John M. Warman

Visible light sensitisation of titanium dioxide using a phenylene vinylene polymer

Abstract The photophysical properties of thin films of poly[2-methoxy-5-(2′-ethyl-hexyloxy), para -phenylene vinylene] (MEH-PPV) on TiO 2 substrates have been investigated. Evidence for dissociation of excitons at the MEH-PPV/TiO 2 interface and electron injection into the conduction band of TiO 2 has been found from current/voltage characteristics. The open circuit voltage is to 0.92 V for these cells and an energy conversion efficiency of 0.15% is found for white light. Capacitance measurements follow the Mott–Schottky relationship and reveal an acceptor density of N A ≈9×10 17 cm −3 for the polymer layers. From analysis of the luminescence as a function of the film thickness, an exciton diffusion length of 20±3 nm for MEH-PPV is derived.

Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylene vinylene)

The nature of the charge carriers in ‘conducting’ polymers is of considerable interest at present,, largely on the basis of the technological potential of these materials for use as the semiconducting layer in field-effect transistors (FETs) and the emissive layer in light-emitting diodes (LEDs). One of the main outstanding questions concerns the relative importance of intra- versus inter-chain charge transfer in determining the overall rate of charge transport. Here we apply the pulse-radiolysis time-resolved microwave conductivity technique to dilute solutions of a soluble dialkoxy derivative of the semiconducting polymer poly(phenylene vinylene), PPV, by which means we determine the one-dimensional intra-chain mobilities of electrons and holes on isolated polymer chains free from inter-chain interactions. The values so obtained—0.5 and 0.2 cm2 V−1 s−1 respectively—are considerably larger than the mobilities measured previously for bulk PPV-based materials. This suggests that considerable improvement in the performance characteristics (in particular switching time and maximum current) of organic FET and LED devices should be possible if material purity and structural order can be better controlled.

Photon-induced molecular charge separation studied by nanosecond time-resolved microwave conductivity

Abstract The application of the time-resolved microwave conductivity (TRMC) technique to the quantitative measurement of charge separation in flash-photolysed molecular systems is desecribed. The apparatus required, background theory and problems involved in deriving absolute values related to the charge-separation process are fully discussed. The use of the technique is illustrated by experiments carried out on the flash photolysis of 4-dimethylamino-4′-nitrostilbene (DMANS) in several solvents. Values of the dipole moments of the T 1 and S 1 states have been estimated and for S 1 are in good agreement with literature values derived in other ways. Other parameters important to the understanding of the photophysics of DMANS are also derived.

Rapid Charge Transport Along Self-Assembling Graphitic Nanowires

36 O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/0101-0036

The study of the transient conductivity of pulse irradiated dielectric liquids on a nanosecond timescale using microwaves

17.50+.50/0 Adv. Mater. 1998, 10, No. 1 [47] R. Schaffler, D. Hariskos, M. Kaiser, M. Ruckh, U. Ruhle, H. W. Schock, Cryst. Res. Technol. 1996, 31, 543. Note that while Cu vacancies create acceptors and hence promote p-type behavior, a decrease of the Cu contents promotes n-type behavior. For sizable deviations from stoichiometry, the point defect model is no longer applicable and detailed phase-diagram data should be used instead. [48] When using Na coming from the SLG without augmentation by Na precursor in the growth of laterally non-uniform samples, Ruckh et al. [34] found that the oxygen (and sodium) content is not maximal for Cu-poor compositions. This suggests that in this case, at least, it may be controlled by other factors. Homogenization (or lack thereof) of electrical performance in this case was not examined by Ruckh et al.

Charge-transfer absorption and emission resulting from long-range through-bond interaction; exploring the relation between electronic coupling andelectron-transfer in bridged donor-acceptor systems.

Abstract The method of measuring conductivity changes in pulse irradiated liquids, on a ns timescale, by microwave absorption is described. Quantitative relationships between the measured absorption signal and the change in conductivity are derived for several cell designs. The method is compared with optical absorption and DC conductivity techniques for the detection of ion formation. The value of the product of the free ion yield and the electron mobility in 2.2.4 trimethylpentane (iso-octane) is found to be 1.62 × 10 -4 m 2 V -1 s -1 (100 eV) -1 . Electron-positive ion recombination in iso-octane is found to obey the Debye relation.

