All-Solution-Processed Quantum Dot Light Emitting Diodes Based on Double Hole Transport Layers by Hot Spin-Coating with Highly Efficient and Low Turn-On Voltage.

Poly( N, N'-bis(4-butylphenyl) N, N'-bis(phenyl)benzidine) (poly-TPD) and poly(9-vinylcarbazole) (PVK) are both traditional hole transport layers (HTLs) in quantum dot light emitting diodes (QLEDs). Nevertheless, the low highest occupied molecular orbital of poly-TPD and poor hole mobility of PVK always result in poor performance of QLEDs when individually used as HTLs. Unfortunately, fabricating stepwise HTLs with poly-TPD and PVK faces technical problems until now. We demonstrate an effective method to construct the stepwise poly-TPD and PVK HTLs by utilizing the technique of hot spin-coating PVK in m-xylene on poly-TPD film. During this hot spin-coating process, the underlying poly-TPD remains unwashed and undamaged, benefiting the all-solution-processed QLED fabrication. The optimized all-solution-processed QLED with stepwise poly-TPD and PVK HTLs shows a maximum external quantum efficiency (EQE) of 15.3% and a maximum luminance of 17 110 cd/m2 with a low turn-on voltage ( Von) of 1.75 V. The maximum EQE is about 6.6 times higher than that of the reference QLED using a cold spin-coating process. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the hot spin-coating stepwise co-HTLs based QLEDs. This work manifests the positive effect on performance boost by a hot spin-coating strategy toward stepwise co-HTLs formation and paves the way to fabricate highly efficient all-solution-processed light emitting diodes.