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/m, which is comparable to that of the reported inverted red QLED with the highest efficiency.