Residual Halide Groups in Gilch-Polymerized Poly(<i>p</i>-phenylene-vinylene) and Their Impact on Performance and Lifetime of Organic Light-Emitting Diodes

Abstract

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.