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Vladimir Bulović

• Title: Professor
• Department: Department of Electrical Engineering and Computer Science

Postdoctoral Associates

• Trisha Andrew
• Ronny Costi
• Parag Deotare
• Yu Gu
• Dong Kyun Ko
• Katey Lo
• Andrea Maurano
• Katherine Stone
• William Tisdale
• Annie Wang

Collaborators

• Prof. Kathy Aidala, Mt. Holyoke
• Prof. Trisha Andrew, U. Wisconsin
• Prof. Tayo Akinwande, M.I.T.
• Prof. Nageh Allam, Amer.U.Cairo
• Prof. Marc Baldo, M.I.T.
• Prof. Moungi Bawendi, M.I.T.
• Dr. Ted Bloomstein, Lincoln Lab.
• Prof. Silvija Gradečak, M.I.T.
• Prof. Jeff Lang, M.I.T.
• Prof. Richard Lunt, Michigan State
• Prof. Martin Schmidt, M.I.T.
• Prof. Tim Swager, M.I.T.
• Prof. William Tisdale, M.I.T.

Graduate Students

• Matthew D’Asaro (EECS)
• Gleb Akselrod (Physics)
• Patrick Brown (Physics)
• Wendi Chang (EECS)
• Eletha Flores (EECS)
• Christina Hanson (Chem)
• Joel Jean (EECS)
• Apoorva Murarka (EECS)
• Farnaz Niroui (EECS)
• Tim Osedach (Harvard Univ.)
• Sarah Paydavosi (EECS)
• Sulinya Ramanan (EECS)
• Jill Macko (DMSE)
• Thomas Mahoney (EECS)
• Yasuhiro Shirasaki (EECS)
• Katherine Song (EECS)
• Geoffrey Supran (DMSE)
• Mun Ee Woo (DMSE)

Support Staff

• Monica Pegis, Administrative Assistant

Selected Publications

(see onelab.mit.edu for full listing)

Shirasaki, Y., Supran, G.J., Tisdale, W.A., V. Bulović, “Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes,” Phys. Rev. Lett., 110, 217403 (2013).

Mashford, B.S., Stevenson, M., Popovic, Z., Hamilton, C., Zhou, Z., Breen, C., Steckel, J., V. Bulović, Bawendi, M., Coe-Sullivan, S., Kazlas, P.T.,

“High-efficiency quantum-dot light-emitting devices with enhanced charge injection,” Nature Photonics, Accepted (2013).

Akselrod, G.M., Young, E.R.,Bradley, M.S., V. Bulović, “Lasing through a strongly-coupled mode by intra-cavity pumping,” Optics Express, 21, 12122-12128 (2013).

Costi, R., Young, E.R., V. Bulović, Nocera, D.G. “Stabilized CdSe-CoPi Composite Photoanode for Light-Assisted Water Oxidation by Transformation of a CdSe/Cobalt Metal Thin Film,” ACS Appl. Mater. Interfaces, 5, 2364-2367 (2013).

Jean, J., Chang, S., Brown, P.R., Cheng, J.J., Rekemeyer, P.H., Bawendi, M.G., Gradecak, S., V. Bulović “ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells,” Adv. Mater., 25, 2790-2796 (2013).

Chang L-Y, Lunt,R.R., Brown, P.R., V. Bulović, Bawendi, M.G. “Low-Temperature Solution -Processed Solar Cells Based on PbS Colloidal Quantum Dot/CdS Heterojunctions,” Nano Lett., 13, 994-999(2013).

Osedach, T.P., Andrew, T.L., V. Bulović, “Effect of Synthetic Accessability on the Commercial Viability of Organic Photovoltaics,” Energy & Environ. Sci., 6, 711-716 (2013).

Shirasaki, Y., Supran, G.J., Bawendi, M.G., V. Bulović, “Emergence of Colloidal Quantum-Dot Light-Emitting Technologies,” Nature Photonics, 7, 13-23 (2013).

Song, K.W., Costi, R., V. Bulović, “Electrophoretic Deposition of CdSe/ZnS Quantum Dots for Light-Emitting Devices,” Adv. Mater., 25, 1420-1423 (2013).

Park, H., Chang, S., Jean, J., Cheung, J.J, Araujo, P.T., Wang, M., Bawendi, M.G., Dresselhaus, M.S., V. Bulović, Kong, J., Gradecak, S. “Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells,” Nano Lett., 13, 233-239 (2013).

Arias, D.H., Stone, K.W., Vlaming, S.M., Walker, B.J, Bawendi, M.G., Silbey, R.J., V. Bulović, Nelson, K.A. “Thermally-Limited Exciton Delocalization in Superradiant Molecular Aggregates,” J. Phys. Chem. B., 117, 4553-4559 (2013).

Barr, M.C., Carbonera, C., Po, R., V. Bulović, Gleason, K.K. “Cathode buffer layers based on vacuum and solution deposited poly(3,4- ethylenedioxythiophene) for efficient inverted organic solar cells,” Appl. Phys. Lett., 100, 183301 (2012).

Borrelli, D.C., Barr, M.C., V. Bulović, Gleason, K.K. “Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition,” Solar Energ. Mater. & Solar Cells, 99, 190-196 (2012)

Barr, M.C., Howden, R.M., Lunt, R.R., V. Bulović, Gleason, K.K. “Top-Illuminated Organic Photovoltaics on a Variety of Opaque Substrates with Vapor-Printed Poly(3,4-ethylenedioxythiophene) Top Electrodes and MoO3 Buffer Layer,” Adv. Energ. Mater., 2, 1404-1409 (2012).

Panzer, M.J., Aidala, K.E., V. Bulović “Contact printing of colloidal nanocrystal thin films for hybrid organic/quantum dot optoelectronic devices,” Nano Rev., 3, 16144 (2012).

Jadhav, P.J., Brown, P.R., Thompson, N., Wunsch, B., Mohanty, A., Yost, S.R., Hontz, E., Van Voorhis, T., Bawendi, M.G., V. Bulović, Baldo, M.A. “Triplet Exciton Dissociation in Singlet Exciton Fission Photovoltaics,” Adv. Mater., 24, 6169-6174 (2012).

Osedach, T.P., Iacchetti, A., Lunt, R.R., Andrew, T.L., Brown, P.R., Akselrod, G.M., V. Bulović, “Near-infrared photodetector consisting of J-aggregating cyanine dye and metal oxide thin films,” Appl. Phys. Lett., 101, 113303 (2012).

Bozyigit, D., Wood, V., Shirasaki, Y., V. Bulović, “Study of field driven electroluminescence in colloidal quantum dot solids,” J Appl. Phys., 111, 113701 (2012).

