MTL Annual Research Report 2011

Vladimir Bulovic

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

Collaborators

K. Aidala, Mt. Holyoke
A. Akinwande, MIT
N. Allam, Masdar Inst.
M. A. Baldo, MIT
M. Bawendi, MIT
T. Bloomstein, Lincoln Lab.
S. Gradečak, MIT
F. Jaworski, Raytheon
J. H. Lang, MIT
M. A. Schmidt, MIT
T. Swager, MIT

Postdoctoral Associates

T. Andrew
R. Costi
Y. Gu
K. Lo
R. Lunt
C. Packard
K. Stone
J. Summers
W. Tisdale
A. Wang
E. Young

Graduate Students

G. Akselrod, Physics
P. Brown, Physics
E. Flores, EECS
A. Iacchetti, Polytech. Milan
A. Murarka, EECS
T. Osedach, Harvard Univ.
S. Paydavosi, EECS
S. Ramanan, EECS
J. Rowehl, DMSE
Y. Shirasaki, EECS
G. Supran, DMSE
M. E. Woo, DMSE

Support Staff

M. Pegis, Admin. Asst.

SELECT Publications

(see onelab.mit.edu for full listing)

Barr, M.C., Rowehl, J.A., Lunt, R.R., Xu, J., Wang, A., Boyce, C.M., Im, S.G., V. Bulović, Gleason, K.K., “Direct monolithic integration of organic photovoltaic circuits on unmodified paper,” Advance Materials (2011).

Packard, C.E., Aidala, K.E., Samanan, S., V. Bulović, “Patterned removal of molecular organic films by diffusion,” Langmuir (2011).

Wood, V., Panzer, M.J., Bozyigit, D., Shirasaki, Y., Rousseau, I., Geyer, S., Bawendi, M.G., V. Bulović, “Electroluminescence from nanoscale materials via field-driven ionization,” Nano Letters (2011).

Brown, P.R., Lunt, R.R., Zhao, N., Osedach, T.P., Wanger, D.D., Chang, L-Y., Bawendi, M.G., V. Bulović, “Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using MoO₃ interfacial layer,” Nano Letters (2011).

Walker, B.J., Dorn, A., V. Bulović, Bawendi, M.G., “Color-selective photocurrent enhancement in coupled J-aggregate/nanowires formed in solution,” Nano Letters (2011).

Murarka, A., Packard, C., Yaul, F., Lang, J., V. Bulović, “Micro-contact printed MEMS,” IEEE 24th International Conference on Micro Electro Mechanical Systems, 292-295 (2011).

Young, E.R., Costi, R., Paydavosi, S., Nocera, D.G., V. Bulović, “Photo-assisted water oxidation with cobalt-based catalyst formed from thin-film cobalt metal on silicon photoanodes,” Energy & Environmental Science 4, 2058-2061 (2011).

Lunt, R.R., V. Bulović, “Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications,” Applied Physics Letters 98, 113305 (2011).

Nausieda, I., Ryu, K.K., Da He, D., Akinwande, A.I., Bulović, V., Sodini, C.G., “Mixed-Signal Organic Integrated Circuits in a Fully Photolithographic Dual Threshold Voltage Technology,” IEEE Transactions on Electron Devices 58, 865-873 (2011).

Ren, S.Q., Zhao, N., Crawford, S.C., Tambe, M., Bulović, V., Gradečak, S., “Heterojunction Photovoltaics Using GaAs Nano-wires and Conjugated Polymers,” Nano Letters 11, 408-413 (2011).

Paydavosi, S., Abdu, H., Supran,G. J., V. Bulović, “Performance Comparison of Different Organic Molecular Floating Gate Memories,” IEEE Transactions on Nanotechnology 10, 594-599 (2011).

Osedach, T.P., Zhao, N., Geyer, S.M., Chang, L.Y., Wagner, D.D., Arango, A.C., Bawendi, M.G., V. Bulović, “Interfacial Recombination for Fast Operation of a Planar Organic/QD Infrared Photodetector,” Advanced Materials 22, 5250-5254 (2010).

Akselrod, G.M., Tischler, Y.R., Young, E.R., Nocera, D.G., V. Bulović, “Exciton-exciton annihilation in organic polariton microcavities,” Physical Review B 82, 113106 (2010).

Bradley, M.S., V. Bulović, “Intracavity optical pumping of J-aggregate microcavity exciton polaritons,” Physical Review B 82, 033305 (2010).

Panzer, M.J., Aidala, K.E., Anikeeva, P.O., Halpert, J.E., Bawendi, M.G., V. Bulović, “Nanoscale Morphology Revealed at the Interface Between Colloidal Quantum Dots and Organic Semiconductor Films,” Nano Letters 10, 2421-2426 (2010).

Zhao, N., T.P. Osedach, L.-Y. Chang, S.M. Geyer, D. Wanger, M.T. Binda, A.C. Arango, M.G. Bawendi, V. Bulović, “Colloidal PbS Quantum Dot Solar Cells with High Fill Factor,” ACS Nano 4, 3743-3752 (2010).

Metal Oxide Transistors for Large Area Electronics

Authors: A. Wang, V. Bulovic, C. G. Sodini, A. I. Akinwande
Sponsorship: DARPA, SRC/FCRP C2S2

Optically transparent, wide band gap metal oxide semiconductors are a promising candidate for large area flexible electronics. Because most commercially available flexible substrates, particularly polymer substrates, cannot withstand the high temperature processing (>400°C) required for traditional silicon device fabrication, the development of new materials and devices that can be processed at low temperatures in a scalable manner is needed. Metal oxide semiconductors have been demonstrated to retain high carrier mobilities even in the disordered, amorphous state obtained when processed at near-room temperatures^[1]^[2]. Compared to amorphous silicon field effect transistors (FETs), which are the dominant technology used in display backplanes, metal-oxide-based FETs have been demonstrated with higher charge carrier mobilities, higher current densities, and faster response performance^[3]^[4].

It has been shown both in simulation and by experiment that FET threshold voltage (V_T) can be modified simply by changing the channel layer thickness, without requiring the additional complexity of multiple channel materials or different dopings. In this project we have developed a low temperature (~100°C), scalable lithographic process for top-gate, bottom-contact amorphous metal oxide-based FETs using parylene, a room temperature-deposited CVD polymer, as gate dielectric. Figure 1 shows a micrograph of an array of FETs fabricated with different channel lengths. The baseline process was extended to enable the integration of FETs with different threshold voltages on the same substrate. The availability of FETs with different threshold voltages enables the implementation of enhancement/depletion (E/D) logic circuits that have faster speeds and smaller device areas than single-V_T topologies. Using the two-V_T lithographic process, we fabricated and characterized integrated E/D inverters and ring oscillators that operate rail-to-rail at supply voltages as low as V_DD = 3V. An example inverter characteristic is plotted in Figure 2. These results demonstrate the potential for low V_DD metal oxide-based integrated circuits fabricated in a low temperature budget, fully lithographic process for large area electronics.

: Figure 1: An array of metal oxide FETs with varying channel lengths fabricated in a low-temperature-budget (~100°C) process. The lithographic process enables integration of these FETs into large-area electronic circuits.

