Efficiently Improved Photoluminescence in Cesium Lead Halide Perovskite Nanocrystals by Using Bis(trifluoromethane)sulfonimide

Abstract

Cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) nanocrystals (NCs) have attracted enormous attention because of their great potential for optoelectronic applications, such as light-emitting diodes (LEDs). However, the photoluminescence and surface ligands of CsPbX3 NCs have a great impact on their device applications. Herein, we report a molecular superacid of bis­(trifluoromethane)­sulfonimide (TFSI), which could boost the photoluminescence in the metal halide perovskite nanocrystals. In particular, the photoluminescence quantum yield (PLQY) of CsPbI3 nanocrystals could be greatly improved from 28.6% to near 100% with the superacid treatment. The improved PLQY in CsPbX3 nanocrystals is mainly contributed from the surface passivation based on the characterizations. The CsPbX3 nanocrystals were further modified with PMMA, which could greatly improve their stability while preserving high photoluminescence and good dispersion. The use of superacid combined with a polymer for improving the photoluminescence and stability in CsPbX3 provides an alternative strategy for optoelectronics.

Quantum Efficiency Enhancement of Lead-Halide Perovskite Nanocrystal LEDs by Organic Lithium Salt Treatment

“…Furthermore, XPS can serve as another proof for successful ligand exchange. Similar to Liu et al, we find a fluorine peak (F 1s) in the XPS spectrum of the LiTFSI-treated NC film (see Figure S6d), which could not be observed in the native sample . However, because of the low X-ray intensity used in our experiments, nitrogen and sulfur peaks could not be identified.…”

“…We do not find a significant difference in NC arrangement and coverage, indicating that the addition of LiTFSI to the NC solutions preserves a certain fraction of the native oleylamine and oleic acid ligands, as schematically shown in Figure . It is important to note that we do not see a significant shrinkage of the NC size upon LiTFSI treatment, in contrast to the findings of Liu et al, where they observe etching of the NC surface by the acidic (H)­TFSI . These films were subsequently studied by photoluminescence, photoelectron spectroscopy, density functional theory modeling, and in PeLEDs.…”

“…Here we use the organic TFSI – anion, proving that a halide-free passivation is possible. As sulfonic moieties have been reported to fill halide vacancies, a similar interaction from LiTFSI seems plausible and has already been reported. ,, In particular, TFSI – has been suggested to act as surface passivation by coordinating with Pb and Cs cations on the NC surface. Additionally, suppressed hysteresis of the j – V characteristics (see Figure S7c) implies that ions have been demobilized to some extent.…”

“…Additionally, suppressed hysteresis of the j – V characteristics (see Figure S7c) implies that ions have been demobilized to some extent. Although neither a significant shrinkage of the NCs nor a spectral blue-shift has been observed, as reported by Liu et al upon (H)­TFSI treatment, an analogous chemical reaction may still take place on the NC surface: CsPbBr 3 + LiTFSI → CsPbBr 2 TFSI + LiBr. The main difference to the (H)­TFSI passivation is that the resulting lithium halide salt is not as volatile as the corresponding halide acid (HBr in this case), effectively blocking a prolonged decomposition of the NC surface, thus avoiding shrinkage.…”

“…However, because of the low X-ray intensity used in our experiments, nitrogen and sulfur peaks could not be identified. Nevertheless, following the consideration of Liu et al, it can be concluded that TFSI – is attached at the surface of the NCs …”