Conducting and semiconducting polymers are important materials in the development of printed, flexible, large area
electronics such as flat panel displays and photovoltaic cells. There has been rapid progress in developing conjugated
polymers with high transport mobility required for high performance field effect transistors (FETs), beginning with
mobilities around 10-5cm2/Vs to a recent report of 1cm2/Vs for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno
[3,2-b]thiophene) (PBTTT). In this work, the electrical properties of PBTTT are studied at high charge densities both as the
semiconductor layer in FETs and in electrochemically doped films to determine the transport mechanism. We show that
data obtained using a wide range of parameters (temperature, gate-induced carrier density, source-drain voltage and
doping level) scale onto the universal curve predicted for transport in the Luttinger Liquid description of the onedimensional
"metal", where fermions along the 1D chain collectively behave as bosons, and where charge and spin are
decoupled.
The overall power conversion efficiency of organic solar cells depends on many factors, some of which such as photon absorption, charge carrier photogeneration, separation and transport are intrinsic properties of the active material. The use of low-bandgap conjugated polymers in polymer/fullerene bulk heterojunctions improves the spectral overlap between the polymer absorption and the solar irradiance spectrum, and is therefore a promising route toward increased light harvesting and higher power conversion efficiency of polymer photovoltaics. We present our studies on the optical and charge transport properties of a novel low-bandgap conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with an optical energy gap of Eg=1.46 eV. The combination of steady-state and transient photoconductivity with photoinduced absorption measurements has allowed us to investigate the charge carrier photogeneration and charge transport mechanisms in pristine PCPDTBT and PCPDTBT:PCBM interpenetrating networks, and to compare them to the P3HT and P3HT:PCBM model systems. The picture of the photophysics of PCPDTBT:PCBM emerging from these studies is very similar to that of P3HT:PCBM blends. We discuss the potential of PCPDTBT as a new material for high efficiency polymer solar cells.
Ground glass nodules (GGNs) have proved especially problematic in lung cancer diagnosis, as despite frequently being malignant they have extremely slow growth rates. In this work, the GGN segmentation results of a computer-based method were compared with manual segmentation performed by two dedicated chest radiologists. CT volumes of 8 patients were acquired by multi-slice CT. 21 pure or mixed GGNs were identified and independently segmented by the computer-based method and by two readers. The computer-based method is initialized by a click point, and uses a Markov random field (MRF) model for segmentation. While the intensity distribution varies for different GGNs, the intensity model used in MRF is adapted for each nodule based on initial estimates. This method was run three times for each nodule using different click points to evaluate consistency. In this work, consistency was defined by the overlap ratio (overlap volume/mean volume). The consistency of the computer-based method with different initial points, with a mean overlap ratio of 0.96±0.02 (95% confidence interval on mean), was significantly higher than the inter-observer consistency between the two radiologists, indicated by a mean overlap ratio of 0.73±0.04. The computer consistency was also significantly higher than the intra-observer consistency of two measurements from the same radiologist, indicated by an overlap ratio of 0.69±0.05 (p-value < 1E-05). The concordance of the computer with the expert interpretation demonstrated a mean overlap ratio of 0.69±0.05. As shown by our data, the consistency provided by the computer-based method is significantly higher than between observers, and the accuracy of the method is no worse than that of one physician’s accuracy with respect to another, allowing more reproducible assessment of nodule growth.
White emission from polymer light-emitting diodes (PLEDs) is demonstrated by spin-casting polymer blends from solution containing poly(9,9-dioctylfluorenyl-2.7-diyl) (PFO) and tris (2,5-bis-2'-(9',9'-dihexylfluorene) pyridine) iridium (III), Ir(HFP)3. The white electrophosphorescence PLEDs exhibit luminance of 1.2 x 104 cd/m2 at 17 volts and luminous efficiency of 4.3 cd/A at current density of 5.2 mA/cm2. Because a single semiconducting polymer, PFO, was used as the common host for red, green and blue emission, the color coordinates, the color temperatures and the color rendering indices of the white emission are insensitive to the brightness, applied voltage and applied current density.
We report on the anisotropic photoluminescence (PL) properties of stretch-oriented free standing films of poly(p-phenylene-vinylene) (PPV) at different temperatures. The PL quantum efficiency is strongly dependent on the pump polarization; it is higher when the pump is polarized perpendicularly to the polymer chain orientation. Independently of the pump polarization, we find that the PL emission spectra are mainly polarized along the polymer chain axis. The PL spectra show high-energy features, close to the onset of the HOMO-LUMO transition, that are significantly affected by self-absorption of the emitted light in the optically thick samples as well as by refractive effects at the polymer-air interface. In order to clarify the origin of these features, we have made a detailed characterization of the anisotropic optical constants of the PPV film. The optical constants have been derived from polarized reflectance and transmittance measurements and were used for the renormalisation of the PL spectra using the Fresnel equations. Frank-Condon analysis for the absorption oscillator strength and for the corrected emission spectra suggests that two different emitting states contribute to the optical properties. The connection of these states with film morphology and intermolecular interactions is described.
High performance electrophosphorescent light emitting diodes (LEDs) were demonstrated by using conjugated polymers, poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), PFO end-capped with hole-transporting moieties (HTM), PFO-HTM, and PFO end-capped with electron-transporting moieties (ETM), PFO-ETM, as the hosts and the organometallic emitter, tris-[2,5-bis-2'-(9,9'-dihexylfluorene) iridium] [Ir(HFP)3] as the guest. Electrophosphorescent LEDs fabricated from PFO, PFO-HTM, and PFO-ETM as the hosts emit red light with turn-on voltage around 5V, luminances (L) of 2040 cd/m2, 1937 cd/m2 and 2487 cd/m2 at 290 mA/cm2 (16 V), and luminance efficiencies (LE) of 1.40 cd/A, 1.38 cd/A and 1.80 cd/A at 4.5 mA/cm2 for PFO, PFO-HTM, and PFO-ETM, respectively. The results demonstrate that high performance electrophosphorescence can be obtained from conjugated polymer-based LEDs that are fabricated by processing the active materials directly from solution.
Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo- excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to h(upsilon) = 6.2 eV indicate an apparent increase in the photoconductivity at h(upsilon) > 3- 4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of (pi) - (pi) * absorption, behavior that is inconsistent with a large exciton binding energy.
We demonstrate ultrafast (100 fs) carrier generation in poly(phenylene vinylene), PPV, and poly[2-methoxy-5-(2- ethyl-hexyloxy)-(phenylene vinylene)], MEH-PPV, by using femtosecond transient spectroscopy in the mid-IR in order to probe the infrared active vibrational active (IRAV) modes. The 10% carrier generation quantum efficiency in MEH-PPV with no electric field applied, implies primary photogeneration of charge carriers. The recombination dynamics in PPV and MEH-PPV indicate that the carrier lifetime is sensitive to the strength of the interchain interaction.
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