Halide vacancy passivation in cesium lead halide perovskite nanocrystals with mixed halide compositions: the impact of prolonged reaction time

Abstract

Perovskite nanocrystals offer a significant advantage in covering the entire visible spectrum with outstanding color purity by simply tuning their halide compositions, making them highly desirable for light-emitting diode applications with excellent color reproducibility. However, mixed halide perovskite nanocrystals often suffer from instability issues, such as halide segregation under various stimuli. In this study, we present a straightforward synthetic approach to produce stable blue-emitting perovskite nanocrystals with mixed halide compositions. Specifically, we synthesized blue-emitting CsPbBr_xCl_3−x nanocrystals in the presence of impurity metal halides, such as ZnBr₂, as halide sources. Our findings show that the reaction can be prolonged for over 90 min at elevated temperature when ZnBr₂ is added as a bromine source, while the addition of PbBr₂ fails to extend the reaction. In CsPbBr_xCl_3−x nanocrystals synthesized in the presence of ZnBr₂, it turned out that the photoluminescence quantum yield gradually increases due to effective passivation of halide vacancies over time during the reaction. In addition, air and thermal stabilities, which are strongly associated with the compositions, were significantly enhanced. Finally, we demonstrate blue light-emitting diodes based on these nanocrystals, exhibiting greatly suppressed phase segregation even under high operating voltage.

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Introduction

All-inorganic cesium lead halide perovskite NCs exhibit remarkable optical properties, such as high photoluminescence quantum yield (PL QY), narrow emission linewidth, defect tolerance, and easy solution processability. Additionally, their widely tunable spectra, which can be achieved through engineering the halide compositions, make them ideal for display applications requiring high color purity.^1–3

For electroluminescent light-emitting diodes (LEDs), it is essential to achieve efficient emission of the three primary colors: red, green, and blue. Among these, the development of blue-emitting materials has posed the greatest challenge, both for conventional semiconductor quantum dots and for perovskite NCs.^4,5 While green and red perovskite NCs have achieved exceptionally high PL QY (∼100%) and external quantum efficiency (EQE) exceeding 20% in devices, blue-emitting perovskite NCs have lagged behind, displaying lower PL QY and poorer device EQE.^6–8 This limitation is mainly attributed to the defect intolerance of chloride-based compositions compared to bromide and iodide, and insufficient passivation strategies targeting halide defects in blue-emitting perovskite NCs.^9–13 Additionally, instability arising from phase segregation remains a significant issue, since mixed-halide compositions, which are vulnerable to phase segregation, are typically required to achieve blue emission in all-inorganic perovskite NCs.^14–19

Recent advancements have been made in blue-emitting perovskite NCs through post-synthetic modifications. For instance, Gao et al. demonstrated effective passivation of blue-emitting perovskite NCs by treating CsPbCl₃ NCs with octylammonium hydrobromide, resulting in CsPbBr_xCl_3−x NCs with a PL QY exceeding 90%. Efficient operation of blue-emitting LEDs based on these NCs was also achieved.²⁰ Similarly, the incorporation of polyethylene glycol dimethacrylate into post-treatment solutions has proven to be an effective dual strategy for defect passivation in perovskite films. This method resulted in enhanced performance of blue 3D mixed-halide perovskite LEDs with high EQE. Additionally, device lifetime saw a significant improvement, achieving a duration five times longer than devices without post-synthetic defect passivation.²¹ Zou et al. reported notable CIE stability in blue-emitting perovskite NCs achieved through a post-treatment process utilizing a functional organotrichlorosilane. This methodology effectively enhances the Cl⁻ concentration derived from HCl within the perovskite lattice, leading to a spectral blueshift with mitigation of ion migration, which contributes to improved device stability.²² Although post-synthetic processes somewhat enhance the performance and stability of blue-emitting perovskite NCs, they often involve complex steps that can lead to challenges such as irreversible degradation.²³ In this respect, a simpler synthetic protocol without employing post-synthetic treatment has been desired.²⁴

