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Conjugated semiconducting block copolymers are interesting organic materials for possible photovoltaic device applications because of their capability to form self-organized nanostructures and the tunability of their electronic and optical properties as a function of the relative lengths of the constituent monomers. In this work, we report the results of our investigation of polymer bulk heterojunction photovoltaic devices based on the block copolymer [MEH-PPV]-co-[biphenylene vinylene] derived from the widely used poly[2-methoxy-5-[2′-ethyl-hexyloxy]-1,4-phenylene-vinylene] (MEH-PPV) as an electron donor material in conjunction with the common electron acceptor molecule [6,6]-phenyl C61 butyric acid methyl ester, a soluble derivative of C 60 . We find that the morphology and photovoltaic characteristics of these devices are strongly influenced by the blend concentration and thickness of the electro-active organic layer. Our initial results demonstrate that a power conversion efficiency of 2.4% is achieved with these materials.

When viewed at frequencies below about 8 THz (250 cm-1; 30 meV) the ab-plane optical conductivity of the cuprate superconductors (in their normal state) is well described by a Drude model. Examples include optimally-doped YBa2Cu3O7-δ and Bi2Sr2CaCu2O8; even the underdoped phases have a Drude character to their optical conductivity. A residual Drude-like normal fluid is seen in the superconducting state in most cases; the scattering rate of this quasiparticle contribution collapses at Tc.

In clean metallic superconductors, 100% of the mobile carriers participate in the condensate, so that the London penetration depth (which measures the electromagnetic screening by the superconductor) indicates charge densities comparable to those inferred from the free-carrier plasma frequency. In the cuprates, this is not the case, even though penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-doped part of the phase diagram. Optical measurements, which permit independent determination of the total doping-induced spectral weight and the superfluid density, show that in optimally doped materials only about 20% of the doping-induced spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials, the superfluid fraction is even smaller. This result implies extremely strong coupling for these superconductors.

The temperature (6{endash}350 K) and magnetic-field dependence (0{endash}7.5 T) of the phononic Raman spectra in the layered manganites, R{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} (R=La,Nd), reveal anomalous behaviors of the Mn and equatorial O{sub xy}{sup B{sub 1g}} modes arising from local octahedral distortions and coupling to the Mn{sup 3+} e{sub g} electrons. These results provide the evidence for polaron formation and its interplay with electrical transport and magnetism in the layered manganites. {copyright} {ital 1998} {ital The American Physical Society}

Polarized Raman spectra of undoped and doped ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{MnO}}_{3}$ single crystals have been studied in the temperature range from 5 to 423 K ($M=\mathrm{Sr},$ Ca). The spectral intensity is mainly related to scattering by optical phonons at $q\ensuremath{\sim}0$ (for $x\ensuremath{\approx}0$) and the density of vibrational states for $xg0.1.$ Raman spectra from the undoped ${\mathrm{LaMnO}}_{3}$ structure distorted by the static Jahn-Teller effect are consistent with the orthorhombic ${D}_{2h}^{16}$ space group. Raman spectra of the doped ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{MnO}}_{3}$ crystals have weakly resolved features and intensities comparable to those of the second-order spectra in cubic media. The anomalies in the phonon spectra of doped crystals were explained by the doping dependence of the ionic radius of a La/Sr site. The most intense ${B}_{1g}$ mode in ${\mathrm{LaMnO}}_{3}$ crystal has an abrupt frequency shift near ${T}_{N}=140\mathrm{K},$ which corresponds to a paramagnetic to canted-antiferromagnetic phase transition.

We report polarized Raman spectra of La1−xSrxMnO3 crystals (x = 0, 0.1, 0.2 and 0.3) in the temperature range from 5 K to 300 K. The small distortion of the nearly cubic lattice of doped La1−xSrxMnO3 single crystals results in a structured phonon Raman spectrum which is composed of two parts with different contributions depending on the value of doping. The first part is assigned to the distorted non-cubic perovskite lattice and follows the selection rules for a tetragonal structure. This suggests the possible presence of a tetragonal structure in doped La1−xSrxMnO3 single crystals. The second part is mainly due to the density of vibrational states and is attributed to the second-order Raman scattering. The frequency of the A1g-like mode was found to be sensitive to doping but the observed shift is opposite to that expected from the difference in the atomic weight of Sr and La. This mode exhibits also an anomalously large shift of ∼20 cm−1 in the temperature range from 5 K to 300 K. A possible explanation of this behaviour is suggested.

