E.V Emelianova

Charge carrier mobility in doped semiconducting polymers

An analytic model of the equilibrium hopping conductivity in a disordered organic semiconductor at large charge carrier densities is formulated. Calculated dependences of the equilibrium hopping mobility upon the carrier density are compared with recent experimental data obtained on doped poly(3-hexylthiophene) films. Doping is shown to create additional energy disorder due to potential fluctuations caused by the Coulomb field of randomly distributed dopant ions.

Weak-field carrier hopping in disordered organic semiconductors: the effects of deep traps and partly filled density-of-states distribution

An analytic model of the weak-field carrier transport in an energetically disordered and positionally random hopping system is formulated. Within the framework of this model, the carrier mobility can be calculated by either direct averaging of carrier hopping rates or by the use of the effective transport energy concept. It is shown that multiple carrier jumps within pairs of occasionally close hopping sites affect the position of the effective transport level on the energy scale. In good quantitative agreement with experimental data and results of Monte Carlo simulation, the temperature and concentration dependences of the mobility can be almost perfectly factorized, i.e. represented as a product of two functions one of which depends solely upon the temperature while the other governs the dependence upon the density of localized states. The model is also used for the calculation of trap-controlled hopping mobility and for the analysis of hopping transport at high charge-carrier densities.

Space-charge-limited currents in materials with Gaussian energy distributions of localized states

A model is formulated describing trap-controlled space-charge-limited currents (SCLCs) in an organic material with a Gaussian density-of-states (DOS) distribution. It is shown that SCLC is not always controlled by carrier release from localized states around the Fermi level and, therefore, a Gaussian DOS can serve as either shallow or deep distribution of localized states depending upon the carrier and/or current density.

Charge injection versus space-charge-limited current in organic light-emitting diodes

A unified model of hopping carrier injection and space-charge-limited current (SCLC) in disordered organic materials is formulated. It is shown that, contrary to what one would intuitively expect, a metal contact with the injection barrier as high as 1 eV can be still an Ohmic contact at low temperatures providing conditions for the hopping SCLC.

Thermally stimulated luminescence in π-conjugated polymers containing fluorene and spirobifluorene units

Abstract Thermally stimulated luminescence (TSL) is studied experimentally in π-conjugated polyfluorene (PF) and in a polymer containing polyspirobifluorene (PSBF). A narrow TSL peak, whose maximum is located at 45 K, evidences for a weak energy disorder in PF films. A broader TSL peak, shifted to higher temperatures, indicates stronger intrinsic disorder and, in some cases, occurrence of deep traps in PSBF. The dominant contribution from phosphorescence to the afterglow emission and coexistence of phosphorescence and fluorescence in the TSL signal can be explained by the energy splitting of the singlet and triplet states of geminate pairs in PF and PSBF.

Excitons in π-conjugated polymers

Recent quantum chemical calculations and measurements of the singlet-triplet splitting inferred from phosphorescence spectra reinforce the notion that the dominant electronic excitations in π-conjugated polymers are excitons which can undergo bimolecular interactions. Their dissociation into electron-hole pairs requires an energy of about 0.5 eV as evidenced by (i) the action spectrum for delayed fluorescence in a poly-chinoxalin due to geminate pair recombination and (ii) electric field quenching studies of delayed fluorescence in a PPV-derivative. Recent experimental and theoretical photoconductive studies are in accordance with the exciton concept.

Equilibrium carrier mobility in disordered organic semiconductors

Abstract The equilibrium carrier mobility in an energetically disordered and positionally random hopping system is analytically calculated by direct averaging of carrier hopping rates and by the use of the effective transport energy concept. In good quantitative agreement with experimental data and results of Monte-Carlo simulation, the temperature and concentration dependencies of the mobility can be almost perfectly factorized, i.e., represented as a product of two functions one of which depends solely upon the temperature while another one governs the dependence on the density of localized states.

Dopant-assisted charge carrier photogeneration in conjugated polymers

A model of two-step carrier photogeneration in doped conjugated polymers is suggested. On the first step, a dissociation of a relaxed exciton into a Coulombically bound geminate pair (GP) occurs at a charge transfer center which consists of a polymer chain and a nearby dopant or deep trap. On the second step, the GP dissociates into free carriers due to carrier release from the on-chain potential well formed by the Coulomb and external fields. The generation yield is shown to be practically field- and temperature-independent in a polymer with a small on-chain effective carrier mass.

Equilibrium trap-controlled and hopping transport of polarons in disordered materials

Equilibrium trap-controlled and hopping mobility of polarons in disordered materials is calculated analytically. It is shown that, irrespective of both the transport mode and the density-of-states distribution, the activation energy of the equilibrium carrier mobility is a sum of the activation energy as calculated for the same density-of-states distribution without the polaronic effect, and one half of the polaron binding energy.

An electronic model of photoinduced optical anisotropy in chalcogenide glasses

A purely electronic model of photoinduced optical anisotropy in glassy semiconductors is formulated. The model is based on the concept of geminate electron-hole pairs captured by correlated deep traps. The kinetics of photoinduced anisotropy are controlled by the photogeneration, trapping and recombination of geminate pairs. Predictions of the model are shown to be in agreement with experimentally observed basic characteristic features of photoinduced anisotropy.