On the basis of new insights into the reaction mechanism of the so-called Gilch route leading to poly(p-phenylene-vinylene)s (PPVs), the importance of vinyl halide defects for the performance of organic light-emitting diodes (OLEDs) is stressed in the present contribution. It is found that the current density, the luminance, and luminance efficiency are superior for PPVs that were subject to a long-term dehydrohalogenation. In particular, the device lifetime improves by a factor of 200 as long as the halide content is reduced from 0.4 to 0.05 wt %. The results imply that rather the mentioned vinyl halide defect than the often discussed tolane-bisbenzyl (TBB) defect has to be considered when investigating lifetime and performance of OLEDs. The device behavior is analyzed in view of a detailed study of the charge-carrier transport properties. We suggest that the penetration of electrons from the cathode in the PPV leads to a separation of halogen and thus to free halogen anions. The anions can move in the electric field to the contacts where they form a salt with the counterion present in the electrode material. The charge-carrier transport across the respective contact is thus impeded as a consequence of the appearance of a salt-containing interlayer. The proposed mechanism explains the observed differences in device performance and lifetime.
The influence of trap concentration on hole transport is investigated by an optical time-of-flight method for the amorphous small molecule organic semiconductor N,N′-bis(1-naphtyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamin (α-NPD) doped with neutral hole traps by codeposition of 4,4′,4″-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA). α-NPD doped with 120ppm 1-NaphDATA exhibits nondispersive hole transport like undoped α-NPD, but trap-controlled with reduced mobility. The trap depth derived from the mobility decrease coincides with the ionization potential difference of α-NPD and 1-NaphDATA. The transition to dispersive transport for increasing trap concentration to 1160ppm is explained by an energetic relaxation of optically generated charge carriers within a density of states broadened by traps.
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Interpretation of trap-limited mobility in space-charge limited current in organic layers with exponential density of traps J. Appl. Phys. 110, 043705 (2011); 10.1063/1.3622615 Space-charge limited conduction in doped polypyrrole devices J. Appl. Phys. 107, 093716 (2010); 10.1063/1.3373393Field-dependent mobility from space-charge-limited current-voltage curvesThe influence of the spatial distribution of trap states on unipolar space-charge limited current ͑SCLC͒ is investigated experimentally and theoretically. Thin-layered films of the small molecule organic semiconductor N , NЈ-di͑1-naphtyl͒-N,NЈ-diphenylbenzidine ͑␣-NPD͒ are vapor deposited on indium tin oxide, with aluminum as the counter electrode. The small molecule 4,4Ј ,4Љ-tris-͓N-͑1-naphtyl͒-N-͑phenylamino͔͒-triphenylamine ͑1-NaphDATA͒, which creates well-known shallow traps for holes, is used as dopant. The realized organic films consist of three layers, one of which is homogeneously doped. The influence of the spatial position of the doped layer on the current-voltage characteristics of the diodes is examined. Compared to an undoped device, the current density is strongly decreased and varies over orders of magnitude for the different spatial positions of the doped layer. It is shown that traps near the injecting electrode have the most pronounced effect on SCLC. A model for unipolar SCLC through a system of homogeneous layers with different trapping parameters for shallow traps is presented. The model quantitatively describes the experimental data and is used to calculate the spatial distributions of the charge-carrier density and the electric-field strength in the differently doped devices.
The hole transport in poly(p-phenylenevinylene) (PPV) was investigated before and after bipolar electrical stress by the time-of-flight (TOF) method. Bipolar structures similar to organic light emitting diodes (OLEDs) were realized, yet with much thicker layers than usually prevailing in OLEDs. During fatigue, the hole mobility is reduced, the field dependence of the mobility is increased, and the hole transport becomes more and more dispersive. These results go along with the fatigue behavior of thin film OLEDs that were investigated by charge extraction via linearly increasing voltage (CELIV). Even though theoretical simulations could show that both thick- and thin-film PPV-based OLED structures are dominated by holes, the presented results indicate that the existence of electrons leads to degradation during hole transport. A possible reason for an enlarged electron density in the otherwise hole dominated device is suggested.
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