N. Kirova

Singlet exciton binding energy in poly(phenylene vinylene)

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band π-π* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with Eb ≈ 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.

A systematic theory for optical properties of phenylene-based polymers

A complete picture of phenylene-based polymers is developed which unifies features of band and molecular exciton models. It incorporates major experimental findings in direct and photoinduced optical absorption, in stimulated photoemission and photoconductivity. Our theoretical picture is based upon a band description for electronic states, while invoking corrections from Coulomb interactions. We demonstrate the existence of different types of Coulomb excitons and their roles in optical absorption. We show that a low binding energy-emitting exciton also dominates in the fundamental absorption. Contradictions in the current modeling state-of-the-art are displayed and discussed. For poly(phenylene vinylene) (PPV)-type polymers, we give new assignments for the most disputed features; for oligomers we identify new ones as edge states. In applying our model to the poly(phenylene) (PPP) family, more progress is available due to analytical results covering not only spectra but also the oscillator strength. We conclude similar assignments as for PPV and emphasize the delocalized nature of basic features. An important and intriguing peculiarity of PPP- compared with PPV-based polymers is the possible level inversion between excitons. Comparison is made to available optical and electron energy loss spectroscopy (EELS) data. Our conclusions are suggestive for future light-polarized experiments.

Excitations and optical properties of phenylene-based conjugated polymers and oligomers

Abstract We present a combined experimental and theoretical study of the ground and photoexcited optical properties of a model oligomer of PPV, MEH-DSB. Our theoretical picture is based upon a band description of electronic states of PPV oligomers, while invoking corrections from Coulomb interactions. The necessary discrete energy levels at low and intermediate energies appear naturally, in addition to the lower energy delocalized states. On this basis we identify the most important features in direct optical absorption for both high (4–6 eV) and low (2–4 eV) photon energies as well as in photoinduced absorption (PA) and stimulated photoemissions (SE) in MEH-DSB solutions, which represent the limit of noninteracting oligomers. While in agreement with previous interpretations for three absorption peaks (2.74, 4.46 and 6.2 eV), we give a new assignment for the most disputed 3.62 eV one as well as for the two PA peaks.

Theory of electronic states and excitations in PPV

Abstract We present a consistent theoretical picture for optical properties of phenyl based polymers, especially for the PPV family. The model is based upon an analytical solution for the band structure of PPV oligomers, while invoking the dominant Coulomb corrections for electron-hole interactions. The adjustable parameters are only the common shift for the bands centers of gravity and a dielectric susceptibility at small distances. Our picture gives a clear understanding for the origin of all possible transitions in linear and nonlinear optics. We describe both tightly bound localized excitons and excitons of intermediate range (i.e. of both the Frenkel and Wannier-Mott types). The quantitative description of excitons is obtained from the long range Coulomb interactions, We emphasize where the ring torsion plays a role in the overall energy minimization of the excited state. This article provides theory details for the joint article [S. Brazovskii, N. Kirova, A.R. Bishop, V. Klimov, D. McBranch, N.N. Barashkov, J.P. Ferraris, Opt. Mater. 9 (1998) 472], where a complete picture was outlined.

Stability of bipolarons in conjugated polymers

The bipolarons appear to be the main entity for charge collection in conducting polymers. But their existence is challenged by the Coulomb repulsion. We study the stability of a bipolaron (BP) as a function of the strength of the long range Coulomb interaction U with and without impurities for both weak and strong electron-phonon coupling (α). We find that in a free state the BP is stable only for small values of U for the weak coupling limit but stays stable until relatively large U in the strong coupling case. Being bound to a dopant, the bipolaron becomes stable in a wide range of U. Free BPs near the metal-polymer contact also are stabilized due to a screening by image charges. These results are important for charge injection at polymer-metal interfaces in the context of organic light-emitting diodes.

Optics of polymers in the light of solid state physics

We review approach of the solid state physics to the electronic and optical properties of conducting polymers. We consider their generic features as 1D semiconductors as well as the particularities of the phenyl group. We are taking efforts to bring together the solid state theory for polymers and the quantum chemistry of oligomers.

Electric field induced ionization of the exciton in poly(phenylene vinylene)

The exciton binding energy (E b ) and the band gap energy (Eg) of poly(phenylene vinylene), PPV, have been determined by photoconductivity excitation profile spectroscopy as a function of applied electric field. The spectral signature of the exciton is a narrow peak (100 meV full width at half maximum) that emerges just below the band edge upon increasing the external field or the defect density. The exciton peak is observed only for light polarized parallel to the chain axis. The exciton binding energy is obtained from the energy of the exciton peak with respect to the band edge and, independently, from analysis of the field dependence of the exciton dissociation: E b 55 meV.

On the possible superfluidity of bipolarons on the junction surface

Abstract There is a number of materials where bipolarons (BP) are probably the main extrinsic carriers. We discuss a dense system of BP produced by the equilibrium charge injection at the surface of Schottki or MIS junctions. Possible super-conductivity of this system due to the Bose-condensation of BP is studied.

Mechanism describing mobility of solitons in crystalline polyacetylene

Abstract A mechanism has been developed describing the mobility of solitons in crystalline polyacetylene. It is shown that in addition to defects of dimerization, or kinks, there may exist sceletol solitons — twistons, which represent nonlinear acoustic deformations of the polymer skeleton. The twistons and the kink have the same topological charge, both changing the sign of the dimerization. Their mutual interaction via confinement force results in the formation of a mobile particle — a kink-twiston complex. Such a particle moves freely along the chain, but the kink is localized in the vicinity of its center.

Towards the theory of metal-polymer contact

Abstract The band structure and the electric field distribution at the metal - polymer junction are considered. In contrast to a usual picture of band bending near the contact for common semiconductors the branching of bands takes place and inside the former gap the new allowed energy bands appear. The Fermi-level of the metal always lies inside the energy gap of the polymer independent on the value and sign of the voltage applied across the contact. Single-electron (polarons) majority carriers created by the junction injection or by the interband absorption are affected by an effective force, which pulls them to the contact interface. The single-electron minority carriers in the presence of the superstructure of solitons (bipolarons) do reverse their charge by recombining with two solitons (one bipolaron). The depletion layer forms via conversion of bipolarons to polarons. The possible influence of the band branching on the contact barrier and optical properties are discussed.