Polymer Enhanced Stability of Inorganic Perovskite Nanocrystals and Their Application in Color Conversion LEDs

Cesium lead halide (CsPbX 3 , X = Cl, Br, I) nanocrystals (NCs) offer exceptional optical properties for several potential applications but their implementation is hindered by a low chemical and structural stability and limited processability. In the present work, we developed a new method to efficiently coat CsPbX 3 NCs, which resulted in their increased chemical and optical stability as well as processability. The method is based on the incorporation of poly(maleic anhydride- alt -1-octadecene) (PMA) into the synthesis of the perovskite NCs. The presence of PMA in the ligand shell stabilizes the NCs by tightening the ligand binding, limiting in this way the NC surface interaction with the surrounding media. . We further show that these NCs can be embedded in self-standing silicone/glass plates as down-conversion filters for the fabrication of monochromatic green and white light emitting diodes (LEDs) with narrow bandwidths and appealing color characteristics. gold nanoparticles. 23,24 This ligand interdigitates in the carbon chain shell that surrounds the nanoparticle offering an enhanced stability. In the case of a stable NC core, the PMA units can be further polymerized utilizing a diamine, which allows the sealing of the NC core and its transfer to polar solvents. In our experiments, PMA was introduced during the synthesis of the perovskite NCs, before the injection of the Cs-precursor, to avoid any potential disruption of the perovskite NC after its formation. We further show the application of green- and orange-red-emitting CsPbX 3 perovskite NCs embedded in self-standing silicone-based plates as down-conversion filters in monochromatic and WLEDs. The growth of the NCs in the presence of a protecting polymer facilitates their processing by preventing aggregation and minimizing interference with the silicone curing process while better protecting the NC surface both by preventing interaction with the media and by obstructing the ligand dynamic exchange.


INTRODUCTION
Inorganic lead halide perovskite NCs with high luminescence quantum yields (up to >90%), extremely narrow emission bandwidths (< 50 nm), broad emission spectra tunability (from 410 to 700 nm) and short radiative lifetimes (<30 ns) have very recently emerged as a new class of material with outstanding potential for optoelectronic applications. [1][2][3] Compared to classical Cd-based chalcogenide quantum dots (QDs), CsPbX3 (X = Cl, Br, I) offer a very broad and facile adjustable composition versatility together with ample options for shape control, which allow tuning of their emission wavelength throughout the whole visible spectrum. [4][5][6][7][8][9] Among other applications, such highly luminescent and spectrally tunable NCs are ideally suited to produce monochromatic and white LEDs (WLEDs). 10 In this regard, CsPbBr3 NCs have been utilized as the emissive layer in electroluminescent monochromatic QD-LEDs showing exceptionally narrow emission bandwidths and thus high color quality in devices. 11,12 However, while CsPbX3 (X = Br, I) perovskite NCs can be stored in solution, maintaining their luminescence for several months, they suffer from humidity, light and temperature driven degradation in operation conditions and from a low colloidal stability with a related limited processability. 13 In part, this is associated with a highly dynamic ligand binding to the NC surface, 14 which additionally hinders the NCs' post-synthetic purification and further functionalization. When perovskite NCs with highly ionic components and high surface energies come into contact with a polar surface or solvent, they rapidly degrade to their components. Any efforts to exchange the oleic acid coating of the as prepared NCs with ligands of interest providing additional functionality, rendering them water soluble or just protecting them from degradation, inevitably results in the NC degradation. This chemical instability is the bottleneck that limits all technological applications where the NC processing into a composite or layer is required and where NCs are exposed to an external source of energy for long times. Very recently, a step towards better stabilization was achieved by crosslinking of the long-chain ligands oleic acid and oleylamine while attached to the NC surface by highly energetic radiation. 15 Additionally, the interdigitation of octadecene molecules was reported to increase stability and emission properties of organohalide perovskite NCs. 16 This raised the hope that an enforced ligand sphere is able to enhance the stability of inorganic perovskite NC.