: Figure 2: DC inverter transfer characteristic for an enhancement/ depletion inverter fabricated using the two-V_T metal oxide FET process. For both driver and load FETs, W/L = 100 μm/25 μm. At V_DD = 3V, inverter gain is >20 and noise margins are >1V on either side.

K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, pp. 488-492, Nov. 2004. [↩]
J. Robertson, “Disorder and instability processes in amorphous conducting oxides,” Physica Status Solidi B-Basic Solid State Physics, vol. 245, pp. 1026-1032, June 2008. [↩]
R. L. Hoffman, B. J. Norris, and J. F. Wager, “ZnO-based transparent thin-film transistors,” Applied Physics Letters, vol. 82, pp. 733-735, Feb. 2003. [↩]
E. Fortunato, P. Barquinha, G. Goncalves, L. Pereira, and R. Martins, “High mobility and low threshold voltage transparent thin film transistors based on amorphous indium zinc oxide semiconductors,” Solid-State Electronics, vol. 52, pp. 443-448, Mar. 2008. [↩]

Graphene Photovoltaics

Authors: H. Park, J. A. Rowehl, K. K. Kim, V. Bulovic, J. Kong
Sponsorship: Eni S.p.A., National Science Foundation

Organic photovoltaics (OPV) has gained much attention as a possible candidate for the next generation of clean electricity due to organic semiconductors’ high absorption coefficients, light weight and flexibility, and low-cost, high throughput fabrication methods^[1]^[2]. In optoelectronics devices, indium tin oxide (ITO) has been widely used as transparent conducting electrodes. However, the need for a substitute for ITO is ever increasing due to the limited availability of indium on earth; furthermore, device issues like susceptible ion diffusion into the organic films^[3] and mechanical brittleness^[4] limit the applicability of ITO in OPVs. Therefore, an ITO-substitute needs to be developed with these characteristics: low-cost, mechanically robust, transparent, electrically conductive, and ultimately capable of demonstrating performance comparable to ITO-based photovoltaics.

In the past, we have synthesized graphene sheets using Ni thin film (300 nm) as a catalyst layer via atmospheric pressure chemical vapor deposition (APCVD): either single to few-layer graphene sheets or multi-layer graphene sheets (>10 layers). The sheet resistance and optical transmittance obtained from the multi-layer graphene were around 500~1000Ω/sq and 75%, respectively. We further improved the synthesis conditions using copper foil (25 µm) as metal catalyst via low-pressure chemical vapor deposition (LPCVD). This method enabled us to synthesize large area, uniform monolayer graphene (>90%) with improved electrical conductivity (400~500Ω/sq) and optical transmittance (~97%). By transferring several times, we could further improve the quality of graphene electrodes (e.g., 3-layer graphene sheet: 300-400 Ω/sq with >90% transmittance) (Figure 1). We then successfully integrated these graphene sheets into the OPV with overall performance comparable, but slightly inferior, to ITO counterparts, possibly due to the relatively higher sheet resistance. Moreover, due to the hydrophobicity of graphene’s surface, uniform coverage of PEDOT:PSS layer was challenging, which was detrimental to device success rates. Various PEDOT:PSS alternatives were investigated, and it was found that AuCl₃ doping significantly improves the graphene OPV device performances, possibly due to the improved conductivity and the work function tuning of graphene electrodes as well as the PEDOT wettability (Figure 2).

: Figure 1: (a) Morphology of monolayer graphene transferred onto quartz substrate (AFM image). AFM scan size and height bar are 5×5 µm² and 10 nm, respectively. (b) Transmittance of graphene sheets of 1 to 3 layers. As-grown LPCVD-synthesized graphene films are mostly single- layered and each additional layer contributes approximately 2.3% opacity over the range of wavelength. Inset indicates the transmittance at 550 nm as a function of the number of the graphene layers.

: Figure 2: J-V characteristics of organic solar cells with different anodes under dark and simulated AM1.5G illumination at 100 mW/cm². Device performances of various types: (a) different PEDOT layer processing (b) ITO and (c) graphene electrodes are demonstrated. Shown in (d) is the comparison of performances of ITO with modified PEDOT:PSS by O₂ plasma and graphene doped with AuCl₃ (10mM).

C. W. Tang, “2-layer organic photovoltaic cell,” Applied Physics Letters, vol. 48, pp. 183-185, Jan. 1986. [↩]
P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693-3723, 2003. [↩]
A. R. Schlatmann, D. W. Floet, A. Hilberer, F. Garten, P. J. M. Smulders, T. M. Klapwijk, and G. Hadziioannou, “Indium contamination from the indium-tin-oxide electrode in polymer light-emitting diodes,” Applied Physics Letters, vol. 69, pp. 1764-1766, Sep. 1996. [↩]
Z. Chen, B. Cotterell, W. Wang, E. Guenther, and S. J. Chua, “A mechanical assessment of flexible optoelectronic devices,” Thin Solid Films, vol. 394, pp. 202-206, Aug. 2001. [↩]

Integration of Cobalt-based Catalyst with Silicon Photoanodes to Achieve Photo-assisted Water Oxidation

Authors: E. R. Young, R. Costi, S. Paydavosi, D. G. Nocera, V. Bulović
Sponsorship: National Science Foundation

Transformation of solar energy into chemical fuels is an attractive energy conversion transformation to address the intermittency of power generation in photovoltaics, which has been a long-standing challenge for development of solar energy^[1]. Stored solar fuels may then be used when sunlight does not reach the solar panels, enabling continuous, solar-generated energy availability. The fuel cycle addressed in this work is the splitting of water to molecular hydrogen and oxygen, which can be used as fuels or as high-energy precursors in fuel synthesis.

We utilize a cobalt-containing water oxidation catalyst (Co-Pi) that assists the water to oxygen reaction^[2]. Co-Pi, can be formed either via electrodeposition from Co²⁺ ions in aqueous solutions containing potassium phosphate (KPi) or by processing supported thin-film (800 nm thick) cobalt metal anodes in KPi at pH 7. Here, we use doped silicon wafers, Figure 1, as substrates for either electrodeposition of Co-Pi or processing of cobalt metal thin films into Co-Pi catalyst. Photoanode structures of ITO/Si/Co/Co-Pi, ITO/Si/ITO/Co-Pi and ITO/Si/ITO are prepared and tested for photo-assisted water oxidation activity.

Differences in water oxidation performance of each photoanode are particularly clear in Figure 2, which shows a comparison of steady-state current versus applied potential under dark and light conditions. The ITO/Si/ITO electrode passes very little current (10 μA/cm²) compared to the Co-Pi loaded electrodes and shows negligible change between light and dark conditions. The ITO/Si/ITO/Co-Pi electrode exhibits increasing dark current densities as voltage is applied and demonstrates additional increase under illumination (current offset between dark and light conditions reaches 200 μA/cm² at 1.35 V). The ITO/Si/Co/Co-Pi electrode achieves current densities higher than the ITO/Si/ITO/Co-Pi electrode in both dark and light conditions. The dark/light current offset over the catalytic regime of the electrode (between 0.85 V to 1.35 V) increases from 100 μA/cm2 (at 0.85 V) to 1 mA/cm² (at 1.35 V) and even higher. These results demonstrate the importance of material selection and device engineering for harnessing the solar spectrum towards catalytic chemical fuel production. The robust and abundant silicon electrode and direct processing of the cobalt metal produces an improved light-assisted water oxidation device.

