics ,General Materials Science ,Density functional theory ,Lead sulfide ,Thin film ,Ultraviolet photoelectron spectroscopy - 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.
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- 2014
14. Thermally-Limited Exciton Delocalization in Superradiant Molecular Aggregates
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S. M. Vlaming, Dylan H. Arias, Moungi G. Bawendi, Robert J. Silbey, Keith A. Nelson, Brian J. Walker, Vladimir Bulovic, and Katherine W. Stone
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Coherence time ,Condensed matter physics ,Condensed Matter::Other ,Chemistry ,Dephasing ,Exciton ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Surfaces, Coatings and Films ,Coherence length ,Condensed Matter::Materials Science ,symbols.namesake ,Delocalized electron ,Fourier transform ,Chemical physics ,Materials Chemistry ,symbols ,Physical and Theoretical Chemistry ,Spectroscopy ,Biexciton - 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 whereas dynamical fluctuations mediate the exciton coherence length.
- Published
- 2012
15. Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells
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Paulo T. Araujo, Jayce J. Cheng, Sehoon Chang, Jing Kong, Ming-Sheng Wang, Silvija Gradečak, Hyesung Park, Vladimir Bulovic, Mildred S. Dresselhaus, Joel Jean, and Moungi G. Bawendi
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Conductive polymer ,Materials science ,Graphene ,Mechanical Engineering ,Graphene foam ,Nanowire ,Bioengineering ,Nanotechnology ,General Chemistry ,Hybrid solar cell ,Condensed Matter Physics ,law.invention ,Quantum dot ,law ,General Materials Science ,Graphene nanoribbons ,Graphene oxide paper - 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. On the basis of 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.
- Published
- 2012
16. Organic Solar Cells with Graphene Electrodes and Vapor Printed Poly(3,4-ethylenedioxythiophene) as the Hole Transporting Layers
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Karen K. Gleason, Jing Kong, Vladimir Bulovic, Hyesung Park, Rachel M. Howden, and Miles C. Barr
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Materials science ,Organic solar cell ,Polymers ,General Physics and Astronomy ,Nanotechnology ,Chemical vapor deposition ,law.invention ,chemistry.chemical_compound ,Electric Power Supplies ,PEDOT:PSS ,law ,Solar Energy ,General Materials Science ,Work function ,Electrodes ,Dopant ,business.industry ,Graphene ,Electric Conductivity ,General Engineering ,Bridged Bicyclo Compounds, Heterocyclic ,Nanostructures ,Organic Chemistry Phenomena ,chemistry ,Optoelectronics ,Graphite ,business ,Layer (electronics) ,Poly(3,4-ethylenedioxythiophene) - 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(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphene's hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).
- Published
- 2012
17. Bulk Heterojuction Solar Cells Containing 6,6-Dicyanofulvenes as n-Type Additives
- Author
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Trisha L. Andrew and Vladimir Bulovic
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Materials science ,General Physics and Astronomy ,Polymer solar cell ,chemistry.chemical_compound ,Electric Power Supplies ,Organoselenium Compounds ,Solar Energy ,Organic chemistry ,General Materials Science ,Irradiation ,Particle Size ,Electrodes ,business.industry ,Photovoltaic system ,General Engineering ,Equipment Design ,Solar energy ,Equipment Failure Analysis ,Semiconductors ,chemistry ,Chemical engineering ,Electrode ,Nanoparticles ,Polythiophene ,Particle size ,business ,Ternary operation - Abstract
P3HT/PC(61)BM bulk heterojunction solar cells containing varying amounts of different 6,6-dicyanofulvenes (DCFs) were fabricated and characterized. Photovoltaic cells containing ternary mixtures of P3HT, 0.5 equiv of PC(61)BM, and 0.5 equiv of 1,4-dimethyl-2,3-diphenyl-DCF (by weight) displayed average power conversion efficiencies of up to 4.5% under AM 1.5 irradiation, compared to 2.9% for reference P3HT-PC(61)BM solar cells. It was found that 1,4-dimethyl-2,3-diphenyl-6,6-dicyanofulvene could replace up to 50 wt % of PC(61)BM in 1:1 P3HT-PC(61)BM solar cells without sacrificing device performance.