In this study, we developed a simple and novel in situ approach to synthesize high-quality blue-emitting perovskite NCs with mixed halide compositions (Cl and Br) by simply extending the reaction time—a straightforward and effective variable that has often been overlooked, particularly in the synthesis of perovskite NCs. Conventional synthesis of blue-emitting CsPbBr_xCl_3−x NCs using the mixture of PbCl₂ and PbBr₂ could not sustain prolonged reaction times at high temperature. However, we discovered that adding impurity metal halides, such as ZnBr₂ in place of PbBr₂, enabled the synthesis of CsPbBr_xCl_3−x NCs with extended reaction times over 90 min. The resulting NCs exhibited significantly improved PL QY, air stability, and thermal stability compared to control samples. Finally, we fabricated LEDs using the blue-emitting NCs synthesized with prolonged reaction times and without post-treatment, demonstrating greatly reduced phase segregation even under high operating voltages, because of effective passivation of halide vacancies.

Experimental

Materials

Cesium carbonate (Cs₂CO₃, 99%, Thermo Fisher Scientific), zinc bromide (ZnBr₂, 99.9%, Alfa Aesar), lead bromide (PbBr₂, ≥98%, Alfa Aesar), lead chloride (PbCl₂, 99%, Thermo Fisher Scientific), oleic acid (OA, technical grade, Sigma-Aldrich), 1-octadecene (ODE, ≥91.5%, UNIAM), and oleylamine (OLA, >50%, Tokyo Chemical Industry) were purchased.

Preparation of Cs-oleate solution

The original synthetic method for Cs-oleate was optimized from a procedure reported in 2015 with some modifications.²⁵ Specifically, 0.326 g of Cs₂CO₃, 1 mL of OA, and 8 mL of ODE were loaded into a 100 mL three-neck flask and degassed for 1 hour at 110 °C. After degassing, the solution was further heated to 150 °C under an argon (Ar) flow and maintained at this temperature.

Synthesis of PbBr₂-CsPbBr_xCl_3−x NCs

The original PbBr₂-CsPbBr_xCl_3−x NCs were optimized from a conventional hot injection method reported in 2015 with some modifications.²⁵ For the synthesis of pristine PbBr₂-CsPbBr_xCl_3−x NCs, 0.93 g of PbBr₂ (2.532 mmol), 0.624 g of PbCl₂ (2.256 mmol), 6 mL of OA, 6 mL of OLA, and 60 mL of ODE were loaded into a 100 mL three-neck flask. The mixture was degassed at 110 °C for 1 hour. After degassing, the temperature of the solution was increased to 200 °C under Ar, and 2.4 mL of Cs-oleate solution (preheated to 150 °C) was swiftly injected into the precursor solution using a glass syringe. After the injection of Cs-oleate, the reaction proceeded for a controlled duration before the flask was quenched to room temperature in an ice-water bath.

Synthesis of ZnBr₂-CsPbBr_xCl_3−x NCs

For the synthesis of ZnBr₂-CsPbBr_xCl_3−x NCs, 0.57 g of ZnBr₂ (2.532 mmol), 0.624 g of PbCl₂ (2.256 mmol), 6 mL of OA, 6 mL of OLA, and 60 mL of ODE were loaded into a 100 mL three-neck flask. The mixture was degassed at 110 °C for 1 hour. After degassing, the temperature of the solution was increased to 200 °C under Ar, and 2.4 mL of Cs-oleate solution (preheated to 150 °C) was swiftly injected into the precursor solution using a glass syringe. After the injection of Cs-oleate, the reaction proceeded for a controlled duration before the flask was quenched to room temperature in an ice-water bath.

Purification of PbBr₂- and ZnBr₂-CsPbBr_xCl_3−x NCs

Samples of 5 mL of NCs were extracted at various reaction times using a glass syringe and subjected to a series of purification steps. Initially, the samples were centrifuged for 10 min at 10 000 rpm. After centrifugation, the supernatant was discarded, and the precipitate was dried. The dried precipitate was then redispersed in 4 mL of toluene and vigorously mixed. The resulting solution was centrifuged again for 10 min at 13 000 rpm. Following this centrifugation, the supernatant was discarded, and the precipitate was dried once more. Subsequently, the precipitate was redispersed in 3 mL of hexane and thoroughly mixed. The solution underwent a final centrifugation for 10 min at 6000 rpm. After this step, only the supernatant was collected and filtered using a 0.45 μm pore size filter. The temperature was set to 22 °C during centrifugations. All processes were carried out under ambient conditions.

