Linear cross-linkers enabling photothermally cured hole transport layer for high-performance quantum dots light-emitting diodes with ultralow efficiency roll-off

In quantum-dot light-emitting diodes (QLEDs), poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4 '-(N-(4-butylphenyl))] (TFB) has been one of the most widely used hole-transport layer (HTL) materials. However, for solutionprocessed especially ink-jet printed QLEDs, TFB normally suffers from low glass-transition temperature and inter-layer erosions hindering to achieve both high device efficiency and stability. In the present work, three novel linear bis-benzophenone cross-linkers (BPO4, BPO6 and BPO8) were carefully designed and synthesized for constructing compact HTL with ~ 100 % solvent resistance. Under a mild photothermal curing condition of 120 degrees C and 365 nm UV with a power of 16 mW cm-2, the introduction of these cross-linkers into TFB film, even at a low weight concentration of 3.33 %, not only resulted in the HTL with reduced free film volumes and hence compact three-dimensional networks in QLEDs, but also enhanced the hole transport ability, so that better charge balance in QDs layer and improved overall QLED performance could be achieved. In the BPO series which have varying alky lengths, the BPO6 with a medium central alkyl length showed the best thin film state, and thus enabled to improve the QLED device performance with the highest EQE of 16.77 % (20.22 lm W-1, 22.72 cd A-1), which is 20% higher than that of pristine TFB based device. From the peak EQE to the EQE at 20000 cd m- 2, an ultra-low deviation of 1.73 % was observed in the TFB:BPO6 based red QLEDs. The T50 lifetime of TFB:BPO6 based QLEDs is almost twice of that of the pristine TFB based device. Our strategy demonstrates that developing linear cross-linkers for polymer HTL can facilitate forming compact and dense HTL, achieving simultaneous improvements in QLEDs device efficiency and stability.