Synergistic ligand mediated anion exchange of CsPbI3 quantum dots for high performance white LED and anti-counterfeiting

Abstract

Anion exchange is an effective strategy to regulate the composition and optoelectronic properties of perovskite quantum dots (PQDs). Though promising, it is more desirable to synthesize PQDs to avoid the decrease of photoluminescence quantum yield (PLQY). Herein, we developed a ligand mediated anion exchange approach, in which the phase transition from CsPbBr₃ QDs to CsPbI₃ QDs was observed with the introduction of N-Acetyl-l-cysteine (NAC) and 1,3-dimethylimidazolium iodide (DMII) aqueous solution in CsPbBr₃ QDs solution. NAC is expected to create more halogen vacancies in CsPbBr₃ QDs, which provides sufficient adsorption sites for I⁻ ions, resulting in accelerating the anion exchange rate in the process of DMII incorporation. Benefiting from the synergistic ligand mediated anion exchange, high PLQY of 97% and remarkable stability of CsPbI₃ QDs are obtained. Furthermore, a white light-emitting diode (WLED) with a lumen efficiency (LE) of 116.82 lm/W is constructed, showing remarkable stability under continuous operation.

INTRODUCTION

All inorganic lead halide perovskite quantum dots (PQDs) have drawn great interest in the field of photoelectronic devices such as photodetectors,^1–5 light-emitting diodes (LEDs),^6–10 lasers,^11,12 and photovoltaics^13–18 because of the high photoluminescence quantum yield (PLQY), tunable bandgap and high defect tolerance.^19–23 Generally, the band gap of PQDs can be well regulated by controlling the dimension of nanocrystals and the composition of halogen ions, resulting in the different fluorescence.^24–27 However, the uneven size distribution of PQDs can greatly broaden the photoluminescence (PL) peaks, leading to the poor fluorescence. Therefore, how to precisely control the PL spectra of PQDs is of great significance for the synthesis of PQDs.

Anion exchange has been demonstrated as an efficient strategy to regulate the spectra of PQDs by changing the composition of halide ions.^28–31 It is noteworthy that anion exchange is a spontaneous and rapid process due to the higher activation energy and the larger surface reaction constant.^32–36 However, the anion exchange is highly dependent on the chemical potential of the reaction system.³⁷ For example, Parobek and coworkers developed a highly controllable anion exchange method by employing light as a trigger mechanism, which dissociates dihalomethane solvent molecules into halide ions followed by anion exchange.³⁸ The anion exchange reaction can be precisely controlled by changing the intensity or the wavelength of light. Zhou and coworkers proposed an in-situ anion reaction strategy by using a trace amount of KI/N,N-dimethylformamide solution and OAmI/toluene solution to realize accurate spectrum control of the CsPbBr_xI_3−x PQDs (600–680 nm) and the corresponding QLED yielded an external quantum efficiency (EQE) of 18.2%.³⁹ Moreover, Uddin and coworkers reported an anion exchange reaction that converted CsPbCl₃ cubes into anisotropic CsPbX₃ nanosheets by adding a mixture of dodecyl mercaptan and AlX₃ (X = Cl, Br, I) into the CsPbCl₃ nanocrystal solution.⁴⁰ It is worth to note that the high-quality of PQDs are mainly achieved via a post-treated procedure by dissolving the anion source in polar solvents, which can greatly reduce the energy barrier for the anion, accelerating the desorption.³⁹ However, the high ionic nature of perovskite makes PQDs more fragile in polar solvents, which can destroy the crystal structure of PQDs, resulting in the decrease of PL intensity and the shift of emission peak. Thus, it is more challenging to obtain high-quality and high PLQY of CsPbI₃ QDs in the process of the conventional anion exchange.

In this work, we developed a ligand mediated anion exchange approach to synthesize the CsPbI₃ QDs with high PLQY. First, CsPbBr₃ QDs dispersed in hexane are pretreated by N-Acetyl-l-cysteine (NAC) and then 1,3-dimethylimidazolium iodide (DMII) dissolved in water is added for anion exchange. The introduction of NAC is expected to desorb the original long-chain ligand, which creates more vacancy defects and reduces the energy barrier of anion desorption in CsPbBr₃ QDs. In parallel, NAC can effectively coordinate the uncoordinated Pb²⁺ ion, which not only improves the PLQY of QDs, but also enhances the stability of the QDs. Moreover, the incorporation of DMII provides sufficient halogen ions for the QDs to promote the anion exchange. Based on the synergy of NAC and DMII, CsPbI₃ QDs with 97% PLQY and excellent stability are obtained. The corresponding white light-emitting diodes (WLEDs) are constructed, yielding a lumen efficiency (LE) of 116.82 lm/W and a high color rendering index (Ra) of 90.8. Furthermore, the proof-of-concept anti-counterfeiting labels are fabricated and show high anti-counterfeiting capability.

