High-performance pure red perovskite light-emitting diodes utilizing conformational transformation of ionic liquid additive

The performance of perovskite light-emitting diodes (PeLEDs) depends highly on the defect density and carrier transport properties of the perovskite layer. Some long-chain ligands are good at passivating the defects, but they hinder the charge injection into the perovskite because of their insulating nature. Notably, red PeLEDs suffer from spectral shift under bias caused by ion migration inside the mixed cation perovskite layer. This study proposes the use of silver bis(trifluoromethanesulfonyl)imide (AgTFSI) a conducting ionic liquid as a multifunctional additive to the perovskite to overcome the aforementioned problems. The TFSI anion comprises strong electronegative -CF3 and Lewis base -SO2 groups. These functional groups interact with undercoordinated Pb2+, efficiently suppressing defects and reducing nonradiative recombination in the perovskite film. AgTFSI displays two distinct conformations: cisoid and transoid. The opposing orientations of the two functional groups enable the cisoid structure to exclusively engage with the perovskite through the -SO2 groups. In contrast, the semi-linear arrangement of the transoid structure provides accessibility to both -SO2 and -CF3 groups for interaction. Owing to their conformational flexibility, the TFSI anions in AgTFSI undergo a transition from the zigzag cisoid to a more linear transoid structure when added to the perovskite, enabling efficient packing and tight binding with the Pb defects. As a result, the perovskite film shows a twofold increase in photoluminescence quantum yield and decay lifetime. Consequently, the red PeLEDs achieve a maximum external quantum efficiency of 13.52%, nearly double that of the control device. This study confirms AgTFSI's potential as an effective additive for high-performance pure red PeLEDs.