Investigation of anti-solvent induced optical properties change of cesium lead bromide iodide mixed perovskite (CsPbBr3-xIx) quantum dots.

Cesium lead halide (CsPbX3, X=Cl, Br, I) perovskites are a new material system that has attracted a lot of research focus. Its tunable band gap and better thermal stability than organic lead halide perovskite give it the potential for applications in optoelectronic devices such as light-emitting diodes and solar cells. Here we have synthesized CsPbBr3-xIx perovskite quantum dots (QDs) via a solution process, and then have selected three different anti-solvents to purify the product. A significant effect on optical properties of CsPbBr3-xIx was found after the centrifugation process. Up to a ∼40nm shift was observed in mixed halide CsPbBr3-xIx QDs in both absorbance and PL spectra after purification while there was no obvious change in pure CsPbBr3 when it was subjected to the same purification steps. XPS analysis shows that the Br:I ratio of the CsPbBr3-xIx QDs had changed as a result of exposure to the anti-solvent, causing the change of the band gap and shift of the spectra. It is also shown that iodine can be removed more easily than bromine during the anti-solvent purification. Ab-initio simulations of small CsPbBr3-xIx atomic clusters suggest that exposed Cs ions on Cs-terminated facets are the first species to be attacked by hydrophilic molecules, likely dragging halide ions into solution with them to maintain overall charge neutrality in the material. Charge carrier recombination rates were found to be unchanged and all samples maintained a good PL quantum yield which was more than 44%.