The integration of CsPbX3 NCs in color conversion instead of electroluminescent LEDs is considered a more convenient and, in terms of material stability and processability, less demanding strategy to fabricate NC LEDs. With this approach, typical problems occurring in electroluminescent LEDs such as charging and thermal stress are reduced as no current flows through the layer. Still the processability of color conversion LEDs is not straightforward. The conventional procedure to fabricate a color conversion LED using NCs as optical phosphors involves the blending of the NCs with a thermo-curable silicone resin and deposition on top of the pump LED. In this procedure, the presence of hydrophobic organic ligand and solvents hamper the polymerization of the encapsulating resin and can reduce the mechanical stability of the composite. This is especially problematic in lead halide perovskites requiring an excess of organic ligands in solution to prevent their aggregation, 14 which would reduce their QY and introduce scattering thus decreasing light emision. 17 To overcome the hydrophobicity limitation and improve dispersion, ligand exchange processes, the growth of oxide shells or the encapsulation of QDs in polymers, silica or glass matrices have been used in other types of semiconductors (e.g. [18][19][20]. However, the limited stability and processability of perovskite NCs have so far prevented applying similar strategies. In order to facilitate the processing and applicability of perovskite NCs, we developed a method to produce cesium lead halide NCs with improved chemical stability and compatibility for LED encapsulation by in situ addition of poly(maleic anhydride-alt-1-octadecene) (PMA). PMA has been shown to be an ideal ligand for the encapsulation of oleic acid or oleylamine-coated inorganic NCs such as magnetic, semiconductor and gold nanoparticles. 23,24 This ligand interdigitates in the carbon chain shell that surrounds the nanoparticle offering an enhanced stability. In the case of a stable NC core, the PMA units can be further polymerized utilizing a diamine, which allows the sealing of the NC core and its transfer to polar solvents. In our experiments, PMA was introduced during the synthesis of the perovskite NCs, before the injection of the Cs-precursor, to avoid any potential disruption of the perovskite NC after its formation. We further show the application of green-and orange-red-emitting CsPbX3 perovskite NCs embedded in self-standing silicone-based plates as down-conversion filters in monochromatic and WLEDs. The growth of the NCs in the presence of a protecting polymer facilitates their processing by preventing aggregation and minimizing interference with the silicone curing process while better protecting the NC surface both by preventing interaction with the media and by obstructing the ligand dynamic exchange.
Methods: CsPbX3 NCs synthesis: CsPbX3 NCs were prepared following a modified procedure from that reported by Protesescu et al. 2 Lead halide salts (0.376 mmol, green NCs: 138 mg PbBr2, orange-red NCs: 69 mg PbBr2 and 87 mg PbI2) were mixed with 1-octadecene (10 mL) and heated under vacuum (60-100 mTorr) to 90 °C for 2 hours. After switching to Ar atmosphere and heating to 120 °C, oleic acid (1.0 mL) and oleylamine (1.0 mL) were injected. When a clear solution was obtained, poly(maleic anhydride-alt-1octadecene) (430 mg) was added under Ar flow. Vacuum was applied and the temperature was kept constant at 100 o C for 10 minutes. The solution was stirred vigorously (1100 rpm) while the temperature was increased to 175 °C. Once this temperature was reached, Cs-oleate solution (0.8 mL) with a temperature of 130 o C was injected swiftly using a syringe pre-warmed in an oven. This way, the Cs-oleate solution remained clear, whereas partial precipitation of the precursor was observed at lower temperatures. After 15 s the reaction was quenched by cooling with a water bath. Purification: After 15 s the reaction was quenched by cooling with a water bath. The reaction solution was centrifuged at a speed of 6300 rcf for 15 minutes. After decantation of the supernatant and of liquid residues eluting from the resting precipitate while leaving the centrifuge tube rest atop, the NCs were transferred to the glove box and stored in Ar atmosphere for further use (these are our samples after 1 precipitation). In our experience this was not necessary for the samples with PMA but to maintain comparable conditions we stored all samples in the glove box (for an impression of the stability of particles without and with polymer see figure S7). If further cleaning was applied, around 65 mg of sample were dispersed in 1 mL toluene and precipitated by the addition of 0.3 mL acetonitrile and centrifugation at 10000 rpm for three minutes. This was repeated two to three times as indicated.
Filter plate fabrication: Green filters were prepared by drop casting 6.6 mg of NCs in 200 µL of toluene onto window glass with 27 mm diameter and letting toluene evaporate slowly in a semi-closed petri-dish under ambient conditions. To fabricate WLEDs, dried particles of each color (Balanced LED as in Figure 5a: green 7.5 mg, orange 5.1 mg; LED with higher red/orange content as in Figure 6: green 7.6 mg, orange 7.8 mg) were dispersed with 2 drops of hexane, mixed separately with 414 ± 5 mg of liquid silicone Polastosil-2000 and sonicated for 5 minutes. Afterwards, 31 mg of curing catalyst were added and the mixtures were stirred thoroughly. After ten minutes, the mixtures were deposited on spherical optical glass with 28 mm diameter and left to cure for 60 to 75 min. When the silicone was nearly cured another glass slide was carefully put on top of one layer for separation before the second layer containing particles of the other color was stacked on top with the silicone layer facing the stack. The stack was left to cure fully for at least ten minutes more.
