Effects of Injection Current on the Modulation Bandwidths of Quantum-Dot Light-Emitting Diodes

Hua Xiao,Xiangtian Xiao,Kin Seng Chiang,Rui Wang,Kai Wang,Dan Wu

doi:10.1109/ted.2019.2941561

Abstract

This article presents an investigation of the modulation bandwidths of quantum-dot (QD) light-emitting diodes (QLEDs). The QLEDs used in our study are red-emissive CdSe/ZnS QLEDs, which have a structure of indium tin oxide (ITO)/poly(3.4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)/TFB/QD/ZnO/Al and an emitting area of 2 or 4 mm 2 . We find that at a small injection current (below ~10 mA), the effects of the resistance-capacitance (RC) time constant and the carrier lifetime on the bandwidths of the QLEDs are comparable, while at a large injection current, the bandwidths are mainly determined by the carrier lifetime. The response time of the QDs is not a limiting factor. The bandwidths of the QLEDs increase with the injection current and are eventually limited by the damage threshold current of the devices. At the same injection current, the QLED that has a smaller emitting area provides a larger current density, and thus exhibits a larger bandwidth. At an injection current of 28 mA, the 2-mm 2 QLED provides a bandwidth of 11.4 MHz and a luminance value of 156 000 cd/m 2 , and the 4-mm 2 QLED provides a bandwidth of 8.2 MHz and a luminance value of 97 000 cd/m 2 . Our investigation provides a guideline for QLED-bandwidth optimization and useful information for the further development of QLEDs for lighting, display, and communication applications.

Highlights

V ISIBLE light communication (VLC) has attracted much attention as a new wireless communication technology for its many unique advantages, such as immunity to electromagnetic interference, license-free operation, high security, and energy-saving [1], [2]
Organic Light-emitting diodes (LEDs) (OLEDs), which are widely used in mobile phone and TV screens, have been proposed for VLC [6], but their bandwidths are low [7]–[10], though the micro-OLEDs with a fast response of ∼10 ns have been reported [11]
To maintain balanced electron/hole injection and a high EL efficiency, the structure of an EL-based QLED is designed to consist of a hole injection layer (HIL), a hole transport layer (HTL), an emission layer, and an electron transport layer (ETL), which are usually formed with organic [16], inorganic [17], or hybrid materials [18]

Summary

INTRODUCTION

V ISIBLE light communication (VLC) has attracted much attention as a new wireless communication technology for its many unique advantages, such as immunity to electromagnetic interference, license-free operation, high security, and energy-saving [1], [2]. Quantum-dot (QD) LEDs (QLEDs) have emerged as promising light sources for panel display applications because of their many distinct characteristics, such as tunable emission wavelength, high efficiency, wide color gamut, and high monochromaticity [12], [13]. In an EL-based QLED, the QDs serve as the active medium of an LED and the light emission is achieved by injecting the electric current into the LED. The luminance characteristics of our QLEDs compare favorably with those of the OLEDs [21]–[23] To our knowledge, this is the first report of the investigation of the modulation bandwidths of the QLEDs

Structure and Fabrication of QLEDs

Characterization of QDs

Optical and Bandwidth Measurement

RESULTS AND DISCUSSION

CONCLUSION