J.A. Macko, R.R. Lunt, T.P. Osedach, P.R. Brown, M.C. Barr, K.K. Gleason, V. Bulović, “Multijunction organic photovoltaics with a broad spectral response,” Phys. Chem. Chem. Phys., 14, 14548-14553 (2012).

Yaul, F.M., V. Bulović, Lang, J.H. “A Flexible Underwater Pressure Sensor Array Using a Conductive Elastomer Strain Gauge,” Journal of Microelectromechanical Systems, 21, 897-906 (2012).

Yu, J., C.E. Packard, V. Bulović, “Subtractive contact-patterning of molecular organic films,” Organic Electronics, 13, 1779-1784 (2012).

Park, H., R.M. Howden, M.C. Barr, V. Bulović, K. Gleason, J. Kong, “Organic Solar Cells with Graphene Electrodes and Vapor Printed PEDOT as the Hole Transporting Layers,” ACS Nano (2012).

Andrew, T.L., V. Bulović, “Bulk Heterojuction Solar Cells Containing 6,6-Dicyanofulvenes as n-Type Additives,” ACS Nano, 6, 4671 (2012).

Borelli, D.C., M.C. Barr, V. Bulović, K.K. Gleason, “Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition,” Solar Energy Materials and Solar Cells, 99, 190 (2012).

Osedach, T.P., N. Zhao, T.L. Andrew, P.R. Brown, D.D. Wanger, D.B. Strasfeld, L.-Y. Chang, M.G. Bawendi, V. Bulović, “Bias-Stress Effect in 1,2-ETD-Treated PbS Quantum Dot Field-Effect Transistors,” ACS Nano, 6, 3121 (2012).

Bradley, S.M., V. Bulović, “Micron-Scale Molecular Organic Microcavity Arrays Patterned With Thin-Film Contact-Patterning,” IEEE Photonics Technology Letters, 24, 104 (2012).

Murarka, A., S. Paydavosi, T.L. Andrew, A.I. Wang, J.H. Lang, V. Bulović, “Printed MEMS Membranes on Silicon,” IEEE 25th International Conference on Micro Electro Mechanical Systems, 309  (2012).

Paydavosi, S., F.M. Yaul, A.I. Wang, F. Niroui, T.L. Andrew, V. Bulović, J.H. Lang, “MEMS Switches Employing Active Metal-Polymer Nanocomposites,” IEEE 25th International Conference on Micro Electro Mechanical Systems, 180 (2012).

Lobez, J.M, T.L. Andrew, V. Bulović, T.M. Swager, “Improving the Performance of P3HT–Fullerene Solar Cells with Side-Chain-Functionalized Poly(thiophene) Additives: A New Paradigm for Polymer Design,” ACS Nano, 6, 3044 (2012).

Paydavosi, S., K.E. Aidala, P.R. Brown, P. Hashemi, G.J. Supran, T.P. Osedach, J.L. Hoyt, V. Bulović, “Detection of Charge Storage on Molecular Thin Films of Tris(8-hydroxyquinoline) Aluminum (Alq3) by Kelvin Force Microscopy: A Candidate System for High Storage Capacity Memory Cells,” Nano Letters, 12, 1260 (2012).

Akselrod, G.M., B.J. Walker, W.A. Tisdale, M.G. Bawedi, V. Bulović, “Twenty-Fold enhancement of molecular fluoresence by coupling to a J-aggregate critically coupled resonator,” ACS Nano 6, 467 (2012).

Park, H., P.R. Brown, V. Bulović, J. Kong, “Graphene as transparent conducting electrodes in organic photocoltaics: Studies in graphene morphology, hole transporting layers and counter electrodes,” Nano Letters 12, 133-140 (2012).

Absorption and Fluorescence Enhancement of Single Molecules and Quantum Dots Using an Excitonic Antenna

• Authors: G. M. Akselrod, V. Bulović
• Sponsorship: EFRC Center for Excitonics at MIT

Nature presents us with a fundamental mismatch between the “size” of visible light (~250 nm in free space) and the size of the absorption cross-section of molecules, which is ~ 0.1 nm in diameter at room temperature. Therefore, the probability that a single photon will be absorbed by a single molecule is ~10^-7. Since most photonic and optoelectronic devices at some level depend on the absorption and emission of light, it is of great interest to engineer solutions to the limited absorption abilities of single molecules, with applications to single molecule sensing, bioimaging, single photon sources, and down-conversion for lighting. The last decade has seen tremendous interest in the ability of plasmon modes to increase the absorption cross-section molecules, owing to the small wavelength and high field localization of plasmons^[1]. We have developed and demonstrated an alternative, exciton-based approach to absorption enhancement using organic thin films of highly absorbing (α = 10⁶ cm^-1) J-aggregates. Acting as an excitonic antenna, the J-aggregates absorb incident light, the generated excitons diffuse through the film, and they eventually come within a Förster resonant energy transfer radius of the acceptor molecules situated on top of the film, thus transferring the exciton energy (Figure 1)^[2]. In this scheme each acceptor molecule “sees” a large number of J-aggregate donor molecules due to the unusually long exciton diffusion length (~100 nm)^[3],^[4] in J-aggregates. Using this approach we have demonstrated enhancement factors over 1000 for the absorption and fluorescence of ensembles of DCM laser dye molecules (Figure 2a), which compares well with the best enhancements seen with plasmonic approaches^[1]. We have confirmed that the enhancement can be achieved on the single emitter level by performing confocal imaging of single quantum dots coupled to the J-aggregate excitonic antenna (Figure 2b). Next steps include the integration of this approach with active optoelectronic and photonic devices.



Figure 1: Schematic of absorption and fluorescence enhancement using the J-aggregate excitonic antenna.



Figure 2: (a) Fluorescence enhancement of DCM molecules at varying concentrations, showing good agreement with predictions for a J-aggregate exciton diffusion length of 100 nm. Increase in enhancement at lower concentration occurs because each molecule sinks to a larger number of J-aggregate excitons. (b) Enhancement of single colloidal core-shell CdSe quantum dot imaged by confocal microscopy.




