All-solution processed inverted QLEDs with double hole transport layers and thermal activated delay fluorescent dopant as energy transfer medium

Abstract

Highly efficient, all-solution processed inverted quantum dot light-emitting diodes (QLEDs) with high performance are demonstrated by employing poly(9-vinlycarbazole) (PVK) as additional hole transport layer (HTL) and doping it with a blue thermal activated delay fluorescent (TADF) material, 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN). This PVK: 2CzPN composite layer not only optimizes hole injection, but also utilizes the leaked electrons to enhance device luminance by energy transfer process from 2CzPN to quantum dots (QDs). These benefits enable a 20-fold increment for the device current efficiency (from 0.523 cd/A to 11 cd/A) and a 9.9-fold improvement for the maximum luminance (from 3220 cd/m 2 to 35352 cd/m 2 ), compared with those of the standard QLED with poly[(9, 9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) single HTL. In comparison with the QLED with TFB/pristine PVK double HTLs, the device performance of the optimal QLED with PVK: 2CzPN additional HTL are still 40.8% and 61.7% higher in luminance and current efficiency, respectively. • 1. TADF material, 2CzPN, is used as the medium for FRET process, instead of phosphorescent materials in several previous reports. • 2. The inverted devices were fabricated in all solution process without vacuum evaporation method. • 3. Double HTLs scheme was fabricated successfully without solvent erosion, and improved the device performance significantly.