Electrical and Environmental Degradation Causes and Effects in Polyfluorene-Based Polymer Light-Emitting Diodes

We report on the degradation mechanism of polymer light-emitting diodes (PLEDs) kept in the environment with no encapsulation. The device structure is indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/polyfluorene/aluminum (Al) fabricated by a simple solution-processed method. Its physical, optical and electrical properties were measured over time in order to explore factors resulting in device instability. The sustained decrease in current density and electroluminescence, as well as the appearance of dark spots and electrode delamination, reveal that the device performance tends to degrade in the atmosphere gradually. To clarify the effect of the external electric field on the device degradation process, a device with lower threshold voltage was fabricated and its performance was recorded with and without bias voltage. Also, when the Al cathode of a completely ruined blue PLED was re-deposited, the device started to emit green light, confirming the irreversible chemical structural changes occurred in the active layer. Noticeably, in addition to cathode delamination, we observed that the rate of luminescence degradation in fabricated PLEDs is more than two times faster than that of the current density. The detailed study of the stability issue in PLEDs, presented in this work, can have great potential importance in the fabrication of other organic electronic devices such as organic solar cells and organic photodetectors due to similar instability factors.