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Advanced Functional Materials 03/2012; 22(5):1087-1091. · 10.18 Impact Factor
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Advanced Functional Materials 05/2011; 21(13):2580 - 2586. · 10.18 Impact Factor
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ABSTRACT: First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns(-1)) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.
The Journal of Physical Chemistry A 03/2011; 115(12):2515-22. · 2.95 Impact Factor
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ABSTRACT: We report on the effects of replacing both In <sub>2</sub> O <sub>3</sub>: Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA / cm <sup>2</sup> . Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.
Applied Physics Letters 07/2010; · 3.84 Impact Factor
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ABSTRACT: Disordered nanohole arrays were formed in silver films by colloidal lithography techniques and characterized for their surface-plasmon activity. Careful control of the reagent concentration, deposition solution ionic strength, and assembly time allowed generation of a wide variety of nanohole densities. The fractional coverage of the nanospheres across the surface was varied from 0.05-0.36. Electrical sheet resistance measurements as a function of nanohole coverage fit well to percolation theory indicating that the electrical behavior of the films is determined by bulk silver characteristics. The transmission and reflection spectra were measured as a function of coverage and the results indicate that the optical behavior of the films is dominated by surface plasmon phenomena. Angle-resolved transmission and reflection spectra were measured, yielding insight into the nature of the excitations taking place on the metal films. The tunability of the colloidal lithography assembly method holds much promise as a means to generate customized transparent electrodes with high surface plasmon activity throughout the visible and NIR spectrum over large surface areas.
ACS Nano 02/2010; 4(2):615-24. · 10.77 Impact Factor
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ABSTRACT: In organic solar cells, the efficiency of the exciton transport and dissociation across donor-acceptor (D/A) interfaces is controlled by the nanoscale distribution of the donor and acceptor phases. The observation of photoluminescence quenching is often used as confirmation for efficient exciton dissociation but provides no information on the nanoscopic nature of the exciton transport. Here we demonstrate nanoscale imaging of the exciton transport in films consisting of the conjugated polymer poly(3-hexylthiophene) (P3HT, electron donor) blended with the C60 derivative 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM, electron acceptor) by a tunneling luminescence spectroscopy based on atomic force microscopy. The excitonic luminescence is significantly enhanced when the conjugated polymer is coupled to the plasmon excitation at the tip (tip-enhanced luminescence). This effect allows one to dramatically improve the detection efficiency of the excitonic luminescence and, consequently, resolve individual domains of the conjugated polymer in which the exciton will recombine before dissociation at the D/A interface. Under thermal annealing conditions promoting the segregation of the donor and acceptor phases, a clear increase of the luminescence is seen from polymer-rich regions, consistent with domains of dimensions much larger than the exciton diffusion length. The described scanning luminescence microscopy can thus be applied to the optimization of the blends used in solar cells.
Nano Letters 09/2009; 9(11):3904-8. · 13.20 Impact Factor
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ABSTRACT: Pentacene, a model organic semiconductor, is shown to couple with surface plasmon (SP) active silver nanohole films to produce enhanced excited-state absorption. In addition, the dynamics of triplet formation and decay on a subpicosecond time scale are altered due to the coupling of the excited state with the resonant SP, possibly involving the interplay between singlet fission and triplet−triplet annihilation. Shifting the resonance of the SP with respect to the pentacene excitations and introducing a dielectric spacer between pentacene and metal lead to changes in the spectra and dynamics that can be explained qualitatively. These results are compared with recent literature reports of molecule/plasmon hybridization and are placed in context with efforts to utilize SPs for enhanced solar energy conversion.
04/2009;
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Physical Review B (Condensed Matter and Materials Physics). 01/2009; 80:115432--5.
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ABSTRACT: We present a comprehensive study of the effects of doping and temperature on the conductivity of single-walled carbon nanotube (SWNT) networks. We investigated nearly type-pure networks as well as networks comprising precisely tuned mixtures of metallic and semiconducting tubes. Networks were studied in their as-produced state and after treatments with nitric acid, thionyl chloride, and hydrazine to explore the effects of both intentional and adventitious doping. For intentionally and adventitiously doped networks, the sheet resistance (R(s)) exhibits an irreversible increase with temperature above approximately 350 K. Dopant desorption is shown to be the main cause of this increase and the observed hysteresis in the temperature-dependent resistivity. Both thermal and chemical dedoping produced networks free of hysteresis. Temperature-programmed desorption data showed that dopants are most strongly bound to the metallic tubes and that networks consisting of metallic tubes exhibit the best thermal stability. At temperatures below the dopant desorption threshold, conductivity in the networks is primarily controlled by thermally assisted tunneling through barriers at the intertube or interbundle junctions.
