Yu. A. Skryshevskii

Thermally stimulated luminescence in porous silicon

The low-temperature thermally stimulated luminescence (TSL) in both porous silicon (PS) layers on Si substrate and PS powder has been found in the 4.2–300 K temperature range. The dependence of the TSL effect on the wavelength and temperature of an excitation was studied in detail. The shape of TSL glow curves displays the quasicontinuous spectrum of the electron states in the gap of PS and the activation energy is subject to the linear law of E[eV]=0.0029T−0.14 at T>70 K. For the homogeneous PS powder, the TSL glow curves weakly depend on an excitation wavelength. For the PS layer on Si substrate the maximum of the TSL glow curve shifts to the higher temperature both at the excitation by the short wavelength light and by the supplementary exposition to IR illumination. Such behavior is stipulated by the gradient of the PS energy band gap with the depth and, consequently, by the reconstruction of electron states in the band gap. It is shown that the TSL is conditioned by the photoluminescence (PhL) in PS ...

Electronic excitation energy transfer in poly-N-epoxypropyl-3,6-dibromocarbazole

The mechanisms of electronic excitation energy transfer in films of poly-N-epoxypropyl-3,6-dibromocarbazole doped with pyrene, rubrene, and bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)iridium(III) (Btp2Ir(acac)) have been studied in the range T = 5–295 K. It has been established that the mechanisms of transfer depend on the relative position of the S1 and T1 levels of the base and dopant: if the S1 level of the dopant lies between the S1 and T1 levels of the base and its T1 level is below than that of the base (pyrene molecules), only the triplet-triplet transfer takes place, whereas if the S1 level of the dopant lies below the T1 level of the base (pyrene and Btp2Ir(acac) molecules), the triplet-singlet transfer is observed in addition to the triplet-triplet transfer. It has been assumed that the spin-forbidden triplet-singlet energy transfer is allowed because of the strong spinorbit interaction induced by two heavy bromine atoms in chromophores of the polymer. This transfer proceeds through the long-range dipole-dipole interaction at low temperatures and the electron exchange interaction in the vicinity of room temperature.

Phototransformations in polysilane films

The absorption (T=295 K) and luminescence (T=5 and 295 K) spectra of films of alkyl and aryl substituted polysilanes, namely, poly(dihexylsilane) (PDHS) and poly(methylphenylsilane) (PMPS), are investigated as functions of the time and the wavelength of light irradiation at Tirr=5 and 295 K. It is shown that the photodegradation in polysilane films depends on the temperature, irradiation wavelength, and the structure of side substituents. The absorption of light by short chain segments in polysilanes at room temperature leads to competitive processes such as the transfer of excitation energy to longer segments, the scission of σ bonds between Si atoms, and the radiationless dissipation of excitation. It is revealed that, at Tirr=295 K, the photo-degradation of PDHS films is accompanied by the transformation of certain chain segments from a low-temperature trans conformation with an ordered arrangement of side hexyl groups to a high-temperature helical conformation with a disordered arrangement of side groups.

Structural defects in carbazolyl-containing polymers with polymethine dye impurities

Photoluminescence spectra and thermoluminescence curves of pure and doped carbazolyl-containing polymers polyvinyl carbazole (PVC) and poly-N-epoxypropyl carbazole (PEPC) are investigated in the temperature range 5–300 K. The impurities are cationic indocarbocyanines with various lengths (n) of the polymethine chain: HIC (n=1), HID (n=2), HIT (n=3) and squaryl dye HISq with the same polymethine chain length and the same structure of heterocyclic end groups as for the HID dye. It is found that solvation of dye molecules by polar groups of a polymer is accompanied by conformation changes in the polymer, which are considerably enhanced with increasing n, as well as upon a transition from a rigid-chain PVC macromolecule to a more flexible PEPC molecule. As a result, the concentration of structural defects, viz., excimer-forming sites playing the role of traps for singlet excitons and charge carriers, increases considerably in doped PEPC films. This leads to the emergence of a band with a peak at 460 nm in the luminescence spectra of PEPC films with HID and HIT and at 480 nm for films with the HISq impurity, while the thermoluminescence curve for PEPC with HISq acquires an additional band with a peak at 275 K.

Spectral–Luminescence Properties of Molecular Complexes of Photoconductive Polymers with Neutral Organic Acceptors

It has been found that in films of aryl– and alkyl–substituted polysilanes -- poly(methylphenylsilane) and poly(dihexylsilanes) -- the molecules of electron acceptors of tetracyanoquinodimethane, dinitrofluorenone, and trinitrofluorenone form weak complexes with charge transfer from the longest segments of the polymeric chain. The absorption and luminescence spectra of these complexes are similar to the spectra of complexes formed by the carbonyl groups of poly–N–epoxypropyl carbazole with the above acceptors and consist of wide bands situated in the visible range of wavelengths.

Unusual temperature dependences of the photoconductivity and recombination luminescence of amorphous molecular semiconductors doped with ionic dyes

A nonexponential increase in photoconductivity with increasing temperature is discovered for poly(N-epoxypropylcarbazole) (PEPK) films doped with polymethine dyes. It is postulated that traps for nonequilibrium charge carriers form in these films during irradiation and are destroyed as the temperature is raised. Such traps are manifested by broadening of the high-temperature shoulder on the thermally stimulated luminescence (TSL) curves following the preliminary irradiation of PEPK films doped with polymethine and xanthene ionic dyes in the visible or UV range at 250–320 K and by the appearance of a new narrow TSL maximum near the preliminary irradiation temperature. These TSL features disappear after prolonged storage of the films in the dark or heating to higher temperatures.