1. A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Mullen, and W. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nature Photonics, vol. 3, pp. 654–657, 2009. [↩] [↩]
2. G. M. Akselrod, B. J. Walker, W. A. Tisdale, M. G. Bawendi, and V. Bulovic, “Twenty-Fold Enhancement of Molecular Fluorescence by Coupling to a J-aggregate Critically Coupled Resonator,” ACS Nano, vol. 6, pp 467-471, Nov. 2011. [↩]
3. G. M. Akselrod, Y. Tischler, E. Young, D. Nocera, and V. Bulovic, “Exciton-exciton annihilation in organic polariton microcavities,” Physycal Review B, vol. 82,,pp. 1–4, Sep. 2010. [↩]
4. J. Moll, W. J. Harrison, D. V Brumbaugh, and A. A. Muenter, “Exciton annihilation in J-aggregates probed by femtosecond fluorescence upconversion,” Journal of Physical Chemistry A, vol. 104,, pp. 8847–8854, Jan. 2000. [↩]

Contact-printed MEMS Membranes

• Authors: A. Murarka, A. Wang, J. H. Lang, V. Bulovic
• Sponsorship: Deshpande Center for Technological Innovation

It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates^[1][2].

A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion 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, as shown in Figure 1^[3]. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).

These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and  large-area arrays of these devices.



Figure 1: Deflection profiles of gold membranes under electrostatic actuation, obtained via optical interferometry. (a) Deflection of the membrane increases as the voltage applied between the membrane and the Si substrate increases. (b) 13 different diametrical deflection profiles (inset: interferometry image) of a membrane under 15 V actuation are plotted and averaged to show the mean profile. (c) Gold membrane deflection over multiple cavities covered by a single membrane, as the voltage is increased from 1 V to 15 V.



Figure 2: Optical interferometry images of several cavities under 15 V peak-to-peak amplitude sinusoidal voltage actuation at 15 Hz, taken at 30 degree phase intervals. Membrane deflection for only one half of a sine wave is shown in this figure.




1. A. Murarka, C. Packard, F. Yaul, J. Lang, and V. Bulovic, “Micro-contact printed MEMS,” IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), 2011, pp. 292-295. [↩]
2. C. Packard, A. Murarka, E. W. Lam, M. A. Schmidt, and V. Bulovic, “Contact-printed microelectromechanical systems,” Advanced Materials, vol. 22, pp. 1840–1844, 2010. [↩]
3. A. Murarka, S. Paydavosi, T. L. Andrew, A. I. Wang, J. H. Lang, and V. Bulovic, “Printed MEMS membranes on silicon,” IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012, pp. 309-312. [↩]

Organic Light Emitting Devices on Plastic and Paper Substrates

• Authors: K. W. Song, Y. Urakawa, T. S. Mahony, K. K. Gleason, V. Bulović
• Sponsorship: NSF Graduate Research Fellowship, EFRC Center for Excitonics

The successful fabrication of optoelectronics on economical, flexible, and lightweight substrates such as plastic and paper could pave the way for an exciting, novel breed of disposable electronics. Here, we demonstrate phosphorescent organic light emitting devices (OLEDs) on various substrates, including polyethylene terephthalate (PET), tracing paper, and copy paper. Poly(3,4-ethylenedioxythiophene) [PEDOT] is deposited onto the substrates via a dry oxidative chemical vapor deposition (oCVD) process, avoiding the difficulties of processing PEDOT on chemically absorbing substrates using conventional wet methods. The completed OLEDs comprise a bottom 50 nm PEDOT anode, a 5 nm molybdenum oxide (MoO₃) hole injection layer, a 100 nm tris(4-carbazoyl-9-ylphenyl)amine [TCTA] hole transport layer, a 15 nm iridium, tris(2-phenylpyidine) [Ir(ppy)3]/TCTA (10%) emissive layer, a 60 nm 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene [TPBi] electron transport layer, and a top 100 nm lithium fluoride (LiF)/aluminum (Al) cathode (all layers except PEDOT deposited via thermal evaporation at ~10^-6 Torr). The green light output is measured through the bottom anode and substrate and is readily observable by eye. As a comparison, control OLEDs are made on indium tin oxide (ITO)-on-glass substrates, and these devices demonstrate external quantum efficiencies (EQEs) of ~14%. For OLEDs on PEDOT-on-plastic/paper, EQEs of up to 2% on tracing paper and 6% on PET have been measured to date, and there remains much room for improvement. For instance, at present, many devices suffer from electrical shorts that arise either from the underlying roughness of paper fibers or from large iron chloride (FeCl₃) residues from oCVD on the surface of the PEDOT. Current work focuses on improving the reproducibility of devices by solving this shorting problem. Furthermore, the mechanical flexibility of devices will be measured in the near future. Like the rest of the OLED layers and unlike ITO, PEDOT is not prone to brittle fracture under mechanical stress, so it is expected that OLEDs made on PEDOT-on-plastic/paper can remain operational under bending, folding, and/or twisting – an attractive asset for portable electronics.

Mechanical Pressure-induced Solid State Solvation in Organics Thin Films

• Authors: W. Chang, V. Bulovic
• Sponsorship: EFRC Center for Excitonics

Significant technological progress of organic semiconducting structures has led to their commercialization in the form of organic LED displays, solid state lighting, and thin film photovoltaics. A guest:host doping system is often employed, and dicyanomethylene (DCM) class dyes are used extensively as the dopant molecules in both fundamental studies and applications^[1],^[2]. The solvation effect describes the physical reorientation of surrounding solvent molecules due to a change in solute dipole moment during transition, affecting the excitonic energy states of the emitting solute dye molecules. This local dielectric effect has been extensively studied in liquid state^[3] and has been reported in molecular doping studies in solid state^[4],^[5]. In this work we demonstrate and numerically quantify a new method of probing dielectric-dependent excitonic-energy-shifts though application of pressure. Thin films of polystyrene (PS) and Alq₃ doped with laser dye DCM(II) showed bathochromic shift in photoluminescence (PL) under increasing pressure, as shown in Figure 1. Films of 5% doped PS:DCM(II) showed peak energy shifts up to 40 meV under pressure change of 0.4 GPa, which when fitted to solid state solvation theory (Figure 2)^[5],^[6], suggests a PS elastic modulus of 1.2±0.6 GPa. Similarly, films of 1% doped Alq3:DCM(II) under pressure showed peak energy shifts up to 50meV, suggesting an Alq₃ elastic modulus of 0.9±0.4 GPa. The results demonstrate the first numerically quantified solid-state solvation effect due to pressure. This ability to tune exciton energy using an external parameter creates a novel method to explore applications such as controlling exciton diffusion and optimizing optoelectronic device.



Figure 1: Spectral shift in PL in response to pressure applied on a thin film with pressure, indicated in legend, of: (a) .5% doping of DCM(II) in PS and (b) 1% doping of DCM(II) in Alq3. The black line indicates a typical Gaussian fit of the peak PL of PS:DCM(II) film under no pressure. The maximum of the peak wavelength as computed from fitting is indicated for each PL as a corresponding dotted line.