An investigation of electron thermalization in irradiated gases using CCl4 as an electron energy probe

Abstract The electronic absorption- and emission spectra of seven (D)onor-(A)cceptor systems are studied with the general structure D-bridge-A, where the bridge consists of an extended, rigid, saturated hydrocarbon skeleton that separates D and A by distances ranging from 3 to 12 CC σ-bonds. Across bridges with a length up to six σ-bonds sufficient electronic interaction occurs to cause a detectable perturbation of the electronic absorption spectra and for lower homologues this leads to the appearance of discrete intramolecular charge-transfer absorptions with an intensity that is strongly enhanced by intensity borrowing from symmetry-matched local transitions. In the fluorescence spectra discrete charge-transfer (CT) type emission has been detected for bridge lengths up to ten σ-bonds. The radiative transition probability of this CT emission provides a direct measure for the electronic coupling matrix element (H da ) between the charge-separated- and the groundstate. The magnitude of H da is found to decrease exponentially with the number of intervening σ-bonds from 850 cm −1 at 3-bond separation to 17.6 cm −1 at 10-bond separation. Furthermore the rate of charge-recombination in the compounds studied is found to be proportional to the square of H da , thus providing an experimental verification of this often implied “golden rule” relation.

Discotic Liquid Crystalline Semiconductors

The attachment of electrons to CCl4 has been studied in pulse irradiated argon and its admixtures with several atomic and molecular compounds under conditions where the timescale of attachment is comparable with that of electron thermalization. Using the known energy dependence of the rate constant for attachment, the resulting complex electron decay kinetics have been quantitatively related to the energy relaxation process. Values of the energy exchange rate coefficient Ku defined by the relationship d?/dt = −Ku⋅[? − (3/2) kT]⋅NM have been determined for 18 compounds of varying complexity and dipole moment. The values of Ku found encompass the range from 1.3×10−13 cm3 sec−1, for argon, to 1.5×10−8 cm3 sec−1, for acetone. The results are compared with energy exchange parameters available from other sources. Thermalization times and ranges of electrons in irradiated gaseous and liquid systems are calculated and their relevance to radiation chemistry is discussed.

The direct observation of a highly mobile positive ion in nanosecond pulse irradiated liquid cyclohexane

Our works on discotic liquid crystalline semiconductors are reviewed. Some of them show very fast carrier mobilities. Novel sandwich rare earth metal complex, bis[octakis(alkylthio)phthalocyaninato]lutetium(III), shows a mobility of 0.71 cm 2 /Vs, which is the fastest in liquid crystals at the present time, so far as we know. It was also found that novel octaphenoxy- and/or hexaphenoxy-substituted phthalocyanine-based discotic liquid crystals exhibit spontaneous perfect homeotropic alignment in the tetragonal columnar mesophase. The corresponding polymer films showing the perfect homeotropic alignment at room temperature could be successfully prepared. Such marvelous films can be practically utilized as organic carrier transport materials and solar batteries.

DNA: a molecular wire?

Abstract The electrical conductivity induced by pulse irradiation of liquid cyclohexane has been studied by means of microwave absorption. The conductivity in pure cyclohexane, due principally to the excess electron, is reduced to less than 10% of the initial value on addition of 5 × 10 −4 M of the electron scavenger SF 6 . The conductivity remaining after addition of SF 6 is however more than an order of magnitude larger than expected for massive ions in cyclohexane and, since it is almost completely removed by the addition of 4 × 10 −3 M of the positive ion scavenger NH 3 , is attributed mainly to the high mobility of the positive hole in this liquid. The ratio of the electron to hole mobility is determined to be 15. The mean lifetime of the hole under the present conditions is 86 ns. The rate constant for reaction of the hole with NH 3 is determined to be 1.8 × 10 11 M −1 s −1 . From the conductivity remaining after removal of both the electron and the hole the sum of the mobilities of the resulting molecular ions is determined to be 8.4 × 10 −4 cm 2 V −1 s −1 .