A Color Tunable Quantum-Dot Light-Emitting Diode Device Driven by Variable Voltage

A hole transporting layer (HTL) consisting of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)] doped with 4,4'-Bis(9H-carbazol-9-yl)biphenyl was designed for high-performance multicolor quantum-dot light-emitting diode (QLED) fabricated by an all-solution process technique. The band structure of the HTL is tailored via small molecule doping to create a cascade of highest occupied molecular orbital levels (HOMO) from the hole injection layer to the quantum-dot emissive layer (EML). This energy band tailoring significantly overcomes the energy barrier for hole injection and enables multicolor emission from a single device, which consists of mixture of red, green, and blue QDs as EML. The color tunability as a function of applied voltage is the most novel feature of our device. The color tunability was observed at a voltage of 2 V for red emission, 3 V for orange, 4 V for yellow and 6 V for greenish white and a high brightness of 15,000 cd m^-2 was demonstrated for white emission, which is attributed to the efficient and balanced carriers' injection into the EML. The strategy of using tunable HOMO of the HTL in combination with color tunability of the EML as a function of applied voltage, pushes QLED a step closer to practical multicolor display applications.