Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Summary

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Abstract

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al₂O₃. The autoxidized Al:Al₂O₃ cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m^-2, and a maximum current efficiency of 1.15 cd A^-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

Introduction

Light-emitting diodes (LEDs) based on colloidal semiconductor quantum dots have attracted great interest due to their unique advantages, including solution processability, tunable emission wavelength, excellent color purity, flexible fabrication, and low processing cost^1,2,3,4. Since the first demonstrations of QDs-based LEDs in 1994, tremendous efforts have been devoted to engineering the materials and device structures^5,6,7. A typical QD-LED devic....

Protocol

1. Pattern Etching of Indium Tin Oxide (ITO) Glass

1. Cut large pieces of ITO glass (12 cm × 12 cm) into 15 mm wide strips. Clean the ITO glass surface using a dust-free cloth with alcohol.
2. Check the conductive side of the ITO glass with a digital multimeter. Cover the active area of the ITO glass with adhesive tape, so that the active area is 2 mm wide in the middle.
3. Pour the zinc powder on the ITO glass (to a thickness of about 0.5 mm).
4. Pour the hydrochloric acid solution (36 wt%) onto the surface of the ITO glass and allow the ITO glass to completely soak in hydrochloric acid solution, then etch for 15 s.
5. Pour o....

Results

UV-Vis absorption and photoluminescence (PL) spectra were used to record the optical properties of ZnCdS/ZnS graded core/shell-based blue QDs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were collected for the morphologies of QDs (Figure 1). X-ray photoelectron spectroscopy (XPS), electrochemical study, and ultraviolet photoelectron spectroscopy (UPS) were employed to detect the structural properties and energy levels .......

Discussion

The device architecture of the blue QD-LED consists of an ITO transparent anode, a PEDOT:PSS HIL (30 nm), a Poly-TPD HTL (40 nm), a ZnCdS/ZnS QDs EML (40 nm), and an Al:Al₂O₃ cathode (100 nm). Due to the porous character of the Al cathode, we obtained an oxidized Al cathode by exposing it to oxygen. Figure 2e and Figure 2f display the energy level alignment diagrams of QDs layer with Al and Al:Al₂O₃. When the QDs conta.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Indium Tin Oxide (ITO)-coated glass substrate	CSG Holding Co., Ltd.		Resistivity≈10 Ω/sq
Zinc powder	Sigma-Aldrich	96454	Molecular Weight 65.38
Isopropyl alcohol	Beijing Chemical Reagent	67-63-0	Analytically pure
Toluene	Innochem	I01367	Analytically pure
Acetone	Innochem	I01366	Analytically pure
Hydrochloric acid	acros	124210025	1 N standard solution
O-dichlorobenzene	acros	396961000	98+%, Extra Dry
Poly(3,4-ethylenedioxythiophene) doped polystyrene sulfonate (PEDOT:PSS)	H. C.Stark	Clevious P VP Al 4083
Poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine) (Poly-TPD)	Luminescence Technology	LT-N149
Aluminum tris(8-Hydroxyquinolinate) (Alq3)	Luminescence Technology	LT-E401
UV-O cleaner	Jelight Company	92618
Filter	Jinteng	JTSF0303/0304	Polyether sulfone (0.45 μm)
Ultrasonic cleaner	HECHUANG ULTRASONIC	KH-500DE
Digital multimeter	UNI-T	UT39A
Spin coater	IMECAS	KW-4A
Digital hotplate	Stuart	SD160