Switchable Anion Exchange in Polymer-Encapsulated APbX3 Nanocrystals Delivers Stable All-Perovskite White Emitters

We report a one-step synthesis of halide perovskite nanocrystals embedded in amphiphilic polymer (poly(acrylic acid)-block-poly(styrene), PAA-b-PS) micelles, based on injecting a dimethylformamide solution of PAA-b-PS, PbBr2, ABr (A = Cs, formamidinium, or both) and “additive” molecules in toluene. These bifunctional or trifunctional short chain organic molecules improve the nanocrystal–polymer compatibility, increasing the nanocrystal stability against polar solvents and high flux irradiation (the nanocrystals retain almost 80% of their photoluminescence after 1 h of 3.2 w/cm2 irradiation). If the nanocrystals are suspended in toluene, the coil state of the polymer allows the nanocrystals to undergo halide exchange, enabling emission color tunability. If the nanocrystals are suspended in methanol, or dried as powders, the polymer is in the globule state, and they are inert to halide exchange. By mixing three primary colors we could prepare stable, multicolor emissive samples (for example, white emitting powders) and a UV-to-white color converting layer for light-emitting diodes entirely made of perovskite nanocrystals.

encapsulating the NCs was "closed" by adding a mixture of methanol and hexane (with a volume ratio of 1:5) to the toluene dispersion of the PAA-b-PS-Cs 0.5 FA 0.5 PbX 3 (X=Br and Br/Cl) NCs, while only hexane was added in case of PAA-b-PS-Cs 0.5 FA 0.5 Pb(BrI) 3 NCs. The resulting mixture was centrifuged again at 6000 rpm for 5 minutes and the supernatant was discarded. The precipitate was dried in a vacuum oven at 40 o C overnight. The obtained powders were individually ground with a ball mill (SPEX SamplePrep 8000M MIXER/MILL). The green and blue emitting samples were ball milled for 5 minutes, while red emitting sample for 2 minutes. The obtained fine powders were then mixed in an appropriate ratio to get a white emitting NCs powder.
White LED fabrication. The white emitting samples were prepared by mixing red, green and blue emitting powders of PAA-b-PS encapsulated Cs 0.5 FA 0.5 PbX 3 (X=Br and Br/Cl) NCs in an approximate ratio of 1:2:3 in weight (green:blue:red) was added to the water solution of polyvinyl alcohol (PVA, 210 mg/mL). Thereafter, the sample was drop-cast on a quartz substrate and subsequently dried for 15 min at 65°C. Then, white emitting film was placed on a 365 nm LED (maximum power of 1 W) which was used for excitation. A 425 nm long pass filter (Thorlabs) was then placed onto the PVA film to remove the UV excitation light, see the device structure in Figure  S23.
Morphological characterization. TEM images of the NC samples were acquired with a JEOL-1100 transmission electron microscope operating at an acceleration voltage of 100 kV. Samples were prepared by drop casting diluted solutions of NCs onto carbon film-coated 200 mesh copper grids.
Structural characterization. Structural analysis was performed on a PANanalytical Empyrean X-ray diffractometer, equipped with a 1.8 kW CuKα ceramic X-ray tube, operating at 45 kV and 40 mA, and a PIXcel3D 2×2 area detector. A colloidal dispersion of corresponding NCs was drop-cast on a zero-diffraction silicon substrate. All the diffraction patterns reported in this work were collected at room temperature under ambient conditions using parallel beam geometry and symmetric reflection mode. Post-acquisition XRPD data analysis was carried out using the HighScore 4.1 software from PANalytical.