The temperature dependent infrared conductivities of Bi2Sr2CaCu2O8 (Tc ⋍ 82 K) and Bi2Sr2CuO6 (Tc ⪅ 5 K) are compared with non-Fermi liquid approaches to the normal state of the high Tc superconductors. We find reasonable agreement with our data only for ω ⪅ 0.12 eV. We attribute the remaining spectral weight to a low-energy excitation in copper-oxide superconductors doped well above the metal-insulator transition.

The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ has a Drude-like component below ${\mathit{T}}_{\mathit{c}}$. Interpreting the width of this component as the quasiparticle relaxation rate (${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$), we find that ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ decreases dramatically just below ${\mathit{T}}_{\mathit{c}}$, in sharp contrast with the T-linear ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ above ${\mathit{T}}_{\mathit{c}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$. This decrease causes a peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}\ensuremath{\rightarrow}0,T) for T just below ${\mathit{T}}_{\mathit{c}}$, a peak which is due to the scattering rate and not to pair coherence effects, consistent with the lack of a coherence peak in the NMR relaxation rate. This result implies that the excitations which scatter the carriers are suppressed below ${\mathit{T}}_{\mathit{c}}$.

Infrared and far-infrared transmittance measurements were performed on Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (2:2:1:2) and Bi{sub 2}Sr{sub 2}CuO{sub 6} (2:2:0:1). Below {ital T}{sub {ital c}}, a characteristic feature occurs at {omega}=700 cm{sup {minus}1} (12{ital k}{sub {ital B}T{ital c}}) in the 2:2:1:2 material. No similar feature is found in the 2:2:0:1 compound. The 2:2:1:2 material also showed substantial {ital ab}-plane anisotropy.

We report on the near-field optical microscopic (NSOM) imaging of single-crystal Nd1/2Sr1/2MnO3 near the first-order phase transition from the CE-type charge-ordered insulator (COI) to the ferromagnetic metal (FMM) at 165 K. Using polarized light, the measurements are sensitive to twin domains through optical anisotropy and to hysteretic inhomogeneous strain associated with the phase transition. Optical evidence for strain relaxation and multiphase coexistence is observed through the transition from 150 to 165 K. The multiphase coexistence is seen as the formation of FMM "nanoneedles" with widths less than 100 nm and lengths of at least several micrometers at temperatures well below TC. The work demonstrates polarized NSOM as a technique for the study of first-order solid-state phase transformations on the nanometer spatial scale.

Transient dynamics of photoexcited carriers in MEH-PPV, PCBM-C60 and their blend is investigated by THz-TDS using a 400 nm optical pump and terahertz probe. Free and bound carriers are revealed in the measurement of the complex conductivity.

From an organic bulk heterojunction photodetector fabricated from a blend of P3HT/PCBM-C60, we report an external quantum efficiency of 87 plusmn 2 % under an applied bias voltage of -10V, leading to an internal quantum efficiency of 97 %.

We explore poly(9,9-dioctylfluorene-co-bithiophene) copolymer (F8T2) for photovoltaic applications. Devices fabricated with F8T2/C60blend show open-circuit voltage of 1V and power conversion efficiency of 1.15% for operation in the short-wavelength region of the solar spectrum.

In this paper, the authors present Auger electron spectroscopy (AES) studies of Zn1−xMgxO (ZMO) films grown via interrupted atomic-layer deposition (ALD) techniques. The ZMO films were fabricated by alternating ALD deposition of ZnO and MgO layers up to 1000 cycles. Zn1−xMgxO films with progressively decreasing Mg/Zn ratios (Mg/Zn = 1/1, 1/2, 1/3, 1/4, 1/5, 1/6, 1/9, and 2/8, 3/12, 4/16, and 5/20) were fabricated for this study. The AES results exhibit an abrupt drop of Mg composition on the ZMO surface when the Mg/Zn

Polarized Raman scattering by phonons is used to characterize thin films prepared by laser ablation of La1−xCaxMnO3 targets. It was found that, in the temperature range from 6 to 300 K, phonon spectra of La0.7Ca0.3MnO3 films exhibit observable differences from those in bulk materials (microcrystalline ceramics and single crystals). A significant difference was found in the spectra of “as-grown” films compared to those annealed in oxygen at 800 °C. The observed Raman peaks and their linewidths exhibit an irregular temperature dependence near Tc. A correlation of Raman data with magnetization of the sample was also found.

