Thermally Modulated Microfluidic Fabrication of Phase-Tunable Cs₄PbBr₆/CsPbBr₃ Hybrid Perovskite Nanocrystals for White Light-Emitting Diodes

Abstract

All inorganic CsPbBr₃ perovskite nanocrystals (NCs) exhibit outstanding luminescence for optoelectronics, yet poor environmental stability severely restricts their practical deployment. As a stable derivative phase, Cs₄PbBr₆ can effectively improve structural stability. Nevertheless, the rational fabrication of high-quality Cs₄PbBr₆/CsPbBr₃ hybrid NCs remains challenging owing to the lack of straightforward and scalable synthetic strategies. To overcome these hurdles, we synthesize well-defined Cs₄PbBr₆/CsPbBr₃ hybrid NCs via a temperature-controllable continuous-flow microfluidic route. This platform precisely modulates phase composition via systematic temperature tuning across a range of 110–170 °C, producing distinct compositions from Cs₄PbBr₆-dominant to high-purity CsPbBr₃. A direct correlation was elucidated between temperature-induced phase transformation and optical properties. The NCs synthesized at 130 °C exhibited a high photoluminescence quantum yield of 96.24% and bright 521 nm green emission. These NCs were successfully integrated into white light-emitting diodes incorporating a 478 nm blue excitation chip and K₂SiF₆:Mn⁴⁺ red phosphor, which demonstrated excellent color performance with a luminous efficiency of 86.3 lm W⁻¹ and Commission Internationale de l’Éclairage coordinates of (0.2991, 0.3784). This work highlights the potential of continuous-flow microfluidics for precise phase modulation and scalable production of high-quality perovskite NCs, offering a viable route for advanced optoelectronic applications.

1. Introduction

Lead halide perovskite nanocrystals (NCs), exemplified by CsPbBr₃, have garnered extensive research interest due to their exceptional optical and electronic properties, positioning them as ideal candidates for diverse optoelectronic applications [1,2,3,4,5]. Beyond the well-established CsPbBr₃ phase, stoichiometric derivatives like Cs₄PbBr₆ have also emerged as noteworthy materials, owing to their distinct crystal structures and unique physicochemical characteristics [6,7,8,9,10]. Recent investigations have revealed that composite CsPbBr₃/Cs₄PbBr₆ hybrid NCs exhibit enhanced luminescence and stability compared to their single-phase counterparts [11]. However, the controllable synthesis of these sophisticated hybrid structures presents a significant challenge with conventional batch methods, thus highlighting a critical need for novel synthetic strategies.

Despite the growing interest in Cs₄PbBr₆ and CsPbBr₃ hybrid perovskite systems [12,13,14], the controlled modulation of phase transformation remains a significant obstacle for their scalable production and widespread application [15]. Theoretical studies indicate that the evolution of Cs-Pb-Br phases is highly sensitive to reaction parameters [16,17]. However, conventional batch synthesis methods struggle to provide the fine, continuous control required to tune this process effectively, frequently leading to mixed phases, poor reproducibility, and low yields. Recently, various post-synthetic temperature-controlled annealing strategies [18,19,20,21,22] have been developed to regulate the phase composition and crystalline structure of Cs₄PbBr₆/CsPbBr₃ mixed-phase perovskites. These studies have provided valuable insights into temperature-dependent phase evolution and demonstrated the feasibility of thermal modulation for phase engineering. Nevertheless, achieving rapid and spatially uniform phase control remains challenging in conventional batch-based systems due to limited heat transfer and diffusion-controlled processes. Microfluidic reactors, with their inherent ability to maintain uniform thermal environments and precise parameter control [23,24,25,26,27], offer a compelling solution to these challenges. Nevertheless, existing microfluidic studies on Cs-Pb-Br derivatives have predominantly focused on static reaction conditions [28,29], lacking systematic investigations into how dynamic temperature gradients influence phase evolution. Critically, the ability to precisely tailor the phase composition of Cs₄PbBr₆/CsPbBr₃ NCs via microfluidic platforms remains insufficiently explored, and the intricate link between phase structure and optical performance in these hybrid systems is still not fully elucidated.

