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 device is designed to have a three-layered sandwich architecture which consists of a hole transport layer (HTL), an emissive layer, and an electron transport layer (ETL). The choice of a suitable charge transport layer is critical to balance the injected holes and electrons into the emissive layer for radiative recombination. Currently, vacuum-deposited small molecules are widely used as ETL, for instance, bathocuproine (BCP), tris(8-Hydroxyquinolinate) (Alq₃), and 3-(biphenyl-4-yl)-5-(4-tertbutylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ)⁸. However, the unbalanced carrier injection often causes the recombination region shift to ETL, making unwanted parasitic electroluminescence (EL) emission and deteriorating the device performance⁹.

To enhance the device efficiency and environmental stability, solution-processed ZnO nanoparticles were introduced as an electron transport layer instead of the vacuum-deposited small-molecule materials. Highly bright RGB QD-LEDs were demonstrated for conventional device architecture, showing luminance up to 31,000, 68,000, and 4,200 cd m^-2 for emission of orange-red, green, and blue, respectively¹⁰. For an inverted device architecture, high performance RGB QD-LEDs with low turn on voltage were successfully demonstrated with brightness and external quantum efficiencies (EQE) of 23,040 cd m^-2 and 7.3% for red, 218,800 cd m^-2 and 5.8% for green, and 2,250 cd m^-2 and 1.7% for blue, respectively¹¹. To balance the injected charges and preserve the QDs emissive layer, an insulating poly(methylmethacrylate) (PMMA) thin film was inserted between the QDs and ZnO ETL. The optimized deep-red QD-LEDs exhibited high external quantum efficiencies up to 20.5% and a low turn-on voltage of only 1.7 V¹².

Besides, optimizing the optoelectronic properties and nanostructures of QDs also plays a crucial role in boosting the device performance. For instance, highly fluorescent blue QDs with photoluminescence quantum yield (PLQE) up to 98% were synthesized through optimizing the ZnS shelling time¹³. Similarly, high-quality, violet-blue QDs with near 100% PLQE were synthesized by precisely controlling the reaction temperature. The violet-blue QDs-LED devices showed remarkable luminance and EQE up to 4,200 cd m^-2 and 3.8%, respectively¹⁴. This synthesis method is also applicable to violet ZnSe/ZnS core/shell QDs, the QD-LEDs exhibited high luminance (2,632 cd m^-2) and efficiency (EQE=7.83%) by using Cd-free QDs¹⁵. Since blue quantum dots with high PLQE have been demonstrated, high charge injection efficiency in the QDs layer plays another crucial role in fabricating high performance QD-LEDs. By substituting long chain oleic acid ligands to shorten 1-octanethiol ligands, the electron mobility of QDs film was increased two-fold, and a high EQE value over 10% was obtained¹⁶. The surface ligand exchange can also improve the morphology of QDs film and suppress the photoluminescence quenching among QDs. For instance, QDs-LED showed improved device performance by using chemically grafted QDs-semiconducting polymer hybrids¹⁷. Besides, high-performance QDs were prepared through reasonable optimization of the graded composition and thickness of the QDs shell, due to the enhanced charge injection, transport, and recombination¹⁸.

In this work, we introduced a partial autoxidized aluminum (Al) cathode to improve the performance of ZnCdS/ZnS graded core/shell-based blue QD-LEDs¹⁹. The change of the potential energy barrier of the Al cathode was confirmed by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Furthermore, the fast charge carrier dynamics at the QDs/Al and QDs/Al:Al₂O₃ interface were analyzed by time-resolved photoluminescence (TRPL) measurements. In order to further validate the influence of partially oxidized Al on device performance, QD-LEDs with different cathodes (Al only, Al:Al₂O₃, Al₂O₃/Al, Al₂O₃/Al:Al₂O₃, and Alq₃/Al) were fabricated. As a result, high performance pure blue QD-LEDs were demonstrated by employing Al:Al₂O₃ cathodes, with a maximum luminance of 13,002 cd m^-2 and a peak current efficiency of 1.15 cd A^-1. Furthermore, there was no additional organic ETL involved in the device architecture, which can avoid unwanted parasitic EL to guarantee the color purity under different working voltages.

Protocol

1. Pattern Etching of Indium Tin Oxide (ITO) Glass 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. 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. Pour the zinc powder on the ITO glass (to a thickness of about 0.5 mm). Pour the hydrochloric acid solution (36…

Representative 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

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