Figure 2: Fitting experimental results of PS;DCM(II) peak energy shift as function of changed molecular packing density or dielectric susceptibility. Red dashed line indicate the OLM theory of DCM(II) with initial dielectric constant of 2.44 (x0 = 1.44) in polystyrene, and blue points indicate the fitted peak wavelength from the pressure experiment using elastic modulus as a fitting parameter. The plot shows good agreement for a PS elastic modulus of 1.2 0.6GPa.




1. M. Swoboda, Organic Microcavity Laser Dynamics: Fundamentals and Experiments on a DCM:Alq3 Guest: Host-composite System: VDM Verlag, 2008. [↩]
2. Z. Guo, W. Zhu, and H. Tian, “Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors,” Chemical communications (Cambridge, England), vol. 48, no. 49, pp. 6073–84, June 2012. [↩]
3. C. Reichardt, Solvents and Solvation Effects in Organic Chemistry: VCH, 1990. [↩]
4. V. Bulovic, R. Deshpande, M. E. Thompson, and S. R. Forrest, “Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect,” Chemical Physics Letters, no. 308, pp. 317–322, 1999. [↩]
5. C. F. Madigan and V. Bulović, “Solid state solvation in amorphous organic thin films,” Physical Review Letters, vol. 91, no. 24, p. 247403, 2003. [↩] [↩]
6. L. Onsager, “Electric Moments of Molecules in Liquids,” Journal of the American Chemical Society, vol. 58, pp. 1486–1493, 1936. [↩]

Engineering Energy Level Alignment in Lead Sulfide Quantum- Dot Photovoltaics through Ligand Exchange

• Authors: P. R. Brown, J. Jean, T. P. Osedach, D. D. Wanger, N. Zhao, M. G. Bawendi, R. R. Lunt, V. Bulović
• Sponsorship: National Science Foundation, Hertz Foundation, Samsung



Figure 1: (a) Schematic depiction of oleic-acid-coated lead sulfide QDs (PbsS QDs); the oleic acid ligands are exchanged for 1,3-benzenedithiol (1,3-BDT), 1,2-benzenedithiol (1,2-BDT), or ethanedithiol (EDT). (b) Valence band, Fermi level, and conduction band energies determined from ultraviolet photoelectron spectroscopy and absorption spectroscopy for PbS QDs treated with different ligands. (c) Current-voltage curve of PbS QD/C60 donor-acceptor heterojunction photovoltaics, where the PbS QDs are ligand-exchanged with 1,3-BDT (blue curve) or 1,2-BDT (red curve). The insets depict the expected mode of device operation.

Solar cells based on lead sulfide colloidal quantum dots (PbS QDs) have made dramatic improvements in efficiency in recent years, owing in large part to novel surface passivation techniques involving organic or inorganic ligands. In spite of these advances, the influence of these ligand treatments on QD charge transport, doping, and energy level structure is still incompletely understood. Different ligand treatments have been shown to alter the valence band and conduction band binding energies in InAs and CdSe QDs by up to 0.3 eV through modification of QD-ligand interface dipoles^[1][2][3], but the importance of similar energy shifts for electronically relevant ligands in PbS QD solids and the effects of these shifts on photovoltaic device performance have hitherto been unexplored. Through a combination of ultraviolet photoelectron spectroscopy and carrier transport measurements, we demonstrate that even between structurally and chemically similar ligands such as 1,2-ethanedithiol (EDT), 1,2-benzenedithiol (1,2-BDT), and 1,3-benzenedithiol (1,3-BDT) (Figure 1a), sizeable changes in the PbS QD valence band energy and work function can dramatically affect photovoltaic device performance. In particular, 1,2-BDT-treated PbS QDs are found to have a ~ 0.2 eV shallower valence band energy and work function than 1,3-BDT-treated PbS QDs (Figure 1b); this shift necessitates a re-optimization of the energy levels of the electron- and hole-extracting contacts for solar cells employing these ligands (Figure 1c). In addition, 1,2-BDT and 1,3-BDT treatments result in higher power conversion efficiencies than EDT treatment for PbS Schottky junction and ZnO/PbS heterojunction photovoltaics, respectively, despite the fact that BDT-treated films demonstrate a lower charge carrier mobility than EDT-treated films. These results emphasize the importance of optimizing interfacial energy offsets and surface passivation, rather than solely the carrier mobility, in PbS QD photovoltaics.

1. M. Soreni-Harari, N. Yaacobi-Gross, D. Steiner, A. Aharoni, U. Banin, O. Millo, and N. Tessler, “Tuning energetic levels in nanocrystal quantum dots through surface manipulations,” Nano Letters, vol. 8, pp. 678-684, Jan. 2008. [↩]
2. A. M. Munro, B. Zacher, A. Graham, and N. R. Armstrong, “Photoemission spectroscopy of tethered CdSe nanocrystals: Shifts in ionization potential and local vacuum level as a function of nanocrystal capping ligand,” ACS Applied Materials and Interface, vol. 2, pp. 863-869, Mar. 3010. [↩]
3. J. Jasieniak, M. Califano, and S. E. Watkins, “Size-dependent valence and conduction band-edge energies of semiconductor nanocrystals,” ACS Nano, vol. 5, pp. 5888-5902, June 2011. [↩]

Nano-thin Encapsulation of Photovoltaics via Atomic Layer Deposition

• Authors: J. A. Macko, V. Bulovic
• Sponsorship: Eni-MIT Alliance Solar Frontiers Center, MIT Energy Initiative

Device lifetime is a key figure of merit for solar cells and impacts the levelized energy cost ( $/(kW*hr) ) of a photovoltaic module^[1].  Its importance necessitates the encapsulation of solar cells; however, the prevalent commercial encapsulation (polymer thin film plus glass sheet) is inflexible and heavy, thereby increasing the installation and balance-of-systems costs.  In this work we implement a nano-thin metal oxide film deposited by atomic layer deposition (ALD) as an encapsulation layer for organic photovoltaics^[2].  With ALD we deposit a 27-nm nano-thin film of alternating layers of HfOx and AlOx^[3] directly on top of the cathode of photovoltaics composed of either polymer bulk heterojunction or molecular planar-mixed heterojunction photoactive layers (Figure 1). The effect of the ALD process on the device performance and furthermore the dependence of lifetime on photoactive materials choice selection and cathode composition are examined.  While the absolute lifetime of the devices depends on the composition of the photoactive film as well as the cathode, all devices displayed significant increases in lifetime when encapsulated with ALD nano-thin films versus no encapsulation. Molecular OPVs with Al electrodes and ALD nano-thin encapsulant exhibit less than 10% decay in efficiency over 1200 hours storage in air.  As shown in Figure 2, aAddition of ALD nano-thin encapsulant to polymer PVs with Al electrodes increases device lifetime by up to 5,000%.



