Small Molecule-Modified Hole Transport Layer Targeting Low Turn-On-Voltage, Bright, and Efficient Full-Color Quantum Dot Light Emitting Diodes

For an organic–inorganic hybrid quantum dot light-emitting diode (QD-LED), enhancing hole injection into the emitter for charge balance is a priority to achieve efficient device performance. Aiming at this, we employ N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) as the additional hole transport material which was mixed with poly(9-vinylcarbazole) (PVK) to form a composite hole transport layer (HTL) or was employed to construct a TPD/PVK bilayer structure. Enabled by this TPD modification, the green QD-LED (at a wavelength of 515 nm) exhibits a subband gap turn-on voltage of 2.3 V and a highest luminance up to 56 157 cd/m2. Meanwhile, such TPD modification is also beneficial to acquire efficient blue and red QD-LEDs. In particular, the external quantum efficiencies (EQEs) for these optimized full-color QD-LEDs are 8.62, 9.22, and 13.40%, which are 3–4 times higher than those of their pure PVK-based counterparts. Revealed by the electrochemical impedance spectroscopy, the improved electroluminescent efficiency is ascribable to the reductions of recombination resistance and charge-transfer resistance. The prepared QD-LEDs surpass the EQE values achieved in previous reports, considering devices with small-molecule-modified HTLs. This work offers a general but simple and very effective approach to realize the low turn-on-voltage, bright, and efficient full-color QD-LEDs via this solution-processable HTL modification.