Improved performance of inverted quantum dot light-emitting diodes by blending the small-molecule and polymer materials as hole transport layer

Abstract

Abstract Quantum dot light-emitting diodes (QLEDs) have received extensive academic and industrial attention for the exceptional properties of QDs including size-tunable emission wavelengths, narrow emission linewidths and inherent photo-physical stability. It is vital for the QLEDs performance to select appropriate hole transport materials due to the imbalance carrier injection caused by the higher electron mobility in most devices. In this paper, a small molecular material 1,1-bis [4-[N,N′-di (p-tolyl)amino]phenyl]-cyclohexane (TAPC) with high hole mobility was used to physically blend with poly-N-vinylcarbazole (PVK) as the hole transport layer (HTL) by solution processing. In the inverted red QLEDs, when the optimized mass ratio of the mixture HTL (2:1 of PVK:TAPC) was adopted, the device turn-on voltage drops from 4.0 V to 2.4 V, the maximum current efficiency (CEmax) reaches 19.3 cd/A, which is 50% higher than that of the standard devices (with only PVK as HTL), the maximum power efficiency (PEmax) significantly increases from 5.2 lm/W to 16.7 lm/W, increased by more than 3 times, the external quantum efficiency (EQE) reaches 45% enhancement from 8.7% to 12.6%, and the optimized device lifetime is enhanced by 4.5 times. The improvement of performances can be mainly attributed to the addition of TAPC with high hole mobility and suitable energy levels, and the unique charge generation between PVK:TAPC and hole injection layer (HIL) interface. The results demonstrate that the blending of the small-molecule and polymer hole transport materials will be a feasible approach to improve the performance in the QLEDs.