Dielectric relaxation studies of poly(N-vinylcarbazole) and poly(3-chloro-N-vinylcarbazole) including the effects of sorbed oxygen

Abstract

NMR relaxationtime studies of poly(Nvinyl carbazole) and sorbedO2 effectsThe dielectric relaxation spectrum of poly(N-vinylcarbazole) (PVK) and poly(3-chloro-N-vinylcarbazole) (CLPVK) have been investigated in the temperature range -180-+ 240 DC. Four relaxations are observed in PVK. The a relaxation (225°C) is associated with the glass transition of the polymer. On the basis of oxygen-sorption experiments, the y relaxation (--60°C) has been associated with a rotationallibration of the pendant carbazole group. It is also speculated on the basis of this work that an off-axis dipole is induced in the carbazole moiety by the sorbed O 2 producing the dielectrically active motion about the symmetry axis of the carbazole moiety. The 13 (-80°C) and the Il (--160°C) relaxations are assigned as carbazole wagging motions and localized backbone motions, respectively. These assignments are based on comparative work in the literature.PACS numbers: 77.40.

The nature of trapping sites and recombination centres in PVK and PVK–PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

“…Such mechanism is called as the "wet dog" effect [25]. Akuetey and Hirsh [26] based on an analysis of dielectric measurements for PVK made by Pochan et al [27], have found that the -relaxation in this polymer extrapolated to the mHz frequency range should appear at around 130 K. This frequency range corresponds to an equivalent frequency of thermally stimulated processes, if the heating rate is a few K/min [26]. It means that the detrapping which gives rise to the TL maximum at around 130 K could be induced by the -relaxation in PVK.…”

Thermoluminescence of poly(9-vinylcarbazole) modified by substitution with halogens

“…Thus, the TL maxima often correspond well to the onset of molecular motions, what affords evidence for a significant contribution of the molecular motions to the charge carrier detrapping process [13,44]. It is known, on a basis of dielectric spectroscopy of PVK done by Pochan et al [45], that at around 130 K, the c-relaxation corresponding to the limited rotation of carbazol-9-yl groups appears, if the dielectric experiments are performed in the mHz range. This frequency range corresponds to an equivalent frequency of the thermally stimulated processes (TL, and also TSCthermally stimulated currents or TSD -thermally stimulated depolarisation) if the heating rate is a few K/min.…”

“…2b), and amount 390 meV and nearly 100 meV for c-relaxation and d-relaxation, respectively [45]. This is because only an onset of the molecular motions may be sufficient to facilitate the detrapping of the localised charge carriers e.g.…”

“…Also the low-temperature TL maximum might be correlated with the d-relaxation in PVK. This relaxation is attributed to wagging motion of the carbazol-9-yl groups [45]. Such molecular relaxation can also promote detrapping of the charge carriers.…”

Nanoscale heterogeneity and light-emission dynamics in solution-processed phosphorescent organic light-emitting devices

“…The voltage-dependent part of the process more likely corresponds to b-relaxation which occurs at 80 1C and which corresponds to the wagging motion of the carbazole units. 24 The wagging motion of neighbouring carbazoles would significantly change their distance and thus the charge transport between them. Local (nanoscale) heating to 80 1C in the poorly thermally conductive PVK film is plausible since it has been shown that a luminance of 1000 cd m À2 in an OLED can lead to overall temperatures of the active layer of 46 1C.…”