Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.