: Figure 1: Schematic of the silicon P-N junction photovoltaic device used as a photoanode coated with the thin film of Co-Pi catalyst on the P-side and the ITO electrode on the N-side.

: Figure 2: The graph plots the steady-state current density at each applied potential. The ITO/Si/ITO electrode shows minimal activity compared to the Co-Pi loaded electrodes. Under illumination the ITO/Si/Co/Co-Pi electrode has significantly higher current densities than ITO/Si/ITO/Co-Pi, emphasizing that the judicious choice of materials and their sequencing on the photoanodic substrate are of critical importance.

T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, “Solar energy supply and storage for the legacy and nonlegacy worlds,” Chem. Rev. vol. 110, pp. 6474-6502, 2010. [↩]
M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science vol. 321, pp. 1072-1075, 2008. [↩]

Ultrahigh Resolution Fluorescence Microscope Utilizing a J-aggregate Critically Coupled Resonator

Authors: W. A. Tisdale, G. M. Akselrod, V. Bulović
Sponsorship: Institute for Soldier Nanotechnology

Figure 1: Schematic of the scanning near-field optical microscope utilizing FRET and a JCCR. (A) The sample is illuminated from the front of the JCCR through an objective lens. Fluorescence is collected through the same objective and a transparent non-interfering AFM probe tip is used. (B) Magnified cartoon of the tip-sample interaction in (A). The JCCR is frequency-matched to the absorption band of the analyte to be detected (a fluorescing small molecule, nanoparticles, or protein of interest). Light is absorbed in the J-aggregate thin film, and then its energy is transferred to the analyte. A fluorescence moiety is attached to the AFM probe tip. When the tip comes within the Förster radius of the analyte (typically 2-5 nm), fluorescence from the probe is detected in the far field.

The non-destructive chemical mapping of surfaces with single-molecule sensitivity and sub-5-nm lateral resolution is an extraordinary challenge. Optical techniques such as absorption, fluorescence, or Raman spectroscopy are powerful tools for chemical detection because of the unique optical signature that each molecule possesses. However, the maximum resolution of a classical (far-field) light microscope is limited by the fundamental law of diffraction. Typically, the resolving power of visible light cannot be better than ~200-300 nm in the lateral dimensions (x and y) and ~500-800 nm axially (z)^[1]. Unfortunately, almost all biomolecules and chemical agents of interest are smaller than this resolution limit. To overcome these classical limitations, we are developing an ultrabright near-field fluorescence microscope capable of single-molecule detection sensitivity, chemical specificity, and sub-5-nm lateral resolution. Ultrahigh spatial resolution is achieved by utilizing a strongly distance-dependent near-field optical interaction called Förster resonance energy transfer (FRET). FRET is the near-field transfer of energy from a photoexcited fluorophore (the “donor”) to another nearby fluorophore (the “acceptor”). By attaching the acceptor species to a scanning probe tip, donor fluorophores can be spatially located on the sample surface within the Förster radius of the interaction (typically 2-5 nm, see Figure 1). Because single molecule fluorescence signals are weak, we employ an optical structure referred to as a J-aggregate critically coupled resonator (JCCR) to enhance the brightness of the donor^[2]. The JCCR, which consists of a mirror, a λ/4 optical spacer, and a thin film (<15 nm thick) of J-aggregating dye molecules (see Figure 1), serves as a very general platform for efficient light capture and redistribution. Greater than 95% of incident light is absorbed within the thin J-aggregate film and then transferred to the donor species residing on the JCCR surface for subsequent FRET to the scanning probe tip.

L. Novotny, Principles of Nano-Optics. New York: Cambridge University Press, 2006. [↩]
J. R. Tischler, M. S. Bradley, and V. Bulović, “Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film” Optics Letters, vol. 31, pp. 2045-2047, July 2006. [↩]

Phosphor Sensitized Quantum-dot Light-emitting Diodes

Authors: G. J. Supran, Y. Shirasaki, V. Bulović
Sponsorship: DOE Center for Excitonics at MIT

Colloidal quantum-dot light-emitting diodes (QD-LEDs) combine the thin film processability of organic materials with the tunable optical properties conferred by QD size control. Archetypal QD-LED architectures have, to date, typically comprised a monolayer of QDs sandwiched between organic hole- and electron-transporting layers (HTL/ETL) (see Figure 1). Two possible QD excitation mechanisms have previously been proposed (Figure 1): (red) direct carrier injection from organic transport layers into QDs, followed by exciton generation and recombination; and (green) Förster resonant energy transfer (FRET) of an exciton – initially generated in an organic transport layer – to the QD monolayer, followed by exciton recombination. Previous work^[1] has demonstrated QD-LED systems in which the latter process appears to dominate. However, to date, all such architectures have comprised fluorescent small-molecule organics. Electrical excitation in the organic transport layer results in the creation of approximately one singlet exciton for every three triplet excitons^[2] yet the latter typically go to waste because the lack of triplet oscillator strength in fluorophores prevents energy transfer. The possibility of harnessing these triplet states to as much as quadruple the efficiency of QD excitation motivates our ongoing efforts to use a phosphorescent sensitizer as a FRET donor for QDs. Analogous phosphor sensitization reported by Baldo et al^[3] provides precedent for this work.

As a test bed for the study of phosphor sensitization, we have fabricated QD-LEDs (Figure 1) comprising monolayers of lead-sulfide (PbS) QDs (which yield near-infrared emission), obtained by micro-contact printing, previously reported by our group^[4]. Devices with and without phosphor dopants in the hole-transport layer were compared. An increase in external quantum efficiency (EQE) upon introduction of the phosphor sensitizer was observed (Figure 2), though its extent (~ 20 to 50 %) was relatively slight, hinting at the possibility that direct charge injection may also play a role in the excitation of narrow band-gap QDs. To this end, we are currently investigating devices with wider band-gap QDs and employing steady-state and transient photoluminescence and photoluminescence excitation measurements to select complementary donor/acceptor systems.

: Figure 1: Energy band-diagram of QD-LED, illustrating two possible QD excitation mechanisms: (i) (red) direct carrier injection; and (ii) (green) Förster resonant energy transfer.

: Figure 2: EQE of IR QD-LEDs both with and without a phosphor doped hole-transport layer.