Herein, we report a continuous-flow microfluidic approach for the controlled synthesis of Cs₄PbBr₆/CsPbBr₃ hybrid perovskite NCs. By systematically modulating the reaction temperature within our microchannel reactor, we achieved unprecedented precise control over the Cs₄PbBr₆-to-CsPbBr₃ phase transformation. This enabled the synthesis of a series of materials, ranging from mixed-phase composites to high-purity single-phase CsPbBr₃. Specifically, we demonstrate that the phase evolution process is tunable solely through temperature control, highlighting the potential of the microfluidic platform for precise phase modulation. The optimized high-quality hybrid NCs were further integrated into white light-emitting diodes to verify their promising application potential in solid-state lighting and wide-color-gamut display devices.

2. Experimental Section

2.1. Material Preparation and Synthesis

Cesium carbonate (Cs₂CO₃, 99.99%), lead bromide (PbBr₂, 99.99%), oleic acid (OA, 90%), oleylamine (OAm, 80–90%), 1-octadecene (ODE, ≥90%), n-hexane (≥98.0%), and methyl acetate (MeOAc, ≥99.0%) were purchased from Shanghai Aladdin Biochemical Technology Co. (Shanghai, China) and used as received without further purification.

The continuous synthesis of Cs₄PbBr₆/CsPbBr₃ nanocrystals was performed on a custom-built three-stream microfluidic platform [30]. A polydimethylsiloxane microfluidic chip with a 200 μm channel diameter was employed, facilitating efficient molecular-level mixing of three precursor streams. Precursor solutions were prepared by separately loading oleate-capped cesium, lead bromide, and 1-octadecene into three syringes. These solutions were precisely injected into the microfluidic chip at constant flow rates using high-precision syringe pumps, enabling accurate control over the reaction residence time. The microfluidic chip was integrated with a temperature-controlled hot plate, maintaining reaction temperatures systematically from 110 to 170 °C to tune the phase evolution. Following reaction and mixing within the microchannel, the product stream was continuously collected from the chip outlet. The crude colloidal suspension was then subjected to purification: mixing with an equal volume of methyl acetate, followed by centrifugation at 5000 rpm for 20 min. The supernatant was discarded and the precipitate was redispersed in n-hexane. A second centrifugation under identical conditions yielded the precipitate, which was again redispersed in n-hexane. A final brief centrifugation (5000 rpm, 5 min) was performed to remove any residual colloidal impurities, resulting in monodisperse Cs₄PbBr₆/CsPbBr₃ nanocrystal colloids.

2.2. Characterizations

Transmission electron microscopy (TEM) images were acquired using a TECNAI G2-20 microscope (Thermo Fisher Scientific, Hillsboro, OR, USA). Crystalline phases were identified by X-ray powder diffraction (XRD) using a Bruker D8 diffractometer (Bruker, Bremen, Germany). UV–visible (UV–vis) absorption spectra were recorded on a Shimadzu UV-2600 spectrometer (Shimadzu Corporation, Kyoto, Japan). Photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL decay measurements were conducted using an Edinburgh FLS980 spectrophotometer (Edinburgh Instrument, Edinburgh, UK).

3. Results and Discussion

CsPbBr₃/Cs₄PbBr₆ NCs were continuously synthesized on a custom-built microfluidic platform (Figure 1). A split-stream feeding configuration facilitated precise stoichiometric control by adjusting individual syringe pumping rates, ensuring a fixed Cs-to-Pb and OA-to-OAM molar ratio of 2:1 for controlled nucleation. The precursor mixture was then subjected to thermal control within a serpentine microchannel, with four reaction temperatures (110, 130, 150, and 170 °C) evaluated for optimizing nanocrystal formation. The crude colloidal suspension was continuously harvested from the reactor outlet post-reaction.

Figure 1.

Schematic illustration of the temperature-regulated three-stream microfluidic platform for continuous synthesis of Cs₄PbBr₆/CsPbBr₃ perovskite NCs.