The properties of ferroelectric films are known to degrade when subjected to hydrogen in forming gas anneals. Earlier studies have attributed this degradation to the loss of oxygen from these films during these anneals. In this study, we show that though oxygen is lost during forming gas annealing, hydrogen incorporation is the primary mechanism for the degradation of ferroelectric properties. Raman spectra obtained from the forming gas-annealed films show evidence of polar hydroxil [OH−] bonds in the films. The most probable site for hydrogen ions is discussed based on ionic radii, crystal structure, electrical properties, and Raman spectra. We propose that the hydrogen ion is bonded with one of the apical oxygen ions and prevents the Ti ion from switching. Pyroelectric measurements on forming gas-annealed capacitors confirm that the capacitors no longer possess spontaneous polarization.

Under the influence of a 0.05 T magnetic field, 15-nm diameter cobalt nanoparticles covered with surfactants in a colloidal solution assemble into highly constrained linear chains along the direction of the magnetic field. The magnetic-field-induced (MFI) chains become floppy after removal of the field, folding into three-dimensional (3D) coiled structures upon gentle agitation. The 3D structures are broken into smaller units with vigorous agitation. The nanoparticles redisperse into the solvent upon ultrasonic agitation. Optical microscopy and transmission electron microscopy (TEM) are used to characterize the morphologies of the nanoparticle assemblies at various stages of this reversible process. The hysteresis loops and zero-field cooled/field cooled (ZFC/FC) curves reveal the interparticle coupling in the assemblies. MFI assembly provides a powerful tool to manipulate magnetic nanoparticles.

We investigate the dependence of the photovoltaic characteristics of organic photocells on the relative concentration of the donor-acceptor molecular complex. The devices were fabricated using a new [MEH-PPV] - co - [phenylene vinylene] blend with C 60 . We find that the morphology and device performance are strongly influenced by the molar fraction (x) of C 60 in the electroactive layer of the device. The best device was obtained with x = 0.6 and manifested V OC = 0.85 V, J SC = 2.65 mA/cm 2 , FF = 0.42, and η P ext = 1%.

We report the effects of thin-film stoichiometry and morphology on the short-circuit current and open-circuit voltage of organic-based photocells. Performance of these devices depends strongly on the relative concentration in the polymer / C60 blend.

By alternating the gate voltage polarity of a pentacene thin film transistor, we show that the drain current is stabilized and thus the bias stress effect is overcome. This allows for controlled testing of the device sensitivity to environmental conditions. We find that the conductivity of the device decreases on the time scale of seconds when the device is exposed to water vapor, which is manifested through a decrease in mobility and a shift in the threshold voltage. Simple recombination modeling suggests that trapping is the responsible mechanism. However, the effects of water vapor can be reversed by exposing the device to dry nitrogen flow. The time scale for recovery is on the order of 10s of minutes.

We observed a dramatic improvement in the performance of polymer light‐emitting diodes (LEDs) upon light doping of the organic layer. The LEDs betrayed symmetrical electrical and light‐emission characteristics. Their turn‐on voltage is lower and their external quantum and power conversion efficiencies are higher by nearly an order of magnitude when compared with devices that utilized a nominally undoped organic layer. We attributed these results to the modification of the tunneling barrier in metal–polymer–metal junctions due to the presence of an induced polarization electric field associated with the ionized dopant counterions and charged polymer chains.

The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the interplay of the orbital and ferromagnetic double-exchange correlations. Inelastic scattering by charge-order fluctuations dominates the quasiparticle dynamics in the ferromagnetic-metal state. The scattering is suppressed at low frequencies, consistent with the opening of a charge-density wave pseudogap., Comment: 10 pages, 4 figures

We present infrared and Raman measurements of magnetite (Fe_3O_4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T_V=120K. At temperatures below T_V, we observe a strong gap-like suppression of the optical conductivity below 1000 cm^-1. The structural aspect of the Verwey transition demonstrates itself by the appearance of additional infrared- and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm^-1 changes abruptly at the transition. For T, Comment: 16 pages, 5 Postscript figures, uses revtex.sty. submitted to Physical Review B

A Raman-scattering investigation of the double-layer manganites. La2−2xSr1+2xMn2O7 (x=0.4,0.5), revealed signatures of a) charge stripe formation and melting, b) small polaron formation, and c) anisotropic quasiparticle dynamics in the ferromagnetic-metallic state. These observations and their implications are discussed in this work.