Spectroscopic measurements.
Optical absorption spectra were recorded using a Varian Cary 300 UV-VIS absorption spectrophotometer. The PL spectra were measured on a Varian Cary Eclipse spectrophotometer using an excitation wavelength (λ ex ) of 350 nm for all the samples. Samples were prepared by diluting NC solutions in toluene, in quartz cuvettes with a path length of 1 cm. Absolute Photoluminescence quantum yields of NC samples were measured using an Edinburgh FLS900 fluorescence spectrometer equipped with a Xenon lamp, a monochromator for steady-state PL excitation, and a time-correlated single photon counting unit coupled with a pulsed laser diode (λ ex = 405 nm, pulse width = 50 ps) for time-resolved PL. The PLQY was measured using a calibrated integrating sphere (λ ex = 350 nm for all samples). For the PLQY measurements, all the NC dispersions were diluted to an optical density of 0.1 ±0.02 at the corresponding excitation wavelength in order to minimize the amount of fluorophore being reabsorbed.
Sample preparation for high flux irradiation experiments. PAA-b-PS encapsulated CsPbBr 3 , Cs 0.5 FA 0.5 PbBr 3 and FAPbBr 3 NCs samples were prepared using AVAc as an additive molecule. The "reference" CsPbBr 3 NCs were prepared using hot injection method developed by Protesescu et al. 3 The toluene dispersions of all samples including the reference sample with the concentration 10-12 mg/mL were mixed with a polystyrene solution (30 mg/mL in toluene). The resulting mixture was precipitated by adding 12 mL of hexane, followed by overnight drying in vacuum oven at 40 °C. Then, the powder was grounded by using a mortar and pestle set and was then placed inside a glass cell.
Laser irritation stability tests. Stability tests were carried out using a continuous wave laser diode for excitation (λex = 445 nm, Oxxius LBX-445-650-HPE-PP, max power 715 mW) coupled with an iris to reduce the circular excitation spot to a diameter of 1mm (spot size was determined with Thorlabs BP209-VIS/M beam profiler). The excitation power was measured after the iris using a Thorlabs PM100D Digital Optical Power meter interfaced with a Thorlabs S121C photodiode. The emission was monitored via a collimator/optical fiber/filter holder/optical fiber assembly coupled with an Ocean Optics HR4000 spectrometer. The collection optics was placed at 45° with the respect to the excitation beam and a long pass filter (λcut-off = 450 nm, Thorlabs FEL0450) was placed in the fiber coupled filter holder. The emission was collected every 60 seconds using the Ocean View software.

DLS measurements.
The hydrodynamic sizes of the particles were measured by Dynamic Light Scattering (DLS) using a Malvern Instruments Zetasizernano series instrument. Prior to each reading, the sample was left for 1 min to equilibrate and triplicate measurements were done for each sample.
NMR measurements. NMR experiments were performed at 298 K on a Bruker AvanceIII 600 MHz spectrometer equipped with 5 mm QCI cryoprobe with z shielded pulsed-field gradient coil. The free polymer sample was prepared by dissolving 8.25 mg of PAA-b-PS powder in 1mL of DMF-d7. The PAA-b-PS micelles sample was prepared by dissolving the same amount of polymer into DMF followed by drop-wise addition of the polymer solution into toluene to induce the micelle formation. Thereafter, the micelles were collected by adding hexane in access to the crude solution followed by centrifugation at 5000 rpm for 5 minutes. The supernatant was discarded and the dried precipitate was redispersed in d-8 toluene. For the liquid state NMR measurements, the samples were transferred into 5 mm disposable NMR tubes (Bruker). Before each acquisition, automatic matching and tuning (both on 1H and 13C) and homogeneity were adjusted.
In the 1 H-NMR experiments, 128 transients were accumulated after having applied a 90 degree of flip angle excitation pulse, with relaxation delay of 30 s, over a spectral width of 20.55 ppm (offset at 10 ppm).
In the 13 C NMR spectra (inverse gated 1 H decoupling), 10752 transients were collected after a 30-degree pulse, with 32768 of digit points, an interpulses delay of 3.5 s, over a spectral width of 240.1 ppm (offset at 100 ppm).
An apodization exponential function equivalent to 0.1 and 15 Hz were applied to the 1 H and 13 C FIDs respectively, before the Fourier transform.
The 1 H-13 C HSQC (multiplicity edited Heteronuclear Single Quantum Coherence, with the selection of CH 2 and CH/CH 3 in opposite phases, graphically represented in blue and red respectively) was performed with 32 FIDs, 2048 data points, 256 increments, over a spectral width of 15.02 ppm for 1 H and 165.7 ppm for 13 C (transmitter frequency offsets at 7.45 and 74.6 ppm, respectively).
The 1 H-13 C HMBC (Heteronuclear Multiple Bond Correlation) was acquired with 128 FIDs, 2048 data points, 128 increments, over spectral width of 15.02 ppm for 1 H and 222.1 ppm for 13 C (transmitter frequency offsets at 7.48 and 99.8 ppm, respectively).
All spectra were referred to the not deuterated residual solvent peaks, at 7.09 and 129.2 ppm (toluene-d 7 ), and at 8.03 and 163.2 ppm (DMF-d 6 ), for 1 H and 13 C, respectively.

