Sky-blue LEDs with a maximum EQE of 5.02% and high brightness are obtained with perovskite nanocrystals treated with CuCl2.

Light emitting diodes (LED) based on halide perovskite nanocrystals (NC) have received widespread attention in recent years. In particular, LEDs based on CsPbBr3 NCs were the object of special interest. Here, we report for the first time green LED based on CsPbBr3 NCs treated with ammonium thiocyanate solution before purification with polar solvent. The champion device fabricated based on the treated CsPbBr3 NCs showed high efficiency and high stability during operation as well as during storage. A study on morphology and current distribution of NC films under applied voltages was carried out by conductive atomic force microscopy, giving a hint on efficiency roll-off. The current work provides a facile way to treat sensitive perovskite NCs and to fabricate perovskite NC-based LED with high stability. Moreover, the results shed new light on the relation between film morphology and device performance and on the possible mechanism of efficiency roll-off in NC LED.

Passivation of defects in halide perovskite using phosphine oxide or alkyl-phosphonate has recently obtained a few remarkable achievements. However, effective application of phosphine oxide or alky-phosphonate in passivating perovskite quantum dots (QDs) are seldom reported due to solubility issue or difficulty of amount control. In this work, two bifunctional organic molecules containing phosphine oxide groups, 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) and 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13), are deposited on QDs films by thermal evaporation. The molecules, both as passivation agents as well as electron transporting materials, exhibit stark contrast in passivating QDs and in light-emitting diodes (LEDs) performance. A competition between charge transfer and defect passivation between the QDs and the molecules is proposed. In film, electron transfer from the QDs to PO-T2T dominates and quench the QDs, while the passivation effect of PO-T2T on the QDs dominates in driving device and enhances luminance of the LEDs. In contrast, passivation effect of SPPO13 on the QDs dominates both in films and in LEDs. A maximum EQE of 2.67% is obtained for the pure-blue LED based on SPPO13-passivated QDs films. This work provides a guide on the selection of passivation agents based on phosphine oxide and a promising passivation method for high-efficient perovskite QD LEDs.