Air stability test under ambient conditions

The glass substrates were coated with NCs under ambient conditions. Subsequently, the samples were stored in ambient air at room temperature. Characterization studies, including PL QY, were performed over time to assess the stability and performance of the NCs.

Thermal stability test under ambient conditions

NCs were coated onto glass substrates via drop casting. All processes were conducted under ambient conditions. Subsequently, the samples were heated on a hot plate set to 70 °C, and the PLQY was measured over time.

Materials characterization

Photoluminescence quantum yield measurements were performed using an integrating sphere (Quantaurus, Hamamatsu Photonics). The crystal structure of the perovskites was determined by X-ray diffraction using a diffractometer (Miniflex 600, Rigaku). The samples for X-ray diffraction analysis were prepared by drop-casting the NC solutions onto conductive Au/Cu-deposited silicon wafer substrates under ambient conditions. X-ray photoelectron spectroscopy measurements were conducted using an X-ray photoelectron spectrometer (ESCALAB 250Xi). For X-ray photoelectron spectroscopy analysis, the samples were prepared by drop-casting the NC solutions onto indium tin oxide substrates under ambient conditions. Transient photoluminescence was measured with 0.01 mW pump power, 0.1 mm beam diameter, and 310 nm wavelength. The pump beam was generated using an optical parametric amplifier (ORPHEUS, LIGHT CONVERSION) with a 1030 nm seed beam (PHAROS, LIGHT CONVERSION), which has a 200 fs pulse duration and a 200 kHz repetition rate. Photoluminescence from the sample was measured using a photon counting detector (PDM, Micro Photon Devices) and a time-correlated single-photon counting module (PicoHarp 300, PicoQuant). Transmission electron microscopy analysis to obtain nanoscale crystalline information of the perovskites was carried out using a Talos L120C.

Device fabrication

Patterned ITO glass was ultrasonically cleaned with isopropyl alcohol (IPA) for 5 min. Then, the substrate was treated with oxygen plasma for 3 min. Poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) was spin-coated at 4000 rpm for 30 s and annealed at 120 °C for 5 min in air. The substrate was placed into a N₂-filled glove box and annealed at 180 °C for 10 min. Then, poly[(9,9-dioctylfluorenyl-2,7-diyl) co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) (5 mg mL⁻¹) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) (1 mg mL⁻¹) were dissolved in m-xylene, spin-coated at 3000 rpm for 30 s and annealed at 180 °C for 30 min. Perovskite NCs (25 mg mL⁻¹ in octane) were spin-coated at 2000 rpm for 30 s and annealed at 80 °C for 15 min. Then, 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm) and LiF/Al electrodes (1 nm/100 nm) were deposited using thermal evaporation. The LED device was encapsulated by sealing with Norland Optical Adhesive (NOA) 8610 B epoxy and a cover glass.

Device characterization

The EL spectra and luminance–current density–voltage curves of the device were measured using a Keithley 2400B sourcemeter and a CS-2000 spectroradiometer (Konica Minolta).

Results and discussion

We synthesized all-inorganic CsPbBr_xCl_3−x perovskite nanocrystals (NCs) with mixed halide compositions using a modified approach.²⁵ Initially, a mixture containing PbBr₂, oleic acid, oleylamine, and 1-octadecene (ODE) was prepared. The mixture solution was degassed at 110 °C and then heated to 200 °C under an inert atmosphere for the synthesis of NCs. Subsequently, a preheated Cs-oleate solution was injected into the lead halide mixture solution, allowing the reaction to proceed for a controlled duration (denoted hereafter as PbBr₂-CsPbBr_xCl_3−x NCs). Previously, the effect of impurity metal halides in perovskite NCs with homogeneous halide compositions has been widely studied.²⁶ However, the effect of such impurities in mixed halide systems has been nearly lacking. Therefore, to investigate the effect of different metal halides, we replaced PbBr₂ with other metal halides. For example, ZnBr₂ was added in place of PbBr₂, and the CsPbBr_xCl_3−x NCs were synthesized following the same procedure as for the PbBr₂-CsPbBr_xCl_3−x NCs (denoted hereafter as ZnBr₂-CsPbBr_xCl_3−x NCs). The synthetic procedures for each type of perovskite NC are schematically shown in Scheme S1.†