To observe the thermal-triggered crystal phase evolution, TEM characterizations were carried out on products obtained under four gradient temperatures. Figure 2 systematically revealed temperature-dependent phase transformation from Cs₄PbBr₆ to CsPbBr₃. Synthesized samples exhibited distinct morphologies including spherical nanoparticles of hexagonal Cs₄PbBr₆ [31] and cubic CsPbBr₃ [32], and such morphological differences show a direct correlation with reaction temperature. At 110 °C, insufficient thermal energy hindered Cs₄PbBr₆ lattice rearrangement [33] and suppressed the evolution toward CsPbBr₃ formation, yielding primarily spherical Cs₄PbBr₆. Elevated temperatures within the microchannel accelerated precursor decomposition and atomic diffusion, facilitating in situ transformation. Notably, at 150 °C, precisely controlled thermal energy dramatically enhanced phase evolution toward CsPbBr₃ formation, significantly boosting CsPbBr₃ conversion efficiency and volume fraction. The inset images show bright green photoluminescence from all colloids in n-hexane under UV irradiation, confirming the CsPbBr₃ luminescent properties. This controlled phase evolution demonstrates the effective thermal regulation capability of the microfluidic reactor for controlling phase transformation. To clarify the microstructure and phase distribution of the hybrid NCs, high-resolution transmission electron microscopy (HRTEM) analyses was performed (Figure S1). The NCs mainly consist of separately distributed CsPbBr₃ and Cs₄PbBr₆ phases rather than a typical raisin-bread-like composite structure, in which one phase is randomly embedded within another. However, localized contact regions between CsPbBr₃ and Cs₄PbBr₆ can be observed. In these contacted regions, distinct lattice fringes with d-spacings of 3.01 Å and 4.43 Å are identified, corresponding to the (200) plane of CsPbBr₃ and the (113) plane of Cs₄PbBr₆, respectively. These lattice-resolved interfacial contacts provide direct evidence for the formation of nanoscale Cs₄PbBr₆/CsPbBr₃ heterointerfaces, confirming the hybrid nature of the synthesized NCs.

Figure 2.

TEM images of Cs₄PbBr₆/CsPbBr₃ hybrid NCs synthesized at distinct reaction temperatures. Panels (a–d) correspond to synthesis temperatures of 110 °C, 130 °C, 150 °C and 170 °C, respectively. Insets in each panel display the corresponding colloidal solutions under UV excitation, showing tunable green luminescence.

To further confirm the temperature-induced phase transition from optical absorption signatures, UV–vis absorption measurements were implemented for all hybrid samples. Figure 3a presents the UV–vis absorption spectra of Cs₄PbBr₆/CsPbBr₃ hybrid NCs synthesized via microfluidic continuous flow. These spectra validate the Cs₄PbBr₆-to-CsPbBr₃ solid-state phase transformation previously concluded from TEM characterization. At 110 °C, a dominant absorption peak at 315 nm confirms the prevalence of the Cs₄PbBr₆ crystalline phase [34], with negligible evidence of CsPbBr₃ generation. As the reaction temperature is gradually elevated, the increased thermal energy accelerates lattice rearrangement [35,36] and atomic diffusion within the microchannel, thereby facilitating the in situ conversion of Cs₄PbBr₆ into CsPbBr₃. This process is evidenced by a continuous growth in absorbance near 510 nm. The systematic evolution of these dual characteristic absorption bands [10] quantitatively demonstrates that elevated temperatures effectively boost the phase-transition kinetics from the intermediate Cs₄PbBr₆ to the target CsPbBr₃ perovskite, further reinforcing that this microfluidic platform enables precise control over hybrid crystalline composition.

Figure 3.

(a) UV–vis absorption spectra of samples obtained at 110 °C, 130 °C, 150 °C and 170 °C. (b) Corresponding steady-state photoluminescence spectra with a vertical dashed line marking the central emission peak position.

Figure 3b presents PL emission spectra of hybrid NCs. All four tested samples exhibit sharp and symmetric PL emission peaks centered around 520 nm, indicated by the vertical dotted line. This emission wavelength is highly consistent with the band-edge absorption feature [37,38] of CsPbBr₃ observed in the preceding UV–vis spectra. As the synthetic temperature was stepwise-elevated from 110 to 170 °C, a gradual red-shift in the PL peak was observed. This slight red-shift may be attributed to the increasing content of CsPbBr₃ within the hybrid nanocrystal system.