ACS Nano 10/2008; 2(9):1968-76. · 10.77 Impact Factor
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ABSTRACT: Random silver nanohole films were created through colloidal lithography techniques and metal vapor deposition. The transparent electrodes were characterized by uv-visible spectroscopy and incorporated into an organic solar cell. The test cells were evaluated for solar power-conversion efficiency and incident photon-to-current conversion efficiency. The incident photon-to-current conversion efficiency spectra displayed evidence that a nanohole film with 92 nm diameter holes induces surface-plasmon-enhanced photoconversion. The nanohole silver films demonstrate a promising route to removing the indium tin oxide transparent electrode that is ubiquitous in organic optoelectronics.
Applied Physics Letters 06/2008; 92(24):243304-243304-3. · 3.84 Impact Factor
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ABSTRACT: Plasmon-active silver nanoparticle layers were included in solution-processed bulk-heterojunction solar cells. Nanoparticle layers were fabricated using vapor-phase deposition on indium tin oxide electrodes. Owing to the increase in optical electrical field inside the photoactive layer, the inclusion of such particle films lead to increased optical absorption and consequently increased photoconversion at solar-conversion relevant wavelengths. The resulting solar energy conversion efficiency for a bulk heterojunction photovoltaic device of poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester was found to increase from 1.3%±0.2% to 2.2%±0.1% for devices employing thin plasmon-active layers. Based on six measurements, the improvement factor of 1.7 was demonstrated to be statistically significant.
Applied Physics Letters 01/2008; 92(1):013504-013504-3. · 3.84 Impact Factor
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ABSTRACT: Thin films of single-wall carbon nanotubes were used as the transparent top electrical contact in Cu(In,Ga)Se2- based solar cells. Specifically, we demonstrate that thin layers of carbon nanotubes in combination with insulating polymer layers can effectively replace the metal oxide layers typically used in polycrystalline thin-film solar cells. Replacing the standard n-type ZnO layer with a thin film of carbon nanotubes yielded energy conversion efficiencies up to 13%. The optical and electrical transport properties of the single-wall carbon nanotubes suggest that suitable applications for these materials include multiple-junction solar cells, thermophotovoltaics, and other applications benefiting from a p-type transparent conductor with high near-infrared transmission.
09/2007;
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ABSTRACT: The authors investigate the localization of photons emitted at the tip during scanning tunneling microscopy measurements on atomically flat gold substrates. Emission patterns of the plasmon-mediated luminescence exhibit distinct features that are assigned to the localized modes of the surface plasmon (LSP) confined to the tunneling gap and propagating modes (PSP) coupled to the LSP by the optical cavity beneath the tip. Tunneling luminescence spectroscopy reveals that the plasmon localization at the tip increases when modes of higher energy are excited. Acquisition of local emission patterns allows us for the simultaneous imaging of LSP and PSP modes.
Applied Physics Letters 05/2007; 90(19):193109-193109-3. · 3.84 Impact Factor
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ABSTRACT: In scanning tunneling microscopy (STM), confinement of surface plasmons to the optical cavity formed at the metallic tunneling gap stimulates the emission of light. We demonstrate that quantum dots (QDs) found in such a cavity give rise to discrete, observable transitions in the tunneling luminescence spectrum due to the resonant extinction of the plasmon. The observed resonances represent a fingerprint of the QD and occur at the optical band gap owing to the nearly simultaneous transfer of carriers from both sides of the tunneling gap to the QD. The resonant quenching of surface plasmons enables a new imaging technique, dubbed plasmon resonance imaging, with a spatial resolution potentially similar to that of STM and the energy resolution of optical spectroscopies. This detection and imaging strategy is not restricted to QDs, being of great interest to an entire spectrum of nanostructures, from molecular assemblies and biomolecules to carbon nanotubes.
Nano Letters 01/2007; 6(12):2833-7. · 13.20 Impact Factor
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ABSTRACT: The dependence of the electron transport and recombination dynamics on the internal surface area of mesoporous nanocrystalline TiO2 films in dye-sensitized solar cells was investigated. The internal surface area was varied by altering the average particle size in the films. The scaling of the photoelectron density and the electron diffusion coefficient at short circuit with internal surface area confirms the results of a recent study (Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 202106) that transport-limiting traps are located predominately on the surfaces of the particles. The recombination current density was found to increase superlinearly (with an exponent of 1.40 +/- 0.12) with the internal surface area. This result is at odds with the expected linear dependence of the recombination current density on the surface area when only the film thickness is increased. The observed scaling of the recombination current density with surface area is consistent with recombination being transport-limited. Evidence is also presented confirming that photoinjected electrons recombine with redox species in the electrolyte via surface states rather than from the TiO2 conduction band.
The Journal of Physical Chemistry B 01/2007; 110(50):25174-80. · 3.70 Impact Factor
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ABSTRACT: Bulk heterojunction organic photovoltaic devices have been fabricated by blending phenyl-cored thiophene dendrimers with a fullerene derivative. A power conversion efficiency of 1.3% under simulated AM1.5 illumination is obtained for a four-arm dendrimer, despite its large optical band gap of 2.1 eV. The devices exhibit an increase in short-circuit current and power conversion efficiency as the length of the arm is increased. The fill factors of the devices studied are characteristically low, which is attributed to overly uniform mixing of the blend.