While previous studies on perovskite NCs typically employed very short reaction times (∼10 s), other types of NCs have shown that prolonged reaction times can result in higher-quality NCs.^27–29 Based on this insight, we hypothesized that extending the reaction time might similarly improve the quality of perovskite NCs. For example, the passivation of NCs might be improved by allowing enough reaction time for crystal formation. Based on the hypothesis, we explored the effect of prolonged reaction times on the properties of the synthesized perovskite NCs.

Fig. 1(a) presents the PL peak wavelength data of PbBr₂-CsPbBr_xCl_3−x and ZnBr₂-CsPbBr_xCl_3−x NCs as a function of reaction time. For the PbBr₂-CsPbBr_xCl_3−x NCs, a redshift in the PL peak was observed until the reaction time reached 20 min. However, extending the reaction time beyond 20 min resulted in the permanent precipitation of the NCs. In contrast, the ZnBr₂-CsPbBr_xCl_3−x NCs showed a continuous redshift in the PL peak up to 90 minutes without any signs of precipitation. Notably, the PL QY of ZnBr₂-CsPbBr_xCl_3−x NCs was markedly increased with prolonged reaction time, whereas the PL QY of PbBr₂-CsPbBr_xCl_3−x NCs remained largely unchanged across different reaction times (Fig. 1(b)).

Fig. 1 (a) PL peak wavelength of ZnBr₂- and PbBr₂-CsPbBr_xCl_3−x NCs as a function of reaction time. (b) PL QYs of ZnBr₂- and PbBr₂-CsPbBr_xCl_3−x NCs depending on reaction time.

To explore the effect of reaction time on the size and morphology of CsPbBr_xCl_3−x NCs, TEM analysis was carried out. Fig. 2(a) and (b) display the TEM images of PbBr₂-CsPbBr_xCl_3−x NCs at reaction times of 10 s and 20 min, respectively. The morphology remained unchanged as the reaction time increased, and notably, the size of the PbBr₂-CsPbBr_xCl_3−x NCs also showed no significant variation despite different reaction times. A similar trend was observed for the ZnBr₂-CsPbBr_xCl_3−x NCs (Fig. 2(c) and (d)), where the PL spectra exhibited a pronounced redshift with increasing reaction time, yet the size of the NCs remained constant throughout the reactions (Table S1 and Fig. S1†). These findings strongly suggest that the observed spectral redshift of both PbBr₂- and ZnBr₂-CsPbBr_xCl_3−x NCs (Fig. 1(a)) was primarily due to anion exchange from Cl to Br, rather than a weakened quantum confinement effect due to an increase in particle size as the reaction time was extended.

Fig. 2 TEM images of PbBr₂-CsPbBr_xCl_3−x NCs with the (a) shortest (10 s) and the (b) longest (20 min) reaction times. TEM images of ZnBr₂-CsPbBr_xCl_3−x NCs with the (c) shortest (10 s) and the (d) longest (90 min) reaction times. The scale bars are 20 nm.

Fig. 3(a) shows the X-ray diffraction (XRD) patterns of ZnBr₂-CsPbBr_xCl_3−x NCs. As the reaction time increased, the XRD peaks gradually shifted closer to the reference pattern of CsPbBr₃. The results clearly indicate that anion exchange reaction from Cl to Br occurs during the prolonged reaction and are consistent with the data shown in Fig. 1(a). XRD patterns of PbBr₂-CsPbBr_xCl_3−x NCs displayed a similar trend to those of ZnBr₂-CsPbBr_xCl_3−x NCs, indicating the progress of the anion exchange reaction from Cl to Br (Fig. S2†). However, quantitative elemental analysis revealed distinctly different behavior between the two systems.

