Advances in Alternating Current Electroluminescent Devices

Abstract

radiatively at the active emissive layer driven by constant-voltage or direct current (DC). However, the DC-driven mode for OLEDs and QLEDs limits their practical applications. One reason is that the unidirectional DC flow may lead to unfavorable charges accumulation at high current density. Furthermore, the power losses are unavoidable as the DC-driven devices require power converters and rectifiers when connected to the 110/220 V at 50/60 Hz alternating current (AC) power sources. OLEDs are also particularly sensitive to dimensional variations accompanied by the generation of leakage currents at such imperfections, which is unfavorable for their application in flexible electronics. Consequently, AC-driven EL devices have attracted attention as promising alternatives to DC-driven EL devices for a variety of applications. [25][26][27][28][29][30][31][32][33] AC-driven EL devices are primarily composed of electrodes, an emissive layer and a single-or multilayer of insulating dielectric without the critical requirement for energy band matching, which facilitates their application in large-scale displays and flexible devices. [34] The device structure of AC-driven EL devices can be mainly divided into three kinds: i) AC-driven thin film electroluminescent (AC-TFEL) devices; ii) AC-driven lightemitting devices (AC-LEDs), and iii) AC-driven light-emitting field effect diodes (AC-LEFETs). Under the AC electric field, light generation is based on either the hot-electron impact excitation mechanism or the exciton recombination mechanism, depending on the device configuration. Despite the different operation principles, AC-driven EL devices have shown specific Alternating current (AC)-driven electroluminescent (EL) devices have recently attracted attention as potential alternatives to direct current (DC)-driven organic light-emitting diodes (OLEDs), as they have the great advantage of easy integration into the AC power system of 110/220 V at 50/60 Hz without complicated back-end electronics. However, the high driving voltage and low power efficiency inherent to AC-driven EL devices limit their widespread application. While researchers have made some remarkable progress in this field, the underlying causes during the development process remain to be explored. The strategies for improving the performance of AC-driven EL devices with different configurations, such as the conventional sandwiched structure and multilayer-based light-emitting devices, are summarized in this review. For example, it is crucial to enhance the effective electric field around the emitters for AC-driven thin film electroluminescent (AC-TFEL) devices, while the unbalanced generation/injection of charge carriers is the main limiting factor for the performance of AC-driven light-emitting devices (AC-LEDs). The recent advances in AC-driven EL devices, with some new configurations or new-type emitting materials, are presented by category. The challenges and opportunities for the further development of AC-driven EL devices are also discussed.

Recent Progress of Fiber Shaped Lighting Devices for Smart Display Applications—A Fibertronic Perspective

“…Additionally, the ACEL device does not require a costly switching mechanism or intricate backend electronics in the circuits when it is connected to a general alternating current power source, 110/220 V at 50/ 60 Hz. This makes it cost‐effective and energy saving, because the power losses from power converters and rectifier when the device is matched with a 110/220 V at 50/60 Hz power supply can be avoided . However, the high driving voltages to emit sufficient brightness have still limited the practical use of portable applications, especially in wearable electronics.…”

“…Given this unique emission mechanism, the structure of ACEL devices can have only a single emissive layer centered between two electrodes, similar to LECs. Also, the electrodes' work functions are not needed for charge injection …”

Three-phase electric power driven electroluminescent devices

“…In this case, the mechanism for the device operation is different from the previously discussed inorganic TPEL and the light emission occurs from the radiative recombination of excitons in the EML rather than the impact excitation of emitters by hot electrons 16 . The above observations can be explained as follows.…”

“…The researchers mainly focus on the light-emitting materials and structures of such devices, whereas there are very few studies on their driving sources. Recently, alternating current (AC)-driven electroluminescent (EL) devices have attracted increased attention and are regarded as promising alternatives to traditional direct current (DC)-driven EL devices 15 , 16 . There are several fundamental reasons for that.…”

“…First, for injection DC-driven EL devices operated at high current density, the electroluminescence is markedly limited by triplet–triplet or triplet–charge annihilation. In contrast to that, the continual reversal of applied electric field in AC-driven EL devices can help to avoid charge accumulation, which may reduce triplet-exciton annihilation at high current densities 16 , 17 . Second, the introduction of an insulating dielectric layer can prevent electrochemical reaction between the electrode and the emissive layer, protecting the device degradation from the moisture and oxygen in the atmosphere 18 – 20 .…”