Enhancing performance in blue quantum-dot light-emitting diodes by using a deep HOMO carbazol-phenyl-triazine small molecule blended hole transporting layer

Abstract

Carrier transport balance is crucial for fabricating high-performance quantum-dot light-emitting diodes (QLEDs). However, due to the deep highest occupied molecular orbital (HOMO) and large bandgap of the blue CdSe/CdS/ZnS QDs, a mismatched energy level with common organic polymer hole materials leads to large hole injection barriers at the interface of the transport layers (HTLs) and quantum dot emitting layers (QD EMLs), reducing the performance of the blue QLEDs. Hence, to achieve highly efficient blue QLEDs, in this paper, bandgap modification engineering is applied to enhance the performance of blue CdSe/CdS/ZnS QLEDs by blending the deep HOMO bipolar host materials 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ) (6.13 eV) and PVK (5.70 eV) as the HTL. Compared to devices with neat PVK as the HTL, the QLEDs with 20 wt% TCPZ blended PVK (20 wt% TCPZ:PVK) as the HTL present a reduced turn-on voltage from 3.40 V to 2.90 V, improved maximum brightness from 7856.3 cd m⁻² to 24 413 cd m⁻², and an increased current efficiency (CE) and external quantum efficiency (EQE) from 3.22 cd A⁻¹ and 6.36% to 8.20 cd A⁻¹ and 16.33%. Moreover, the blended HTL devices show a lower efficiency roll-off at higher brightness (20 wt% TCPZ:PVK HTL:EQE = 10.14%@L = 7000 cd m⁻²; PVK HTL:EQE = 2.25%@L = 7000 cd m⁻²). The results demonstrate that the enhanced performance of blue QLEDs can be attributed to an improved hole injection at heterogeneous interfaces and facilitated the charge transfer balance between electrons and holes in blue QLEDs due to the presence of π–π interactions of carbazole groups in PVK and TCPZ.