Temperature-Dependent Dynamic Behaviors of Organic Light-Emitting Diode

A systematic investigation of temperature-dependent dynamic behaviors of NPD-Alq₃ organic light-emitting diodes (OLEDs) is carried out. Through an in-depth numerical analysis, it has been found that the luminance decreases and consequently the turn-on voltage increases with decreasing temperature due to a reduction of thermally activated hopping speed, which retards the rise of electroluminescence (EL) upon turn-on as well as the discharge upon turn-off of OLEDs. Most importantly, however, the device efficiency is literally raised as the temperature decreases, a direct consequence of enhanced charge-balance factor. It is also demonstrated that the EL delay upon turn-on is mostly determined by the electron transport through the electron transport layer (ETL), while the fast EL decay (short-lived EL tail) upon turn-off is mainly by the rapid discharge of the steep pileup of carriers at the NPD/Alq₃ interface. The long-lived EL tail is shown to be more pronounced under lower temperatures. In response to a train of voltage pulses, the delay of EL occurring for the first voltage pulse has vanished for the subsequent pulses regardless of temperature due to space charges remaining inside the device after turn-off (in the "off-state"). However, it appears that the pulse-to-pulse interference by the space charge effects is more significant under lower temperatures