Polyethylenimine Insulativity-Dominant Charge-Injection Balance for Highly Efficient Inverted Quantum Dot Light-Emitting Diodes

Quantum dot (QD) light-emitting diodes (QLEDs) with an inverted architecture suffer from charge-injection imbalance and severe QD charging, which degrade device performance. Blocking excess electron injection into QDs is crucial for efficient inverted QLEDs. It is observed that polyethylenimine (PEI) has two opposite effects on electron injection: one is blocking electron injection by its intrinsic insulativity and the other one is promoting electron injection by reducing the work function of ZnO/PEI. In this work, the insulating nature of PEI has been dominantly utilized to reduce electron injection and the charge-injection balance is realized when PEI becomes thicker and blocks more excess electrons. Furthermore, PEI contributes to QD charging suppression and results in a smoother surface morphology than that of ZnO nanoparticles, which is beneficial for leakage current reduction and QD deposition. As a result, the optimized QLED with 15 nm PEI shows a 2.5 times improved efficiency compared to that of the QLED without PEI. Also, the QLED possesses the maximum external quantum efficiency and current efficiency of 16.5% and 18.8 cd/A, respectively, accompanied with a low efficiency roll-off of 15% at 1000 cd/m², which is comparable to that of the reported inverted red QLED with the highest efficiency

Architectural Engineering of Nanowire Network Fine Pattern for 30 μm Wide Flexible Quantum Dot Light-Emitting Diode Application

Replacing rigid metal oxides with flexible alternatives as a next-generation transparent conductor is important for flexible optoelectronic devices. Recently, nanowire networks have emerged as a new type of transparent conductor and have attracted wide attention because of their all-solution-based process manufacturing and excellent flexibility. However, the intrinsic percolation characteristics of the network determine that its fine pattern behavior is very different from that of continuous films, which is a critical issue for their practical application in high-resolution devices. Herein, a simple optimization approach is proposed to address this issue through the architectural engineering of the nanowire network. The aligned and random silver nanowire networks are fabricated and compared in theory and experimentally. Remarkably, network performance can be notably improved with an aligned structure, which is helpful for external quantum efficiency and the luminance of quantum dot light-emitting diodes (QLEDs) when the network is applied as the bottom-transparent electrode. More importantly, the advantage introduced by network alignment is also of benefit to fine pattern performance, even when the pattern width is narrowed to 30 μm, which leads to improved luminescent properties and lower failure rates in fine QLED strip applications. This paradigm illuminates a strategy to optimize nanowire network based transparent conductors and can promote their practical application in high-definition flexible optoelectronic devices