Applied Physics Letters 09/2006; 89(10):103524-103524-3. · 3.84 Impact Factor
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ABSTRACT: We present a new thin-film solar cell structure in which the traditional transparent conductive oxide electrode (ZnO) is replaced by a transparent conductive coating consisting of a network of bundled single-wall carbon nanotubes. Optical transmission properties of these coatings are presented in relation to their electrical properties (sheet resistance), along with preliminary solar cell results from devices made using CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> thin-film absorber materials. Achieving an energy conversion efficiency of >12% and a quantum efficiency of ~80% demonstrate the feasibility of the concept. A discussion of the device structures will be presented considering the physical properties of the new electrodes comparing current-voltage results from the new solar cell structure and those from standard ZnO/CdS/Cu(In,Ga)Se<sub>2</sub>/Mo solar cells
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: The mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, alters the open-circuit photovoltage and short-circuit current of dye-sensitized solar cells was investigated. The influence of tetrabutylammonium chenodeoxycholate on dye loading was studied under a variety of conditions in which the TiO2 films were exposed to the sensitizing dye and coadsorbent. Photocurrent--voltage measurements combined with desorption studies revealed that adding chenodeoxycholate reduces the dye loading by as much as 60% while having a relatively small effect on the short-circuit photocurrent. Calculations along with measurements showed that even at low loading, enough dye is present to absorb a significant fraction of the incident light in the visible spectrum. In concurrence with the observations of others, we find evidence for weakly and strongly adsorbed forms of the dye resulting from either different binding conformations or aggregates. The most strongly adsorbed dyes are less susceptible to displacement by chenodeoxycholate than those that are weakly adsorbed. While having no observable effect on dye coverage, multiple exposures of a TiO2 film to a dye solution substantially increased the fraction of strongly adsorbed dye as judged by the resistance of the adsorbed dye to displacement by chenodeoxycholate. Measurements of the open-circuit voltage as a function of the photocharge density, determined by infrared transmittance, showed that chenodeoxycholate not only shifts the conduction band edge to negative potentials, but also significantly increases the rate of recombination. The net effect of adding chenodeoxycholate is, however, to improve the photovoltage.
The Journal of Physical Chemistry B 01/2006; 109(49):23183-9. · 3.70 Impact Factor
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ABSTRACT: The effect of lithium intercalation on the transport dynamics and recombination kinetics in dye-sensitized nanoparticle TiO2 solar cells at lithium levels below 5 atom % was investigated by photocurrent and photovoltage transient and spectroelectrochemical techniques. Titanium dioxide films were doped electrochemically in the dark and under illumination. It was discovered that when Li+ is present in the electrolyte, lithium intercalates irreversibly into dye-sensitized TiO2 films at open circuit (ca. −0.7 V) under normal solar light intensities. Photocurrent transients of doped nonsensitized TiO2 films indicate that lithium doping decreases the diffusion coefficient of electrons through the nanoparticle network. Photocurrent and photovoltage transients of sensitized TiO2 films provide the first evidence that electron transport limits recombination with the redox electrolyte in working cells. As the Li density in the films increases, the diffusion and recombination times of photoelectrons increase proportionately, indicating a causal link between electron transport and recombination. The electron diffusion coefficient in dye-sensitized solar cells exhibits a power-law dependence on photocharge at all concentrations of inserted lithium in the TiO2 film. With increasing doping, the dependence of the electron diffusion coefficient on the photocharge becomes stronger, a phenomenon attributed to widening of the exponential conduction band tail resulting from disorder induced by randomly placed lithium defects in TiO2. The photovoltaic characteristics of dye-sensitized solar cells are largely unaffected by lithium intercalation, implying that intercalation has only a small effect on the charge collection efficiency and the rate of recombination. A simple model is presented that explains the observed transport-limited recombination. The results suggest that increasing the electron transport rate will not significantly improve the solar cell performance.
09/2003;
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ABSTRACT: The light harvesting efficiency of dye-sensitized photoelectrodes was enhanced by coupling a TiO(2) photonic crystal layer to a conventional film of TiO(2) nanoparticles. In addition to acting as a dielectric mirror, the inverse opal photonic crystal caused a significant change in dye absorbance which depended on the position of the stop band. Absorbance was suppressed at wavelengths shorter than the stop band maximum and was enhanced at longer wavelengths. This effect arises from the slow group velocity of light in the vicinity of the stop band, and the consequent localization of light intensity in the voids (to the blue) or in the dye-sensitized TiO(2) (to the red) portions of the photonic crystal. By coupling a photonic crystal to a film of TiO(2) nanoparticles, the short circuit photocurrent efficiency across the visible spectrum (400-750 nm) could be increased by about 26%, relative to an ordinary dye-sensitized nanocrystalline TiO(2) photoelectrode.
Journal of the American Chemical Society 06/2003; 125(20):6306-10. · 9.91 Impact Factor
