Effect of PVK mixed TAPC as hole transport layers on device performance of red quantum-dot light-emitting diodes

A series of red quantum-dot light-emitting diodes (QLEDs) are developed using poly(9-vinlycarbazole) (PVK) mixed 4,4′-Cyclohexylidenebis [N, N-bis(4-methylphenyl) benzenamine] (TAPC) as hole transport layer (HTL) by all-solution processing. The current density-voltage characteristics of carrier-only devices with emissive layer (EML) shows that the PVK: TAPC composite HTL exhibits the increase of current density at the same voltage with increasing the mixing concentration of TAPC. And the hole and electron currents are almost identical as the mixing concentration of TAPC is equal to 20%. The corresponding red QLED achieves the highest performance with a maximum luminance of 289 490 cd/m2, a maximum CE of 26.8 cd/A, a maximum EQE of 17.9% and a narrow full width at half-maximum (FWHM, 26 nm). The T50 lifetime is up to 65 min, which is more than 8 times longer than that of QLED with PVK as HTL at an initial luminance of 100 000 cd/m2. Furthermore, the efficiency roll-off is significantly reduced in the developed QLEDs. For instance, even at a very high brightness over 50 000 cd/m2, the QLEDs can still maintain a high CE of 26.6 cd/A and an EQE of 17.4%. The results show that the PVK: TAPC composite HTL builds energy level cascade to some degree and improve the hole transport capacity, leading to better charge carrier transfer balance. Consequently, the device performance is promoted.