- Published
- 2012
18. Inorganic–Organic Hybrid Solar Cell: Bridging Quantum Dots to Conjugated Polymer Nanowires
- Author
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Vladimir Bulovic, Shenqiang Ren, Matthew J. Smith, Silvija Gradečak, Sung-Keun Lim, Liang-Yi Chang, Moungi G. Bawendi, Ni Zhao, and Jing Zhao
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Materials science ,business.industry ,Mechanical Engineering ,Energy conversion efficiency ,Nanowire ,Bioengineering ,Nanotechnology ,General Chemistry ,Hybrid solar cell ,Quantum dot solar cell ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Polymer solar cell ,Multiple exciton generation ,Nanocrystal ,Quantum dot ,Optoelectronics ,General Materials Science ,business - Abstract
Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.
- Published
- 2011
19. Electroluminescence from Nanoscale Materials via Field-Driven Ionization
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Vanessa Wood, Ian Rousseau, Scott M. Geyer, Vladimir Bulovic, Deniz Bozyigit, Yasuhiro Shirasaki, Moungi G. Bawendi, and Matthew J. Panzer
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Luminescence ,Nanostructure ,Materials science ,Surface Properties ,Bioengineering ,Nanotechnology ,Electroluminescence ,law.invention ,law ,Electrochemistry ,General Materials Science ,Particle Size ,Thin film ,business.industry ,Mechanical Engineering ,Direct current ,Membranes, Artificial ,General Chemistry ,Condensed Matter Physics ,Nanostructures ,Solid-state lighting ,Field desorption ,Optoelectronics ,Charge carrier ,business ,Excitation - Abstract
The high degree of morphological and energetic disorder inherent to many nanosized materials places limitations on charge injection into and transport rates through thin films of these materials. We demonstrate electroluminescence achieved by local generation of charge that eliminates the need for injection of charge carriers from the device electrodes. We show electroluminescence from thin films of nanoscale materials that do not support direct current excitation and suggest a mechanism for the charge generation and electroluminescence that is consistent with our time-averaged and time-resolved observations.
- Published
- 2011
20. Nanoscale Morphology Revealed at the Interface Between Colloidal Quantum Dots and Organic Semiconductor Films
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Jonathan E. Halpert, Moungi G. Bawendi, Polina Anikeeva, Matthew J. Panzer, Katherine E. Aidala, and Vladimir Bulovic
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Materials science ,Nanostructure ,business.industry ,Mechanical Engineering ,Bioengineering ,Heterojunction ,Nanotechnology ,General Chemistry ,Condensed Matter Physics ,Organic semiconductor ,Semiconductor ,Quantum dot ,Monolayer ,General Materials Science ,Thin film ,business ,Contact print - Abstract
The degree of interpenetration at the interface between colloidal quantum dots (QDs) and organic semiconductor molecules commonly employed in hybrid light-emitting devices (QD-LEDs) has been examined using tapping-mode atomic force microscopy. Both phase separation-driven and Contact Printing-enabled QD/semiconductor heterojunction fabrication methodologies lead to significant QD embedment in the underlying organic film with the greatest degree of QD penetration observed for QD monolayers that have been contact printed. The relative performance of QD-LEDs fabricated via three different methods using the same materials set has also been investigated.
- Published
- 2010
21. Air-Stable Operation of Transparent, Colloidal Quantum Dot Based LEDs with a Unipolar Device Architecture
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Vanessa Wood, Vladimir Bulovic, Jean-Michel Caruge, Moungi G. Bawendi, Matthew J. Panzer, and Jonathan E. Halpert
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Ceramics ,Materials science ,Light ,Polymers ,Surface Properties ,Electrons ,Bioengineering ,Nanotechnology ,Electroluminescence ,law.invention ,Colloid ,law ,Materials Testing ,Quantum Dots ,General Materials Science ,Transparent ceramics ,business.industry ,Air ,Mechanical Engineering ,Nanostructured materials ,Equipment Design ,General Chemistry ,Condensed Matter Physics ,Nanostructures ,Quantum dot ,Optoelectronics ,business ,Light-emitting diode - Abstract
We report a novel unipolar light-emitting device architecture that operates using direct-current, field-driven electroluminescence of colloidally synthesized quantum dots (QDs). This device architecture, which is based only on transparent ceramics and QDs, enables emission from different color QDs and, for the first time, constant QD electroluminescence during extended operation in air, unpackaged.