Characterization: Structural and chemical characterization: X-ray diffraction analyses were carried out on Bruker AXS D8 ADVANCE X-ray diffractometer with Cu-Kα radiation (λ= 0.15406 Å). Reference patterns were calculated from crystallographic data of the cubic and orthorhombic phases reported in ref. 21 by CaRIne software. Size and shape of the initial NCs were examined by transmission electron microscopy (TEM) using a ZEISS LIBRA 120, operating at 120 kV. High resolution TEM (HRTEM) and scanning TEM (STEM) images were recorded using an FEI Tecnai F20 TEM microscope, equipped with a high angle annular dark field (HAADF) detector, operated at 200 kV. For TEM analysis the NCs were dispersed in toluene and drop casted onto a carbon-coated copper grid. Field-emission scanning electron microscopy (SEM) was carried out on an Auriga Zeiss at 5.0 kV. Quantitative elemental analysis was performed by means of energy dispersive X-ray spectroscopy (EDX) within the SEM. Metal contents were determined with Perkin Elmer inductively coupled plasma (ICP) instruments using mass spectrometry (NexIon 350D) for Cs and atomic emission spectroscopy for Pb (Optima 8300) . Samples synthesized in reactions with an upscaling factor of two for all substances were purified as reported earlier. 14 Of each purified and dried sample (2 per batch) around 30 mg were attacked by adding 5 mL of HNO3 and 3 mL of H2O2 and heated until 210 °C in a microwave. Finally, the samples were diluted to 50 mL with ultrapure water and measured.
For optical characterization the samples were diluted in hexane and analyzed using a quartz cuvette with 1 cm path length on a Cary 300 Bio UV-vis Spectrophotometer and a Cary Eclipse Fluorescence Spectrophotometer. Quantum yields were determined according to literature procedures using Rhodamine 6G and Fluorescein as standards. 22 Each QY was determined from measurements in three NC batches produced under the same conditions and with at least 2 measurements per batch.
Nuclear Magnetic Resonance (NMR) measurements were recorded on a Bruker Avance III HD Spectrometer operating at a 1H frequency of 500.26 MHz and equipped with a BBFO-Z probe. For NMR measurements batches scaled up with a factor 2 were produced and purified by two cycles of precipitation and redispersion with acetonitrile/toluene before drying and redispersing them in deuterated toluene. The sample temperature was set to 298.2 K. One dimensional (1D) 1H and 2D NOESY (Nuclear Overhauser Effect Spectroscopy) spectra were acquired using standard pulse sequences from the Bruker library. For the quantitative 1D 1H measurements, 64k data points were sampled with the spectral width set to 20 ppm and a relaxation delay of 30 s. NOESY mixing time was set to 300 ms and 4096 data points in the direct dimension for 512 data points in the indirect dimension were typically sampled, with the spectral width set to 10 ppm. Diffusion measurements (2D DOSY) were performed using a double stimulated echo sequence for convection compensation and with monopolar gradient pulses. (ref. J. Magn. Reson. 2009, 198, 121−131) Smoothed rectangle gradient pulse shapes were used throughout. The gradient strength was varied linearly from 2 to 95% of the probe's maximum value in 64 increments, with the gradient pulse duration and diffusion delay optimized to ensure a final attenuation of the signal in the final increment of less than 10% relative to the first increment. For 2D processing, the spectra were zero filled until a 4096−2048 real data matrix. Before Fourier transforma on, the 2D spectra were mul plied with a squared cosine bell function in both dimensions, the 1D spectra were multiplied with an exponential window function. The diffusion coefficients were obtained by fitting the appropriate Stejskal- Tanner with the gyromagnetic ratio of the observed 1 H nucleus γ, the gradient pulse length δ, the gradient strength g, the diffusion time ∆ and the diffusion coefficient D. The shape factor ξ depends on the gradient shape but this is usually already corrected for in the gradient strength value by the Bruker software, TOPSPIN. The ST equation for other pulse programs is only slightly different and the impact on the diffusion coefficient is very small in the case of long diffusion times (Δ>250 ms). In a real DOSY experiment, values of ∆ and δ are set while varying g to derive the decay curve.
