Bias-Dependent Generation and Quenching of Defects in Pentacene

We describe a defect in pentacene single crystals that is created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at Ev + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (108 s-1) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.Comment: 10 pages, 3 figure

Photovoltaic and optical properties of a polymer - PbS nanocomposite

Photovoltaic and optical properties of a polymer-PbS nanoparticles in polyvinyl alcohol (PVA) films and the effect of annealing in air at various temperatures on these properties are studied in this work. In the range of 1.0 to 3.7 eV, absorption spectra of PbS nanocomposite films are correctly fitted by five Gaussian bands peaking at 1.5, 1.84, 2.41, 3.06, and 3.58 eV. A characteristic feature of the films is the presence of green and blue luminescence with intensity maxima at 2.25 and 3.02 eV, respectively. The luminescence intensity strongly depends on the excitation energy and the annealing temperature in air. The highest intensity of the green luminescence is observed under the excitation with 2.35 eV radiation, whereas the intensity of the blue luminescence peaks under 3.46 eV excitation. After annealing at 370 K, the intensity of the green luminescence decreases a few times whereas the intensity of the blue emission increases. The photovoltaic sensitivity of the PbS nanocomposite films depends on the annealing conditions, attaining the maximum after annealing in air at approximately glass transition temperature of PVA (370 K). The spectral range of the photosensitivity of PbS nanocomposite films is wider than that of CdSe- and CuInS₂-polymer nanocomposite films

Non-Exponential Decays in First-Order Kinetic Processes. The Case of"Squeezed Exponential"

Kinetics of processes, in which the reaction rate increases with conversion, is discussed and illustrated with an example of the chemical reaction of isomerization of an azobenzene derivative in a liquid crystalline matrix. A simple phenomenological model is put forward explaining the effect by dynamic changes of interactions between the reacting species and the matrix

Non-Exponential Decays in First-Order Kinetic Processes. The Case of"Squeezed Exponential"

Kinetics of processes, in which the reaction rate increases with conversion, is discussed and illustrated with an example of the chemical reaction of isomerization of an azobenzene derivative in a liquid crystalline matrix. A simple phenomenological model is put forward explaining the effect by dynamic changes of interactions between the reacting species and the matrix

Trapping of Charge Carriers in Organic Molecular Materials: Phthalocyanine Thin Films Revisited

The technique of temperature-modulated space-charge-limited currents was employed to study the energetic distribution of local states in thin films of metal-free phthalocyanine. Four well-defined local levels were detected within the energy range 0.5-0.3 eV, accompanied by an onset to a manifold of shallower states. The morphology of the films, depending on the deposition rate and changing from polycrystalline to nearly amorphous, affects the densities of local states but not their energies

Trapping of Charge Carriers in Organic Molecular Materials: Phthalocyanine Thin Films Revisited

The technique of temperature-modulated space-charge-limited currents was employed to study the energetic distribution of local states in thin films of metal-free phthalocyanine. Four well-defined local levels were detected within the energy range 0.5-0.3 eV, accompanied by an onset to a manifold of shallower states. The morphology of the films, depending on the deposition rate and changing from polycrystalline to nearly amorphous, affects the densities of local states but not their energies

Phase transitions in ferroelectric nonachlorodiantimonate [(CH 3)3NH]3Sb2Cl9 studied by calorimetric and dielectric methods

Phase transitions occurring in a ferroelectric crystal tris(trimethylammonium)nonachlorodiantimonate (III) (TMACA) were studied by calorimetric, dielectric and pyroelectric methods. Anomalies of dielectric and thermal properties were found around 200 K and at ca. 364 K. The anomalies in the low-temperature region exhibit a pronounced relaxational character. A detailed study of the behaviour of TMACA around 364 K clearly indicates that in fact TMACA undergoes two closely lying phase transitions, one of them being probably of the first order and the other of the second order. The temperature of the second order transition was determined to amount to (364.0 ± 0.3) K irrespective of the method employed, whereas the first order transition temperatures are detected by the dielectric measurements ca. 1 K below those found from the calorimetric measurements. An unusual sequence of the phase transitions was observed in DSC measurements : both on cooling and on heating the samples through the transition region, the second order phase transition precedes the first order one. The onset to ferroelectricity was found to be associated with the first order phase transition.Nous avons étudié des transitions de phase dans les cristaux de tris (triméthylammonium) nonachlorodiantimonate (III) (TMACA) par microcalorimétrie différentielle (DSC), ainsi que par des mesures des propriétés diélectriques et pyroélectriques. A 364 K, TMACA présente une transition depuis une phase basse température ferroélectrique vers une phase haute température paraélectrique. Une anomalie a été aussi observée à environ 200 K, associée probablement à une relaxation. Les résultats obtenus montrent que la transition à 364 K présente en fait deux transitions très rapprochées, l'une étant probablement du premier ordre et l'autre du second ordre. La transition du deuxième ordre est observée à la même température (364.0 ± 0,3) K quelle que soit la méthode utilisée, alors que celle du premier ordre est décelée par mesure de la constante diélectrique un degré plus bas que par calorimétrie. Celle-ci montre de plus un phénomène inhabituel, qu'on opère en programmation de température croissante ou décroissante : dans les deux cas, la transition du second ordre précède la transition du premier ordre. L'étude de la réponse pyroélectrique du cristal montre que le passage à l'état polaire est gouverné par la transition du premier ordre

Dielectric Study of Pure Polymer and Mixed Monomer-Polymer Single Crystals of pTS Diacetylene: a Comparison with Neutron Scattering Results

Temperature and frequency dependences of the electric permittivity in mixed monomer-polymer single crystals of the diacetylene pTS (2,4-hexadiynylene bis(p-toluenesulphonate)) are reported. The dependences were determined in the temperature range 100-250 K, and in the frequency range 100 Hz - 1 MHz. Two types of single crystals were studied: nearly-fully polymerized ones, and those containing ca. 20% of polymer in the monomer matrix. The dynamics is characterized by a high-frequency response (v ≫ 3 GHz) which corresponds to the soft mode analysed previously by the Raman and neutron inelastic scatterings. At low frequencies (v ≤ 1 kHz), another component is observed, its dielectric intensity peaking around the transition temperature in the fully polymerized crystals. The results are consistent with the results obtained from NMR and neutron scattering experiments