- Published
- 2009
22. Selection of Metal Oxide Charge Transport Layers for Colloidal Quantum Dot LEDs
- Author
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Jonathan E. Halpert, Moungi G. Bawendi, Vanessa Wood, Vladimir Bulovic, Jean-Michel Caruge, and Matthew J. Panzer
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Materials science ,business.industry ,General Engineering ,Oxide ,General Physics and Astronomy ,Nanotechnology ,Electron ,Electroluminescence ,law.invention ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,chemistry ,Quantum dot laser ,law ,Quantum dot ,visual_art ,visual_art.visual_art_medium ,Optoelectronics ,General Materials Science ,Ceramic ,Luminescence ,business ,Light-emitting diode - Abstract
We investigate the effect of the electronic energy level positioning, conductivity, and morphology of metal oxide charge transport layers on the performance of light emitting devices (LEDs) that consist of a colloidally synthesized quantum dot (QD) luminescent film embedded between electron and hole injecting ceramic layers. We demonstrate that understanding of these material properties and their effect on charging processes in QDs enables the systematic design of higher efficiency QD-LEDs and excitation of QDs with different emission colors using the same device structure.
- Published
- 2009
23. Electrostatic Formation of Quantum Dot/J-aggregate FRET Pairs in Solution
- Author
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Jonathan R. Tischler, Brian J. Walker, Vladimir Bulovic, Jonathan E. Halpert, Moungi G. Bawendi, Wenhao Liu, and Gautham Nair
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Resonant inductive coupling ,Photoluminescence ,Chemistry ,business.industry ,medicine.disease_cause ,Molecular physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,Förster resonance energy transfer ,Dynamic light scattering ,Quantum dot ,medicine ,Optoelectronics ,Physical and Theoretical Chemistry ,Spectroscopy ,business ,J-aggregate ,Ultraviolet - Abstract
We report the formation and photonic properties of CdSe/ZnS quantum dot (QD)/J-aggregate donor−acceptor Forster resonant energy transfer (FRET) pairs that are electrostatically bound and dispersed in water. Assembly occurs when the positively charged dye, 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (TTBC), binds electrostatically to QDs that are coated with a negatively charged amphiphilic polymer. QD/J-aggregate FRET pairs display the broadband absorption in the visible and the ultraviolet (UV) part of the spectrum typical of quantum dots, along with the narrow emission linewidths characteristic of J-band emitters (∼12 nm full width at half-maximum). We use dynamic light scattering, transmission electron microscopy, photoluminescence spectroscopy, and photoluminescence lifetimes to conclude that the size of the aggregates formed in the presence of QDs is between 1−6 nm. We find the FRET radius of the QD/J-aggregate to be 5.1 nm. We further demonstrate electrostatic binding as a ge...
- Published
- 2009
24. Alternating Current Driven Electroluminescence from ZnSe/ZnS:Mn/ZnS Nanocrystals
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Moungi G. Bawendi, Vladimir Bulovic, Vanessa Wood, Matthew J. Panzer, and Jonathan E. Halpert
- Subjects
Materials science ,business.industry ,Mechanical Engineering ,Doping ,Wide-bandgap semiconductor ,Bioengineering ,General Chemistry ,Electroluminescence ,Condensed Matter Physics ,Zinc sulfide ,chemistry.chemical_compound ,chemistry ,Nanocrystal ,visual_art ,visual_art.visual_art_medium ,Optoelectronics ,General Materials Science ,Ceramic ,Thin film ,business ,Visible spectrum - Abstract
We present a novel technique for room temperature, solution-based fabrication of alternating current thin-film electroluminescent (AC-TFEL) devices using phosphor-doped nanocrystals. Synthesis for stable ZnSe/ZnS:Mn/ZnS nanocrystals that exhibit a quantum yield of 65 +/- 5% is outlined, and their electroluminescence is demonstrated in structures consisting of only wide band gap ceramic layers. Both the nanocrystal and the ceramic films have minimal absorption across the visible light spectrum, enabling us to demonstrate transparent AC-TFEL devices.