LED testing: The measurements were carried out under ambient conditions. For green LEDs a Thorlabs' Mounted High-Power LED M365L2 UV pump with 0.55 W (3.42 V, I= 156.4 mA) was applied to illuminate the filter. The detection of the resulting luminescence was performed by means of an ISP 500 integrating sphere and a SPECTRO 320 PMT (Photomultiplier tube) based spectrometer, both from Instrument Systems. WLEDs were evaluated in an ISP 2000 integrating sphere coupled to a CAS 120 CCD-based spectrometer, both from Instrument Systems. The blue pump (450 nm) was run at 5.48 V and 58.8 mA, which implied a total power consumption of 0.31 W. The total irradiated filter area had a diameter of 25 mm. It is worth noticing that the switch from 365 nm to 450 nm as pump wavelength responded to the necessity of complementing the emission from the filter (by covering the short wavelength range) in the research of white light. In addition, it should be noted that both spectrophotometric systems are equivalent. The change from one system to the other is related to the fact that, for mechanical reasons, the blue light from the 450 nm pump was more easily coupled to the ISP 2000 sphere. Figure 1 shows representative TEM micrographs of the cubic green-emitting CsPbBr3 and cubic/partially squared orange-red-emitting CsPbBr1.6I1.4 NCs obtained in the presence of PMA (CsPbX3-PMA) following the procedure above described. Fig. 1C and 1D respectively shows an atomic resolution HAADF STEM micrograph of one NC and its corresponding power spectrum, confirming the cubic crystal structure of the NC. The prepared CsPbX3-PMA NCs thus exhibited the same crystal structure and a very similar emission maximum as those prepared in the same conditions by the unmodified synthesis protocol. CsPbBr3-PMA and CsPbBr1.6I1.4-PMA NCs had average edge lengths of 9 ± 1 nm and 11 ± 2 nm outside the quantum confinement regime, 2 first absorption maxima at 504 and 561 nm and corresponding emission wavelengths of 513 and 586 nm.

RESULTS AND DISCUSSION
SEM-EDX and ICP analyses revealed the CsPbX3-PMA NCs to have slightly non-stoichiometric compositions, with an excess of lead and halide providing atomic ratios Cs:Pb:Br = 1:1.2:4 for green-emitting NCs and Cs:Pb:Br:I = 1:1.1:2:1.4 for orange-red NCs (see supporting information for details). This off-stoichiometry may arise from the fact that the reaction is carried out under an excess of the two elements which may entangle with the polymer. An excess of halide may further be attributed to its role in the surface capping of the NCs, where it constitutes the negative counterpart of oleylammonium ions that stabilize the NCs in solution. 14 The NC quantum yield (QY) increased slightly but reproducibly with the polymer addition for both the green-emitting CsPbBr3 NCs, from 49 ± 4 % to 53 ± 4 % , and the red-emitting CsPbBr1.6I1.4 NCs, from 84 ± 3 % to 88 ± 1 % . We believe the reason behind this slight increase is a better passivation of the trap states by the more stable ligand shell. 14 To investigate this point further, we studied the composition of the ligand shell by means of 1 H solution NMR and IR spectroscopy (Figures S5 and S6). The surface study by means of 1 H solution NMR showed the presence of oleylammonium bromide or oleylammonium oleate as surface ligands, in both the samples produced with and without the presence of PMA. The overlapping of the resonances between these ligands and the PMA, prevented us to directly identify the presence of PMA in the 1 H NMR spectrum. Additionally, the similarity of the spectrum of PMA to the ligands and the fact that the anhydride ring seems to open during the reaction providing more -COOH and -COOgroups that are positioned close to oleic acid/oleate signals, did neither allow to unambiguously identify the presence of PMA on the samples by IR spectroscopy. However, Diffusion Ordered NMR Spectroscopy (DOSY) clearly revealed a change of the dynamic behaviour of those ligands in the presence of PMA. Lower diffusion coefficients and thus a solvodynamic radius closer to the NC size, were systematically obtained for all samples containing PMA ( Table 1). This indicates that when polymer is added to the solution, ligands are indeed more tightly bound to the NCs and it indirectly proves the presence of PMA on the NPs surface. While original ligands are still present, we presume that PMA is entangled on the ligand shell and acts as a contingency wall reducing the kinetics of the dynamic stabilization. On the other hand, NMR analysis showed that adding PMA after the synthesis was not so effective. Adding PMA to a pure CsPbBr3 sample after the synthesis combined with sonication 10 min and purification with acetonitrile resulted in larger diffusion coefficients when compared with