Figure 1: Schematics of the device architectures studied in this work: polymer bulk heterojunction and small molecular planar-mixed heterojunction. The devices were fabricated with either Ag or Al electrodes and with or without the ALD nano-thin encapsulant.



Figure 2: Effect of ALD encapsulation on time-resolved power conversion efficiency of polymer cells stored in air or in an inert environment. The cells last <1 day when exposed to air without encapsulation but maintain power generation after up to 1200 hours in storage with implementation of ALD nano-thin encapsulant.




1. R. R. Lunt, T. P. Osedach, P. R. Brown, J. A. Rowehl, and V. Bulović, “Practical Roadmap and Limits to Nanostructured Photovoltaics,” Advanced Materials, vol. 23, no. 48, pp. 5712–5727, Nov. 2011. [↩]
2. A. Bernardi, J. A. Rowehl, R. Po, A. O’Mahony, P. Scudo, and V. Bulovic, “Encapsulation of Organic Solar Cells with ALD deposited HfOx/AlOx films,” presented at International Conference on Science and Technology of Synthetic Metals, July 2012. [↩]
3. C.-Y. Chang and F.-Y. Tsai, “Efficient and air-stable plastics-based polymer solar cells enabled by atomic layer deposition,” Journal of Materials Chemistry, vol. 21, no. 15, p. 5710, 2011. [↩]

Applications of Nanoimprinted Structures In Organic Photovoltaics

• Authors: E. Flores, R. Howden, K. Gleason, V. Bulovic
• Sponsorship: MIT

Small-molecule organic photovoltaic cells (OPVs) have the potential to be a low-cost, flexible power conversion solution to many energy problems^[1]. These OPVs take advantage of an extremely thin active layer, which enables this flexibility and reduces material volume. However, it is this thin quality that calls for improved power conversion efficiency compared to traditional silicon solar cells. Thin films suffer from reduced optical path lengths, which hinder light absorption and hence, power conversion efficiency. Many designs have been proposed to improve light absorption^{[2][3][4][5][6]}. We present a  novel light-trapping substrate geometry for OPVs that is based on a conformally coated, subwavelength-textured substrate design, which is intended to substantially increase optical path lengths. The subwavelength nature of these nanocones/nanowedges decouples the light propagation from the exciton diffusion path. This decoupling is an optimized situation for efficient charge transfer. Enhanced power absorption into the OPV active layer has been demonstrated via numerical computation methods, including COMSOL FEM (shown in Figure 1) and Lumerical FDTD. We will address the challenge to fabricate a working device by using nanoimprinting to create the structures in a conductive polymer, in which the nanoimprinting process is optimized to maintain good electrical properties of the patterned conductive film. We will also present an alternative approach that utilizes a conformal coating of the organic conductor PEDOT on the pre-patterned nanostructures, as shown in Figure 2. Uniform and conformal PEDOT coverage over the nanoscale features was achieved using an all-dry deposition process^[7].



Figure 1: Model of power flow and absorption of single nanowedge unit cell. Arrows designate the Poynting vector of time-averaged power flow. Absorbing active layer is sandwiched between layers of glass. Incident light is TE polarized and varied from 300 to 900 nm.



Figure 2. All-dry deposited PEDOT is shown to conformally coat a nanoscale feature. This uniform thickness over sharp features is a substantial improvement over the non-conformal coating properties of spun-coat PEDOT:PSS on features of the same scale.




1. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, p. 3693, Apr. 2003. [↩]
2. V. Karagodsky, F. Sedgwick, and C. J.  Chang-Hasnain, “New Physics of Subwavelength High Contrast Gratings,” Quantum Electronics and Laser Science Conference OSA Technical Digest Optical Society of America, 2011. [↩]
3. S. Mallick, N. P. Sergeant, M. Agrawal, J. Lee, and P. Peumans, “Coherent light trapping in thin-film photovoltaics,” Materials Research Society Bulletin, vol. 36, no. 06, pp. 453 – 460, 2011. [↩]
4. K. Nalwa, J. M. Park, K. M. Ho, and S. Chaudhary, “On realizing higher efficiency polymer solar cells using a textured substrate platform,” Advanced Materials, vol. 23, pp. 112-116, 2011. [↩]
5. A. Raman, Z. Yu, and S. Fan, “Dielectric nanostructures for broadband light trapping in organic solar cells,” Optics Express, vol. 19, pp. 19015-19026, 2011. [↩]
6. S. B. Rim, S. Zhao, S. R. Scully, M. D.  McGehe, and P. Peumans, “An effective light trapping configuration for thin-film solar cells,” Applied Physics Letters, vol. 91, p. 243501, 2007. [↩]
7. S. GACS Nano. vol. 2(9), pp. 1959-67, 2008. [↩]

Micron- and Submicron-thick Parylene Substrates for Transfer Printing and Solar Applications

• Authors: A. I. Wang, A. Murarka, F. Niroui, J. Jean, J. H. Lang, V. Bulović
• Sponsorship: NSF

Transfer printing of thin metal films enables the fabrication of both planar and suspended membrane electrodes for microelectromechanical (MEMS) sensors and actuators in an additive process. In addition, transfer-printed metal films can be used to form abrupt junctions between organic and metal layers. In contrast, conventional deposition processes such as evaporation or sputtering can cause the metal atoms to penetrate into underlying organic layers.

We have developed a solvent-free transfer printing method using micron- or submicron-thick films of a robust, flexible polymer, parylene-C, as a carrier membrane. The transparent parylene films are initially deposited by chemical vapor deposition onto a rigid or semi-rigid handle substrate for ease of handling during fabrication. Metal and/or organic layers are subsequently deposited and patterned on top. The entire stack is then peeled away from the handle in a continuous sheet and transferred to the receiving substrate. After transfer, the ultrathin parylene carrier may be left in place or removed by oxygen plasma.

Using this method, we have demonstrated electrostatically-actuated gold membrane-covered cavity arrays for microspeakers with larger areas (>1cm²) than previously possible using a solvent-assisted contact transfer printing method^[1]. The solvent-free method has also been employed to deposit metal electrodes on top of solvent-sensitive organic layers in metal-molecule-metal structures for tunneling nanoelectromechanical switches^[2].