- Published
- 2009
25. Synthesis of J-Aggregating Dibenz[a,j]anthracene-Based Macrocycles
- Author
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Vladimir Bulovic, Timothy M. Swager, Jonathan R. Tischler, Julian M. W. Chan, and Steve E. Kooi
- Subjects
chemistry.chemical_classification ,Anthracene ,Dibenz(a,j)anthracene ,Alkyne ,General Chemistry ,Conjugated system ,Photochemistry ,Biochemistry ,Fluorescence ,Catalysis ,chemistry.chemical_compound ,symbols.namesake ,Colloid and Surface Chemistry ,Monomer ,chemistry ,Stokes shift ,symbols ,Physical chemistry ,Glaser coupling - Abstract
Several fluorescent macrocycles based on 1,3-butadiyne-bridged dibenz[a,j]anthracene subunits have been synthesized via a multistep route. The synthetic strategy involved the initial construction of a functionalized dibenz[a,j]anthracene building block, subsequent installation of free alkyne groups on one side of the polycyclic aromatic framework, and a final cyclization based on a modified Glaser coupling under high-dilution conditions. Photophysical studies on three conjugated macrocycles revealed the formation of J-aggregates in thin films, as well as in concentrated solid solutions (polyisobutylene matrix), with peak absorption and emission wavelength in the range of lambda = 460-480 nm. The characteristic red-shifting of the J-aggregate features as compared to the monomer spectra, enhancement in absorption intensities, narrowed linewidths, and minimal Stokes shift values, were all observed. We demonstrate that improvements in spectral features can be brought about by annealing the films under a solvent-saturated atmosphere, where for the best films the luminescence quantum efficiency as high as 92% was measured. This class of macrocycles represents a new category of J-aggregates that due to their high peak oscillator strength and high luminescence efficiency have the potential to be utilized in a variety of optoelectronic devices.
- Published
- 2009
26. Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer
- Author
-
David C. Oertel, Youfeng Xu, Moungi G. Bawendi, Vladimir Bulovic, and Alexi C. Arango
- Subjects
Materials science ,Cadmium selenide ,Open-circuit voltage ,business.industry ,Mechanical Engineering ,Bilayer ,Bioengineering ,Nanotechnology ,Heterojunction ,General Chemistry ,Condensed Matter Physics ,chemistry.chemical_compound ,chemistry ,Quantum dot ,Photovoltaics ,General Materials Science ,Quantum efficiency ,Thin film ,business - Abstract
We demonstrate a bilayer photovoltaic device consisting of a heterojunction between colloidal cadmium selenide (CdSe) quantum dots (QDs) and a wide band gap organic hole-transporting thin film of N,N'-diphenyl-N,N'-bis(3-methylphenyl)[1,1'-biphenyl]-4,4'-diamine (TPD) molecules. The active light-absorbing film of QDs is nondestructively printed onto TPD using microcontact stamping. Indium-tin-oxide (ITO) provides the top contact. The resulting device structure can accommodate different size QDs, produces an exceptionally large open circuit voltage (0.8 V) for an architecture with symmetric electrodes, and yields an internal quantum efficiency of 10% at the first QD absorption peak.
- Published
- 2009
27. Contact Printing of Quantum Dot Light-Emitting Devices
- Author
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Moungi G. Bawendi, Leeann Kim, Seth Coe-Sullivan, Polina Anikeeva, Vladimir Bulovic, and Jonathan S. Steckel
- Subjects
Materials science ,Mechanical Engineering ,Bioengineering ,Nanotechnology ,General Chemistry ,Electroluminescence ,Condensed Matter Physics ,law.invention ,law ,Quantum dot ,Deposition (phase transition) ,General Materials Science ,Thin film ,Contact print ,Hybrid material ,Critical dimension ,Light-emitting diode - Abstract
We demonstrate a solvent-free contact printing process for deposition of patterned and unpatterned colloidal quantum dot (QD) thin films as the electroluminescent layers within hybrid organic-QD light-emitting devices (QD-LEDs). Our method benefits from the simplicity, low cost, and high throughput of solution-processing methods, while eliminating exposure of device structures to solvents. Because the charge transport layers in hybrid organic/inorganic QD-LEDs consist of solvent-sensitive organic thin films, the ability to avoid solvent exposure during device growth, as presented in this study, provides a new flexibility in choosing organic materials for improved device performance. In addition, our method allows us to fabricate both monochrome and red-green-blue patterned electroluminescent structures with 25 microm critical dimension, corresponding to 1000 ppi (pixels-per-inch) print resolution.
