Transient current-voltage characteristics of organic light-emitting diodes (OLEDs) made from both conjugated polymers and low molecular-weight materials show h ysteresis e ects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N'-diphenyl-N,N'-bis(1naphtyl)-1,1'-biphenyl-4,4'-diamine (NPB) with Ca and indium-tin oxide as electrodes. To clarify the origin of this peculiarity n umerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such a s m e asuring conditions, trap characteristics, basic doping level, mobility and injec-

Articles you may be interested inTemperature-dependent electroluminescence spectra of poly(phenylene-vinylene) derivatives-based polymer light-emitting diodes J. Appl. Phys. 97, 034505 (2005); 10.1063/1.1845580Determination of traps in poly(p-phenylene vinylene) light emitting diodes by charge-based deep level transient spectroscopy Using temperature dependent impedance spectroscopy in a broad frequency range (10 Ϫ1 -10 7 Hz͒, we have found that the ac behavior of indium-tin oxide ͑ITO͒/poly-͑p-phenylene-vinylene͒ ͑PPV͒/aluminum light-emitting diodes shows several features which cannot be described by the usual simple double RC circuit representing a depleted junction region and an undepleted bulk. Instead, our measurements in combination with a theoretical modeling suggest that the PPV bulk is composed of a highly doped region at the ITO interface and a region with lower doping at a higher distance to the ITO. Moreover, the boundary between these two regions is not sharp but there is a gradual change in dopant concentration. The large frequency range allowed us to identify two distinct processes corresponding to the PPV bulk and a third one to the junction. The bulk relaxation frequencies correspond to the characteristic dielectric relaxation frequencies of charge carriers in the high and low conducting sublayers and are proportional to the respective conductivities. The magnitude and activation energy of the relaxation time correlates well with results obtained from temperature dependent DC conductivity measurements. For ITO substrates we obtain activation energies of 0.4 eV and room temperature conductivity of about 10 Ϫ7 and 10 Ϫ9 S/cm for the high and low conducting sublayers, respectively. On gold substrates only one bulk process and no junction process with an activation energy of about 0.6 eV and a corresponding conductivity of 3ϫ10 Ϫ11 S/cm at room temperature is observed. The Schottky junction has been studied by temperature dependent capacitance-voltage spectroscopy at a low frequency of 0.16 Hz. The obtained acceptor dopant concentration from 1/C 2 plots varies from 1.4ϫ10 17 at room temperature to 6.9ϫ10 16 cm Ϫ3 at 200 K. Assuming a density of states between 5ϫ10 20 and 5ϫ10 21 cm Ϫ3 for the valence band the temperature dependent acceptor dopant density can be described with an acceptor ionization energy between 0.16 and 0.2 eV.

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It has been shown in a recent analysis of the temperature dependence of the dc conductivity of the quasi-one-dimensional conductor (8uoranthene)2PFs that in spite of the occurrence of the Peierls transition to a charge-density-wave ground state (formally implicating polarons as excitations), the dc conduction is essentially due to electron-hole transport in bands and acoustical phonon scattering of the carriers. The theory allows for the determination of the temperature dependence of the Peierls gap below and the Huctuating pseudogap above the transition temperature. Our dc-conductivity measurements con6rm that a common temperature dependence occurs in organic radical cation salts and in inorganic materials from the groups of the blue bronzes and the transition metal tetrachalcohalogenides. These materials are rather di8'erent especially with respect to the nature of the states forming the conduction band and the filling of the latter. Here we reduce the needed information on the band structure to a minimum connected with optical data and extend the theory to the case of a gap small compared to k&T. The theory is applied to (Fa)2PFs, Kp, spMo03, and (TaSe4)2I as representatives of the above-mentioned groups of materials. From the measured conductivity data the temperature dependence of the Peierls gap below and the pseudogap above the transition temperature are determined as well as several conductivity-related quantities. Similarities and difFerences of the investigated materials are discussed.

The mutational spectrum of deafness in Indochina Peninsula, including Vietnam, remains mostly undetermined. This significantly hampers the progress toward establishing an effective genetic screening method and early customized rehabilitation modalities for hearing loss. In this study, we evaluated the genetic profile of severe-to-profound hearing loss in a Vietnamese pediatric population using a hierarchical genetic analysis protocol that screened 11 known deafness-causing variants, followed by massively parallel sequencing targeting 129 deafness-associated genes. Eighty-seven children with isolated severe-to-profound non-syndromic hearing loss without family history were included. The overall molecular diagnostic yield was estimated to be 31.7%. The mutational spectrum for severe-to-profound non-syndromic hearing loss in our Vietnamese population was unique: The most prevalent variants resided in the MYO15A gene (7.2%), followed by GJB2 (6.9%), MYO7A (5.5%), SLC26A4 (4.6%), TMC1 (1.8%), ESPN (1.8%), POU3F4 (1.8%), MYH14 (1.8%), EYA1 (1.8%), and MR-RNR1 (1.1%). The unique spectrum of causative genes in the Vietnamese deaf population was similar to that in the southern Chinese deaf population. It is our hope that the mutation spectrum provided here could aid in establishing an efficient protocol for genetic analysis of severe-to-profound hearing loss and a customized screening kit for the Vietnamese population.

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