Highly Luminescent and Water-Resistant CsPbBr₃–CsPb₂Br₅ Perovskite Nanocrystals Coordinated with Partially Hydrolyzed Poly(methyl methacrylate) and Polyethylenimine

Abstract

All inorganic lead halide perovskite nanocrystals (PNCs) typically suffer from poor stability against moisture and UV radiation as well as degradation during thermal treatment. The stability of PNCs can be significantly enhanced through polymer encapsulation, often accompanied by a decrease of photoluminescence quantum yield (PLQY) due to the loss of highly dynamic oleylamine/oleic acid (OLA/OA) ligands. Herein, we propose a solution for this problem by utilizing partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and highly branched poly(ethylenimine) (b-PEI) as double ligands stabilizing the PNCs already during the mechanochemical synthesis (grinding). The hydrophobic polymer of h-PMMA imparts excellent film-forming properties and water stability to the resulting NC−polymer composite. In its own turn, the b-PEI forms an amino-rich, strongly binding ligand layer on the surface of the PNCs being responsible for the significant improvement of the PLQY and the stability of the resulting material. Moreover, the introduction of b-PEI promotes a partial phase conversion from CsPbBr 3 to CsPb 2 Br 5 to obtain CsPbBr 3 /CsPb 2 Br 5 nanocrystals with a core− shell-like structure. As-prepared PNCs solutions are directly processable as inks, while their PLQY drops only slightly from 75% in colloidal solution to 65% in films. Moreover, the final PNC−polymer film exhibits excellent stability against water, heat, and ultraviolet light irradiation. These superior properties allowed us to fabricate a proof of concept thin film OLED with h-PMMA/b-PEI-stabilized PNCs as an easily processable, narrowly emitting color conversion composite material. KEYWORDS: CsPbBr 3 −CsPb 2 Br 5 , partially hydrolyzed PMMA, highly branched PEI, high photoluminescence quantum yield, stability B ecause of their excellent photophysical properties, such as adjustable band gaps, high molar extinction coefficients, and excellent charge−transfer performance, all-inorganic cesium lead halide perovskite nanomaterials CsPbX 3 (X = Cl, Br, or I) have been perfect candidates for many optoelectronic applications, such as solar cells, 1 LEDs, 2,3 lasers, 4 photodetectors, 5 field effect transistors (FETs), 6 and Xray scintillators. 7 However, moisture, heat, and oxygen make perovskite nanomaterials suffering from poor stability. 8,9 For example, they dissolve in polar solvents, such as water, due to the ionic nature of the material itself. Additionally, perovskite nanomaterials easily undergo phase transitions and decompose

Mechanosynthesis of polymer-stabilized lead bromide perovskites: insight into the formation and phase conversion of nanoparticles

“…Perovskite NPs stabilized by h-PMMA/b-PEI were prepared based on our previous work with some modifications [36]. Specifically, mixtures of equivalent molar amounts of PbBr2 and CsBr were ground by a ball mill.…”

“…2). Based on the comparative analysis of the results provided by XRD and electron microscopy, we propose a possible mechanism of the formation and phase transformation of these particles during grinding, which is discussed below and schematically presented in After adding h-PMMA/PEI solved in DCM to the milling jar, most h-PMMA remains in the solvent phase of DCM; however, most PEI bounds quickly to PbBr2 in the solid phase, which is expected considering the different binding forces between the corresponding polymer (h-PMMA or PEI) and the solid phase [36,37]. The h-PMMA may form micellar templates for the growth of NPs due to the interactions between the hydrophobic ester bond and hydrophilic carboxylic acid group (or carboxylic acid anion) in the non-polar DCM.…”

“…The highly branched PEI, which is rich in amino-groups (about 600 amino groups per PEI-25K molecule), could form a complex with PbBr2 in solid phase, due to the strong interaction between the vacant orbital from Pb 2+ and the lone electron pair of nitrogen from PEI [36,37]. This complexation will further result in the redistribution of the material in the reaction mixture into the PbBr2-rich phase of CsPbBr3-xPbBr2/PEI and the rest, i.e., the CsBr-rich phase of CsPbBr3-yCsBr, in accordance to Reaction (1) CsPbBr3 (solid) + PEI  CsPbBr3-xPbBr2/PEI (in solid) + CsPbBr3-yCsBr (in solid) (1) The values of x and y in Reaction (1) reflect an additive influence of both the molecular weight and the amount of PEI molecules.…”

“…In our recent work, we reported that the CsPbBr3-CsPb2Br5 NPs coordinated by partially hydrolyzed PMMA (h-PMMA) and low-molecular-weight (800 g/mol) poly(ethylenimine) (PEI-0.8K) exhibited impressive water and UV resistance as well as thermal stability while maintaining high PL QY [36]. We attributed this attractive improvement to the synergetic influence of the surface modification with strong binding ligands, polymer-encapsulation, and proper heterostructuring of the emitting and non-emitting perovskite-phases.…”

“…Herein, attempting to further understand and master this system, we performed systematic in-depth investigations and found that although addition of the highly branched PEI improved the PL QY, the system investigated did not tolerate further increasing of the PEI-0.8K content and reached its limits by the concentrations reported in [36]. The application of a high-molecular-weight (25,000 g/mol) PEI (further referred to as PEI-25K) ligand in the NP synthesis allowed us to overcome this limitation.…”