Parylene films can also serve as ultrathin, lightweight substrates for organic photovoltaics (OPVs). Their chemically inert and insoluble nature enables the use of common vapor- and solution-phase methods for thin-film deposition, including thermal evaporation and layer-by-layer spin-casting. Since the full active layer stack in an OPV is itself less than a micron thick, moving to thinner, lighter substrates could significantly reduce total weight and cost^[3]. Although our current devices absorb strongly in the visible wavelength range, further materials and architecture engineering could yield a fully transparent solar cell on parylene with exceptional flexibility and versatility for optoelectronic applications.

1. A. Murarka, S. Paydavosi, T. Andrew, A. Wang, J. Lang, and V. Bulović, “Printed MEMS membranes on silicon,” in Proc. IEEE 25th Int. Conf. Micro Electro Mechanical Systems (MEMS), Paris, France, 2012, pp. 309-312. [↩]
2. F. Niroui, A. I. Wang, E. M. Sletten, T. M. Swager, J. H. Lang, and V. Bulović, “Tunneling Nanoelectromechanical Switches Based on Organic Thin Films,” MTL Annual Report, 2013. [↩]
3. M. Kaltenbrunner, M. S. White, E. D. Glowacki, T. Sekitani, T. Someya, N. S. Sariciftci, and S. Bauer. “Ultrathin and lightweight organic solar cells with high flexibility,” Nat. Comm., 3:770 doi: 10.1038/ncomms1772 (2012). [↩]

Recombination Dynamics of Charge Carriers in Nanostructured Solar Cells

• Authors: A. Maurano, P. Brown, J. Rowehl, V. Bulović
• Sponsorship: ENI-MIT Solar Frontier Center

Nanostructured solar cells attract increasing attention as a promising photovoltaic (PV) technology^[1]. Generation of free charge carriers in nanostructured PV devices occurs at the electron donor-acceptor interface, analogous to a pn-junction interface in traditional crystalline Si solar cells. Recombination at this interface constitutes a major pathway of charge carrier loss. Characterizing and controlling recombination dynamics are key to informing design of novel device architectures. Recombination parameters also enable comparisons between different device architectures.

This work uses the transient optoelectroic techniques^[2] to probe recombination mechanisms under standard operating conditions in four solar cells. Figure 1 shows the cells: a lead sulfide quantum dot and zinc oxide (QD PbS:ZnO) pn-heterojunction, a chloroaluminium phthalocyanine and fullerene (ClAlPc:C₆₀) planar heterojunction, a tetraphenyldibenzoperiflanthene and C₆₀ (DBP:C₆₀) planar heterojunction, and a DBP and C₆₀ planar mixed heterojunction (DBP:( DBP/C₆₀):C₆₀). Figure 2a shows charge carrier lifetimes as functions of the open-circuit voltage. Varied carrier lifetimes may arise from interface morphologies: e.g.,, the slower recombination transients observed in the DBP:C₆₀ devices may be due to their intrinsic planarity. We measure charge carrier density as a function of the open circuit voltage, as in Figure 2b.  Combining carrier lifetime and charge carrier density allows us to calculate recombination rates and assign the major recombination to a dominant mechanism. In all devices, the relevant recombination is a geminate recombination of excitons or standard SRH recombination (PbS:ZnO solar cells) and a non-geminate recombination between two free charges. Investigating the type of recombination is critical for designing solar cells with improved efficiency.



Figure 1: Schematics of nanostructured PV architectures studied in this work. Left to right: QD PbS:ZnO pn-heterojunction (inset shows cartoon of QD passivated by organic ligand on ZnO), ClAlPc:C60 planar heterojunction with ClAlPc molecular structure, DBP:C60 planar heterojunction with DBP molecular structure, and DBP:( DBP/C60):C60 planar mixed heterojunction. All devices are sandwiched between an electron and hole transporting layer such as bathocuproine and molybdenum oxide and ITO and Au as electrodes.



Figure 2: a) Charge carrier lifetime and b) charge carrier density as function of open circuit voltage for ClAlPc:C60, DBP:C60, (DBP:( DBP/C60):C60), and PbS:ZnO.

 

1. Anonymous, “A sunny outlook,” Nature Photonics, vol. 6, no. 3, p. 129, Mar. 2012. [↩]
2. C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, and J. R. Durrant, “Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell,” Applied Physics Letters, vol. 92, p. 3, 2008. [↩]

Tunneling Nanoelectromechanical Switches Based on Organic Thin Films

• Authors: F. Niroui, A. I. Wang, E. M. Sletten, T. M. Swager, J. H. Lang, V. Bulović
• Sponsorship: NSF Center for Energy Efficient Electronics Science

With the silicon-based electronics reaching physical limits that inhibit continued improvements in device performance, much research has been directed towards nanoelectromechnical (NEM) switches as a promising alternative. NEM switches exhibit abrupt switching behavior and near-zero leakage current but suffer from large actuation voltages and failure due to stiction^[1][2][3]. To overcome these challenges, this work presents a NEM switch, squitch, that operates based on electromechanical modulation of tunneling current through a nanometer-thick organic thin film (Figure 1). The switching process is initiated by the electrostatic compression of the organic film sandwiched between conductive contacts. As the organic layer is compressed, the tunneling distance is reduced, leading to an exponential increase in the tunneling current. The presence of the organic layer prevents direct contact between the electrodes. Furthermore, when the electrostatic force is removed, the deformed organic layer provides the restoring force necessary to displace the top electrode to the off-state position, helping to mitigate stiction.

Theoretical analysis of two- and three-terminal devices suggests the possibility of achieving large on-off current ratio exceeding 6 orders of magnitude, sub-1-V actuation voltage, and nanosecond switching time. Various fabrication techniques have been explored to form the metal-organic-metal junction with the organic layer created mainly through use of a self-assembled monolayer of thiolated molecules. While the bottom metallic contact is commonly formed through thermal evaporation, the top electrode can be fabricated through evaporation, nanotransfer printing of a metallic thin film, or assembly of graphene sheets. An example of a metal-organic-metal junction fabricated through nanotransfer printing of a gold (Au) top electrode is shown in Figure 2. In addition to device fabrication, our current work is focused on investigating the electromechanical response of organic-based tunneling junctions through simultaneous use of electrical and optical measurements.



Figure 1: Schematic diagram of the squitch switching mechanism. An applied electrostatic force compresses the active organic layer, reducing the tunneling width and resulting in an exponential increase in current.



Figure 2: Cross-sectional SEM of an example metal-organic-metal structure formed with an evaporated Au bottom electrode, self-assembled poly(ethylene glycol) dithiol organic layer, and nanotransfer printed Au top electrode.