P. O. Anikeeva, C. F. Madigan, J. E. Halpert, M. G. Bawendi, and V. Bulović, “Electronic and excitonic processes in light-emitting devices based on organic materials and colloidal quantum dots,” Physical Review B, vol. 78, pp. 085434-1 – 085424-8, Aug. 2008. [↩]
M. A. Baldo, D. F. O’Brien, M. E. Thompson, and S. R. Forrest, “The excitonic singlet-triplet ratio in a semiconducting organic thin film,” Phys. Rev. B, vol. 60, pp. 14422–14428, Nov. 1999. [↩]
M. A. Baldo, M. E. Thompson, and S. R. Forrest, “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer,” Nature, vol. 403, pp. 750-753, Feb. 2000. [↩]
L. Kim, P. O. Anikeeva, S. A. Coe-Sullivan, J. S. Steckel, M. G. Bawendi, and V. Bulović, “Contact printing of quantum dot light-emitting devices,” Nano Letters, vol. 8, pp. 4513-4517, Oct. 2008. [↩]

Exciton Delocalization and Disorder in J-aggregates

Authors: K. W. Stone, D. H. Arias, K. A. Nelson and V. Bulovic
Sponsorship: Department of Energy, EFRC Excitonics Center

J-aggregates, which are self-assembling molecular nanostructures^[1], have unique optical properties resulting from coherent coupling of their molecular components. For instance, J-aggregate films a few nanometers thick can strongly absorb light, making them interesting candidates for applications involving strong coupling in a microcavity^[2]. In a J-aggregate, the peak absorption wavelength is red-shifted and narrowed with respect to the peak absorption wavelength of its molecular components due to dipole-dipole coupling, which produces a collective excitation, called an exciton, that is delocalized over the nanostructure^[3]. Static and dynamic disorder in the aggregate affects the number of molecules over which the exciton is delocalized, therefore broadening the absorption line width. We use 2D Fourier transform optical spectroscopy to quantify the exciton size and determine its relationship to inhomogeneous (static) and homogeneous (dynamic) broadening.

Figure 1(a) shows a 2D correlation spectrum of a film of BIC^[4] J-aggregates deposited on a sapphire substrate and cooled to 6 K in a cryostat. The negative-going (blue) feature indicates bleaching of the exciton transition while the positive-going (red) feature indicates excited state absorption to a two-exciton state. The energy separation of the two features is related to the exciton size^[5], similar to the model for a quantum-mechanical “particle-in-a-box.” Figure 1(b) shows the dependence of the exciton size on its absorption energy. Figure 1(c) shows that the average exciton size given by the pump-probe spectrum (17 molecules) does not reflect the value of the energy-dependent delocalization since the pump-probe peaks are broadened due to the inhomogeneity of the film. As the temperature increases, the exciton size decreases, as Figure 2(a) shows, due to increased exciton-phonon scattering. The homogeneous line width also increases as the temperature increases, as in Figure 2(b), while the inhomogeneous line width remains constant, indicating that dynamic disorder influences the exciton size more strongly.

: Figure 1: (a) 2D correlation spectrum of BIC J-aggregates. The black lines indicate the minima and maxima of the spectral feature with respect to the exciton absorption energy. (b) Coherence length, N, as a function of exciton absorption energy. The coherence length of the lowest energy exciton is 48 molecules. (c) Projection of the 2D correlation spectrum to the exciton emission energy axis (equivalent to a pump-probe spectrum) yields an exciton coherence length of 17 molecules.

: Figure 2: The dependence of the exciton size (a) and homogeneous (squares) and inhomogeneous (asterisks) line widths (b) on temperature.

E. Jelley, “Spectral absorption and fluorescence of dyes in the molecular state,” Nature, vol. 138, pp. 1009-1010, Dec. 1936. [↩]
J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, and V. Bulovic, “Solid-state cavity QED: Strong coupling in organic thin films,” Organic Electronics, vol. 8, pp 94-113, Jan. 2007. [↩]
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The anionic J-aggregating molecule 5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-1,3-dihydro-3-(3-sulfopropyl)-2H-benzimidazol-2-ylidene]-1-propenyl]-1-ethyl-3(3-sulfopropyl)-1H-benzimidazolium, inner salt, sodium salt (CAS No. 28272-54-0) was dip-coated onto the substrate using a layer-by-layer deposition method along with the cationic polymer poly(diallyldimethylammonium chloride). The present sample had four bilayers of the cationic polymer and anionic dye. [↩]
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Resonant Cavity Effect on Colloidal Quantum-dot Light Emitting Diodes: An Experimental and Theoretical Study

Authors: Y. Shirasaki, V. Wood, Y. R. Tischler, G. J. Supran, V. Bulović
Sponsorship: Institute for Soldier Nanotechnologies, DOE Center for Excitonics at MIT

Colloidal quantum dots (QDs) are semiconductor nanoparticles that can be used in monolayers in low operating-voltage, non-lithographically fabricated, thin film LEDs exhibiting narrow-band emission^[1]. Applications such as optical communication, spectroscopy, and sensing could benefit greatly from thin film devices that offer near-monochromatic, directed, and intense emission. The goal of our project is to demonstrate the feasibility of such devices by integrating a QD-LED into a planar resonant cavity (RC), akin to previous reports of RC structures^[2]^[3], to enhance the intensity, directionality, and monochromaticity of the QD electroluminescence (EL).

The RC structure that we investigate (Figure 1) consists of a standard QD-LED^[4] grown on top of a distributed Bragg reflector (DBR). An 8-pair DBR with an average reflectivity of 98 % between wavelengths of 575 nm and 625 nm is used to investigate the RC effect on a QD-LED. With this structure, we achieve a narrowed emission spectrum, which is evident when comparing spectra and photographs of the QD-LED and the RC QD-LED EL (Figure 2). The full width half maximum of the EL spectrum is reduced from 40 nm to 8 nm upon addition of the cavity.

To corroborate our experimental findings and to optimize the design of an RC QD-LED, we simulated the effect of the RC on the EL of the QD film. We first calculate the enhancement or suppression of the electric field at the location of the QD film for different wavelengths and different angles and then use Fermi’s golden rule to determine the transition rates for each of these conditions. As shown in Figure 2, the simulation is in good agreement with the experimental result, suggesting that the effect we observe is indeed an RC effect.

: Figure 1: Schematic of RC QD-LED.

: Figure 2: Comparison of EL spectra and photographs of RC QD-LED and QD-LED. The RC QD-LED exhibits marked spectral narrowing compared to the QD-LED.

S. Coe, W. K. Woo, M. Bawendi, and V. Bulović, “Electroluminescence from single monolayers of nanocrystals in molecular organic devices,” Nature, vol. 420, pp. 800-803, Dec. 2002. [↩]
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C. Wilmsen, H. Temkin, and L. Coldren, Vertical Cavity Surface Emitting Laser, Cambridge: Cambridge University Press, 1999. [↩]

Integration of Small Organic Molecules in Flash Memory Devices

Authors: S. Paydavosi, K. Aidala, P. Brown, G. J. Supran, P. Hashemi, T. P. Osedach, J. L. Hoyt, V. Bulović
Sponsorship: SRC/FCRP MSD

As demands for high storage density, high chip memory capacity, and decreasing process costs continue to mount, conventional flash memory has found it challenging to continue scaling because of the minimum tunnel oxide thickness and poor charge retention due to defects in the tunneling oxide, necessitating modification in the implementation of the flash memory technology^[1]^[2].

