Abstract
A systematic investigation of temperature-dependent dynamic behaviors
of NPD-Alq<sub>3</sub> 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/Alq<sub>3</sub> 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.
© 2006 IEEE
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