Facile purification protocol of CsPbBr3 nanocrystals for light-emitting diodes with improved performance

Abstract

Light-emitting diodes (LEDs) based on CsPbBr3 nanocrystals (NCs) show excellent performance in terms of external quantum efficiency (EQE) and brightness. Removal of impurities after NC synthesis is a necessary and critical step to achieve such high performance. In fact, the optical and electronics properties of CsPbBr3 NCs are greatly affected by how the NCs are purified from the synthetic by-products, unreacted reagents, and organic ligand excess. Perovskite NC purification protocols must consider the ionic nature of the inorganic core, the labile nature of the ligand to surface bond, and the dynamic equilibrium between the surface-interacting and free form organic ligands to prevent NC damaging with subsequent loss of photoluminescence. Nowadays, the most employed purification protocol involves NC precipitation based on a diminished non-polar environment achieved by adding a low-dielectric constant miscible solvent to the NC solution (i.e., ethyl acetate to toluene), followed by redispersion of the precipitate in a storage solvent. Here, we explored the possibility to precipitate impurities left in the NC solution exploiting variations in solubility at decreasing temperature: by taking NC solutions at −20 °C overnight and by collecting the respective supernatant we obtain CsPbBr3 NC solutions which leads to LEDs with improved EQE. The freezing process at −20 °C gives rise to NC solutions that retain the pristine optical and electronic properties while the LED performances display a 3 times increase in terms of EQE (EQEmax = 8.9%) with respect to NCs treated with anti-solvent only. Finally, we show that our purification protocol can be applied to CsPbBr3 NCs obtained via different synthetic methods, thus demonstrating the versatility of our facile purification protocol.