One-step fabrication of ZnO electron-transporting layers for perovskite light-emitting diodes with sub-bandgap turn-on voltage

Efficient charge injection into the emitters, which can reduce the voltage loss at interfaces, is a prerequisite for high-performance light-emitting diodes, with low voltage operation. Here we develop a sol–gel ZnO (s-ZnO) electron-transport layer for the perovskite light-emitting diodes (PeLEDs) with green-emission formamidinium lead bromide (FAPbBr₃) perovskites as emitters. Polyethylenimine (PEI) is mixed into the s-ZnO precursor as a modifier, which not only promotes the wettability of s-ZnO films, but also passivates the defects of s-ZnO without sacrificing their electrical conductivity. As a result, highly efficient FAPbBr₃ films are obtained on the s-ZnO films. The maximum current efficiency of PeLED with s-ZnO:PEI electron-transporting layer reaches 13.5 cd/A, 45% higher than that based on pristine s-ZnO without PEI modifier. Benefiting from the outstanding charge-transport properties of s-ZnO and high-quality perovskite film, the turn-on voltage of the s-ZnO based PeLEDs is only 1.9 V, much lower than the band-gap voltage (~ 2.3 V) of FAPbBr₃.