S. Nespurek

Photoconductivity of Crystalline Iodoform I.

Abstract Steady state photoconduction in iodoform has been studied between 210–400 nm. The photoconduction threshold is at 345 nm whereas the crystal absorption extends up to 480 nm. The photoconductivity action spectrum shows a steep rise of the photo carrier generation efficiency in the range 345–290 nm and a plateau at shorter wavelengths. Current-voltage characteristics measured under steady illumination show a square dependence of the photocurrent on applied voltage showing the influence of space charge effects. Electron photocurrents are three orders of magnitude lower than hole photocurrents. The lower limit of the photogeneration quantum yield is estimated as 6 × 10−3 charge carriers/photon with an applied field of 1 × 104 V/cm.

Polysilylenes: charge carrier transport and photogeneration

Charge carrier transport and photogeneration in polysilylenes, specifically in poly[methyl(phenyl)silylene], are discussed. Electronic properties are influenced by electron delocalization along the silicon backbone. A model of disordered polarons seems to be adequate to describe the charge carrier transport properties. Dissociation of ion pairs in external electric field, which can be described in terms of the Onsager theory of geminate recombination, seems to be the main mechanism of the free charge carrier photogeneration. Polar additives or polar side groups attached to the polymer backbone locally modify the ionization energy of the polymer segments and energies of electrostatic interactions of charge carriers and the dipoles. These changes result in the creation of local states for charge carriers and in the formation of a potential well structure. If the side groups are photochromic species changing their dipole moments upon illumination, then a light-driven molecular switch can be proposed.

The influence of dipolar species on charge carrier transport in a linear polysilicon

Effect of polar substituents and polar dopants on the transport of charge carriers in poly[methyl(phenyl)silanediyl] (PMPSI) was studied. The transport in undoped PMPSI can be explained within the theory of disordered polarons which postulates that the temperature and field dependence of the charge carrier mobility contain contributions both from the dynamic disorder, i.e. the polaron barrier, and from the environment-dependent static disorder. Polar moieties, chemically attached to the main polymer chain, give rise to a decrease of the on-chain mobility, due to the formation of local electronic polarization states. On the macroscopic level, these moieties and polar dopants give rise to a decrease of the drift (hopping) mobility due to a broadening of the distribution of the density of transport states as a consequence of the electrostatic interaction of the charge carrier with the dipole.

The electronic properties of the interface between nickel phthalocyanine and a PEDOT:PSS film

Thin films of nickel phthalocyanine (NiPc) are deposited on an as-received and on a mildly sputtered PEDOT:PSS film, spin coated on fluorine tin oxide coated glass. The electronic properties of the PEDOT:PSS surface, both as loaded and upon thermal treatment and sputtering, as well as of the interfaces between NiPc and PEDOT:PSS are studied by x-ray and UV photoelectron spectroscopies in order to investigate both the electronic and the chemical properties of the materials. Surface analysis of the PEDOT:PSS films showed that upon sputtering the insulating PSS film is removed leading to lower work function, as well as to an increase of the density of occupied states close to the Fermi level. The investigation of the interfaces between NiPc and PEDOT:PSS revealed charge transfer and a pinning of the Fermi level across the interface. The hole injection barrier was found significantly lower compared with that for the NiPc/Au interface, indicating that the presence of the PEDOT:PSS layer facilitates the carrier...

Liquid Crystal Alignment on Photoinduced Species in Poly[methyl(phenyl)silylene] Film

Abstract The first observation of alignment of a liquid crystal deposited on a poly[methyl(phenyl)silylene] (PMPSi) film irradiated with linearly polarized light is reported. The effect seems to be controlled by the angular-dependent photodegradation caused by the photoinduced scission of the Si-Si bonds in the polymer backbone connected with the formation of other photoproducts preferentially oriented along the light polarization.

Thermally stimulated technique employed to reactions in polycrystalline photochromic systems

Abstract The paper presents preliminary results of measurements of the energetics and kinetics of thermally driven reverse reaction in polycrystalline photochromic systems: substituted dihydropyridines and their thiopyrane analogue. The technique of differential scanning calorimetry (DSC) was employed aiming at obtaining a qualitative information concerning the relative energies of the ground states of the stable and metastable formsof the photochromic systems. Moreover, a DSC experiment can be regarded as a thermally stimulated process and analysed in a way allowing one to extract information about the energy barrier associated with the thermal process. A more detailed study was performed on 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine: two processes were identified, their activation energies amounting to (115 ± 20) and (60 ± 20) kJ/mol. The DSC results were supplemented with those obtained from UV-VIS spectroscopy.

Gas Sensing Device Based on Phthalocyanine LB Films

Abstract An investigation of the electrical conductivity of films consisting of multilayers of tetra-t−butyl-substituted copper phthalocyanine (TTBCuPc) films exposed to various levels of nitrogen oxides in air (within the ppm range) was carried out. The TTBCuPc films were deposited, using the Langmuir-Blodgett (LB) technique, onto glass substrates. Gold interdigital electrodes (ca. 2μm thick, 100 μm wide and 100 μm spaced) were evaporated on the top of the films. High reproducibility and reasonable sensitivity of the sensors to the nitrogen oxide at a relatively low temperature (313 K) suggest that TTBCuPc may be a viable material for gas sensing devices.

Determining local state densities in spatially inhomogeneous samples from steady-state space-charge-limited currents

The effect of spatial inhomogeneity of the distribution of local levels on the reliability of the determination of the densities of these states from steady-state space-charge-limited current-voltage characteristics is discussed. Numerical calculations of correction factors show that experimentally determined apparent densities of local states are close to spatially averaged values. In some cases, information about their spatial distribution can be deduced from measurements of the thickness dependence of the apparent density of states. >

The 'compensation rule' in steady-state space-charge-limited currents

The conditions leading to the occurrence of the compensation rule in space-charge-limited current studies is discussed. It is shown that this effect is not fortuitous but that it can arise whenever a convergence point exists on current-voltage characteristics. Conditions leading to the appearance of a convergence point in such characteristics are discussed in terms of continuous and quasi-discrete trapping levels. The convergence point is shown to result when the concentration of a discrete (or nearly discrete) trap remains constant.

Grafted polybutadiene for fast retrieval of optical information

The goal of the investigations was to measure the dynamics of molecular reorientation in polybutadiene backbone based polymer grafted with photochromic thiols. Measurements were performed in a typical degenerate two- and four-wave mixing experiments, in which time constants of dynamic diffraction grating formation were determined. Holographic gratings which were inscribed in polymeric liquid crystalline films show switching times (build-up and decay) within the 20–70 ms range. An excellent reversibility of the recording-erasure process is reported.