AC-driven Electroluminescent Devices Research Articles

White-emission from ZnS:Eu incorporated in AC-driven electroluminescent devices via ultrasonic spray pyrolysis

In this work, white-emitting-alternating-current-thin film electroluminescent (w-ACTFEL) devices are demonstrated using europium-doped zinc sulfide (ZnS:Eu) and zirconium oxide (ZrO2) as the emissive and dielectric layers, respectively. These films were deposited by the ultrasonic spray pyrolysis technique on antimony-doped tin oxide glass substrates, forming a standard metal-insulator-semiconductor-insulator-metal (MISIM) architecture. 10 kHz sinusoidal voltages activated the white-EL of the devices. The colorimetric characteristics were investigated for three amplitudes of the applied voltage. The emission of the devices is made up of wide and narrow bands with peaks corresponding to violet, blue, green and red light, which together produce the resulting white light. According to the colorimetric analysis, this white light is close to the standard D65 CIE illuminant with a minimal dominant blue component. The variation in voltage amplitude induces small changes in the visual characteristics of the EL emission. The white-EL emission of these MISIM devices is attributed to the electron-impact excitation and subsequent relaxation of the excited levels of Eu2+ and Eu3+ impurities, and defect levels in the sublayer regions adjacent to the ZrO2–ZnS:Eu interfaces.

Alternating current electroluminescence from GaN-based nanorod light-emitting diodes

Alternating current (AC)-driven electroluminescent (EL) devices have attracted widespread attention due to their special advantages. Under the trend of nanoscale miniaturization in optoelectronics, AC-driven EL devices with nanoscale are highly desirable to be applied to nano-optoelectronic integrated circuits. In this work, GaN-based nanorod LEDs (nLED) are used as nanoscale emitters to fabricate AC-driven nano-EL device, namely, AC-nLED. Two configurations including single-dielectric AC-nLED and double-dielectric AC-nLED without carrier injection are demonstrated. The electrical and optical characteristics of single-dielectric AC-nLED and double-dielectric AC-nLED are demonstrated, which have completely different frequency responses. The working mechanisms and origin of the difference in frequency response are discussed. Our work presents an alternative device configuration for obtaining AC-driven nano-EL devices, which are expected to be used in nano-photonic systems and nano-pixel light-emitting displays.

Performance Limits of an Alternating Current Electroluminescent Device.

The use of an alternating current (AC) voltage is a simple, versatile method of producing electroluminescence from generic emissive materials without the need for contact engineering. Recently, it was shown that AC-driven, capacitive electroluminescent devices with carbon nanotube network contacts can be used to generate and study electroluminescence from a variety of molecular materials emitting in the infrared-to-ultraviolet range. Here, performance trade-offs in these devices are studied through comprehensive device simulations and illustrative experiments, enhancing understanding of the mechanism and capability of electroluminescent devices based on alternating as opposed to direct current (DC) schemes. AC-driven electroluminescent devices can overcome several limitations of conventional DC-driven electroluminescent devices, including the requirement for proper alignment of material energy levels and the need to process emitting materials into uniform thin films. By simultaneously optimizing device geometry, driving parameters, and material characteristics, the performance of these devices can be tuned. Importantly, the turn-on voltage of AC-driven electroluminescent devices approaches the bandgap of the emitting material as the gate oxide thickness is scaled, and internally efficient electroluminescence can be achieved using low-mobility single-layer emitter films with varying thicknesses and energy barrier heights relative to the contact.

Effect of Free Carriers on ac-Driven Electroluminescent Devices with Hydrogenated Amorphous Silicon Carbide Thin Films

The benefit of free carriers on ac-driven electroluminescence (EL) in hydrogenated amorphous silicon carbide (a-SiC:H) thin films has been investigated by observing the photoenhanced EL and analyzing of EL in multilayers. The EL intensity was found to be increased with the intensity of the illuminating laser with a sub-band gap energy. This has been correlated with photoluminescence quenching. The EL quenching at higher electric fields observed only with irradiation has been attributed to the effect of a combination of an increased number of free carriers and interaction of impact-generated electron-hole pairs. Moreover, the reduction in the threshold voltage of a-SiC:H/hydrogenated amorphous carbon (a-C:H) multilayers has been found to be related to the a-SiC:H thickness. The slope of the linear increase in EL intensity with ac voltage depends on the multilayer sequence. Both observations can be explained using a microscopic model based on impact excitation by free carriers.

Luminescence of amorphous Si xC 1-x:H and its applications

A mechanism of photoluminescence in hydrogenated amorphous silicon-carbon alloy is discussed on the basis of the Stokes shift, decay time and polarization memory of luminescence. The ac-driven electroluminescent devices are shown as one of applications of this unique material. An observation of a quantum size effect in a heterostructure consisting of ultrathin layers with different compositions indicates a possibility of future application of this material to a novel light-emitting device.