We report the results of a detailed polarization and temperature dependence study of the Raman scattering from La1.2Sr1.8Mn2O7. The Raman spectra reveal three general spectral features. First, there are sharp peaks due to long-wavelength optic phonons. Phonons, attributed to the distortion of the MnO6 octahedra, reveal an anomalous behavior which correlates with the transition from a paramagnetic-insulating (PI) to a ferromagnetic-metallic (FM) phase at a critical temperature Tc. Second, there is an electronic continuum that is suppressed for ω < 500 cm−1 at T ≪ Tc. Third, broad peaks between 400 cm1 to 800 cm−1 seen in the PI state, surprisingly, disappear in the FM state. The implications of these results are discussed.

We investigate the effects of interfacial layers on the photovoltaic properties of bi- layer organic heterojunction photovoltaic devices. The devices were fabricated using aluminum phthalocyanine chloride (AlPcCl) as electron donor and fullerene (C60) as electron acceptor. Two types of interfacial layers inserted between the transparent indium-tin-oxide anode and the AlPcCl layer were investigated: PEDOT:PSS and MoO3. We find that these interfacial layers have a strong influence on the device open-circuit voltage (VOC). The effects of temperature and illumination intensity on VOC were explored. C 2011 Society of Photo-Optical Instrumentation

Infrared measurements in superconductors offer the opportunity to study the superconducting energy gap, the dynamics of the conduction electrons, and the nature of low-energy excitations in the material. The infrared transmittance of single crystals of Bi2Sr2CaCu2O8 has been measured and the optical properties determined by Kramers-Kronig analysis. The normal-state data are compared to both one- and two-component descriptions of the optical conductivity. Both approaches allow for a qualitative description of the T-dependent far-infrared conductivity and the nearly T-independent midinfrared conductivity. Below T c the samples show almost zero absorption at low frequencies, with an onset around 150 cm-1. At higher frequencies, there is a characteristic maximum in the transmittance at ~ 700 cm-1 and corresponding structure in σ 1(ω). Although these features are revealed clearly below T c , we conclude that neither should be assigned to the superconducting gap.

Magnetotransport and far-infrared spectroscopy are combined to probe the electronic states in the vicinity of the magnetic-field-induced metal-insulator transition in n-type GaAs. Resonant absorption lines, identified as the shallow-donor 1s-2p transitions by their selection rules, magnetic-field dependence, and temperature dependence are observed even in the metallic state. These results demonstrate that, near the metal-insulator transition, the electrons are in the donor impurity band, which is split off from the conduction band.

Two important issues for the infrared properties of high-T c materials are the superconducting energy gap and the nature of the strong midinfrared absorption. In this work we discuss the infrared reflectance of epitaxially-grown laser-deposited films of YBa2Cu3O7−δ, PrBa2Cu307−δ, and Y1−xPrxBa2Cu3O7−δ. (x = 0.2, and 0.4).

We investigate the effects of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and LiF interfacial layers on the efficiency of organic bulk heterojunction photodetectors fabricated with blends of regioregular poly(3-hexylthiophene-2,5-diyl) as electron donor and {6}-1-[3-(methoxycarbonyl)propyl]-{5}-1-phenyl-[5,6]-C61 as electron acceptor. At high reverse bias voltage, we find that the efficiency of these devices is limited only by the rate of creating excitons in the organic layer. Upon correction for the optical loss at the interfacial layers, the internal quantum efficiency can approach 100% in this highly disordered network of donor-acceptor molecular heterojunctions.

As carriers are introduced into the cuprates (by doping the insulating “parent” compounds) spectral weight appears in the optical spectrum at photon energies below the charge-transfer gap. This spectral weight increases as the doping level increases. Magnetic penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-doped part of the phase diagram. Optical measurements allow independent determination of the total doping-induced spectral weight and the superfluid density. These measurements, made on cuprates with transition temperatures from 40 to 110 K, find that in optimally doped materials only about 20% of the doping-induced spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials, the superfluid fraction is even smaller. This result implies extremely strong coupling for these superconductors.