- Published
- 2008
28. Planarization in Electrochemically Fabricated Nanodimensional Films
- Author
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Vladimir Bulovic, Conor F. Madigan, Ethan Howe, Pavel Kornilovitch, and Peter Mardilovich
- Subjects
Materials science ,Fabrication ,Oxide ,Nanotechnology ,Electrochemistry ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Metal ,chemistry.chemical_compound ,General Energy ,chemistry ,Chemical-mechanical planarization ,visual_art ,System parameters ,visual_art.visual_art_medium ,Physical and Theoretical Chemistry ,Thin film ,Nanoscopic scale - Abstract
Recent demonstrations of electrochemical fabrication of nanodimensional, alternating metal and metal oxide films open a pathway to nanoscale templating with high-quality interfaces and high uniformity over macroscopic surface areas. Planarization during electrochemical oxidation is the critical enabling feature of this growth process. Here we present a theory and simulation of this planarization phenomenon applicable to a wide range of initial surface profiles and material systems. We describe the impact of different system parameters on the rate of planarization for both the exposed oxide surface and the internal metal oxide interface. Finally, we show that our simulations are consistent with experimental measurements of Ta 2 O 5 electrochemically grown on Ta thin films.
- Published
- 2008
29. NiO as an Inorganic Hole-Transporting Layer in Quantum-Dot Light-Emitting Devices
- Author
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Jonathan E. Halpert, Jean-Michel Caruge, Moungi G. Bawendi, and Vladimir Bulovic
- Subjects
Electron density ,Materials science ,Condensed matter physics ,business.industry ,Mechanical Engineering ,Non-blocking I/O ,Bioengineering ,General Chemistry ,Electroluminescence ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,law.invention ,Condensed Matter::Materials Science ,Semiconductor ,Quantum dot ,law ,Optoelectronics ,General Materials Science ,Quantum efficiency ,business ,Layer (electronics) ,Light-emitting diode - Abstract
We demonstrate a hybrid inorganic/organic light-emitting device composed of a CdSe/ZnS core/shell semiconductor quantum-dot emissive layer sandwiched between p-type NiO and tris-(8-hydroxyquinoline) aluminum (Alq3), as hole and electron transporting layers, respectively. A maximum external electroluminescence quantum efficiency of 0.18% is achieved by tuning the resistivity of the NiO layer to balance the electron and hole densities at quantum-dot sites.
- Published
- 2006
30. Blue Electroluminescence from Oxadiazole Grafted Poly(phenylene-ethynylene)s
- Author
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Vladimir Bulovic, Timothy M. Swager, Craig Breen, and Sandra Rifai
- Subjects
Nanostructure ,Materials science ,Polymers ,Color ,Oxadiazole ,Bioengineering ,Conjugated system ,Electroluminescence ,Electrochemistry ,law.invention ,chemistry.chemical_compound ,law ,Materials Testing ,Polymer chemistry ,General Materials Science ,chemistry.chemical_classification ,business.industry ,Mechanical Engineering ,Equipment Design ,General Chemistry ,Polymer ,Condensed Matter Physics ,Poly(phenylene ethynylene) ,Nanostructures ,Equipment Failure Analysis ,chemistry ,Luminescent Measurements ,Optoelectronics ,business ,Light-emitting diode - Abstract
Blue poly(phenylene-ethynylene) (PPE) electroluminescence is achieved in a single layer organic light emitting device. The polymeric system consists of an oxadiazole grafted PPE, which combines the necessary charge transport properties while maintaining the desirable efficient, narrow light-emitting properties of the PPE. Incorporation of a pentiptycene scaffold within the PPE structure prevents ground-state and excited-state interactions between the pendent oxadiazole units and the conjugated backbone.
- Published
- 2005
31. Layer Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition
- Author
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Alfonso Reina, Xiaoting Jia, John Ho, Daniel Nezich, Hyungbin Son, Vladimir Bulovic, Mildred S. Dresselhaus, and Jing Kong
- Subjects
Mechanical Engineering ,General Materials Science ,Bioengineering ,General Chemistry ,Condensed Matter Physics - Published
- 2009
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