J. Sworakowski

Use of space‐charge‐limited current measurements to determine the properties of energetic distributions of bulk traps

A simple analytical method is put forward in this paper, enabling one to extract quantitative information about an arbitrary energetic distribution of traps for current carriers from a single experimental space‐charge‐limited current‐voltage characteristic. The applicability of the method is illustrated with computer‐simulated characteristics, calculated for various typical distributions of traps. The effect of the nonstability of the current, nonuniformity of the spatial distribution of traps, and temperature on the accuracy of the determination of the parameters of traps is briefly discussed.

On the Origin of Trapping Centres in Organic Molecular Crystals

Abstract Traps for current carriers in molecular crystals may be due either to imperfections of the crystal structure (structural traps) or to chemical impurities introduced into the lattice (chemical traps). In the present paper simple hypotheses are presented explaining the formation of traps of the two groups. Perturbations of the crystal structure are not localized in one unit cell, but may extend over several farther cell. The local values of the polarization energy of the crystal lattice will change near to the centre of the defect, thus the “perturbed” molecules can act as traps for currentcarriers.Energetic distributions of traps as calculated under such conditions are foundto be in a good agreement with those determined experimentally. Trapping states may be also formed by impurity molecules having either greater electron affinity or smaller ionization energy than the host ones. The depth of the chemical traps estimated from the relationships given in the paper has been compared with the experime...

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.

New theoretical insight into the thermal cis–trans isomerization of azo compounds: Protonation lowers the activation barrier

The detailed mechanism of thermal cis–trans inversion of protonated diazenes and azobenzenes has been investigated for the first time using ab initio Hartree–Fock (HF), density functional theory (DFT), second order Mo/ller–Plesset (MP2), and complete active space self consistent field (CASSCF) approaches. Protonation of one of the nitrogens at the N=N bond leads to a considerable (5–20 kcal/mol) reduction of the activation barrier. The magnitude of this effect depends on the electron withdrawing nature of the substituents and the differential stabilization of the transition state by positively charged functional groups. In the case of 4-phenylazopirydine protonated at pyridine nitrogen, the barrier is dramatically reduced to just 7 kcal/mol. This result establishes the lowest (close to an experimental estimate) ab initio value for the cis–trans isomerization for a substituted azobenzene. The structures of neutral and protonated transition states are similar, and the correlation effects for molecular param...

Molecular current modulator consisting of conjugated polymer chain with chemically attached photoactive side groups

A scheme for an optically controlled current modulator is put forward, based on properties of a conducting polymer containing photochromic moieties chemically attached to the main chain. The idea is supported with results of calculations of local states for charge carriers formed in the vicinity of dipolar species in molecular materials, and with measurements of the drift mobilities in polysilane copolymers containing polar benzaldehyde 2,4-dinitrophenylhydrazone groups.

Charge carrier transport on molecular wire controlled by dipolar species: towards light-driven molecular switch

Abstract Charge carrier transport on molecular wire (conjugated polymer chain) is limited by polaron formation and by Coulomb interactions between a charge localized on a chain and another charge moving on the same or a neighboring chain. The ‘Coulomb limitation’ results from a feature characteristic of one-dimensional systems in which carriers cannot be scattered. Polar side groups attached to the molecular wire locally modify the ionization energy of the polymer segments and energies of electrostatic interactions of charge carriers on the wire and the dipoles. These changes result in the formation of a potential well structure and in the creation of local states for charge carriers. If the side groups are photochromic species changing their dipole moments upon illumination, then a light-driven molecular switch can be constructed.

4 Electrical properties of organic materials

Recent research into the electrical properties of low-molecular weight organic solids and polymers is reviewed. Conditions for an effective injection from electrodes into organic solids, and recent views on the photogeneration of charge carriers in photoactive polymers are briefly discussed. The transport of charge carriers in organic materials is reviewed, focusing on disordered (polycrystalline and amorphous) materials. Particular attention is paid to materials potentially applicable as elements of electronic devices: thin film field-effect transistors, and light-emitting and photovoltaic devices.

Dielectric properties of single crystals of the substituted diacetylene pTS: effect of the solid-state polymerization and phase transitions

Abstract Dielectric permittivities of the polymerizable organic solid, p TS diacetylene have been measured between 115 and 330 K in the directions parallel and perpendicular to the direction of polymer chain growth. The upper phase transition in monomer, polymer and mixed crystals at various stages of the solid-state polymerization manifests itself as a maximum in the temperature dependence of e measured in the direction parallel to the molecular stacks, being particularly well pronounced in fully polymerized crystals. The transition was identified as an antiferroelectric one, the sublattice polarization being most probably the order parameter. The lower phase transition could be observed only in monomer and monomer-rich crystals as a shoulder on the e( T ) dependence. This transition could be detected only in the crystals containing less than ≈ 10–15% of polymer. The dielectric permittivity was found to be independent of frequency up to 3 GHz. The polymerization results in changes of the dielectric permittivity. In samples where the direction of measurements coincides with the b axis, these changes follow the monomer-polymer conversion curve.

Trapping States Formed by Structural Defects in Molecular Crystals

Abstract Results of calculations of the energetic distributions of trapping states in simple molecular crystals are given in the paper. A crystal with primitive regular unit cells has been taken into account, and three kinds of simple isolated linear defects have been considered: an edge dislocation, a linear array of vacancies, and a defect formed by a linear array of molecules larger than the host ones. Equilibrium positions of molecules in the vicinity of the defects have been calculated describing the intermolecular interactions by the Lennard-Jones type potential function. Local values of the polarization energy and hence depths and distributions of traps formed by molecules neighbouring to the defects have been calculated in the ion-dipole approximation. It has been shown that the formation of trapping states by thermal vibrations can be neglected as being a small effect.

Effect of polar molecules on the transport and localization of charge carriers in molecular materials

The influence of polar species on the transport of charge carriers in molecular solids is reviewed. Guest molecules with permanent dipole moments introduced into molecular crystals can act as traps localizing carriers provided certain energetic conditions are fulfilled. In this case their polar character may manifest itself in a field-dependent decrease of the trap depth. Moreover, irrespective of the positions of their energy levels, polar impurities may locally modify the polarization energy thus creating traps on neighbouring molecules. The effect of polar dopants in disordered molecular solids can be described within the model due to Bassler: the presence of polar species broadens the density-of-states function thus decreasing the effective mobility of charge carriers.