Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

Abstract

In the study of hybrid quantum dot light-emitting diodes (QLEDs), even for state-of-the-art achievement, there still exists a long-standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn 1−x Cd x Se/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn 1−x Cd x Se-based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m −2 , very low efficiency roll-off at high current density (EQE ≈25% @ current density of 150 mA cm −2 ), and operational lifetime extended to ≈1 800 000 h at 100 cd m −2 . These extraordinary performances make this work the best among all solution-processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2-ethylhexane-1-thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.

Progress of display performances: AR, VR, QLED, and OLED

“…With these advantages, the external quantum efficiency (EQE) of Cd-based QLEDs is continuously increasing. Recently, > 30% EQE was reported through shell growth control and modification of the surface ligands of the red-emitting Cd-based QDs [45]. As shown in Figure 1 and Table 5, the maximum EQEs of red-, green-, and blue-emitting QLEDs reached 30.9, 25.04, and 19.5%, respectively [45,51,54].…”

Highly Efficient Near-Infrared Light-Emitting Diodes Based on Chloride Treated CdTe/CdSe Type-II Quantum Dots

“…As an alternative material, II-VI semiconductor quantum dots (QDs) demonstrate unique superiorities as NIR emitters, due to high PL QY, easily tunable size-dependent emissions, lowcost solution processability and scalable production of highquality QDs (Kwak et al, 2012;Shirasaki et al, 2012;Chen et al, 2013;Qin et al, 2013;Zhang et al, 2019). Recent advances in visible LEDs based on II-VI QDs (especially type I structure) have already satisfied the requirements for display and solid-state lighting (Dai et al, 2014;Yang et al, 2015;Zhang et al, 2017;Li et al, 2019;Shen et al, 2019;Song J. et al, 2019). In particular, very recently, the novel strategy that optimizes shell materials to get a better energy level matching with the highest occupied molecular orbital (HOMO) of the hole transport layers facilitates their industrialization (Yang et al, 2015;Li et al, 2019;Shen et al, 2019).…”

“…But these long aliphatic ligands act as barriers for charge carrier injection or extraction and lower potential performance of QDs in many optoelectronic devices (Zanella et al, 2013;Page et al, 2014). To solve this issue, on one hand, short-chain thiol ligands [such as 1-octanthiol, tris(mercaptomethyl) and 2-ethylhexane-1-thiol] are used to replace the long-chain ligands by the solutionphase ligand exchange method to improve carrier mobility, which has been demonstrated in the high-performance visible QLEDs (Shen et al, 2015;Li et al, 2016;Song J. et al, 2019). On the other hand, embedding QDs in a high-mobility hybrid perovskite matrix has been demonstrated to be effective by Gong et al (2016).…”

Improved Efficiency of All-Inorganic Quantum-Dot Light-Emitting Diodes via Interface Engineering

“…Quantum dots (QDs) have many advantages including high color purity, high photoluminescence (PL) quantum yield (QY), and high stability, which make them promising luminescent materials for light-emitting diodes (LEDs) (Anikeeva et al, 2009;Bae et al, 2013;Shirasaki et al, 2013;Shen et al, 2015;Chen et al, 2018;Cao et al, 2019;Zhang et al, 2019). Recently, the performance of QD LEDs (QLEDs) has been improved greatly, the external quantum efficiencies (EQEs) for tricolor QLEDs have all surpassed 20%, with peak EQEs of 30.4% for red, 22.9% for green, and 19.8% for blue QLEDs, respectively Shen et al, 2019;Song et al, 2019). At present, highly efficient QLEDs are mainly based on hybrid organic-inorganic structure, in which poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is widely used as the hole injection layer (HIL); poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (Poly-TPD), poly{9,9dioctylfluorene-co-N-[4-(3-methylpropyl)]diphenylamine} (TFB), or poly(N-vinyl carbazole) (PVK) are adopted as the hole transport layer (HTL); and zinc oxide (ZnO) nanoparticles (NPs) are used as the electron transport layer (ETL) (Qian et al, 2011;Dai et al, 2014;Zhang et al, 2019).…”