Extended literature review on polymer encapsulated lead halide pervoskite NCs
Various studies have studies have shown that embedding perovskite NCs in different polymeric matrices significantly improves their stability. Among those, encapsulation of individual MHP NCs is most the appealing and gain signifincat attention of the scientific community in recent past. Hou et al. back in 2017 used polystyreneblock-poly-2-vinylpyridine (PS-b-P2VP) micelles to prepare PS-b-P2VP coated CsPbX 3 NCs. 4 This strategy was later adopted by Lohmuller et al. to prepare the PS-b-P2VP coated MAPbX 3 NCs. 5 In both studies, the nanoreactors were prepared initially by dissolving P4VP-b-PS in toluene, followed by the addition of the corresponding halide salts (such as CsX, MAX and PbX 2 ) sequentially. Given a lower solubility of metals salts in toluene, Hou et al. noted that the mixture of nanoreactors and metal halide salts (PbBr 2 ) needed to be kept under continuous stirring for at least two weeks to achieve 0.1 mg/mL loading of the corresponding salt. This however makes the procedure reported by Hou et al. and Lohmuller et al. et al. not only time consuming but also not much useful, as it leads to a very low yield of the final material. In terms of stability, Hou et al. reported that PS-b-P2VP capped CsPbBr 3 NCs completely lost their PL in a mixture of methanol and toluene (with a volume ratio of 10 to 1) within 2 minutes upon dispersion (see Figure 3 of their manuscript). Lohmuller et al., on the other hand, reported that the PS-b-P2VP encapsulated MAPbBr 3 NCs have improved stability against moisture, water and weak UV irradiation. The high stability of perovskite NCs against a short chain alcohol such as methanol remains an unachieved goal so far. This is most likely due to the fact that methanol is less hydrophilic and has a lower surface tension compared to water, hence it will have higher tendency to penetrate through the PS protective layer which consequently makes it more detrimental for the perovskite NCs compared even to water. This was indeed observed experimentally on the PS-b-P2VP capped perovskite NCs prepared by Hou et al.
Lin et al. recently used a multiarm star-like amphiphilic triblock copolymer, namely poly(4-vinylpyridine)-blockpoly(tert-butyl acrylate)-block-polystyrene (denoted P4VP-b-PtBA-b-PS), to encapsulate perovskite NCs. 6 In that study, a three-step synthesis was used to prepare MAPbX 3 NCs coated with a SiO 2 middle layer and PS as the outer shell. Due to the unique molecular structure of the polymer, the micelles are made of 21 chains of triblock copolymer (1 st segment of P4VP for MAPbX 3 nucleation, 1 middle segment of PtBA to yield PAA upon the thermolysis to bestow the capability to grow a silica shell and last outer segment of PS to improve the stability) linked together through a knot at the end of the P4VP block. Although this approach is elegant, the micelles made of multi-arm star-like amphiphilic triblock copolymers have a rather low density of PS shell (21 chains per micelles) in comparison to the conventional self-assembled micelles made of linear diblock copolymers. This in turn provides a low barrier against a polar environment. Indeed, the authors observed that the growth of the SiO 2 shell between the PS layer and the MAPbX 3 layer is crucial to achieve a good stability against water. Also in that case, the stability against short chain alcohols was not addressed. Similarly, in another study reported by the same group, Liu et al. used a star-like molecular bottlebrush trilobe, poly(2-hydroxyethyl methacrylate)-graft-(poly(acrylic acid)-block-partially cross-linked polystyrene (denoted PHEMA-g-(PAA-b-cPS)) to act as a polymeric nanoreactor. 7 The authors reported that the CsPbBr 3 is formed within the PAA block while PS block acts as the protective layer. Owing to its trilobe structure, such polymer will not be able to self-assemble to form a classical micellar structure due to the constraint of the chain. This, in turn, will create some defects in the protective PS layer (not uniform hairy shell), thus allowing the penetration of solvents through the PS shell. The authors indeed reported that polymer coated NCs could withstand 20-30% volume of methanol for less than 20 days. Further increase in the methanol volume (40 % or higher) resulted in a complete loss of PL instantly upon dispersion.       Figure  S7.   Figure S1 and S6. The measurements were performed in toluene dispersions. The PL decays were fitted with the following expression: Fit = A + B1*exp(-t/T1) + B2*exp(-t/T2) + B3*exp(-t/T3) (values reported in Table S1). Table S2: PL lifetime data extracted from the fittings of plots reported in Figure S8 of PAA-b-PS encapsulated NCs. The reported amplitude and intensity weighted average lifetime were calculated using the following expressions:

