Efficient and Stable Red QLEDs via 4-TBP-Doped Hole Transport Layer Engineering

The insufficient charge carrier mobility in the hole transport layer (HTL) and the energy level mismatch with the emissive layer have long been critical factors limiting the electroluminescence performance and carrier balance of quantum dot light-emitting diodes (QLEDs). To address this issue, we employed 4-tert-butylpyridine (4-TBP) as a dopant in poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine))] (TFB) as the HTL. By modulating the highest occupied molecular orbital energy level of TFB and enhancing the hole mobility, 4-TBP doping effectively reduces the hole injection barrier and suppresses interfacial charge accumulation. The optimized TFB:4-TBP HTL significantly improves hole injection efficiency in red QLEDs, leading to a decrease in turn-on voltage from 1.74 V to 1.65 V and an increase in external quantum efficiency (EQE) from 22.21 to 26.47%. Moreover, at an initial luminance of 1000 cd m-2, the QLED with the TFB:4-TBP HTL reaches a T95 operational lifetime of 11,708.67 h, which is 2.4 times longer than that of the undoped one. 1.22- and 1.37-fold improvements in EQEmax are obtained for green and blue QLEDs, respectively. This strategy demonstrates that 4-TBP doping, combined with interface engineering, provides an effective method to achieve efficient and stable hole transport, offering new insights for carrier balance control in solution-processed QLEDs.