Synergistic Regulation of Hole and Electron Transport Layers for Efficient Injection Balance in Deep Blue Quantum Dot Light-Emitting Diodes

The insufficient hole injection and excess electron injection unfortunately result in inferior performance in blue QLEDs. To study the predominant factors for hole injection, six different hole transport materials were investigated. The smaller energy barrier between the hole transport layer (HTL) and QDs dominates the injection, and high hole mobility further accelerates the process, which qualifies 4,4′-bis(3-vinyl-9H-carbazol-9-yl)-1,1′-biphenyl (CBP-V) as the most suitable HTL matrix. Simultaneously, moderate electron mobility reduction is also needed, which could be realized by PVP doping in the ZnO electron transport layer (ETL). The highly hole-mobile poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB) was wrapped in the cross-linked CPB-V framework to further improve hole mobility and hole injection. The high level charge balance after the matching of 20 wt % TFB:CBP-V and 0.75 wt % PVP:ZnO enables high-performance blue QLEDs, with the EQE reaching 17.11% and the lifetime of 216 h at 100 cd/m2. Our investigation builds a general principle of both HTL and ETL material regulation for high-level injection balance.