Molecular organic materials exhibit fascinating electronic properties that motivate their hybridization with traditional silicon-based memory devices in order to continue memory scaling. A floating gate consisting of a thin film of molecules would provide the advantage of a uniform set of identical nanostructured charge storage elements with high molecular area densities (e.g., 8 × 10¹³ cm^-2, which can result in a several-fold higher density of charge-storage sites as compared to QD memory and even SONOS devices)^[3]^[4]. Additionally, the discrete charge storage in such nano-segmented floating gate designs limits the impact of any tunnel oxide defects to the charge stored in the proximity of the defect site.

In order to study the memory behavior of organic molecules, we inject electrons/holes into the molecules by applying negative/positive bias to a conductive atomic force microscopy (AFM) tip (Pt probe tip with 10-nm radius) in contact with the organic layer (Figure 1). During the charge injection phase, the tip is brought into contact with the sample surface by reducing the amplitude setpoint to 0.5 V. The stored charges within molecules can be detected from surface potential mapping of the sample by Kelvin force microscopy (KFM). Figure 1 shows the KFM image of the charged spots by applying 9V tip bias. The minimal temporal decay of injected charges and their corresponding lateral spreading indicate highly localized charge distribution, suggesting potential use of small organic molecules in multi level trap based molecular flash memory cells with high storage capacity. The calculated stored charge density within molecules is shown in Figure 2.

: Figure 1: The schematic of the prepared sample and KFM image of the written charge dots into a 5-nm-thick molecular layer on a 4-nm-thick oxide layer.

: Figure 2: Calculated stored charge density from the measured surface potential assuming uniform distribution of charge across the thickness of the trapping layer.

P. Pavan, R. Bez, P. Olivo, and E. Zanoni, “Flash memory cells-An overview,” Proc. IEEE., vol. 85, no. 8, pp. 1248-1271,1997. [↩]
International Technology Roadmap for Semiconductors, ITRS. (2007). [Online]. Available: http://www.itrs.net. [↩]
P. K. Singh, R. Hofmann, K. K. Singh, N. Krishna, and S. Mahapatra “Performance and reliability of Au and Pt single-layer metal nanocrystal flash memory under NAND (FN/FN) operation,” IEEE Trans. Electron Devices, vol. 56, no. 9, Sep. 2009. [↩]
S. Paydavosi, H. Abdu, G. J. Supran, V. Bulović, “Performance Comparison of Different Organic Molecular Floating Gate Memories,” IEEE Trans. Nanotechnology, vol. 10, no. 3, May 2011. [↩]

A Low-loss Squeezable Micro-Electro-Mechanical Switch

Authors: S. Paydavosi, F. Yaul, A. Murarka, T. L. Andrew, A. Wang, J. H. Lang, V. Bulović
Sponsorship: National Science Foundation

It has been known for several decades that polymers doped with conducting particles, for example silicone nickel nano-particles, will exhibit a dramatically decreasing resistivity as the polymer is compressed. It is possible that the conductivity will vary by 12 orders of magnitude over a 40% strain^[1]. Such composites conduct via tunneling from particle to particle, and the tunneling currents grow exponentially as the particles become closer together. These composites have already been used in applications from tactile sensors to fuses.

In this study we use the composites as the active element in an electronically-controlled switch. The Squeezable electronically controlled switch, referred to here as a “squitch,” is shown in Figure 1. In this embodiment, the squitch is a three-terminal device, with its terminals labeled as per the comparable terminals in a MOSFET. The central component of the device is the doped polymer labeled “Squitch Material” connected to drain and source electrodes. As fabricated, the squitch material would be a poor conductor, permitting little if any electron current to flow from the source to drain. That is, the resistance of this conduction path would be very large, putting the squitch in an off state. By applying voltage to the gate electrode, either positive or negative, an electric field is developed between the gate and the source. This electric field causes the gate to be attracted to the source, thereby compressing the squitch material. As the squitch material is compressed in the vertical direction, it begins to conduct, putting the squitch in an on state.

Thus, the squitch is a voltage controlled conductor, much the same as a FET or a BJT but with very large on-to-off conduction ratio and subthreshold swing (S) < 60 mV/dec, which allows for more aggressive supply voltage scaling and improvement in the energy efficiency.

: Figure 1: SEM image of nickel-silicone composite and cross section schematic of a three-terminal squitch. More than 7 orders of magnitude change in the conductivity of the composite was observed over a 20% strain.

: Figure 2: The initial demonstration of the squitch, showing 4 orders of magnitude change in the current by applying voltage to the gate terminal.

D. Bloor, K. Donnelly, P. J. Hands, P. Laughlin, and D. Lussey, “A metal–polymer composite with unusual properties,” J. of Phys. D: Appl. Phys., 38 (2005) 2851–2860. [↩]

Bias Stress Effect in PbS Quantum Dot Field Effect Transistors

Authors: T. P. Osedach, N. Zhao, T. Andrew, P. R. Brown, D. B. Strasfeld, D. D. Wanger, L.-Y. Chang, M. G. Bawendi, V. Bulović
Sponsorship: Institute for Soldier Nanotechnologies, CMSE

In recent years, there has been an increasing interest in the use of PbS and PbSe quantum dots (QDs) for optoelectronic device applications. Much remains poorly understood about charge transport in thin films of these materials, however. In several recent studies of lead chalcogenide QDs in field effect transistor (FET) structures, a fast bias stress effect, or shift in threshold voltage (ΔV_th) under prolonged gate bias, was observed^[1]^[2]^[3]^[4]. This effect manifests as a stretched-exponential decay in the drain-source current (I_ds) and is usually attributed to screening of the gate field by the trapped charge. The instability associated with bias stress precludes many practical applications of FETs and complicates the characterization of their performance. Moreover, the dynamics of the stressing process in QD-FETs have not been quantitatively characterized and uncertainty remains regarding the physical origin for the trapped charge, such as whether it accumulates at the QD/dielectric interface or within the QD film itself.

We investigated the bias stress effect in ethanedithiol (EDT)-treated PbS QD films in a top-gate bottom-contact field effect transistor (FET) configuration (see Figure 1). Devices exhibit ambipolar operation with typical mobilities on the order of μ_e = 4×10^-3 cm²V^-1s^-1and μ_h = 3×10^-4 cm²V^-1s^-1. A fast stress effect is observed in both n-channel and p-channel operation (Figure 2). Similar stress and recovery characteristics are observed in devices made with a variety of dielectrics, including parylene-C, PMMA and polystyrene, suggesting that the bias stress effect is a bulk phenomenon and not due to the semiconductor/dielectric interface. At room temperature, stress and recovery characteristics are found to be consistent with the presence of deep electron traps and shallow hole traps with densities on the order of at least 0.5 traps-per-QD. Study of bias stress at low temperature (T=78 K), however, reveals a freeze-out of the effect which cannot be simply explained by the presence of traps.

: Figure 1: Schematic of top-gate bottom-contact device structure.

: Figure 2: Drain-source current during stress and recovery for a typical device.

