CsPbBr3 Nanocrystals Prepared Using Block Copolymer Micelles for LEDs

We systematically investigated how block copolymer (BCP) templating influences CsPbBr₃ nanocrystals inside polystyrene-block-poly(2-vinylpyridine) (PS_m-b-P2VP_n) micelles in benzene (BEN), toluene (TOL), oxylene (OXY), and 1,3,5-trimethylbenzene (TMB) for light-emitting diode (LED) applications. Each organic solvent features a benzene ring attached to different numbers of methyl groups, thus providing varying solvent qualities for the formation of CsPbBr₃ nanocrystals. We found that solvent quality plays a crucial role in fabricating CsPbBr₃ nanocrystals with a superior photoluminescence (PL) performance and long-term stability. The micellization strength of PS_m-b-P2VP_n is strongest in TMB but weakest in BEN, suggesting that increasing the number of methyl groups attached to benzene enhances micellization. Furthermore, increasing the number of methyl groups attached to benzene also yields three positive effects: increased dissociation, complexation, and coordination interaction of PbBr₂ with P2VP cores. Consequently, each PS_m-b-P2VP_n micelle can capture a higher content of [PbBr₃]⁻ complexes in TMB. Complexation of PbBr₂ followed by coordination interaction with P2VP is also critical because the growth of CsPbBr₃ nanocrystals inside individual micelles involves binding of Cs⁺ cations with [PbBr₃]⁻ complexes rather than with PbBr₂ nanocrystals. However, [PbBr₃]⁻ complexes are not crystalline but amorphous and are not uniformly distributed within the P2VP cores. When a small amount of CsBr was added, a portion of [PbBr₃]⁻ complexes transformed to form CsPbBr₃ perovskite nanocrystals (PNCs) with small size and polydisperse size distribution, coexisting with abundant active [PbBr₃]⁻ and [PbBr₄]^2– complexes. Furthermore, variations in the molecular weights and compositions of PS_m-b-P2VP_n have minimal influence on the size of the CsPbBr₃ nanoparticles. Instead, the CsBr content plays a more decisive role in controlling nanoparticle size, primarily due to the limited solubility of CsBr, an all-inorganic precursor, in methanol.