Fig. 3 (a) XRD patterns of ZnBr₂-CsPbBr_xCl_3−x NCs with various reaction times (reference patterns: JCPDS PDF#54-0752 and JCPDS PDF#84-0438 for CsPbBr₃ NCs and CsPbCl₃ NCs, respectively). (b) Anion (Br + Cl) to cation (Cs + Pb) atomic ratios of ZnBr₂- and PbBr₂-CsPbBr_xCl_3−x NCs.

Quantitative composition analysis data obtained from X-ray photoelectron spectroscopy (XPS) are shown in Fig. 3(b). For the PbBr₂-CsPbBr_xCl_3−x NCs, the total anionic compositions remained largely unchanged with varying reaction times, indicating that the anion exchange reaction exclusively occurred without effectively filling halide vacancies. This lack of halide vacancy passivation is crucial, as it explains why there was no significant increase in PL QY during the reaction from 10 s to 20 min. In contrast, the ZnBr₂-CsPbBr_xCl_3−x NCs showed a substantial increase in the total halide content over time, suggesting that halide supplementation to the NCs occurred in addition to the exchange of Cl for Br. It is noteworthy that the distinct interaction mechanism of ZnBr₂ with the NCs, compared to PbBr₂, may lead to additional halide incorporation into the NCs. This halide enrichment contributed to the effective passivation of halide vacancies, which in turn increased the PL QY of the NCs. As a result, the ZnBr₂-CsPbBr_xCl_3−x NCs achieved significantly higher PL QY with prolonged reaction times. Previous studies suggest that substituting the B site cation with a smaller cation can enhance structural stability by forming shorter bond lengths that can suppress octahedral rotation and tilting.^30–33 We indeed observed partial substitution of Pb to Zn in ZnBr₂-CsPbBr_xCl_3−x NCs, possibly due to lattice compatibility between the CsPbBr_xCl_3−x host lattice and Zn²⁺ ions (Fig. S4 and S5†). This substitution should result in a more stable octahedral structure that could persist under prolonged reactions at high temperature, facilitating halide vacancy passivation during reactions.

We conducted nuclear magnetic resonance (NMR) spectroscopy to reveal the surface of CsPbBr_xCl_3−x NCs. To better understand the surface structures, we employed a selective ligand exchange with methyl acetate.³⁴ In previous studies, a selective ligand exchange strategy using methyl acetate was employed to distinguish surface ligands, as methyl acetate molecules selectively replace oleate ligands through a hydrolysis reaction. In the NMR spectra of the as-prepared PbBr₂-CsPbBr_xCl_3−x NCs, a broad alkene resonance around 5.6 ppm was observed (Fig. 4(a)). However, this resonance disappeared completely after selective ligand exchange with methyl acetate (Fig. 4(b)), indicating that the surface termination of PbBr₂-CsPbBr_xCl_3−x NCs should be predominantly cesium oleate. In contrast, for the ZnBr₂-CsPbBr_xCl_3−x NCs, the broad resonance at 5.6 ppm persisted even after repeated selective ligand exchange with methyl acetate (Fig. 4(c) and (d)), suggesting that the surface termination consisted of oleylammonium halide. Additionally, broad resonances at 7.5 ppm and 4.1 ppm were detected, corresponding to α and β protons of oleylammonium molecules, respectively.^35–37 These findings indicate that the surface termination of ZnBr₂-CsPbBr_xCl_3−x NCs is anion-rich oleylammonium. This anion-rich surface may contribute to effective passivation of halide defects at the surface of perovskite NCs.

Fig. 4 ¹H NMR spectra of PbBr₂-CsPbBr_xCl_3−x NCs purified (a) without methyl acetate and purified (b) with methyl acetate. ¹H NMR results of ZnBr₂-CsPbBr_xCl_3−x NCs purified (c) without methyl acetate and purified (d) with methyl acetate.