Detailed NMR analysis
We performed detailed liquid state nuclear magnetic resonance (NMR) spectroscopy to confirm that the micelles have an outer shell of PS and an inner core of PAA. The NMR measurements include 1 H, 13 C, 2D 1 H-13 C, Heteronuclear Single Quantum Coherence (HSQC) and 1 H-13 C Heteronuclear Multiple Bond Correlation (HMBC). The experiments were performed on the starting free PAA-b-PS, empty PAA-b-PS micelles and PAA-b-PS encapsulated NCs. The free polymer sample was prepared by dissolving 8.25 mg of PAA-b-PS powder in 1mL of DMF-d7. The PAA-b-PS micelles sample was prepared by dissolving the same amount of polymer into DMF followed by drop-wise addition of the polymer solution into toluene to induce the micelle formation. Thereafter, the micelles were collected by adding hexane in excess to the crude solution, followed by centrifugation at 5000 rpm for 5 minutes. The supernatant was discarded and the dried precipitate was redispersed in d-8 toluene. The PAA-b-PS encapsulated NCs sample was prepared under the same conditions as those used for the empty micelles, but with the addition of metal S14 bromides (CsBr and PbBr 2 ) and the solution containing the additive molecules. For NMR measurements, the samples were transferred into 5 mm disposable NMR tubes (Bruker).
We initially performed the 1 H, 13 C, 2D 1 H-13 C, HSQC and 1 H-13 C HMBC NMR analysis on the starting free PAA-b-PS in DMF-d 7 as a solvent, see the results in Figure 3 c, d and Figure S11. DMF is a nonselective solvent for the polymer, therefore both the PAA and PS components of the polymer should be detectable through 1 (Figure 3d, top panel). These evidences indicate that the PAA block has a very poor mobility in toluene (it remains inaccessible to the solvent) and the only contribution to the high resolution NMR peaks is due to the PS moiety. We further recorded the 1 H NMR spectrum of the PAA-b-PS encapsulated NCs in toluene-d 8 and compared it with the 1 H NMR spectrum of the empty micelles (see Figure S13). The spectrum of the empty PAA-b-PS micelles has peak profiles and signal widths at half height that are identical to those of the micelles with the NCs inside, thereby confirming the similar structure of the polymer micelles in both cases. These measurements overall confirm that PAA forms the core of the micelles encapsulating the perovskite NCs while PS forms the outer shell. Overall, our NMR analysis verifies the structural conformation of the polymer, which consists of PAA as a rigid core and a PS outer shell, the latter exposed to the solvent thus more flexible.