T. S. Mentzel, V. J. Porter, S. Geyer, K. MacLean, M. G. Bawendi, and M. A. Kastner, “Charge transport in PbSe nanocrystal arrays,” Physical Review Letters B, vol. 77, no. 7, pp. 075316, 2008. [↩]
M. Law, J. M. Luther, Q. Song, B. K. Hughes, C. L. Perkins, and A. J. Nozik, “Structural, optical, and electrical properties of PbSe nanocrystal solids treated thermally or with simple amines,” Journal of the American Chemical Society, vol. 130, no. 18, pp. 5974-5985, 2008. [↩]
J. M. Luther, M. Law, Q. Song, C. L. Perkins, M. C. Beard, and A. J. Nozik, “Structural, optical, and electrical properties of self-assembled films of PbSe nanocrystals treated with 1,2-ethanedithiol,” ACS Nano, vol. 2, no. 2, pp. 271-280, 2008. [↩]
Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H. W. Hillhouse, and M. Law, “Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids,” Nano Letters, vol. 10, no. 5, pp. 1960-1969, 2010. [↩]

Micro-contact Printed MEMS

Authors: A. Murarka, 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. We present a new method for fabricating thin (140-nm-thick) suspended metal films in MEMS using micro-contact printing. These films can be utilized in pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of MEMS sensors.

Our approach to MEMS fabrication involves the use of a stamp and a donor viscoelastic transfer pad that is coated with an organic release layer and a thin film of metal. The stamp consists of a layer of patterned polydimethylsiloxane (PDMS) atop a glass slide that is coated with a layer of electrically conducting indium tin oxide (ITO). The surface of this patterned PDMS stamp is placed in contact with the thin metal film on the donor transfer pad, and then the stamp is rapidly peeled away, picking up the metal film. The metal film ends up bridging the gaps in the patterns of the PDMS stamp, forming a capacitive MEMS structure. A continuous film of metal is lifted onto the stamp only if the stamp is peeled off the transfer pad rapidly.

This process avoids the use of solvents and etchants, eliminating the need for deep reactive-ion etching and other harsh chemical treatments. Solvent absence during fabrication also avoids the detrimental effects of MEMS stiction that can result during wet processing. MEMS fabrication on flexible polymeric substrates is also possible due to the absence of elevated temperature processing.

Thin films up to 0.78 mm² in area have been fabricated using the aforementioned process, as shown in Figure 1. These MEMS devices are actuated electrostatically to demonstrate the deflection of 25-μm-diameter films (see Figure 2).

: Figure 1: An optical microscopy image of the pressure sensor fabricated using the contact lift-off mesa-transfer process. The gold parallelogram is the pressure sensor. About 1024 circular cavities are covered by this parallelogram gold membrane, which is 140 nm thick. The underlying PDMS on ITO-Glass is patterned with 25-µm- diameter circular cavities with about 5-µm to 7-µm spacing between the circles.

: Figure 2: Three capacitance-voltage sweeps for a single device. The increase in the capacitance of the device with voltage indirectly demonstrates deflection of the gold membrane, which bows under the electrostatic force to decrease the gap spacing over the air cavities in the PDMS spacer layer, thus increasing the capacitance.

A. Murarka, C. Packard, F. Yaul, J. Lang, and V. Bulovic, “Micro-contact printed MEMS,” in 2011 IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), pp. 292-295, 23-27 Jan. 2011.
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.

Transparent Organic Photovoltaics for Window Applications

Authors: R. Lunt, V. Bulovic
Sponsorship: EFRC Center for Excitonics, Department of Energy

The low energy density of solar illumination necessitates deployment of solar technologies over large surface areas in order to capture enough of the sun’s energy to offset a significant portion of non-renewable energy consumption. The obstacle of large-area deployment could be overcome with development of a low-cost, transparent, photovoltaic (PV) technology that can be integrated onto window panes in homes, skyscrapers, and automobiles, enhancing the functionality of already utilized transparent surfaces. Since glass, structural framing, and inverters comprise nearly 40% of thin-film PV module prices^[1], integrating solar cells into windows can also reduce effective installation costs. Presently, window glass used in automobiles and in architectural installations^[2] requires 70-80% and 55-90% transmission to visible light, respectively. Previous efforts to construct semitransparent devices have focused on the use of thin active layers (or spatially segmented films) with light absorption focused in the visible spectrum and therefore have been limited to either low efficiencies or low average visible-light transmissivity since both parameters cannot be simultaneously optimized.

We demonstrate an additive transparent organic PV technology (see Figure 1a) that can similarly retain the glass transparency, where the non-transmitted light in the near-infrared is utilized for power generation. To do this, we exploit the excitonic character of organic and molecular semiconductors to produce photovoltaic architectures with structured absorption, i.e., exhibiting minima and maxima that are uniquely distinct from the band-absorption of their inorganic counterparts. Combining these PVs with selective high-reflectivity near-infrared mirrors, we demonstrate a prototype transparent device with a power conversion efficiency of several percent (Figure 1b) while also permitting more than 60% transmission of visible light through the entire device^[3]. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting (see Figure 2), making them highly useful for distributed and point-of-source utilization of solar energy.

: Figure 1: (a) Schematic of the transparent organic photovoltaic architecture. (b) Measured power efficiency (η_p) and average visible transparency (AVT) of the devices as a function of the cathode thickness.

: Figure 2: (a-b) Picture of the “Teton mountains” displayed on an LCD screen with the fully assembled transparent solar cell in front of the picture where the cell design is shown in (a). (c) Picture of the series-integrated transparent OPV powering an LCD clock.

K. Zweibel, “The terawatt challenge for thin film photovoltaics,” Thin Film Solar Cells, Hoboken, NJ, John Wiley and Sons Ltd., 2006, pp. 427-459. [↩]
C. Tuchinda, S. Srivannaboon, and H. W. Lim, “Photoprotection by window glass, automobile glass, and sunglasses,” Journal of the American Academy of Dermatology, vol. 54, pp. 845-854, 2006. [↩]
R. R. Lunt and V. Bulovic, “Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications,” Applied Physics Letters, vol. 98, pp. 113305:1-3, 2011. [↩]

Paper-thin and Paper-based Organic Photovoltaics

Authors: R. Lunt, M. C. Barr, J. Rowehl, K. K. Gleason, V. Bulović
Sponsorship: ENI, MIT Energy Initiative

There is emerging interest in the ability to produce low-cost and lightweight solar cells and other electronics on flexible, stretchable, and foldable substrates. Rigid glass or silicon substrates in the current designs represent a large fraction of the overall module cost^[1] and also restrict how and where modules can be deployed. Thus, a shift to the design of modules specifically on low-cost substrates could open untapped locations for solar deployment, including formats that are ubiquitous in our society (e.g., textiles, window curtains, printed paper documents, and wall paper). For example, photovoltaic (PV) devices fabricated directly on common fiber-based paper substrates are also foldable and rollable for storage and portability, easily shaped for three-dimensional applications, and able to be stapled to the roof structures or glued onto walls. To this end, there is significant interest in integrating various electronics to low-cost paper substrates, including transistors, storage devices, and displays^[2]^[3]. In our work, we examine the use of oxidative chemical vapor deposition (oCVD)^[4]^[5] in conjunction with organic photovoltaics to fabricate PV cells^[6] directly on fiber-based and paper substrates that are both flexible and foldable^[7]. For example, in Figure 1 we show a paper PV cell that was folded into a paper airplane (see corresponding video^[8]). Furthermore, we have designed paper-based, monolithically series-integrated arrays that are capable of powering common electronics such as small-format LCD displays under ambient light as shown in Figure 2, with shelf lifetimes greater than several months. These demonstrations allow us to rethink how and where lightweight and potentially low-cost photovoltaics can be deployed.