To quantify the temperature modulation effect on luminescence performance, we tracked the variation in emission peak position and photoluminescence quantum yield across the full thermal gradient. Figure 4a plots the evolution of PL peak wavelength and PLQY with synthesis temperature. From 110 to 150 °C, PL emission wavelength gradually red-shifts from ~514 nm to ~521 nm, then remains nearly constant up to 170 °C. Conversely, PLQY sharply climbs from 62.41% at 110 °C to a peak of 96.24% at 130 °C. Beyond 130 °C, PLQY mildly decreases to 94.05% at 150 °C and 90.95% at 170 °C. This PLQY decline above 130 °C is likely due to excessive temperature causing mild nanoparticle agglomeration and new surface traps [39,40,41,42], increasing non-radiative losses. In addition to these structural effects, the superior PLQY at 130 °C can also be attributed to the optimized Cs₄PbBr₆/CsPbBr₃ phase balance. The presence of the Cs₄PbBr₆ component provides effective interfacial passivation for CsPbBr₃ domains, reducing surface-related defects and suppressing non-radiative recombination pathways. With further increasing temperature, the reduced Cs₄PbBr₆ contribution and enhanced crystal growth may weaken this passivation effect, resulting in increased defect-assisted recombination. TRPL measurements were employed to probe the recombination dynamics of the microfluidic-derived hybrid NCs (Figure 4b). Fitting the decay curves to a biexponential function [43,44] reveals that the fast component (τ₁) represents non-radiative recombination, while the slow component (τ₂) corresponds to radiative recombination. As detailed in the summarized Table 1, at 130 °C, the sample achieved the longest carrier lifetime τ_ave (25.21 ns), the highest radiative recombination rate constant k_r (0.038 ns⁻¹), and the lowest non-radiative rate constant k_nr (0.0015 ns⁻¹). These findings suggest that the precise thermal control enabled by our microfluidic platform fosters a favorable phase transformation, effectively mitigating surface trap states.

Figure 4.

(a) Variation in PL emission peak wavelength and corresponding photoluminescence quantum yield (PLQY) for samples synthesized at temperatures ranging from 110 °C to 170 °C. (b) Time-resolved photoluminescence decay curves of Cs₄PbBr₆/CsPbBr₃ hybrid NCs prepared at 110 °C, 130 °C, 150 °C and 170 °C.

Table 1.

Data summary of TRPL and PLQY spectra of samples synthesized at different temperature conditions.

Temperature (°C)	PLQY	τ1 (ns)	τ2 (ns)	τave (ns)	τr (ns)	kr (ns−1)	knr (ns−1)
110	62.41	13.02	95.33	20.66	33.11	0.030	0.018
130	96.24	14.61	78.68	25.21	26.20	0.038	0.0015
150	94.05	12.32	38.71	20.39	21.68	0.046	0.0029
170	90.95	10.98	32.96	21.71	23.87	0.042	0.0042

To confirm the phase evolution and crystallinity of the microfluidically synthesized Cs₄PbBr₆/CsPbBr₃ hybrid NCs, powder X-ray diffraction (XRD) patterns were recorded across the temperature range of 110–170 °C (Figure 5). These XRD results revealed a clear temperature-dependent phase transformation. At 110 °C, the pattern indicated a mixed phase comprising Cs₄PbBr₆ (with characteristic peaks at 12.6° and 25.4°) [45] and CsPbBr₃ (displaying broadened peaks at 15.2° and 30.6°) [46], suggesting poor CsPbBr₃ crystallinity at this temperature. Upon increasing temperature to 130 °C, Cs₄PbBr₆ signals diminished while CsPbBr₃ peaks intensified and sharpened, signifying improved crystallinity. At 150–170 °C, Cs₄PbBr₆ signals nearly disappeared, and the patterns exclusively matched the standard CsPbBr₃ reference, confirming high-purity perovskite formation. The consistent cubic CsPbBr₃ (100)/(200) plane reflections across all temperatures, coupled with the temperature-dependent attenuation of Cs₄PbBr₆ peaks, demonstrate the controlled phase evolution driven by thermal energy. The temperature-dependent phase evolution is likely associated with a thermally regulated dissolution–recrystallization process during the microfluidic synthesis. In the solution-phase reaction environment, temperature influences precursor dissolution, Pb–Br intermediate conversion, and subsequent nucleation/growth kinetics, thereby regulating the phase equilibrium between Cs₄PbBr₆ and CsPbBr₃. Previous studies have suggested that Cs-based lead halide perovskite phase evolution involves the formation and conversion of Pb–Br complexes, highlighting the critical role of solution-phase precursor chemistry in determining the final phase composition [47,48,49]. At lower temperatures, insufficient reaction kinetics may hinder precursor conversion and CsPbBr₃ crystallization, resulting in relatively poor CsPbBr₃ crystallinity. With optimized thermal energy at 130 °C, accelerated precursor conversion and controlled crystal growth facilitate the formation of highly crystalline CsPbBr₃ domains. Further temperature increase promotes crystal growth and alters the Cs₄PbBr₆/CsPbBr₃ phase equilibrium toward a CsPbBr₃-dominant composition.

