Low-frequency noise as a diagnostic tool for OLED reliability
Organic light emitting diodes (OLED), either based on polymers or small molecules, suffer from early failure: an unpredictable sudden increase in current with a total loss of light output. This work addresses this problem using small-signal impedance measurements and electrical noise techniques. Robust OLEDs show a current noise spectrum proportional to 1/f. OLEDs susceptible to failure have 1/f3/2 and/or may start exhibiting a standard 1/f behavior that rapidly evolves with time (typical 30 minutes) to 1/f1.6. In addition OLEDs susceptible to early failure have a higher DC leakage. It is proposed that a combination of both measurements can be used as a diagnostic tool for OLED reliability in a production line. Insight into the physics of the degradation mechanism is also provided. Unreliable OLEDs exhibit current switching events and optical blinks at wavelengths higher than the polymer band gap electroluminescence. It is proposed that degradation is induced by the appearance of an insulating resistive switching layer. Charge recombination trough this layer is responsible for the optical and electrical blinks.