1. R. Nathanael, V. Pott, H. Kam, J. Jeon, and T. J. K. Liu, “4-terminal relay technology for complementary logic,” in Proc. IEEE Int. Electron Device Meeting Tech. Dig., Dec. 2009, pp. 223–226. [↩]
2. R. Parsa, M. Shavezipur, W. S. Lee, S. Chong, D. Lee, H. S. P. Wong, R. Maboudian, and R. T. Howe, “Nanoelectromechanical relays with decoupled electrode and suspension,” IEEE 24th International Conference on Micro Electro Mechanical Systems, 2011, pp. 1361-1364. [↩]
3. O. Y. Loh, and H. D. Epsinosa, “Nanoelectromechanical contact switches,” Nature Nanotechnology, vol. 7, pp. 283-295, May 2012. [↩]

High-performance Near-infrared Light-emitting Devices using Core-shell (PbS-CdS) Colloidal Quantum-dots

• Authors: G. J. Supran, K. W. Song, G. W. Hwang, R. E. Correa, J. Scherer, E. A. Dauler, Y. Shirasaki, M. G. Bawendi, V. Bulović
• Sponsorship: Center for Excitonics, Assistant Secretary of Defense for Research and Engineering, National Science Foundation, Institute for Soldier Nanotechnology

Near-infrared light sources integrated at room temperature with any planar surface could be realized by harnessing the broad spectral tunability, high brightness, and solution-processability of colloidal quantum-dots (QDs)^[1]. Yet the performance of near-infrared quantum-dot light-emitting devices (NIR QD-LEDs) has remained relatively low until now^[1]. Here we show that PbS-CdS core-shell QDs, synthesized via a Pb-to-Cd cation-exchange reaction^[2] (Figure 1, inset), enhance the peak external quantum efficiency (EQE) of PbS core-only control devices by 50- to 100-fold, up to 4.3 ± 0.3% (Figure 2). By incorporating the core-shell QDs into the novel organic-QD-inorganic hybrid NIR QD-LED architecture depicted in Figure 1, “turn-on” voltages are lowered by 0.6 ± 0.2 V, and per-amp radiant intensities are increased by up to 150 times. Peak EQEs and power conversion efficiencies are more than double those of previous QD-LEDs emitting at wavelengths beyond 1 μm^[3] and are comparable with those of commercial NIR LEDs. We demonstrate that the primary origin of the performance enhancement is passivation of PbS cores by CdS shells against in situ photoluminescence quenching, suggesting that core-shell QDs may become a mainstay of high-efficiency NIR QD-LEDs.

Electrically-driven and wavelength-selectable NIR light sources that can be deposited on any substrate and at lower cost than existing (usually epitaxially grown) NIR-emitters have the potential to enhance existing technologies and to trigger the development of new ones^[1]. Beneficiaries could include optical telecommunications and computing, bio-medical imaging, on-chip bio(sensing) and spectroscopy, night-time surveillance, and other security applications.



Figure 1: The novel organic-QD-inorganic hybrid NIR QD-LED architecture, fabricated at room temperature, employed in this work. The inset is a schematic illustration of the Pb-to-Cd cation-exchange reaction used to convert core-only PbS QDs into core-shell PbS-CdS QDs.



Figure 2: A plot of external quantum efficiency versus current density, demonstrating the ~50- to 100-fold enhancement in QD-LED performance achieved by replacing core-only PbS QDs (black) with core-shell PbS-CdS QDs (green). The inset is a photograph of an array of five QD-LEDs taken with an infrared camera, with the middle device turned on and emitting at a center wavelength of λ=1242 nm .




1. Y. Shirasaki, G. J. Supran, M. G. Bawendi, and V. Bulović, “Emergence of Colloidal Quantum-Dot Light-Emitting Technologies,” Nature Photonics, vol. 7, pp. 13-23, Jan. 2013. [↩] [↩] [↩]
2. J. M. Pietryga et al., “Utilizing the lability of lead selenide to produce heterostructured nanocrystals with bright, stable infrared emission,” J. Am. Chem. Soc., vol. 130, pp. 4879-4885, Mar. 2008. [↩]
3. L. Sun et al., “Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control,” Nature Nanotechnology, vol. 7, pp. 369-373, May 2012. [↩]

Binary Nanoparticle Superlattices Formed from Highly Luminescent Core-Shell Quantum Dots and Their Photoluminescence Properties

• Authors: D. K. Ko, G. W. Hwang, R. E. Correa, G. J. Supran, M. G. Bawendi, V. Bulović
• Sponsorship: EFRC Center for Excitonics, Assistant Secretary of Defense for Research and Engineering, Samsung Advanced Institute of Technology



Figure 1. Large-area (1.5- x 1.5-cm Si/SiO2 substrate) binary nanocrystal superlattices (BNSLs) assembled from 8-nm CdSe/ZnS and 4-nm PbS/CdS core-shell QDs. (a) Fluorescence microscope image (1000x, 595-nm band filter) of BNSLs showing an average grain size of 3.2 um. (b) False color high-resolution transmission microscope (HRTEM) image and small-angle electron diffraction (CL 200 cm) pattern obtained from a single BNSL domain showing a formation of a cub-AB13 structure.

Colloidal semiconductor nanocrystals or quantum dots (QDs) offer size-tunable absorption and luminescence spectra, permitting a broad range of applications in optoelectronics and biomedical imaging. Advances in colloidal synthesis enable the preparation of monodispersed QDs, which can be further used as building blocks to direct self-assembly into closely packed superlattices with well-defined spatial coordination. Superlattices can also be formed from two different materials of QDs, often referred to as binary nanocrystal superlattices (BNSLs)^[1],^[2], providing an excellent materials platform to explore unique photophysical properties arising from electronic and optical interactions between two components.

In this work, we report for the first time BNSLs assembled from highly luminescent 8-nm CdSe/ZnS and 4-nm PbS/CdS core/shell QDs. Single superlattice domains with an average size of 3.2 μm show formation of cub-AB₁₃ structures as confirmed by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction, and structural modeling (Figure 1). We characterized these BNSLs optically via micro-photoluminescence spectroscopy of single superlattice domains formed on a Si/SiO₂ substrate. We probed resonant energy transfer from CdSe/ZnS to PbS/CdS QDs, enabled by the spectral overlap of their emission and absorption spectra, respectively. A combination of spectral and time-resolved dynamics was used to elucidate differences in energy transfer between amorphous and ordered superlattice arrays; these differences will be further discussed in the context of a novel QD light-emitting diode. The spectral tunability of QDs combined with controlled nearest-neighbors and interparticle distances in their superlattice assemblies allow us to understand and potentially engineer excitonic energy flow in these unique material systems.

1. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, and C. B. Murray, “Structural diversity in binary nanoparticle superlattices,” Nature, vol. 439, pp. 55-59, Jan. 2006. [↩]
2. S. A. Claridge, A. W. Castleman, S. N. Khanna, C. B. Murray, A. Sen, and P. S. Weiss, “Cluster-assembled materials,” ACS Nano, vol. 3, pp. 244-255, Feb. 2009. [↩]

Nano-patterning of J-aggregate Thin Films

• Authors: P. B. Deotare, G. M. Akselrod, V. Bulovic
• Sponsorship: EFRC Center for Excitonics

We show patterning of thin J-aggregate^[1] films using nano-imprinting. Unlike the conventional microfabrication techniques, such as direct-write or lift-off processes, which can be detrimental to polymers, we explore the possibility of using nano-imprinting technology for patterning thin film polymers. A 4.5 molecular layer of J-aggregate is grown using layer-by-layer deposition of thiacyanine dye coadsorbed over ployallylamine hydrocholoride (PAH) polyelectrolyte on a polydimethylsiloxane (PDMS) substrate. To allow for PAH’s poor adhesion with PDMS, the first PAH adsorption step was extended to 30 minutes to allow extra time for the initial adsorption via hydrophobic interactions^[2]. This reduced adhesion property also helped in the pattern transfer. A master stamp consisting of nano-pillars is fabricated using ebeam lithography on SU8 resist spun on a silicon substrate. A 100-nm oxide layer is sputtered on the stamp to increase the adhesion between the j-aggregate film and the stamp. The stamp is then pressed gently on the j-aggregate thin film for about 10 s. Fluorescence microscope imaging is used to confirm the transfer process. Figure 1 shows the bright-field image of the patterned stamp used for the stamping process while Figure 2 shows the fluorescence image of the same area after pattern transfer. The results confirm a clean transfer of j-aggregate films onto the tip of the pillars. This successful high-yield, large-area patterning process not only opens new possibilities of designing devices in polymer thin films but also allows us to study and control excitons in such materials.



Figure 1: Brightfield image showing the patterned structure on a silicon substrate. The patterns were defined using ebeam lithography on SU8 resist followed by sputtering of 100 nm of silicon dioxide.



Figure 2: Fluorescence image of the area shown in Figure 1. The results confirm the high yield of the transfer process.




1. E. E. Jelley, “Molecular, Nematic and Crystal States of I: I Diethyl-Cyanine Chloride,” Nature, vol. 139, pp. 631-631, Apr. 1937. [↩]
2. S. M. Bradley, “Highly absorptive thin films for integrated photonic devices,” Master’s thesis, Massachusetts Institute of Technology, 2006. [↩]

ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells

• Authors: J. Jean, S. Chang, P. R. Brown, J. J. Cheng, P. H. Rekemeyer, M. G. Bawendi, S. Gradečak, V. Bulović
• Sponsorship: Samsung, NSF

Here we demonstrate a solution-processed ordered bulk heterojunction quantum dot (QD) solar cell with AM1.5G power conversion efficiencies of up to 4.9%, the highest reported performance for a QD solar cell based on ZnO^[1]. Solid-state QD photovoltaics (QDPVs) constitute the most recent addition to the NREL photovoltaic efficiency charts, with most reports focusing on an inverted heterojunction between a wide-bandgap n-type metal oxide (ZnO or TiO₂) and p-type lead sulfide (PbS) QDs. Despite rapid improvements in performance over the last several years, these devices remain limited by a narrow depletion width and hence a fundamental trade-off between light absorption and carrier collection. We find that vertically aligned arrays of ZnO nanowires can orthogonalize absorption and collection, as shown in Figure 1. The nanowires penetrate into the photoactive QD film and increase the effective depletion width, allowing efficient extraction of photogenerated charges from deep within the film. After optimizing the nanowire growth and device fabrication processes, we show that incorporating nanowires boosts both the photocurrent and the efficiency of planar QDPVs by over 35% (Figure 2).

The primary contribution of this work is the integration of bottom-up-grown 1-D ZnO nanostructures with PbS nanocrystals, the leading material system for QDPVs. While ZnO/PbSe^[2] and TiO₂/PbS^[3] bulk heterojunction devices have been demonstrated, we report the first ZnO/PbS bulk heterojunction QD solar cell and show that a bottom-up solution process can produce nanostructure alignment and device performance on par with that of top-down synthetic processes. The simple hydrothermal ZnO nanowire growth process^[4] is well characterized and compatible with a variety of substrate and electrode materials at low temperatures^[5]. Our work on scalable bottom-up processing of ZnO nanowire-based QD solar cells suggests that 1-D nanostructures may be the key to enhancing the efficiency and hence the economic viability of QDPVs.



Figure 1: (a) Schematic and (b) energy band diagram at short-circuit (cross-section along A-A’) of an ordered bulk heterojunction QDPV device. An array of ZnO nanowires decouples light absorption from carrier collection, extending the effective depletion width throughout a thick QD film.



Figure 2: J-V characteristics under AM1.5G illumination for representative planar (black) and nanowire-based (red) QDPV devices. Incorporating ZnO nanowires boosts the short-circuit current density (JSC) and the power conversion efficiency (PCE) by 50% and 35%, respectively. Black crosses indicate the maximum power point.




1. J. Jean et al., “ZnO nanowire arrays for enhanced photocurrent in PbS quantum dot solar cells,” Adv. Mater., vol. 25, no. 20, pp. 2790-2796, May 2013. [↩]
2. K. S. Leschkies et al., “Nanowire-quantum-dot solar cells and the influence of nanowire length on the charge collection efficiency,” Appl. Phys. Lett., vol. 95, no. 19, Nov. 2009. [↩]
3. I. J. Kramer et al., “Ordered nanopillar structured electrodes for depleted bulk heterojunction quantum dot solar cells,” Adv. Mater., vol. 24, no. 17, pp. 2315-2319, Mar. 2012. [↩]
4. L. E. Greene et al., “General route to vertical ZnO nanowire arrays using textured ZnO seeds,” Nano Lett., vol. 5, no. 7, pp. 1231-1236, June 2005. [↩]
5. H. Park et al., “Graphene cathode-based ZnO nanowire hybrid solar cells,” Nano Lett., vol. 13, no. 1, pp. 233-239, Dec. 2012. [↩]