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CC3.01 - Metal Electromigration through Transparent Conductors - Monitoring an OLED Failure Mechanism 
April 7, 2015   1:30pm - 1:45pm

Operational lifetime is a crucial performance feature for many electronic devices such as organic light emitting diodes (OLEDs). One of the many failure mechanisms which limit OLED lifetime is the electromigration of metallic components of the electrode materials, forming short circuits through the device architecture. Metals such as silver, copper or aluminium are e. g. used in the highly conductive shunting lines which are necessary to reduce resistive losses in the transparent electrodes of large area OLED panels. A direct study of the effects of metal electromigration in devices, however, is complicated by the interference with other failure mechanisms, as well as by the very localised formation of short circuits, which renders the application of imaging techniques highly challenging. In this contribution, we present a study of silver and copper electromigration through PEDOT:PSS films, a conductive polymer which is frequently used as replacement for indium tin oxide as transparent electrode material in OLEDs. Due to the experimental setup, electromigration can be studied isolated from other phenomena, and in addition, the formation of short circuits by metal dendrite growth can be easily monitored using electrical current monitoring and optical microscopy. We have studied both experimentally and theoretically the influence of various parameters such as the electric field strength, electrode material and intrinsic electrical conductivity of the PEDOT:PSS on the speed of short circuit formation and on the appearance of the formed metal dendrites. Silver was found to migrate at a much higher rate than copper, and a high current density through the PEDOT:PSS was found to increase the migration rate, which is consistent with the assumption of metal migration being induced by the force exerted on the metal atoms by the moving charge carriers. The results from our study have been correlated with lifetime studies in functional OLED devices. Currently, our experimental setup is used to develop strategies to suppress or slow down metal electromigration and thus improve OLED lifetime.

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