N. I. Ostapenko

Photoinduced Metastability and Degradation of Poly[Methyl (Phenyl)Silylene] as seen by Thermoluminescence

Abstract Photodegradation of poly[methyl(phenyl)silylene] (PMPSi) was studied by low-temperature thermoluminescence and photoluminescence. It has been found that this process is nearly reversible at moderate UV exposure; the reverse reaction could be accelerated thermally. The activation energy of the annealing process was determined as 0.65 eV. Different character of the photodegradation was found under the excitation with 366 nm and 250–280 nm light at room temperature (i.e., within [sgrave]-[sgrave]* and π-π* transitions, respectively), σ-σ* excitation leads to the formation of deep charge carrier traps (Et = 0.45 eV) of photodestructive origin which are associated with photoscission of Si bonds in the backbone. The presence of electron acceptor dopants strongly inhibits the trapping ability of these traps.

Clusters of dipole charge-carrier capture centers in organic crystals

Abstract Clusters of dipole charge-carrier capture centers were studied in anthracene crystals doped with carbazole. The microscopic model for the dipole centers of the different structure is proposed. The thermoluminescence of the anthracene crystal doped with carbazole was investigated in the temperature range of 4.2–300 K. It was shown that thermoluminescence bands became structured with the increase of the impurity concentration. The observed structure is explained by the emergence of dipole charge-carrier capture centers created by clusters of impurities, which consist of one, two or more molecules. The authors carried out theoretical calculations both for immobile carriers and taking into account their delocalisation. The theoretical results are in good agreement with experimental ones.

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.

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.