High-performance quantum dot light-emitting diodes with hybrid hole transport layer via doping engineering

Here, we report on the hybrid hole transport materials 4,4'-bis-(carbazole-9-yl)biphenyl (CBP) or poly-N-vinylcarbazole (PVK) doped into poly(4-butyl-phenyl-diphenyl-amine) (Poly-TPD) as the hybrid hole transport layer (HTL) to tailor the energy band alignment between hole injection layer (HIL) and quantum dot (QD) light emitting layer in order to realize efficient quantum dot light emitting diodes (QLEDs) in all solution-processed fabrication. Compared to the pristine Poly-TPD based device, it is found that the electroluminescence (EL) performance of QLEDs can be significantly improved by 1.5 fold via addition of CBP into Poly-TPD, which can be attributed to the lowered highest occupied molecular orbital (HOMO) level of Poly-TPD to reduce the energy barrier between HTL and valance band (VB) of QDs. Thus, after doping small molecules into polymer under optimized proportion (Poly-TPD:CBP = 2:1 by weight), the hole transport rate can be balanced, facilitating the carrier injection from HTL to QDs and enhancing the efficiency of QLEDs. As a result, a maximum luminance, a maximum current efficiency and a maximum power efficiency of 7600 cd/m2, 5.41 cd/A and 4.25 lm/W can be obtained based on this variety of hybrid HTL employed QLEDs.