Flexible Double-Side Red Emission Quantum Dot Light-Emitting Diodes via van der Waals Epitaxy of Nanometer-Thick Metal Transparent Electrodes on Two-Dimensional MoS₂

Double-side-emission (DSE) CdSe@CdZnSeS/ZnS quantum dot (QD) light-emitting diodes (DSE-QLEDs) with transparent nanometer-thick metal electrodes were achieved by monolayer (ML) MoS 2 -mediated van der Waals (vdW) epitaxy. Via the transfer of a wafer-area MoS 2 film from sapphire to polyethylene terephthalate (PET) substrates and ZnO nanoparticle (NP) layer surfaces, flexible red DSE-QLEDs with the structure of MoS 2 /Au/M-PEDOT:PSS/poly-TPD/QD/ZnO NP/MoS 2 /Al can be readily obtained by spin-coating and thermal evaporation. The unique van der Waals epitaxy on MoS 2 surfaces greatly helps the lateral film formation for both nanometer-thick Au films on flat PET substrates and Al films on the rough ZnO layer surface. The conductive Au and Al films grown on MoS 2 surfaces act as effective anode and cathode electrodes for current injections of the QLEDs, respectively, while the ML MoS 2 contributes to the balance of injected charges by suppressing the electron injection rate. Finally, with the 10 nm-thick Au anode and 15 nm-thick Al cathode, our flexible DSE-QLEDs exhibit a subthreshold turn-on voltage of 1.8 V, a maximum luminance of ∼83000 cd/m 2, and a current efficiency of 31.9 cd/A, in which approximately 36% of total luminance is contributed from the Al cathode side emission.