purified samples synthesized in the presence of PMA (table 1). Besides, the addition of PMA under such circumstances presented a photoluminescence decay already visible by eye. This underlines the importance of adding PMA during the synthesis and not after. Solvodynamic size (nm) CsPbBr3 89 9.4 ± 1.5 6.9 CsPbBr3-PMA 64 9.7 ± 1.7 9.5 CsPbBr3 (PMA)* 90 11 ± 2 6.8 * PMA added after synthesis A main challenge to fabricate color conversion QD-WLEDs is to increase durability by preventing the NC degradation with a prolonged LED operation period. QD-LED degradation can be the result of a loss of QD luminescence when integrated into the device, due to photo-, thermal-or chemical degradation and to aggregation during processing due to incompatibility between the surface organic ligands and the polymer matrix. We tested the effect of the PMA on the stability of CsPbX3 NCs when used as down converters of UV light. For this application, a toluene solution of CsPbX3-PMA NCs was spread on 27 mm diameter glass substrates followed by slow evaporation of toluene in a semi-closed container to obtain a homogeneous layer.
These plates were placed on top of a commercial 365 nm UV-emitting LED delivering a radiometric power of 100 mWopt.. Under these conditions the CsPbBr3-PMA NCs emitted at 528 nm with a full-width at half maximum of 17 nm. The green-emitting LED without PMA had a color saturation of 0.944, a luminous efficacy of radiation of 255 lm/Wopt. and a luminous efficacy of 2.5 lm/Welec. (1 mWopt../Welectr.). Devices incorporating CsPbBr3-PMA NCs provided a color saturation increased to 0.988 and a much higher luminous efficacy of radiation up to 562 lm/Wopt, associated to its spectral proximity to the maximum of the photopic curve (683 lm/Wopt, at 555 nm) and the partial UV light absorption by the polymer. The luminous efficacy was, in contrast, slightly lower, 1.8 lm/Welec.. Figure 2 shows the temporal evolution of the spectra obtained from two filters produced from the same amount of CsPbBr3 NCs and thoroughly purified CsPbBr3-PMA NCs under continuous illumination by a 365 nm pump in ambient conditions. This pump was operated at 58.8 mA forward current and provided an optical power of 100 mWopt. at sample level. In both cases, the luminescence decreased with time and shifted to longer wavelengths. The stability of the emission signal was significantly higher for the CsPbBr3-PMA NCs, with 60% of peak area remaining after twelve operation hours, compared to CsPbBr3 NCs with no PMA protection, decreasing down to 21%. X-ray diffraction (XRD) analysis ( Figure 3) suggested that the NCs grow and partially rearrange crystallographically during long-term optical excitation. This growth and rearrangement was evidenced after 22 h operation for CsPbBr3 NCs and after 3 days for CsPbBr3-PMA NCs and was indicated by a narrowing of the XRD peaks following an extended illumination period. This growth and a rearrangement of the atoms through a phase transition would also explain the observed emission spectral red-shift. TEM micrographs in figure S8 show the grown crystals. The polymer effectively slowed down these processes, but it did not completely prevent them. The initial NC deposited on a Si wafer by drop casting exhibit a cubic crystal structure with characteristically broad reflexes, no significant differences were observed for NC films with and without PMA. After the stability tests smaller peak widths indicate crystal growth and new peaks can be assigned to orthorhombic CsPbBr3 (*references were calculated based on data from ref. 21  To investigate the performance of CsPbX3 NCs in color conversion WLEDs, we combined green-emitting CsPbBr3-PMA and red-emitting CsPbBr1.6I1.4-PMA NCs with emission peaks at 520 and 620 nm on independent free-standing plates. Owing to the sensitivity of perovskite NCs, a room temperature encapsulation and a remote-type architecture where a color conversion plate is placed on top of the LED at a distance was found to be more suitable for implementing perovskite-based color conversion LEDs. 26 NCs were embedded into a silicone matrix (Polastosil-2000) which was mixed with a catalyst at room temperature, spread on an optical glass substrate and left to cure. Heat-induced curing processes, widely applied in two-component silicone, proved inapplicable due to destruction of the perovskite NCs. To provide further protection from humidity and facilitate handling, the filter was covered by another glass layer when the silicone was mostly but not fully cured. The applied silicone is not only an easy to handle as well as cost effective matrix but also through adhesion forces towards the glass leads to a homogeneous distribution of the layer without any bubbles. Epoxy curing used for sealing perovskite solar cells