All-solution-processed colour-tuneable tandem quantum-dot light-emitting diode driven by AC signal

We demonstrate a novel structure for a quantum-dot light-emitting diode (QD-LED) with wide-range colour-tuneable pixels, fabricated via full solution processing. The proposed device has a symmetrical structure produced via stacking of an inverted-structure diode with a green QD emission layer (EML) and normal-structure diode with a red QD EML. It is an electron-only device; however, a charge generation layer in the middle of the device generates holes for the formation of excitons. Depending on the polarity of the applied voltage, either the bottom inverted unit or the top normal unit is operated, thereby emitting green or red light, respectively. The working mechanism of the device is investigated via analysis of the charge generation mechanism and carrier transport path. In addition, the colour tunability is verified using a simple alternating current (AC) driving scheme; the duty cycle modulation of the AC signal enables fine colour adjustment over a broad range, from pure green to pure red. Thus, our colour-tuneable QD-LED with vertically stacked independently operated sub-pixels can open a promising pathway towards cost-effective ultra-high-resolution displays.