: Figure 1: A paper airplane with integrated photovoltaic wings, folded from a ~50-cm² sheet of tracing paper that was first patterned with the oCVD-based device structure.

: Figure 2: Monolithically series-integrated, lamination-encapsulated paper PV array that is capable of powering an LCD clock under ambient lighting conditions.

K. Zweibel, “The terawatt challenge for thin film photovoltaics,” Thin Film Solar Cells, Hoboken, NJ, John Wiley and Sons Ltd., 2006, pp. 427-459. [↩]
U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, and H. Klauk, “Organic electronics on banknotes,” Advanced Materials, vol. 23, pp. 654-658, 2011. [↩]
D. Tobjörk and R. Österbacka, “Paper electronics,” Advanced Materials, vol. 23, pp. 1935-1961, 2011. [↩]
M. E. Alf, A. Asatekin, M. C. Barr, S. H. Baxamusa, H. Chelawat, G. Ozaydin-Ince, C. D. Petruczok, R. Sreenivasan, W. E. Tenhaeff, N. J. Trujillo, S. Vaddiraju, J. J. Xu, and K. K. Gleason, “Chemical vapor deposition of conformal, functional, and responsive polymer films,” Advanced Materials, vol. 22, pp. 1993-2027, 2010. [↩]
S. H. Baxamusa, S. G. Im, and K. K. Gleason, “Initiated and oxidative chemical vapor deposition: a scalable method for conformal and functional polymer films on real substrates,” Physical Chemistry Chemical Physics, vol. 11, pp. 5227-5240, 2009. [↩]
C. W. Tang, “2-Layer Organic Photovoltaic Cell,” Applied Physics Letters, vol. 48, pp. 183-185, 1986. [↩]
M. C. Barr, J. A. Rowehl, R. R. Lunt, J. J. Xu, A. Wang, C. M. Boyce, S. G. Im, V. Bulovic, and K. K. Gleason, “Paper-thin, organic photovoltaic circuits fabricated directly on ubiquitous, everyday substrates,” Advanced Materials, accepted for publication. [↩]
Available: http://web.mit.edu/newsoffice/component/mitmultimedia/?type=video&videoid=233 [↩]

Improved Current Extraction from Colloidal Quantum Dot Photovoltaics Using Metal Oxide Transport Layers

Authors: P. R. Brown, N. Zhao, R. R. Lunt, T. P. Osedach, M. G. Bawendi, V. Bulović
Sponsorship: Department of Energy

Solar cells based on lead sulfide colloidal quantum dots (PbS QDs) have demonstrated recent improvements in both efficiency and stability^[1]. With bandgaps tunable from 0.5 eV to 1.7 eV, PbS QDs offer infrared spectral responsivity surpassing that of organic materials and provide a potentially facile route to multijunction architectures. Because of their high elemental abundance and amenability to ambient-atmosphere solution-phase deposition and operation, PbS QDs represent a promising candidate for expansion into large-area production if further efficiency enhancements may be realized. Recent studies on ZnO/PbS np-heterojunction solar cells have identified the presence of a reverse-bias Schottky barrier at the top PbS/anode contact as a primary loss mechanism in PbS QD photovoltaics (Figure 1b)^[2]. This barrier inhibits the extraction of photogenerated holes from the p-type PbS active layer, leading to reductions in the open circuit voltage (V_OC), fill factor (FF), and efficiency.

Our research demonstrates that this Schottky barrier may be attenuated through the incorporation of a molybdenum oxide (MoO₃) interlayer between the PbS QDs and the top-contacted anode (Figure 1c)^[3]. The high-work-function MoO₃ layer pins the Fermi level of the top contact, resulting in a high V_OC (0.59 ± 0.01 V) regardless of the anode work function. Corresponding increases in short-circuit current (J_SC) and FF enable 1.5-fold, 2.3-fold, and 4.5-fold enhancements in photovoltaic device efficiency for Au, Ag, and ITO anodes, respectively (Figure 2), and result in a power conversion efficiency of 3.5 ± 0.4 % for a device employing a gold anode. The effective decoupling of device performance from the identity of the anode material allows for greater flexibility in the design of the photovoltaic device architecture and, by enabling the use of transparent electrodes without loss in V_OC, represents a step toward demonstration of stacked multijunction quantum dot photovoltaics.

: Figure 1: (a) Flat-band energy alignment of the ZnO/PbS heterojunction photovoltaic device. Illumination under normal operation is incident from the ZnO side. (b) Proposed band-bending in the MoO₃-free device, showing the depleted np-heterojunction at the ZnO/PbS interface and a hole-current-limiting Schottky contact at the PbS/anode interface. (c) The removal of the Schottky barrier following the insertion of MoO₃ between PbS and the anode material.

: Figure 2: J-V characteristics under illumination of ITO/ZnO/PbS/MoO₃/anode devices with varying thicknesses of MoO₃, where the anode is Ag (blue circles), Au (red squares), or ITO (green triangles). The roll-over in light current observed at +0.5 V for the device employing an ITO anode without MoO₃ is ascribed to the presence of a Schottky junction at the anode contact. The addition of MoO₃ alleviates this roll-over and simultaneously increases the J_SC, V_OC, FF, and efficiency.

J. Tang and E. H. Sargent, “Infrared colloidal quantum dots for photovoltaics: Fundamentals and recent progress,” Advanced Materials, vol. 23, pp. 12-29, Sep. 2010. [↩]
J. Gao, J. M. Luther, O. E. Semonin, R. J. Ellingson, A. J. Nozik, and M. C. Beard, “Quantum dot size dependent J-V characteristics in heterojunction ZnO/PbS quantum dot solar cells,” Nano Letters, vol. 11, pp. 1002-1008, Feb. 2011. [↩]
P. R. Brown, R. R. Lunt, N. Zhao, T. P. Osedach, D. D. Wanger, L.-Y. Chang, M. G. Bawendi, and V. Bulovic, “Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer,” Nano Letters, vol. 11, pp. 2955-2961, June 2011. [↩]

Ternary Mixtures for Improved Performance in P3HT/PCBM Bulk Heterojunction Solar Cells

Authors: T. L. Andrew, V. Bulović
Sponsorship: Eni

Photovoltaic cells containing nanoscale phase separated mixtures of a poly(thiophene) donor and a fullerene acceptor in the active layer have proven to be a attractive class of low-cost solar energy harvesting devices. The benchmark polymer solar cell (PSC) is one fabricated with poly(3-hexylthiophene) (P3HT) and PC_nBM (n = 61, 71, Figure 1), which displays power conversion efficiencies of approximately 5%^[1]. Extensive optimization of the polymeric donor component has lead to the evolution of PSCs with power conversion efficiencies of approximately 8%^[2]; however, soluble fullerene derivatives remain the primary choice for the donor component in bulk heterojunctions. We have demonstrated that select small-molecule acceptors can serve as fullerene substitutes in P3HT/PC₆₁BM bulk heterojunctions and can increase the power conversion efficiencies of the resulting solar cells (Figure 2). Ternary mixtures containing 6,6-dicyanofulvenes^[3], such as DCF (Figure 1), yield average power conversion efficiencies of 4%. Moreover, ternary mixtures containing stable organic radicals, such as TEMPO and DPPH, also augment the performance of P3HT–PC₆₁BM solar cells and result in power conversion efficiencies of up to 3.4%.

