Device Characteristics of Inverted Red Colloidal Quantum-Dot Light-Emitting Diodes Depending on Hole Transport Layers

We investigated the dependence of the device characteristics of inverted red colloidal quantum dot light-emitting diodes on the hole transport layer. Three different hole transport materials, 4,4′-bis(carbazole-9-yl)biphenyl, 4,4,′4″-tri(N-carbazolyl)triphenylamine, N, N′-bis(naphthalen-1-yl)-N, N′-bis(phynyl)-2,2′-dimethylbenzidine, and six different hole transport layer structures were used for comparing the devices’ performances. The turn-on voltage of the devices was dominated by the energy level difference between the lowest unoccupied molecular orbital of the hole-injection layer (molybdenum trioxide) and the highest occupied molecular orbital of the adjacent hole transport material. The hole mobility as well as the energy level difference between the valence band of the quantum dot and the highest occupied molecular orbital of the adjacent hole transport material was significant factors for high luminance and efficiency. Among the considered devices with six different hole transport layer structures, the device with a single 4,4′-bis(carbazole-9-yl)biphenyl layer as a hole transport layer exhibited the best performance, with a peak efficiency of 5.56% at ~10 mA/cm². All of the devices exhibited nearly the same main emission peak at ~641 nm and a narrow full-width-half-maximum of ~34 nm, and their International Commission on Illumination 1931 color coordinates were very deep red, nearly the same as the BT.2020 red color coordinate of (0.708, 0.292).