To understand the impact of reaction time on carrier dynamics in perovskite NCs, we conducted time-resolved photoluminescence spectroscopy (TRPL) using time-correlated single-photon counting. The resulting decay curves were fitted with a tri-exponential decay function,³⁸ and the corresponding data are summarized in Table S2 and Fig. S3.† Each decay time component corresponds to a specific recombination process: Auger recombination or trap-assisted charge recombination (τ₁), bi-molecular radiative recombination (τ₂), and a slow radiative recombination process (τ₃). Additionally, A₁, A₂, and A₃ represent the fractional contributions of the fast (A₁), intermediate (A₂), and slow (A₃) components, respectively.³⁹

Overall, the average PL lifetime of ZnBr₂-CsPbCl_xBr_3−x NCs was markedly longer compared to the average PL lifetime of PbBr₂-CsPbBr_xCl_3−x NCs. This fact likely indicates a much lower defect density in the ZnBr₂-CsPbBr_xCl_3−x NCs, leading to more efficient radiative recombination.^40–42 To understand the carrier dynamics more deeply, we extracted the radiative recombination rate (k_r) and non-radiative recombination rate (k_nr) from the data. The average lifetime (τ_avg) reflects the contributions of both k_r and k_nr, and related parameters can be calculated using the following equation:^43,44

The PL QY can be calculated from the ratio of k_r to the total recombination rate. From eqn (1) and (2), k_r can be derived as:

The parameters extracted from TRPL data are summarized in Table 1. We compare the relaxation ratio (k_r/k_nr) to investigate the carrier dynamics of NCs. In PbBr₂-CsPbBr_xCl_3−x NCs, both k_r and k_nr increased when the reaction time was increased from 10 s to 20 min. Therefore, the relaxation ratio, k_r/k_nr, of PbBr₂-CsPbBr_xCl_3−x NCs exhibited only a slight difference, resulting in similar PL QY even at different reaction times (10 s and 20 min), as shown in Fig. 1(a). In stark contrast, in ZnBr₂-CsPbBr_xCl_3−x NCs, k_nr noticeably decreased, whereas k_r increased as the reaction time was extended, possibly due to effective halide passivation in ZnBr₂-CsPbBr_xCl_3−x NCs, which underwent longer reactions (Fig. 3(b)). As a result, we observed a considerable increase in k_r/k_nr, and thus enhanced PL QY as the reaction time increases in ZnBr₂-CsPbBr_xCl_3−x NCs.

Table 1 Parameters extracted from TRPL data of perovskite NCs with different reaction times

Perovskite NCs	τavg [ns]	kr [μs^−1]	knr [μs^−1]	kr/knr
PbBr2-CsPbBrxCl3−x 10 s	11.5	15.6	71.3	0.22
PbBr2-CsPbBrxCl3−x 20 min	6.9	39.0	105.0	0.37
ZnBr2-CsPbBrxCl3−x 10 s	18.7	12.8	40.6	0.32
ZnBr2-CsPbBrxCl3−x 20 min	19.8	19.2	31.4	0.61
ZnBr2-CsPbBrxCl3−x 50 min	19.9	21.1	29.2	0.72
ZnBr2-CsPbBrxCl3−x 90 min	18.3	28.0	26.8	1.04

