High‐Performance Cadmium‐Free Blue Quantum Dot Light‐Emitting Devices with Stepwise Double Hole‐Transport Layers

Abstract

ZnSe/ZnS core/shell quantum dots (QDs) are environmental‐friendly blue light‐emitting material, which can easily achieve deep blue emission upon external excitation. However, its deep valence band (VB) and numerous defect states remain handicap to realize sufficient performance of quantum dot light‐emitting diodes (QLEDs). In this work, high‐performance cadmium‐free ZnSe/ZnS QLEDs by constructing a double organic hole‐transport layer (HTL) to obtain carrier balance are presented. The double HTLs, which consist of poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)diphenylamine) (TFB) and 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT), can suppress the accumulation of electrons between the HTL and the emissive layer (EML), leading to more hole and electron recombination luminescence in QD layer. In addition, the C8‐BTBT layer is conducive to improve the uniformity of QDs film. Thus, the resulting device achieves an external quantum efficiency of 7.23% with TFB/C8‐BTBT double HTLs, which is almost 150% higher than that of traditional devices based on a single hole‐transport layer (4.84%). The authors anticipate that these results can provide a guidance for the optimization of cadmium‐free blue QLEDs.

Research Progress of Heavy-Metal-Free Quantum Dot Light-Emitting Diodes

“…In addition, the study of multi-hole transport layer structures has also become one of the exploration directions. For example, Luo et al [99] prepared a high-performance Cd-free blue ZnSe/ZnS QLEDs by using a TFB/C8-BTBT dual hole transport layer, as shown in Figure 6; its maximum EQE was 7.23%, which is nearly 150% higher than traditional devices based on a single hole transport layer. Research has shown that introducing a TFB/C8-BTBT dual hole transport layer can effectively reduce the charge accumulation between the HTL and QD emission layer, and improve hole injection in Cd-free blue light QLEDs.…”

Enhancing the efficiency and stability of ZnSe pure blue quantum dot light-emitting diodes via ionic liquid doping

“…6,7 However, the practical applicability of Cd-based QDs is limited due to environmental concerns. Consequently, recent research has shifted its focus to Cd-free substitutes, including zinc selenide (ZnSe) 12–15 zinc selenide telluride (ZnSeTe), 16–18 indium phosphide (InP), 19,20 and copper indium sulfide (CIS). 21,22 ZnSe-based QDs emerge as promising candidates for Cd-free blue QLEDs due to their narrow full-width at half-maximal values (FWHM) and a broad bandgap of 2.7 eV.…”

“…Contemporary QLEDs, representing the current state of the art, utilize organic polymeric materials such as poly(4-butylphenyl-diphenylamine) (poly-TPD), ([9,9-dioctylfluoren-2,7-diyl)- co -(4,4′-( N -(4- sec -butylphenyl)diphenylamine))] (TFB), and poly( N -vinyl carbazole) (PVK) in hole transport layers (HTLs), while inorganic zinc oxide (ZnO) or Mg-doped ZnO nanoparticles (NPs) are employed in electron transport layers. 12–15,24 However, the integration of organic HTLs and inorganic electron transport layers introduces substantial energy barriers for charge carrier injection, along with disparities in electron/hole transport mobilities. 15 This combination results in charge accumulation at the interface and interfacial exciton quenching, factors that ultimately diminish operational lifetime.…”

“…This endeavor involves enhancing hole mobility and mitigating the significant energy barrier between the valence band of ZnSe QDs and the highest occupied molecular orbital (HOMO) of polymeric HTLs. 14,32,33 Addressing this alignment challenge is crucial for achieving optimal performance in ZnSe-based QLEDs.…”

“…38,39 Furthermore, enhancing device properties can be achieved by integrating cross-linkable units into traditional polymer side chains. 40 Techniques such as using double HTLs 14,41 and blending small molecules 12 into the quantum dot emissive layer have also proven effective. Notably, doping ionic liquid (IL) into the polymer hole transport material (HTM) stands out as a straightforward and viable method to reconfigure the polymeric HTL, thereby enhancing the characteristics of QLEDs.…”