Improving the Performance of Quantum Dot Light-Emitting Diodes by the Enrichment of a Fluorinated Component on Top of a Hole Transport Layer

The hole transport layers are crucial to the device performance of quantum dot light-emitting diodes (QLEDs) due to the huge hole-injection barrier between the work function of the anode and the valence band of II-VI-type quantum dots (QDs). In this study, we developed a heterogeneous hole transport layer (HTL) consisting of two conjugated polymers with almost identical conjugated backbones and successfully applied it in QLEDs. The introduction of trifluoromethyl groups simultaneously modified surface energy, energy levels, and hole transport mobility, thus altering the heterojunction of HTL/QD and enhancing the device's performance. Based on an optimized weight ratio of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-4,4 '-(N-(4-s-butylphenyl)diphenylamine)] (TFB)/TF-DCF3 = 7:3, the maximum current efficiency of a red QLED significantly improved from 14.1 cd A(-1) for the reference device based on TFB as the HTL to 23.1 cd A(-1). According to X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, the blend film with a weight ratio of TFB/TF-DCF3 = 7:3 exhibited a gradient component distribution, with TF-DCF3 enriched on the top surface. Our research sheds light on the advantage of a hole transport layer with a fluorinated component and its application in a QLED and provides an approach to fine-tune the component's vertical distribution of HTL.