Preparation of efficient quantum dot light-emitting diodes by balancing charge injection and sensitizing emitting layer with phosphorescent dye

Abstract

The balance between electron and hole injection is critical for obtaining high efficiency in quantum dot light-emitting diodes (QLEDs). Herein, 1,4-dioxane was employed as a solvent for poly(9-vinylcarbazole) (PVK) in stepwise co-HTL (HTL: hole transport layer)-based red QLEDs with poly[bis(4-phenyl)(4-butylphenyl)amine] (poly-TPD)/PVK, enabling a 41.5% enhancement in external quantum efficiency (EQE) compared to that of the control device without a PVK layer. Based on the high-performance co-HTL device, the blue phosphorescence dye bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) was incorporated into PVK for further improving the device efficiency. Consequently, at a blend ratio of 10 : 2 in volume, the co-HTL QLED with a mixed PVK : FIrpic layer showed the highest maximum current efficiency (CE_max) of 18.2 cd A⁻¹ and EQE_max of 12.2%, which was 30% and 28.4% higher than that of the co-HTL device without the FIrpic mixture, respectively. Compared to the control device with only poly-TPD as the HTL, the luminous efficiency nearly doubled. The substantial performance improvements are mainly attributed to the stepwise energy level alignment of the HTLs, uniform morphology of the PVK:FIrpic films obtained from the orthogonal solvent 1,4-dioxane, and efficient nonradiative energy transfer (NRET) from PVK : FIrpic (10 : 2) to the QDs. Thus, this study offers a new practical method to promote the performance of QLEDs and eventually their commercial use in display and lighting applications.