Figure S11. 1 H (A), 13 C (B), 1 H-13 C HSQC (C) and 1 H-13 C HMBC (D) NMR spectra of the free PAA-b-PS polymer dissolved in DMF-d7
The spectra reveal all the characteristic 1 H and 13 C peaks of the PAA-b-PS structure, along with their peak assignments, and 1 H-13 C HMBC cross correlations. HMBC allows the possibility to identify the correlation between 1 H and distant 13 C (1-2 bonds). Here, HMBC confirm unambiguously the assignment of peak 3 of PAA moiety, through the HMBC cross correlations (dashed lines in the figure S11 D) between the signals 3 and 4, and the signal 2, whose 13 C resonance (at 176.4 ppm) is uniquely attributed to a C=O group of the acidic function of PAA moiety.

Figure S12: 1 H (A), 13 C (B), 1 H-13 C HSQC (C) and 1 H-13 C HMBC (D) NMR spectra of empty PAA-b-PS micelles dispersed in toluene-d8.
The spectra reveal all the characteristic 1 H and 13 C peaks of the PS structure along with their unambiguous peak assignments and 1 H-13 C correlation (2D NMR). In toluene-d8, the 1 H and 13 C signals of PAA block are missing. These evidences indicate that PAA moiety has a poor mobility in toluene, which is consistent with a rigid core conformation.

Figure S13: the 1 H NMR spectra of A) the PAA-b-PS encapsulated CsPBBr3 NCs micelles and B) empty PAA-b-PS, in toluene-d8.
The inset shows the aromatic region expanded. Broad aromatic peak corresponding to signals 7,7' and 8 of PS moiety of polymer, is clear distinguishable from not deuterated residual Tolene-d8 peaks (sharp).