RESULTS AND DISCUSSION

The process of ligand mediated anion exchange is displayed in Figure 1A. Here, CsPbBr₃ QDs were synthesized by hot injection method. NAC was added into CsPbBr₃ QD solution followed by stirring for 5 min. As shown in Figure 1B, the color of NAC-CsPbBr₃ QD solution changes from green to tan with the fluorescence intensity decrease after the introduction of NAC. In contrast, the color of CsPbI₃ QD solution changes from green to deep red feature and displays bright red light under 365 nm irradiation with the addition of DMII aqueous solution.

FIGURE 1. (A) Schematic illustration of ligand mediated anion exchange process. (B) Photographs of pristine CsPbBr3 QD solution, NAC-CsPbBr3 QD solution, and CsPbI3 QD solution under daylight and 365 nm irradiation.

Field emission transmission electron microscope (TEM) were employed to observe the morphology and size distribution of QDs. As shown in Figure S1, the QDs show cubic feature before and after anion exchange, which indicates that the ligand exchange and anion exchange show negligible influence on the morphology of QDs. Furthermore, NAC-CsPbBr₃ QDs and CsPbI₃ QDs show the increased dimension with an average size of 8.56 ± 1.50 and 8.63 ± 1.20 nm, respectively, compared with that of the pristine CsPbBr₃ QDs at 8.23 ± 1.18 nm, which is ascribed to the Ostwald ripening after ligand exchange. In addition, to confirm the introduction of iodine element, QDs after anion exchange were analyzed by Energy dispersive x-ray spectroscopy (EDS). The corresponding EDS mapping of Cs, Pb, and I are shown in Figure 2A–D, in which the I element is uniformly distributed in CsPbI₃ QDs. In parallel, the structure of all samples was characterized using x-ray diffraction (XRD). It is found that both CsPbBr₃ QDs and NAC-CsPbBr₃ QDs show cubic phase (PDF#75-0412), which indicates that NAC has no apparent effect on the phase structure and crystallization behavior of CsPbBr₃ QDs. Meaningfully, a slight peak shift toward low angle is observed after the incorporation of DMII aqueous solution, further confirming that CsPbBr₃ QDs are transformed into cubic CsPbI₃ QDs (PDF#76-8588) (Figure 2E).

FIGURE 2. (A) TEM image of CsPbI3 QDs. EDS mapping of (B) Cs, (C) Pb, (D) I in CsPbI3 QDs. (E) XRD patterns, (F) normalized UV–vis spectra, (G) normalized PL spectra, and (H) FTIR spectra of pristine CsPbBr3 QDs, NAC-CsPbBr3 QDs, and CsPbI3 QDs.

In order to evaluate the effect of the treatment on the optical properties of CsPbBr₃ QDs, normalized UV–vis and PL spectra were conducted. As shown in Figure 2F, the absorption spectrum of CsPbI₃ QDs shows an obvious increase with the addition of DMII solution. In parallel, the emission peak position of CsPbI₃ QDs presents a red-shift behavior from 512 to 699 nm (Figure 2G). Besides, Full Width at Half Maximum (FWHM) is changed from 23 to 51 nm, which is consistent with the reported literatures.^20,41,42 Figure S3 shows UV and PL spectra with the addition of different content of DMII. It is found that with the increase of DMII content, red-shift behavior of absorption peak and PL emission peak is clearly observed in both UV and PL spectra, which indicates that more Br⁻ ions are exchanged by I⁻ ions with the introduction of more DMII. As the content of DMII reaches to 150 and 200 mg, the perovskite QDs show good reproducibility with no discernible red-shift in both absorption peak and PL emission peak, which indicates the anion exchange process is completed. The resultant PLQY of CsPbI₃ QDs after anion exchange reaches 97%, an increment of 56% relative to the pristine CsPbBr₃ QDs (PLQY = 41%). As shown in Figure S4 and Table S1, the CsPbI₃ QDs synthesized in this work shows higher performance in comparison with other anion exchange works.

Surface chemistry of QDs was investigated by Fourier transform infrared spectroscopy (FTIR). As shown in Figure 2H, the wavenumber of 1542 and 1402 cm⁻¹ correspond to the antisymmetric and symmetric stretching vibrations of COO⁻, respectively, in pristine CsPbBr₃ QDs. However, the anti-symmetric and symmetric stretching vibration peaks of COO⁻ are slightly blue-shifted with the decreased intensity in the case of NAC-CsPbBr₃ QDs, which indicate the content of oleic acid ions at the surface of CsPbBr₃ QDs is greatly reduced.⁴³ Furthermore, the addition of DMII solution is expected to provide excessive I⁻ ions, in which the original Br⁻ ions are exchanged by I⁻ ions quickly and part of the metal ions are taken away. In order to realize the charge balance in the system, the COO⁻ ions are expected to be desorbed. As a result, the reaction system is transformed into CsPbI₃ QDs. These results suggest that NAC is adsorbed at the surface of CsPbI₃ QDs after ligand exchange and anion exchange.

