Design of the Hole-Injection/Hole-Transport Interfaces for Stable Quantum-Dot Light-Emitting Diodes

Abstract

Extensive efforts have been devoted to improving the operational performance of quantum-dot light-emitting diodes (QLEDs). However, the fundamental understanding of the relationship between the design of the hole-injection layer (HIL)/hole-transporting layer (HTL) interface and the operational stability of QLEDs is limited. Here, we demonstrate that in the operation of red QLEDs, the leakage electrons induce in situ electrochemical reduction reactions of the polyfluorene HTLs, which in consequence create trap states and deteriorate charge-transport properties. We invoke an oxygen-plasma treatment on the PEDOT:PSS HILs, resulting in HIL/HTL interfaces with enhanced hole-injection properties. This simple method leads to more efficient exciton generation in the QDs layer and mitigated leakage electron-induced degradation of the HTLs, enabling red-emitting QLEDs with improved operational performance, i.e., high external quantum efficiency of >20.0% at a brightness ranging from 1000 to 10 000 cd m–2 and a long T 95 operational lifetime of ∼4200 h at 1000 cd m–2.

Shelf‐Stable Quantum‐Dot Light‐Emitting Diodes with High Operational Performance

“…The PEDOT:PSS‐coated substrates were treated by oxygen plasma for 4 min before transferred into a nitrogen‐filled glovebox (O 2 < 1 ppm, H 2 O < 1 ppm). [ 44 ] TFB (in chlorobenzene, 12 mg mL −1 ), QDs (in octane, 15 mg mL −1 ), and Zn 0.9 Mg 0.1 O nanoparticles (in ethanol, 30 mg mL −1 ) were layer‐by‐layer deposited by spin coating at 2000 r.p.m. for 45 s. The TFB layers were baked at 150 °C for 30 min before the deposition of the QD layers.…”

Effect of Air Exposure of ZnMgO Nanoparticle Electron Transport Layer on Efficiency of Quantum-Dot Light-Emitting Diodes

“… 23 Accumulated electrons tend also to leak into the adjacent TFB layer and cause electrochemical reactions deteriorating hole transport in TFB which is essential for device stability. 24 …”

“…This is explained by electron leakage into HTL and is confirmed by parasitic TFB signal emerging at a short-wavelength shoulder of the QDs PL spectrum ( Figure S7a,b ), which is however quickly quenched as a result of electrochemical reduction of TFB. 24 In QLED with 1 ML thick QDs layer, this effect is still severe, and after the voltage is increased above ∼3.4 V, the HTL signal disappears irreversibly ( Figure S8a ). That voltage can be considered as a threshold above which electrons start to leak.…”

“…Electrochemical degradation is most likely related to poor stability of ligands, 38 but other mechanisms including QDs etching by oxygen or PL quenching by trap states induced in TFB by electron leak are also possible. 14 , 24 …”

Electrically Pumped QD Light Emission from LEDs to Lasers

“…3e). 23,24 These excess electrons could be creating nonradiative recombination centers within the HTL, leading to severe quenching of excitons and physical degradation of the HTL. As a result, these research groups targeted improving hole injection efficiency in red QLEDs to inhibit the overflowing of electrons into the HTL.…”