Tenfold Enhancement in Light Emission from Low-Temperature Operational Doped OLEDs by Detrapping Super-Long-Lived Trapped Charges

Although doping organic fluorophores into host matrix is a common way to obtain high-efficiency organic light-emitting diodes (OLEDs), trapped charges are often observed on dopant molecules that are considered to be detrimental to the further enhancement of their photoelectric performances. Surprisingly, trapped charges with a super-long lifetime of >2 h are detected in doped OLEDs operated at 20 K, which has never been discovered previously in the literature. However, the observations demonstrate that the longer lifetimes of these trapped charges are primarily governed by the larger spacing distances between trapped electrons and holes, rather than being solely determined by the well-accepted energy-level depth of trap states. More amazingly, compared with the device driven only by a conventional constant voltage source, a tenfold enhancement in light emission is achieved in low-temperature operational doped OLEDs powered by an alternating positive and negative pulse voltage, because the applied negative pulse voltage can assist these super-long-lived trapped electrons and holes to detrap and then recombine with each other for producing enhanced light-emission. Therefore, this study clarifies the dynamic behaviors of trapped charges and paves the pathway for obtaining high-performance OLEDs working in low-temperature circumstances such as outer-space or aerospace fields.