was not compatible with the NCs, as the NCs degraded upon contact with the resin, most likely due to unfavorable reactions with hydroxyl groups. Sandwiching the layers between glasses further opens the possibility of totally sealing the NCs and prevents any release of toxic Pb compounds. Apart from these design considerations, we discarded the blending of the two types of NCs in the same layer due to fast composition equilibration in solution that is not prevented by the ligand keeping but ion permeable PMA (see Supporting information, Figure S7). Instead we sandwiched the NCs in silicone curing between two glass plates or stacks of two layers between three glass slides. Each silicone layer between two glasses was measured to cause a loss of optical emission of about 6%, part of this amount of light could be gained with modified LED design that redirects backscattered light in combination with more specific curing strategies reducing the difference in refractive indexes. Color conversion plates containing the CsPbX3-PMA NCs were mounted over a blue-emitting 446 nm pump. The maximum current, 58.8 mA providing 100 mWopt., was applied to test the device stability. Figure 4 shows the electroluminescence spectra obtained in ambient conditions of a WLED having the red-emitting filter stacked on top of the green-emitting one with one glass slide in between. The initial luminous efficacy of radiation (LER) of the WLED was 353 lm/WOpt, which is within the range for optimum white light generation. 10 However, the luminous efficacy in terms of conversion of electricity into light (LE) of 4.5 lm/Welec..leaves room for improvement. Within 5 minutes the color temperature shifted from warm/neutral white light with 3665 K to 9300 K and the CRI decreased from 72.4 to 66.9 as a consequence of the degradation of the red component. Even with the improvement accomplished by the presence of the protecting PMA, in ambient conditions and with a relatively high forward current of 60 mA, the unstable red component limited the device durability. In iodide-based perovskite NCs photo activated atomic rearrangement is facilitated by the higher energetic gain of transition between the highly emissive cubic and less active orthorhombic phase of crystals containing iodine. 2,25,27 The reason for this lies in the intrinsic properties of the anions. The lower electronegativity of iodine and its larger size makes it less stable in the cubic CsPbX3 structure, and thus the iodide-based perovskite decomposes faster than the bromide based one.(ref) In thin films recent studies show that doping with chloride ions improves the stability of the crystal phase.(ref) So far, solution based doping of the iodide containing NCs has proved evasive but future advances in this direction may help to amend the instability of the red component. During irradiation the emission of the green filter increased as the red component decreased, indicating a substantial reabsorption of the emitted green light by the red component that may be solved by advanced filter design. Figure 5 displays the EL spectra of a CsPbX3-PMA WLED with higher red content under different driving currents ranging from 5 to 60 mA taken over a period of time of 6 minutes. A non-linear increase of the red component was observed, caused by simultaneous degradation that counteracts the trend of increasing emission with higher currents. This again points out the vulnerability of the red component in comparison to the green one but on the other hand confirms the reliability of the green layer. Figure 5. Electroluminescence spectra of a CsPbX3-PMA WLED with increasing applied current.

CONCLUSIONS
CsPbX3 NCs were synthesized in the presence of PMA. The addition of the polymer led to an increase of QY and optical stability of CsPbX3 NCs under illumination in ambient conditions which is associated to a tighter bonding of the ligands in the presence of PMA as observed by 1 H NMR spectroscopy. Owing to the choice of emission wavelengths and the small emission bandwidths of these NCs, color conversion LEDs with high luminous efficacies of radiation and thus bright colors were achieved. The initial values of warm/neutral white light obtained are promising, although the bottleneck of an all-CsPbX3 color converter WLED is the orange or red component obtained with a mixed bromide/iodide composition, which even in the presence of PMA shows a limited lifetime. While further advances in NC surface passivation in combination with component encapsulation strategies and advanced LED designs are necessary, our approach suggests that growing the NCs with a compatible polymer in situ during the reaction can facilitate their further processing and may be the key for further stabilization and encapsulation procedures.