: Figure 1: Structures of PC₆₁BM, a soluble fullerene acceptor, and three substitutes for fullerene-based acceptors in polymer solar cells.

: Figure 2: J-V characteristics of photovoltaic cells fabricated with ternary mixtures containing P3HT; 50 wt% PC₆₁BM; and 50 wt% of either DCF, TEMPO, or DPPH.

G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-Fullerene Bulk-Heterojunction Solar Cells” Adv. Mater., vol. 21, pp. 1323-1338, Nov. 2009. [↩]
H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, and G. Li, “Polymer solar cells with enhanced open-circuit voltage and efficiency” Nature Photon., vol. 3, pp. 649-653, June 2009. [↩]
T. L. Andrew, J. R. Cox, and T. M. Swager, “Synthesis, Reactivity, and Electronic Properties of 6,6-Dicyanofulvenes” Org. Lett. vol 12, pp. 5302-5305, Sept. 2010. [↩]

Polariton Lasing by Intra-cavity Pumping and Applications to Ultra-fast Switching

Authors: G. M. Akselrod, E. R. Young, M. S. Bradley, V. Bulovic
Sponsorship: Department of Energy Center for Excitonics, Hertz Foundation, National Science Foundation

Figure 1: (a) Non-resonant pumping scheme that is impeded by exciton annihilation, preventing polariton build up in the well. (b) Intra-cavity pumping scheme that populates polaritons along the entire dispersion curve. (c) Device structure. (d) Spectra showing the overlap of the emission of the DCM pump with the absorption of the strongly coupled J-aggregate.

The strongly coupled states of light and matter in microcavities known as polaritons offer exciting possibilities for the study of condensation, superfluidity, and other condensed matter phenomena and show promise as a radically new class of optoelectronic devices based on the macroscopic coherence of light and matter^[1]. In particular, organic materials allow for strong coupling and polariton lasing to be achieved at room temperature and with substantially reduced requirements for cavity quality factor^[2]^[3]. In our work, we demonstrate room temperature polariton lasing in a lambda-thick microcavity where a highly absorbing thin film of J-aggregates serves as the strong coupling material (Figure 1c). Typically, polaritons in the cavity are created by excitation with non-resonant laser pulses. Subsequently, the hot excitons couple to the cavity and move along the polariton dispersion towards the bottom of the polariton trap (Figure 1a). However, a loss mechanism known as annihilation is present in organic materials which prevents a high density of polaritons from being formed in the trap, and hence polariton condensation (lasing) cannot be achieved. We employ a new pumping scheme known as intra-cavity pumping that circumvents annihilation losses inherent to organic materials at high excitation densities (Figure 1b) by pumping the polariton mode from within the cavity using another organic material (the laser dye DCM) (Figure 1c and 1d). Using this flexible cavity architecture, polariton lasing at room temperature has been achieved. Current work focuses on the application of the polariton laser as a low-energy, ultra-fast all-optical switch and on a better understanding of the properties of the polariton condensate.

H. Deng and Y. Yamamoto, “Exciton-polariton Bose-Einstein condensation,” Reviews of Modern Physics, vol. 82, pp. 1489-1537, May 2010. [↩]
S. Kéna-Cohen and S. R. Forrest, “Room-temperature polariton lasing in an organic single-crystal microcavity,” Nature Photonics, vol. 4, pp. 371-375, Apr. 2010. [↩]
J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, and V. Bulovic, “Solid state cavity QED: Strong coupling in organic thin films,” Organic Electronics, vol. 8, pp. 94-113, 2007. [↩]

20-fold Enhancement of Molecular Fluorescence by Coupling to a J-aggregate Critically Coupled Resonator

Authors: G. M. Akselrod, B. J. Walker, W. A. Tisdale, M. Bawendi, V. Bulovic
Sponsorship: Department of Energy Center for Excitonics, Hertz Foundation, National Science Foundation

Figure 1: (a) Enhancement structure schematic. (b) Absorption and emission spectra of materials in the system. (c) Absorption enhancement of J-aggregates due to the critically coupled resonator. (d) Spectrum of DCM emission for control sample and when coupled to JCCR. (e) Photograph of sample under ambient light. (f-g) Photographs of control DCM sample and DCM on JCCR.

The efficient absorption and emission of light is of fundamental importance to the operation of optoelectronic devices such as solid-state lighting, lasers, and photodetectors. The properties that determine the brightness of molecules and other luminescent species—absorption cross-section and emission quantum yield – are fundamental to the material and cannot be readily changed. It is thus highly desirable to have a device or general strategy for decoupling the absorption and emission properties of a luminescent thin film, so that the brightness of a lumiphore can be increased without changing its spectral emission properties or increasing the intensity of light for photoexcitation. Towards this end, work over the past decade has focused on the enhancement of fluorescence by coupling luminophores to metal nanoparticles, such as gold^[1].

Here we propose an alternative, purely exciton-based, approach to the enhancement of luminophore emission by coupling the luminophore to a highly absorbing resonant optical structure. The enhancement structure is based on a thin film of strongly absorbing J-aggregates placed at the anti-node of the electric field at a distance λ/4 away from a mirror, where λ is the wavelength of incident light. This placement results in destructive interference between the light reflected from the J-aggregate film and light that is reflected by the mirror and passed by the film. The structure (Figure 1a), here referred to as a J-aggregate critically coupled resonator (JCCR), can absorb up to 97% of the incident light^[2]. The optical energy, localized in the form of excitons in the J-aggregate film, can then be coupled to luminophores deposited on the JCCR by Förster resonant energy transfer (FRET). As a result, the JCCR acts as a platform for dramatically enhancing the optical absorption cross-section of the target luminophore, leading to a subsequent enhancement in the emission. Such an architecture forms a highly general platform for fluorescence enhancement. A wide range of luminophores, both organic and inorganic, with emission spanning the visible and near IR, can be enhanced by simply choosing the appropriate J-aggregate material. In this work, we show a dramatic 20-fold enhancement in the emission of the laser dye DCM by utilizing the JCCR enhancement structure.

J.-H. Song, T. Atay, S. Shi, H. Urabe, and A. V. Nurmikko, “Large enhancement of fluorescence efficiency from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons,” Nano Letters, vol. 5, pp. 1557-61, Aug. 2005. [↩]
J. R. Tischler, M. S. Bradley, and V. Bulovic, “Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film,” Optics Letters, vol. 31, pp. 2045-2047, 2006 [↩]