Surface Modification of ZnO Nanocrystals with Conjugated Polyelectrolytes Carrying Different Counterions for Inverted Perovskite Light-Emitting Diodes.

In this work, bromide ions (Br - ) on the conjugated polyelectrolytes (CPEs) were converted to tetrafluoroborate (BF 4 - ) or hexafluorophosphate (PF 6 - ) ions through anion exchange. The three CPEs (PFN-Br, PFN-BF 4 , and PFN-PF 6 ) were utilized solely for surface modification of zinc oxide nanocrystals (ZnO NCs). The ionic groups on CPEs can form permanent dipoles to facilitate charge injection from ZnO NCs to cesium lead bromide (CsPbBr 3 ) NC emitters, therefore promoting luminescent properties of inverted perovskite light-emitting diodes (PeLEDs). The experimental results reveal that ZnO NC films were smoothened by CPEs that allowed flat deposition of the perovskite active layers; moreover, the improved contact between ZnO and perovskite layers was beneficial for reducing leakage current, as verified in the dark current measurement of devices. In addition, the incorporation of CPEs helped to passivate the defects of ZnO NC films and prolong the carrier lifetime of CsPbBr 3 NCs. PeLEDs based on different CPEs were then constructed and evaluated. The device based on PFN-Br showed the highest brightness and current efficiency, and the one based on PFN-BF 4 exhibited better current efficiency over PFN-Br under the low current density below 160 mA/cm 2 . This is the first report using fluorene-based CPEs with Br - , BF 4 - , or PF 6 - groups to modify the properties of ZnO and CsPbBr 3 NCs for the construction of inverted PeLEDs so far. Our experiments explored new kinds of CPEs on the surface modification of ZnO NCs and device performance of PeLEDs.