Figure 5.

Stacked X-ray powder diffraction patterns of Cs₄PbBr₆/CsPbBr₃ hybrid NCs synthesized at 110 °C, 130 °C, 150 °C and 170 °C. Standard reference diffraction peaks of pure Cs₄PbBr₆ (PDF #73-2478) and pure CsPbBr₃ (PDF #54-0752) are displayed at the bottom and top of the plot, respectively, for phase identification.

Benefiting from the optimal luminescence performance of the 130 °C-synthesized composite NCs, we further explored their practical optoelectronic application as light-emitting phosphors. Recent studies have demonstrated that Cs₄PbBr₆/CsPbBr₃ hybrid systems can achieve excellent LED performance through phase engineering and vacuum thermal evaporation strategies, highlighting the importance of precise phase control for optoelectronic applications [50,51,52]. In comparison, our continuous-flow microfluidic approach provides a solution-processable route for the synthesis of colloidal hybrid nanocrystals with tunable phase composition, offering an alternative strategy for phosphor-based LED applications. The synthesized CsPbBr₃/Cs₄PbBr₆ hybrid NCs, exhibiting a high photoluminescence quantum yield of 96.24% and sharp green emission centered at ~520 nm, were employed as the green phosphor in a white light-emitting diode (WLED). This WLED device [53,54] was fabricated by integrating these NCs with a 478 nm blue excitation chip and a K₂SiF₆:Mn⁴⁺ red phosphor, resulting in a balanced spectrum for neutral white light emission (Figure 6a). The precise alignment of the green emission with the NCs’ PL peak ensured efficient conversion, contributing a strong green component. The resultant WLED demonstrated a luminous efficiency of 86.3 lm·W⁻¹ and CIE coordinates of (0.2991, 0.3784) at 30 mA (Figure 6b). This high performance, enabled by continuous microfluidic synthesis, highlights the NCs’ potential for next-generation wide-color-gamut displays.

Figure 6.

(a) Electroluminescence spectrum of the fabricated white light-emitting diode device, with an inset photograph showing the operating WLED chip. (b) Corresponding Commission Internationale de l’Éclairage chromaticity diagram.

4. Conclusions

In conclusion, this work reports a temperature-regulated microfluidic strategy for the controlled synthesis of Cs₄PbBr₆/CsPbBr₃ hybrid perovskite NCs with precisely tunable phase compositions. Our continuous-flow platform enables exceptional control over reaction conditions, allowing for systematic tuning of phase evolution from Cs₄PbBr₆ to high-quality CsPbBr₃. Critically, we elucidated the direct correlation between temperature-induced phase transformation and the resulting optical properties of the composites. Our findings reveal that the enhanced luminescence performance is intrinsically linked to an optimized Cs₄PbBr₆ content. The optimized NCs were successfully fabricated into white light-emitting diodes. This work offers a viable route for controlled hybrid perovskite synthesis and highlights their potential for optoelectronic device applications.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/nano16150962/s1, Figure S1: (a) Representative TEM image of the Cs₄PbBr₆/CsPbBr₃ hybrid nanocrystals synthesized at 130 °C, showing a uniform morphological distribution. (b) HRTEM image of a single hybrid nanocrystal, revealing the intimately intergrown interface between the CsPbBr₃ (200) plane (d = 3.01 Å) and the Cs₄PbBr₆ (113) plane (d = 4.43 Å). The clear lattice continuity indicates the formation of a nanoscale heterostructure.

Author Contributions

Conceptualization, C.C. and B.Z.; methodology, Y.N. and S.G.; validation, Y.N.; formal analysis, T.L. and D.S.; writing—original draft preparation, Y.N. and W.L.; writing—review and editing, B.H. and B.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This work was supported by the National Key Technologies Research and Development Program of China (2023YFB3609300), the National Natural Science Foundation of China (62574194, 62341508, 62575276), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2022109), and the State Key Laboratory of Precision Measurement Technology and Instruments of China (Grant No. Pilab2406).