Interface regulation toward low driving voltage perovskite light-emitting diodes

Electroluminescence (EL) at sub-bandgap voltages has been observed in perovskite light-emitting diodes (PeLEDs), which, however, does not appear as a universal phenomenon in current reports, and the mechanism remains in debate. Herein, we verify that the turn-on voltage of PeLEDs depends neither on the carrier injection nor on the energy levels of charge-transport layers, ruling out the Auger-assisted upconversion charge injection effect. The recombination of diffused and thermally generated charge carriers, rather than the high-order effect, is believed to be responsible for the sub-bandgap EL behavior. Moreover, we demonstrate that a critical prerequisite for the sub-bandgap EL is the strong confinement of charge carriers in the emissive layer, which can be achieved by inserting a CdSe/ZnS quantum-dot monolayer at the interface of the perovskite/electron-transport layer. The accumulated holes induced by the CdSe/ZnS monolayer can reduce the electron-injection barrier, thus leading to an observable EL at a sub-bandgap voltage. Our work provides a substantial evidence for the sub-bandgap EL turn-on processes and addresses the debate in PeLEDs.