Fig. 1: Principle and performance of tandem QLED.

a Device structure. The tandem device consists of two vertically stacking QLEDs that are connected by an IZO intermediate electrode (ZMO_B/ZMO_T represents the bottom/top ZnMgO layer). The use of IZO enables the tandem device to operate in either direct current (DC) or alternating current (AC) mode (V_DC/V_AC represents the direct/alternating current voltage). b Schematic optical model of a tandem QLED (EML represents emission layer). To achieve constructive wide-angle interference, the length of both cavities should be simultaneously optimized (D_{B_1}/D_{T_1} and D_{B_2}/D_{T_2} are the distances of the bottom/top recombination zone from the top Al electrode and the bottom ITO electrode, respectively; B-QLED/T-QLED represents the bottom/top QLED in tandem QLED). c Simulated outcoupling efficiencies (OCEs) of both B-QLED and T-QLED as a function of the thickness of ZMO_T and IZO (\(\gamma\): charge balance efficiency). d–h The optical images of the surface of IZO thin films after static spin-casting (d), and dynamic spin-casting with ethanol solvent (e), as well as the electroluminescence (EL) images of B-QLED (f), T-QLED (g), and tandem QLED (denoted as S-QLED, where both B-QLED and T-QLED are connected in series) (h). Scale bars, 1 mm. The J-V-L (i), and EQE-J characteristics (j), of B-QLED (red solid circle), T-QLED (blue solid circle), and S-QLED (purple solid circle), respectively: the EQE of the S-QLED (purple solid circle) are perfectly equal to the summation of those of B-QLED and T-QLED (purple open star). k The photos of a tandem device that was operated in alternating current mode at different driving conditions (also demonstrated in Supplementary Movie 1). Scale bars, 5 mm.