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Previously, it was very well known that perovskite NCs are vulnerable to the ambient environment due to their ionic nature and dynamic surfaces. This vulnerability usually leads to shape and phase transformation when the perovskite NCs are exposed to ambient conditions, finally deteriorating optoelectronic properties.^45–48 Therefore, we investigated the stability of our perovskite NCs under both thermal and ambient conditions. The perovskite NCs were exposed to either ambient conditions or heat as films, and then PL quenching was measured. When the PbBr₂-CsPbBr_xCl_3−x NCs were exposed to elevated temperature around 70 °C, the PL QY was quenched nearly 80% compared to the initial value, accompanied by the emergence of a shoulder peak around 500–510 nm attributed to phase segregation, indicating their high susceptibility to heat (Fig. 5(a)). In the case of ZnBr₂-CsPbBr_xCl_3−x NCs, the sample showed just around 20% quenching compared with the initial value when stored at 70 °C, demonstrating much greater thermal stability compared to PbBr₂-CsPbBr_xCl_3−x NCs (Fig. 5(b)). The air stability also shows a marked contrast between the samples. While PbBr₂-CsPbBr_xCl_3−x NCs exhibited a decrease in PL QY around 60% compared to the initial value (Fig. 5(c)), the ZnBr₂-CsPbBr_xCl_3−x NCs showed nearly comparable PL QY after air storage for 2 weeks (Fig. 5(d)). In addition, a slight redshift resulting from phase segregation was also observed in PbBr₂-CsPbBr_xCl_3−x NCs, whereas ZnBr₂-CsPbBr_xCl_3−x NCs showed no sign of phase segregation after 2 weeks of air exposure. All the stability data shown in Fig. S6† also point to significantly superior properties of ZnBr₂-CsPbBr_xCl_3−x NCs, possibly due to effective passivation of halide vacancies and thus stable structures of ZnBr₂-CsPbBr_xCl_3−x NCs. Partial substitution of B sites from Pb to Zn may also contribute to enhanced stability (Fig. S4†).³⁰

Fig. 5 Thermal stability data of (a) PbBr₂-CsPbBr_xCl_3−x NCs and (b) ZnBr₂-CsPbBr_xCl_3−x NCs. The samples were kept at 70 °C for 2 h. Air stability data of (c) PbBr₂-CsPbBr_xCl_3−x NCs and (d) ZnBr₂-CsPbBr_xCl_3−x NCs. For the air stability test, the samples were stored under ambient conditions for 2 weeks.

We fabricated LEDs employing PbBr₂- and ZnBr₂-CsPbBr_xCl_3−x NCs, and the device performance was investigated (Fig. 6 and Scheme S2†). In LEDs based on perovskite NCs with mixed halide compositions, it has been frequently reported that the presence of halide vacancies leads to halide ion migration within the device, causing phase segregation, which finally results in lower energy emissions.^49,50 In our devices, ZnBr₂-CsPbBr_xCl_3−x NCs synthesized with a relatively short reaction time exhibited significant electroluminescent spectral shift as the applied voltage increased, due of the high density of halide vacancies (Fig. 6(a)). However, as the reaction time increases from 20 min to 90 min (Fig. 6(b)–(d)), the electroluminescent spectral shift was significantly mitigated. Finally, we observed just little spectral shift (∼3 nm) for the LEDs based on the ZnBr₂-CsPbBr_xCl_3−x NCs synthesized with the longest reaction time (90 min), as shown in Fig. 6(d). We believe the considerably suppressed spectral shift resulted from the effective passivation of halide vacancies in ZnBr₂-CsPbBr_xCl_3−x NCs due to prolonged reaction time.

Fig. 6 Normalized EL spectra of ZnBr₂-CsPbBr_xCl_3−x NCs with the reactions of (a) 10 s, (b) 20 min, (c) 50 min, and (d) 90 min, respectively.

In contrast, importantly, the devices using PbBr₂-CsPbBr_xCl_3−x NCs showed significant electroluminescent spectral shift even in NCs with prolonged reaction time (Fig. S7†). The results are in line with the elemental analysis data (Fig. 3(b)), demonstrating the presence of considerable halide vacancies even at the extended reaction in PbBr₂-CsPbBr_xCl_3−x NCs. Also, it is important to note that LEDs fabricated with PbBr₂-CsPbBr_xCl_3−x NCs exhibited marked TFB emission peaks (Fig. S7†). The observed TFB peaks can be attributed to recombination at the interface between the hole transport layer (HTL) and the emissive layer, due to excessive electron accumulation at the interface.⁵¹ In mixed halide perovskite LEDs, phase segregation of narrow bandgap domains occurs near the HTL under the electric field.⁵² This phenomenon can lead to the accumulation of electrons at narrow bandgap domains near the HTL.⁵³ Consequently, HTL emission is readily observed in PbBr₂-CsPbBr_xCl_3−x NCs, indicating that this behavior is a result of significant phase segregation. And the result is also a direct sign of high density of halide vacancies in PbBr₂-CsPbBr_xCl_3−x NCs. We note that previous studies performing halide defect suppression also reported similar results of contrasting TFB emission.⁵⁴ This implies that halide defects in PbBr₂-CsPbBr_xCl_3−x NCs can trigger excessive charge imbalance. Therefore, we believe the reduction of surface defects in the NCs and thus suppressed phase segregation contributed to the enhanced electroluminescent spectral stability of ZnBr₂-CsPbBr_xCl_3−x NC LEDs compared to their PbBr₂-CsPbBr_xCl_3−x counterparts.