Stability tests using various additive molecules
To understand the role of the additive molecules, we initially prepared PAA-b-PS encapsulated CsPbBr 3 , Cs 0.5 FA 0.5 PbBr 3 and FAPbBr 3 NCs by using two different organic molecules as additives, namely phenethylammonium bromide (PEABr, one functional group: ammonium) and 5-aminopentanoic acid (APAc, two functional groups: one carboxylic, one amino) and tested their stability against methanol. Freshly prepared samples were dispersed in methanol (under condition similar to those reported for Figure 3 of the manuscript) and the results are reported in Figure S10. The CsPbBr 3 and Cs 0.5 FA 0.5 PbBr 3 samples prepared using PEABr exhibit significant drop in the PL soon after their dispersion in methanol. The FAPbBr 3 sample suffered the most (nearly the complete loss of PL). On the other hand the samples prepared using APAc remained bright and stable in methanol. As the FAPbBr 3 NCs appeared to be the most sensitive to the type of molecule, we decided that an extensive search for a list of additive molecules that were good stabilizers for the polymer encapsulated NCs should be done indeed on NCs with FAPbBr 3 composition. The results of this search are reported in Figures S15-18).
PAA-b-PS encapsulated FAPbBr 3 NCs were prepared in the presence of various additive molecules having one, two or more functional groups, and either one aliphatic or one aromatic hydrocarbon chain, while all other reaction conditions were kept the same. Qualitative observations regarding the optical properties and stabilities of resulting PAA-b-PS encapsulated FAPbBr 3 NCs, when different additive molecules were used, are summarized in Table S4. Typical molecules that were tested include hexan-1-amine, hexanoic acid, 2-aminoethanethiol, 4-aminobutanoic acid, 5-aminopentanoic acid, (3-aminopropyl)phosphonic acid, 1,4-butandioic acid, 2-amino-3-hydroxypropanoic acid, pyrrolidine-2-carboxylic acid, 2-amino-3methylbutanoic acid, 2-amino-5-(diaminomethylideneamino)pentanoic, 2-aminopentanedioic acid, -2amino-3-sulfhydrylpropanoic acid, 2-amino-3-(1H-imidazol-4-yl)propanoic acid and 2aminotertphthalic acid. Absorption and PL spectra measured in toluene dispersions are reported in Figure  S15 and the photographs of the corresponding samples recorded under UV-lamp are reported in Figure  S16. In toluene dispersions, the sample prepared using molecules having one amino and one thiol group (cysteine) had well defined absorption features but no PL at room temperature. Samples prepared using molecules containing one carboxylic, one thiol and one amino group or one carboxylic and two amino groups were weakly emissive (see Figure S15). Molecules containing more than one amino groups were not useful in the present case (FAPbBr 3 NCs), most likely due to their tendency to deprotonate the FA + cations, compromising the photoluminescence characteristic of corresponding samples, in agreement with previous reports. [8][9] Upon dispersing in methanol, the control sample (prepared without additive molecules) as well as the samples prepared using additive molecules with one functional group (amine, carboxylic acid and their mixture) significantly lost their PL. Notably, the sample prepared using a mixture of two additive molecules having each a single functional group attached to the hydrocarbon chain (hexan-1-amine + hexanoic acid) nearly lost its PL upon dispersion in methanol. On the other hand, the sample prepared using 5-aminopentanoic acid, which contains two functional groups (carboxylic and amino), both attached to the same hydrocarbon chain remains bright and stable. A comparison of these two latter samples shows that additive molecules containing two functional groups are much more effective than mixtures of two types of molecules each containing a single functional group. In general, the samples prepared by using molecules containing at least two or more functional groups remained bright and stable, see Figures S17-S18. The successful molecules had either all acidic (carboxylic, phosphonic) groups, or a S19 combination of acidic and at most one basic (such as amino) groups. Details on PL stability of PAA-b-PS encapsulated FAPbBr 3 NCs prepared by various additives molecules are summarized in Table S4.    Figure S15. Figure S17. Stability of PAA-b-PS encapsulated FAPbBr3 NCs prepared by using additive molecules with different functional groups in methanol. PL spectra recorded on PAA-b-PS encapsulated FAPbBr3 NCs prepared using various additive molecules (reported in Figure S15, S16) and dispersed in methanol.  Figure S15 and S17. Table S4. Optical properties of the PAA-b-PS encapsulated FAPbBr 3 NCs in terms of PL intensity and stability in toluene and methanol using different additives Figure S19: Stability of Cs0.5FA0.5PbBr3 NCs prepared using two different additive molecules. (a) Evolution of PL spectra of NCs prepared by using phenethylammonium bromide (PEABr) showing 56 % loss of PL over 60 min of continuous exposure to laser irradiation. While instead, the samples prepared by using 5-aminopentanoic acid (APAc) evidenced 6% loss of PL for the same period of exposure to laser irradiation (see Figure 4d   NCs deposited from toluene dispersions. Cs0.5FA0.5PbCl3 and Cs0.5FA0.5PbI3 NCs were prepared by halide exchange reactions starting from Cs0.5FA0.5PbBr3 NCs.

S23
Additional data on emitting powders and on the white light emitting layer preparation Figure S21: Comparison of the spectral features of PAA-b-PS encapsulated Cs0.5FA0.5PbX3 NCs powders before (i) and after mixing (ii), evidencing the retention of their native PL peak position in the white emitting blend. A PL spectrum recorded the same powder mixture after 4 weeks of ageing under ambient air shows that the PL peak positions and relative PL intensities remain unchanged (iii). The insets correspond to the photographs of the various samples under UV illumination (excitation light at 365 nm).