Stability Improvement in Quantum-Dot Light-Emitting Devices via a New Robust Hole Transport Layer

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4 '-(N-(4-butylphenyl))-di-phenylamine)] (TFB) is commonly used as the organic hole transport layer (HTL) in high-efficiency Cd-based quantum-dot light-emitting devices (QDLEDs). Despite its good hole transport performance, limitations with its cross-linking properties often result in susceptibility to solvent damage when coating subsequent layers. Here, we investigate the use of a robust thermally cross-linked polymer, 9,9-bis[4-[(4- ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl-N2,N7-diphenyl-9H-fluo-rene-2,7-diamine (VB-FNPD), as an HTL for QDLEDs. The results show that using VB-FNPD instead of TFB can double the electroluminescence half-life (LT50) of the devices, leading to an LT50 of 10,100 h versus only 4900 h for the TFB device at an initial luminance (L0) of 1000 cd m-2. Atomic force microscopy surface scans show that VB-FNPD HTLs have smoother and more uniform morphologies when compared to TFB, which may help improve the quality of the HTL/QD interface and QD film uniformity, both of which are important for long-lived QDLEDs. Steady-state photoluminescence studies on hole-only devices suggest that VB-FNPD is also more stable under hole current flow. Further investigations using capacitance versus voltage on the devices show that replacing TFB by VB-FNPD reduces charge accumulation in the devices, which is likely another factor in the stability improvement.