The device performance of ZnBr₂-CsPbBr_xCl_3−x NCs reached a significantly improved maximum EQE of 0.02% compared to a maximum EQE of 0.004% in PbBr₂-CsPbBr_xCl_3−x NCs (Fig. S8†). Importantly, ZnBr₂-CsPbBr_xCl_3−x NCs exhibited an increase in maximum luminance and EQE with prolonged reaction time, which can be attributed to effective halide passivation and thus improved carrier injection balance. The results strongly suggest the potential for fabricating stable and high-performance blue-emitting LEDs based on mixed halide perovskite NCs.

Conclusions

In conclusion, we have developed a straightforward and effective in situ approach for synthesizing bright and stable all-inorganic perovskite NCs with mixed halide compositions by simply extending the reaction time – an essential but previously underexplored experimental variable in the synthesis of perovskite NCs. Partial replacement of B sites from Pb to Zn by employing impurity metal halides, such as ZnBr₂, stabilizes the CsPbCl_xBr_3−x NCs over longer reaction periods at high temperature, preventing precipitation that would otherwise occur without the impurity metal halides. Our findings reveal that CsPbCl_xBr_3−x NCs synthesized with extended reaction times are halide-rich, suggesting effective passivation of halide vacancies during prolonged reactions. This passivation significantly enhances the optical properties, as demonstrated by static PL QY studies and carrier dynamics investigations depending on reaction times. Additionally, the CsPbCl_xBr_3−x NCs synthesized with the prolonged reaction time exhibit improved air and thermal stability. Notably, LEDs fabricated using ZnBr₂-CsPbCl_xBr_3−x NCs synthesized with extended reaction times demonstrated superior spectral stability during operation, which should be attributed to suppressed phase segregation due to effective passivation of halide vacancies. This synthetic method offers valuable insights for producing high-quality perovskite NCs by simply adjusting the basic reaction variable (reaction time), thereby avoiding the need for complex post-synthetic processes.

Data availability

The data supporting this finding are available in this article and included as part of the ESI.† Raw data are available from the corresponding author upon reasonable request.

Author contributions

Y. K.: investigation, data curation, formal analysis, writing – original draft. S. C.: investigation, data curation, formal analysis, writing – original draft. S. P.: investigation, data curation, formal analysis. M. J. K.: investigation, data curation, formal analysis. Y. K.: methodology, writing – review & editing. G.-M. K.: data curation, formal analysis. D. C. L.: methodology, writing – review & editing. S. N. L.: formal analysis. S. A. S.: formal analysis. C. Y.: data curation, formal analysis. S. L.: data curation. S.-Y. C.: supervision, methodology, writing – review & editing. S. J.: supervision, methodology, writing – review & editing. S. L.: supervision, methodology, writing – review & editing. J. Y. W.: conceptualization, methodology, supervision, project administration, writing – original draft, writing – review & editing. All authors have critically reviewed the manuscript for intellectual content, provided final approval for publication, and agreed to be accountable for all aspects of the work to ensure its accuracy and integrity.

Conflicts of interest

The authors declare that they have no financial or personal conflicts of interest that could have influenced the results presented in this study.

Acknowledgements

This study was supported by grants funded by the Ministry of Trade, Industry, and Energy (RS-2022-00144108, RS-2025-02315917 and RS-2024-00423271) of the Korean Government. This research was also supported by grants from the National Research Foundation of Korea (RS-2024-00339674) funded by the Ministry of Science and ICT, Republic of Korea.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01550b

‡ These authors contributed equally to this work.

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