Exciton dynamics at interfaces of organic semiconductors

We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C₆₀ and pentacene. For C₆₀ thin films epitaxially grown on Au(1 1 1) and Cu(1 1 1) surfaces, we observe both charge transfer and exciton states. For excitons in C₆₀, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

Exciton binding energies in II–VI compound strained layer superlattices

II–VI strained-layer superlattices are very efficient emitters of visible light. The dependence of the luminescence intensity on the excitation power density allows us to characterise the recombination processes involved in the emission. At low temperatures excitonic processes are dominant whereas electron-hole recombinations feature at room temperature. No special evidence of the dual nature of the emission is observed at intermediate temperatures because the optical transitions are broadened by well-width fluctuations. In spite of this we may estimate the exciton binding energy from the temperature dependence of the photoluminescence intensity, as long as the photoluminescence remains excitonic. This is the case for narrow wells in CdS---ZnS superlattices over the temperature range zero to room temperature. The estimated exciton binding energy measured in this way approaches the two dimensional limit but does not exceed it.     

Exciton dispersion law in diamond-like semiconductors

It is shown that in covalent diamond-like semiconductors the minimum of the exciton energy is situated at non-zero exciton momentum if the heavy hole mass is large enough. The depth of the minimum may be of the order of the exciton binding energy. The minimal exciton energy is calculated for several semiconductors with the variational procedure.     

Exciton molecules and exciton liquids in semiconductors

Excitons in many-valley semiconductors form molecules consisting of four and more excitons. The degeneracy factor g of the conduction band in germanium is 8, and in silicon g=12. As in acceptors, the hole ground state in excitons is fourfold degenerate. The same is valid for exciton molecules, because they are quantum objects with spherical symmetry. The exciton binding energy in molecules is close to that in exciton-liquid droplets. Experimental evidence is considered for the existence, besides biexcitons, of stable exciton molecules consisting of three and four, and, possibly, 11 and 12 excitons. Molecules containing from five to ten excitons are apparently unstable. Fiz. Tverd. Tela (St. Petersburg) 40, 929–931 (May 1998)     

Analytical expressions for the binding energies of excitons and acceptors in cubic semiconductors

The effective mass equation for direct excitons and acceptors in cubic semiconductors is solved accurately. Simple analytical expressions for the binding energy of the most important states are given as function of the spherical and cubic coefficients.     

Exciton molecule in semiconductors by phonon-assisted two-photon absorption

Direct creation of bi-excitonic states by photon-assisted two-photon absorption in indirect-gap semiconductors is investigated theoretically. The symmetry of the indirect bi-exciton states and of the phonon used are given for the case of Ge. A numerical application to the case of Si shows that the indirect two-photon absorption coefficient for bi-excitonic α²_I (bi-ex) transitions is several orders of magnitude larger than both indirect two-photon interband, α²_I (band), and excitonic, α²_I (exc), transitions. It becomes smaller than both indirect one-photon interband, α¹_I (band), and excitonic, α¹_I (exc), transitions for available laser intensities. The essential contribution to this enhancement of α²_I (bi-ex) is found to be from the resonance effect in the first process and from both the resonance effect and matrix elements included in the second process.     

Exciton and donor binding energies in quantum-well wires and quantum dots a fractional-dimensional space approach

A simple method for calculating the free-exciton binding energies in the fractional-dimensional-space model for single-quantum-well structure has been extended to quantum-well wires and quantum dots, in which the real anisotropic system is modelled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz. We calculated the ground-state binding energies of excitons and donors in quantum-well wires with rectangular cross sections. Our results are found to be in good agreement with previous variational calculations and available experimental measurements. We also discussed the ground-state exciton binding energy changing with different shapes of quantum-well wires,     

Exciton susceptibility of semiconductors at high laser-excitation levels

The hysteresis behavior of the real and imaginary parts of the susceptibility of a semiconductor in the exciton region of the spectrum is investigated with consideration of the exciton-phonon and elastic exciton-exciton interactions in the pump-probe regime as a function of the intensity and frequency of the strong laser pulse and the frequency of the probe pulse. The conditions for the appearance of suppression of the damping and amplification of the probe pulse are determined. Abrupt red and blue shifts of the spectral position of the exciton absorption band as a function of the pump pulse intensity are predicted. Fiz. Tverd. Tela (St. Petersburg) 40, 934–935 (May 1998)     

Exciton effect in the interband electronic raman scattering in semiconductors

The cross section of the interband electronic Raman scattering in the two band gap and with the allowed dipole transition is calculated. The exciton effect was taken into account.We express our thanks to Prof. Nguyen Van Hieu for suggesting the problem and fruitful discussions.     

Reduced exciton binding energy in organic semiconductors: Tailoring the Coulomb interaction

For organic photovoltaics (OPV) the maximum in obtainable power conversion efficiency is limited by a low semiconductor permittivity and the resulting enhanced Coulomb interaction (CI). This, however, is an aspect rarely addressed in the OPV development. Here, a concept is introduced which allows a reduced CI in organic semiconductors. This is the result of a device structure, which upon exciton formation forces part of the electric field between complementary charges through a high‐k material, resulting in partial field screening and as such a reduced CI. The feasibility of this concept is substantiated by an investigation on the exciton separation efficiency in pentacene devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitons in organic semiconductors

In this article, excitonic effects in organic semiconductors investigated within the framework of many-body perturbation theory are reviewed. As an example for this technologically relevant class of materials the oligoacene series is studied. The electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green's function. This approach allows for the evaluation of the exciton binding energies, which are of major interest concerning the application in organic opto-electronic devices. We start the discussion with a comparison of the Kohn–Sham band structure with recent angular resolved photo-emission data. Starting from this one-electron band structure we focus on the impact of the electron–hole interactions on the optical properties by solving the Bethe–Salpeter equation. We demonstrate the dependence of the exciton binding energy on the molecular size and emphasize the effect of the intermolecular interaction on the exciton binding energies by means of pressure investigations. To cite this article: P. Puschnig, C. Ambrosch-Draxl, C. R. Physique 10 (2009).     

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.     

Exciton binding energy in CdMnTe crystals

We report some results of magnetoabsorption and magnetoreflectivity measurements performed on CdMnTe crystals in magnetic field up to 30 T. The exciton energy at low fields in materials containing magnetic ions is calculated on the basis of the Altarelli-Lipari model and compared with the experimental results. It is shown that spin mixing in degenerate bands strongly affects magnetoabsorption.     

Space-charge-limited magnetoresistance in organic semiconductors

We present a two-site model for large magnetic field effect on current of nonmagnetic organic semiconductor, based on bipolaron formation in the presence of both hyperfine field and an applied magnetic field. Under the framework of space-charge-limited current, we derived our current density and magnetoresistance expression. The magnetoresistance fit well with the Lorentzian empirical law. We discuss all parameters involved in, and elucidate the origin of positive and negative magnetoresistance under our model.