Vladimir Arkhipov

CHARGE INJECTION INTO LIGHT-EMITTING DIODES : THEORY AND EXPERIMENT

An analytic theory is presented for dark injection from a metallic electrode into a random hopping system, e.g., a conjugated polymer or a molecularly doped polymer. It encompasses injection of a charge carrier from the Fermi level of the electrode into tail states of the distribution of hopping states of the dielectric followed by either return of the charge carrier to the electrode or diffusive escape from the attractive image potential. The latter process resembles Onsager-type geminate pair dissociation in one dimension. The theory yields the injection current as a function of electric field, temperature and energetic width of the distribution of hopping states. At high electric fields it resembles that the current calculated from Fowler-Nordheim tunneling theory although tunneling transitions are not included in the theory. Good agreement with experimental data obtained for diode structures with conjugated polymers as a dielectric is found.

Equilibrium carrier mobility in disordered hopping systems

Abstract It is shown that the charge carrier mobility in a positionally and energetically disordered hopping system can be evaluated by averaging either the hopping rates or the hopping times over the thermal equilibrium energy distribution of localized carriers. However, at variance with averaging the hopping rates, averaging the hopping times can be correct only if the energy dependence of the carrier energy relaxation time is also taken into consideration. The temperature and concentration dependences of the equilibrium carrier mobility were calculated by averaging the hopping rates. The consideration was based on the variable-range hopping approach with full account for the interplay between jump distance and energy difference. However, the obtained results prove that, in good quantitative agreement with both Monte Carlo simulations and experimental data, the mobility can be approximated as a product of two functions. The first function depends almost solely upon the temperature and reveals only a weak concentration dependence while the second mainly governs the concentration dependence of the mobility and is almost independent of the temperature.

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.

Hopping model of thermally stimulated photoluminescence in disordered organic materials

Abstract An analytical model of thermally stimulated photoluminescence (TSPL) in a random hopping system is formulated. The model is based on the assumption that TSPL originates from radiative recombination of sufficiently long geminate pairs of charge carriers created during photoexcitation of the sample at a low (helium) temperature. Since TSPL measurements are normally performed after some dwell time the initial energy distribution of localized carriers is formed after low-temperature hopping relaxation of photogenerated carriers and, therefore, first thermally assisted jumps of relaxed carriers are considered as rate-limiting steps in the present model. Predictions of the model are found to be in good quantitative agreement with experimental data on molecularly doped polymers if a double-peak energy distribution of localized states is invoked for these materials. Comparing theoretical results with existing experimental data also reveals a somewhat slower low-temperature energy relaxation of charge carriers in these materials than predicted by the conventional theory of carrier random walk in random hopping systems.

A tandem mechanism of charge-carrier photogeneration in disordered organic materials

A model of tandem charge-carrier photogeneration in disordered organic materials at low excess photon energies above the absorption edge is formulated. It suggests that relaxed singlet excitons dissociate into strongly bound metastable short off-chain geminate pairs. In parallel, triplet excitons can be generated from both primary singlets and short geminate pairs. Owing to a long intrinsic lifetime of triplets, most of them are quenched by geminate pairs and the latter can gain additional energy that facilitates transformation of short pairs into longer ones. This reduces the Coulomb binding energy such that field-assisted full dissociation into free carriers becomes feasible.

Charge carrier photogeneration yield in amorphous materials with long-range potential fluctuations

Random intrinsic electrostatic potentials cause local intrinsic electric fields that must affect the Onsager probability of geminate pair dissociation. If the intrinsic fluctuating field is strong enough the dissociation yield could be close to unity even at weak external electric fields and at low temperatures. An analytic model describing the effect of intrinsic field and potential fluctuations on the carrier photogeneration yield is formulated and compared to observed behavior in a-Si:H and a-Se.

Modeling of Photoinduced Anisotropies in Chalcogenide Glasses

Photoinduced structural changes were recognized early on in the study of the optical properties of chalcogenide glasses, and more than two decades ago it was further noted that these changes could make the glasses optically anisotropic if polarized light is used for the illumination [1], The latter phenomena have been termed vectoral effects.