Face-to-face integrated tandem quantum-dot LEDs with high performance and multifunctionality.

The realization of solution-processed tandem quantum-dot LEDs (QLEDs) remains a technical challenge due to the limitations of inefficient interconnect layer and the damage caused by multiple solution processes. Here, we develop a high performance tandem QLED by face-to-face integrating a top-emitting QLED with a transparent QLED. The top and bottom units can be addressed independently, thereby enabling the tandem QLED to operate in series, parallel, and color-tunable modes for multifunctionality. In series mode, QLEDs demonstrate an ultra-low turn-on voltage of 3.3 V and a record-breaking external quantum efficiency of 60.7%. In parallel mode, QLEDs achieve an impressive brightness of over 4.8 × 10cd m. In color-tunable mode, the color, brightness, and color temperature can all be adjusted with a color tuning range of 114% NSTC. Moreover, both series and parallel connections contribute to an improved stability, resulting in a T lifetime of nearly 30,000 h at 1000 cd m, which is an improvement of approximately 2.8 times over control devices. Our work offers a feasible solution for achieving multifunctional tandem QLEDs with the advantages of solution processed, high efficiency, high brightness, long lifetime and full color tunability for various light source applications.