Ligand‐Exchange of Low‐Temperature Synthesized CsPbBr₃ Perovskite toward High‐Efficiency Light‐Emitting Diodes

Abstract

CsPbX3 perovskite nanocrystals (NCs) are promising light‐emitting materials that have gained significant attention over the past few years. The synthesis of CsPbX3 NCs is usually performed in an inert environment under high temperature conditions; however, this requirement hinders commercial development. Low‐temperature preparation of CsPbX3 NCs with rationally designed surface ligands is crucial to the performance and stability of CsPbX3 light‐emitting diodes (LEDs). In this work, short‐chain ligand‐anchored CsPbBr3 NCs in ambient conditions at room temperature are synthesized and a ligand‐exchange strategy is employed, using longer‐chain acid/amine ligand pairs, resulting in the dramatic improvement of performance and stability of CsPbBr3 NCs LEDs. It is found that CsPbBr3 NCs anchored by oleic acid and oleylamine complex ligands result in the best LED performance, with the maximum luminance of 5033 Cd m−2, current efficiency of 18.6 Cd A−1, external quantum efficiency of 5.4%, turn‐on voltage of 3.2 V, and the best thermal stability under ambient conditions.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

“…[ 252 ] The importance of film quality was further demonstrated when butylamine was used along with short chain propionic acid in a low‐temperature synthesis of CsPbBr 3 NCs for directly fabricating LED devices (ITO/PEDOT:PSS/CsPbX 3 NCs/TPBi/Ca/Ag), which failed to work due to poor film quality . [ 290 ] In order to improve the LED device performances, ligand exchange reactions were performed and LEDs fabricated from OA and OAm passivated CsPbBr 3 NCs exhibited the best device performances (maximum luminance of 5033 cd m −2 , current efficiency of 18.6 cd A −1 , and EQE of 5.4%) despite possessing ligands with the longest carbon‐chain length (C18) tested. [ 290 ] This improvement in device performance was attributed to the NC film quality and superior defect passivation, which was supported by lengthened PL LTs that led to enhanced device luminance.…”

“…[ 290 ] In order to improve the LED device performances, ligand exchange reactions were performed and LEDs fabricated from OA and OAm passivated CsPbBr 3 NCs exhibited the best device performances (maximum luminance of 5033 cd m −2 , current efficiency of 18.6 cd A −1 , and EQE of 5.4%) despite possessing ligands with the longest carbon‐chain length (C18) tested. [ 290 ] This improvement in device performance was attributed to the NC film quality and superior defect passivation, which was supported by lengthened PL LTs that led to enhanced device luminance. [ 290 ] Together, these findings all demonstrate that a balance between NC quality (e.g., stability, defect passivation, and film fabrication) and the insulating nature of the ligands (e.g., charge injection and charge transport) must be reached for improved LED device performances.…”

“…[ 290 ] This improvement in device performance was attributed to the NC film quality and superior defect passivation, which was supported by lengthened PL LTs that led to enhanced device luminance. [ 290 ] Together, these findings all demonstrate that a balance between NC quality (e.g., stability, defect passivation, and film fabrication) and the insulating nature of the ligands (e.g., charge injection and charge transport) must be reached for improved LED device performances. [ 180,252,290,395 ]…”

The impact of ligands on the synthesis and application of metal halide perovskite nanocrystals

“…LED devices fabricated using the NC lms with optimized ligand chain length and bulkiness achieved high EQE of 9.71% that is 16-fold enhancement compared to OA/ OAm-capped CsPbBr 3 NCs. 147 Ye et al 201 investigated an opposite ligand exchange strategy where two short chain ligands (propionic acid and butylamine) anchored to CsPbBr 3 NCs were replaced with longer chain ligands. The ligand exchange was done by adding long chain acid and amine ligands to the NC solution prior to purication.…”

Lead Halide Perovskite Nanocrystals: Room Temperature Syntheses toward Commercial Viability

“…Ye et al adapted Akkerman's approach for LED applications. [70] They added carefully optimized amounts of oleic acid and oleylamine to as-synthesized CsPbBr 3 NCs, followed by ethyl acetate to precipitate the NCs. By redispersing the nanocrystals and repeating the cycle four times, they achieved complete ligand exchange while minimizing excess oleyl ligand in the final NC ink.…”