Dynamic Behaviors of Ultralong-Lived Trapped Charges in Doped Organic Light-Emitting Diodes Operating at Low Temperatures and Their Potential Applications for Time–Temperature Indicators

Trapped charges are commonly observed in doped organic light-emitting diodes (OLEDs) that are especially operating at low temperatures; however, the dynamic behaviors of these charges remain poorly understood, and their potential applications are yet to be explored. Herein, using transient electroluminescence (TEL) technology, a large number of ultralong-lived trapped charges are detected in the doped OLEDs at 20 K. Through systematic studies on the device’s TEL responses, we construct a clear charge-carrier dynamics model for the doped OLEDs working at low temperatures. It reveals that shallow trapped charges can spontaneously detrap under Coulomb interaction, while deep trapped charges are permanently stored at their trap states, as long as the device stays at low-temperature operation. Furthermore, we demonstrate for the first time that the spike always appearing at the TEL rising edge of the doped devices at low temperatures is generated from the radiative recombination of those deep trapped charges released by the applied external electric field. More importantly, we propose a novel application of these OLEDs with ultralong-lived trapped charges as time–temperature indicators (TTIs) for monitoring the product quality of biological agents and specialty chemicals during their low-temperature storage and transportation. Thus, this work not only elucidates the dynamics of trapped charges in low-temperature-doped systems but also expands the potential applications of OLEDs to energy and information storage technologies.