Low turn-on voltage and highly bright Ag–In–Zn–S quantum dot light-emitting diodes†
During the last three decades, there has been significant development of quantum dot light emitting diodes (QLEDs) due to their widespread applications. However, common QLEDs depend on cadmium, which is toxic and limits their all-inclusive applications. To address this challenge, herein, we report a highly efficient and low turn-on voltage Cd-free inverted QLEDs with eco-friendly Zn–Ag–In–S quantum dots as the emissive layers. The device performance is closely related to the properties of the hole transport layer (HTL), which are explored by employing different hole transport materials, namely, 4,4′,4′′-tris(carbazol-9-yl) triphenylamine (TCTA) and N,N′-bis(naphthalen-1-yl)-N,N′ bis(phenyl)-benzidine (NPB). Although the energy levels of the lowest unoccupied molecular orbitals (LUMO) of TCTA and NPB are similar, electron leakage from the QDs to NPB is observed, which deteriorates the device performance. An excellent QLED can be achieved if a thick TCTA layer is introduced next to the QDs. Additionally, the QLED with a stepwise HTL consisting of TCTA/NPB presents a slightly higher current efficiency than the TCTA-only device, revealing that the reduction in holes accumulated at the QDs/HTL interface is beneficial for enhancing the device efficiency. Compared to the previously reported Zn–Ag–In–S based QLEDs, our device exhibits a low turn-on voltage of 2.2 V and the highest luminance (over 2000 cd m−2) and efficiency (larger than 2.1 cd A−1).