A. V. Kukhto

Electroluminescence of Thin Films of Organic Compounds (Review)

This paper is a brief review of investigations on the electroluminescence of organic compounds. It considers the background of the problem, the organic electroactive materials, the structures based on them and the technology of their preparation, the main physical characteristics of electroluminescence (spectra, polarization, volt-luminance and volt-ampere characteristics, kinetics, temperature dependence, quenching, efficiency and operation time, and ways of increasing them). A brief analysis of the investigations on injection of charge carriers, their transport, and the formation of excited states of molecules as well as applications and prospects of development of organic electroactive materials and structures is carried out.

Electrical and luminescence properties of a poly(amidoamine) dendrimer containing naphthalimide

The electrical and luminescence properties of a poly(amidoamine) (PAMAM) dendrimer containing naphthalimide are investigated. The influence of adsorbed oxygen on the electrical conductivity of thermally evaporated organic films is analyzed using cyclic thermal desorption. The results obtained are interpreted within a model of two-level hopping electron transfer according to which one of the two systems of energy levels corresponds to intrinsic states of electrons in PAMAM molecules and the other system is associated with the states of electrons in adsorbed oxygen molecules. The use of the cyclic thermal desorption method in combination with the proposed model makes it possible to estimate the localization length of electrons numerically and to determine the states involved in hopping electron transfer. It is demonstrated that the studied compound in the solid state possesses a high luminescence power over a wide spectral range.

Spectral‐Luminescent Properties of Oxazoles and Oxadiazoles in a Gas Phase on Excitation by Electrons

The energy‐loss spectra of electrons, fluorescence excitation functions, and the fluorescence spectra on excitation of the vapors of a number of oxazoles and oxadiazoles by monokinetic beams of electrons of various energies are determined. In contrast to optical absorption spectra, in the energy‐loss spectra of a number of studied substances a band associated with the S0–T1 singlet‐triplet transition is observed. The π–π*‐type transitions are fixed up to S0–S5 on excitation of molecules by high‐energy electrons, including the region of vacuum ultraviolet. The cross sections of elastic and inelastic collisions of electrons of different energies with POPOP molecules have been measured. The dependences obtained differ substantially from those calculated in the Born approximation. The cross section of elastic scattering is in a rather good correspondence with the geometric section of the molecule.

Spectral‐Luminescence Characteristics of Polyphenyls and Polyacenes in the Gas Phase in Electron‐Beam Excitation

The spectra of electron‐energy loss, excitation functions, and fluorescence spectra in excitation of the vapor of polyphenyls and polyacenes by electron beams of different energies are determined. The influence of successive complication of the molecules under study on these spectral‐luminescence characteristics is tracked. Unlike the optical absorption spectra, in the spectra of electron‐energy loss of all the substances studied one observes a band which is related to the singlet‐triplet transition S0–T1. The transitions up to S0–S5 are recorded in excitation of the molecules by high‐energy electrons, including the region of vacuum ultraviolet. From the functions of fluorescence excitation the authors have determined the excitation thresholds that correlate with the energies of the S1 levels, except for pyrene in which the S0–S1 transition is forbidden and does not show up not only in photon excitation but also in electron‐beam excitation, although the intercombination forbiddenness in the latter case is removed and the S0–T1 band is observed.

Electroluminescence of Belophores in a Wide Spectral Region

The results of spectral investigations of the electroluminescence of triazinestilbene derivatives in the amorphous state and in a polymeric matrix are presented. These compounds also possess transport properties. The use of two layers that contain belophores, i.e., triazinestilbene and naphthalimide, led to radiation in the blue and green spectral regions.

Losses in the Energy of Low-Energy Electrons in Fluorene, Fluorenone, and Diiodofluorenone Vapors

Electron energy loss spectra (EELS) of fluorene, fluorenone, and diiodofluorenone vapors excited by monokinetic electrons of energies 15–50 eV have been obtained. The singlet and triplet absorption bands of these molecules have been calculated. Comparison of these bands with the experimental EELSs and optical absorption spectra has shown that the forbiddenness of singlet-triplet transitions is not completely removed in the process of interaction of molecules with electrons. The presence of heavy iodine atoms in the diiodofluorenone molecule enhances singlet-triplet transitions. Bands of overtones of stretching vibrations of the CH groups of the benzene rings have been detected near the peak of elastic scattering of electrons of the molecules studied.

Electron Energy Loss Spectra of the Organic Complexes of Europium

In the electron energy loss spectra (EELS) of the organic europium complexes Eu3+ (BTFA)3TPPO and Eu3+(Br‐BTFA)3TPPO in a gas phase obtained on excitation by monokinetic beams of electrons of different energies in the range 12–50 eV, we have identified the bands associated with the electron transitions S0–S1, S0–S2, and S0–S3. The connection of these transitions with the structural groups of the complexes is established. The addition of the bromine atom to the phenyl ring of β‐diketonate leads to the rise in the relative intensity of the S0–S2 band. The singlet‐triplet transitions manifest themselves in the region 2.5–3.2 eV and contribute to the S0–S2 band of the electron energy loss spectra.

Spectral-Luminescence Characteristics of Carbazole, Dibenzofuran, and Dinaphthofuran in the Gas Phase in Excitation by Electrons and Photons

The spectra of electron-energy loss, the excitation functions, and the fluorescence spectra in excitation of carbazole, dibenzofuran, and dinaphthofuran by monoenergetic beams of electrons of different energies are determined. The singlet-triplet transitions S0–T1 and the singlet-singlet transitions up to S0–S7 are recorded, which covers the region 2–11 eV. In the spectra of electron-energy loss, bands that refer to the nπast and ππ* transitions are identified. The replacement of the heteroatom of nitrogen by the atom of oxygen in the five-membered ring has no substantial effect on the spectra of electron-energy loss.

Modeling of Electroluminescence of Thin Organic Films

We propose a simple model for describing the electroluminescence in a single-layer thin-film organic electroluminescent cell on the assumption that the luminophore molecules are directly excited by electron impact and are the result of recombination. The calculated luminescence and efficiency curves are in qualitative and quantitative agreement with experimental data. To attain a high efficiency, it is necessary to use cathodes with a low work function and a high Fermi energy and relatively small electric fields.

Conditions for Obtaining Stimulated Radiation in Thin‐Film Organic Electroluminescent Structures

This paper analyzes the possibilities of obtaining induced radiation in single‐layer organic electroluminescent structures on the basis of the proposed model of luminophor molecule excitation by “hot” electrons emitted from the cathode as a result of the tunnel effect. Numerical calculations of the kinetic, spectral, and energy characteristics of radiation in the regime of single‐pass superluminescence and in the presence of positive feedback have been made. It has been concluded that, in principle, it is possible to obtain in such systems induced radiation at actually attainable values of the applied electric‐field strength, and the conditions for this have been analyzed.