X-ray photoelectron spectroscopy (XPS) measurements were performed to analyze the chemical composition of QDs. As shown in Figure 3, two additional peaks are appeared at 618.00 and 629.50 eV, corresponding to the I 3d core level after anion exchange. Compared with the pristine CsPbBr₃ QDs, the two spin-orbit splitting peaks of Pb 4f in CsPbBr₃ QDs shift from 138.06 and 142.93 eV to lower binding energies of 137.50 and 142.37 eV, which is attributed to the stronger coordination between Pb²⁺ ions and S²⁻ ions, leading to the increase of electron cloud density of Pb²⁺ ions and the corresponding decrease of binding energy. Moreover, the binding energy of Pb 4f peaks continuously shift to the lower binding energies of 137.24, 142.11 eV after anion exchange, which is ascribed to the fact that the electronegativity of I is weaker than that of Br, resulting in the further increase of the electron cloud density of Pb²⁺ ions. Correspondingly, the binding energy of Br 3d in CsPbBr₃ QDs also shifts from 67.92 and 68.96 eV to lower binding energies of 67.45 and 68.49 eV, respectively. The binding energy decrease is mainly ascribed to the increase of electron cloud density of Br⁻ ions. On the other hand, the disappearance of Br element after DMII treatment can be explained by the fact that most of Br⁻ ions are replaced by I⁻ ions (Figure 3D). In addition, the stoichiometric ratio of Pb:Br in pristine CsPbBr₃ QDs and NAC-CsPbBr₃ QDs are 1:3.58 and 1:3.88, respectively. In comparison, the stoichiometric ratio of Pb:I in CsPbI₃ QDs is 1:4.07, which provides the rich I⁻ ions at surface of QDs, leading to reduction of the non-radiative recombination originated from surface defects.

FIGURE 3. XPS spectra of pristine CsPbBr3 QDs, NAC-CsPbBr3 QDs, and CsPbI3 QDs. (A) Survey, (B) I 3d, (C) Pb 4f, and (D) Br 3d.

Typically, PQDs are in a relatively stable structure because they are dispersed in the nonpolar solvent and coated with a large number of ligands. To control the anion exchange of CsPbBr₃ QDs, it is necessary to break this stable structure and lower the free energy barrier for the desorption of Br⁻ ions. With the addition of NAC, the COOH group of NAC is expected to deprotonate followed by reacting with the protonated OAm⁺ to form an acid/base complex. Moreover, the complex will desorb from the surface of QDs and take away part of halide ions and metal ions, creating a large number of Br⁻ ion vacancies at the surface of CsPbBr₃ QDs. In addition, the excess I⁻ ion is sufficiently substituted for the Br⁻ ion with the incorporation of DMII. As the reaction proceeds, CsPbI₃ QDs with uniform size and high quality are obtained. It is worth to note that anion exchange is expected to be conducted with a high concentration of halide ions.

In order to verify the necessity of aqueous solutions of NAC and halide ions, four groups of experiments were carried out, including the addition of DMII particles, DMII aqueous solution, NAC powder + DMII particles, NAC powder + DMII aqueous solution. As shown in Figure S2a, unreacted DMII particles are observed at the bottom of the glass bottle and the color of the QD solution shows no apparent change with the addition of DMII particles. Turbid QD solution is observed as the introduction of DMII aqueous solution, which is caused by the aggregation of QDs (Figure S2b). In parallel, the color of QD solution changes from green to orange with the appearance of white particles at the bottom when NAC powder and DMII particles are added (Figure S2c). As shown in Figure S2d, CsPbBr₃ QD solution is treated by NAC and DMII aqueous solution in sequence. After 20 min, it can be obviously observed that the QD solution is delaminated. The upper layer is CsPbI₃ QD solution, and the lower layer is the mixture of phase-change CsPbI₃ QD and deionized water. Based on the above analysis, we believe that NAC treated QD solution is beneficial for the rapid anion exchange.

To further verify the generality of the ligand mediated anion exchange approach, CsPbCl₃ QDs were synthesized. As shown in Figure S5a, NAC and DMIC aqueous solution is introduced to CsPbBr₃ QD solution subsequently. After 20 min stirring, the upper QD solution changes from green to colorless. Moreover, it shows bright dark blue under 365 nm irradiation (Figure S5b). As seen in Figure S6, two additional peaks are observed at 197.29 and 198.89 eV after DMIC processing, which corresponds to the Cl 2p core level. By contrast, the peaks of Pb 4f 7/2 and 4f 5/2 are shifted to 137.77 and 142.64 eV, respectively in CsPbCl₃ QDs. While the peaks of Br 3d 5/2 and 3d 3/2 are shifted to 67.58 and 68.62 eV, respectively, which is due to the fact that Cl is more electronegative than Br, resulting in the reduction of Pb²⁺ ions electron cloud density. As shown in Figure S7, CsPbCl₃ QDs show cubic crystal structure (PDF#80-4468) and uniform distribution with an average size of 7.99 ± 1.24 nm. In addition, the emission peak position of the CsPbCl₃ QDs shows blue shift to 455 nm.

The temperature-dependent PL spectra of CsPbI₃ QDs were evaluated by investigating the exciton binding energy and the electron–phonon interaction. As the temperature increases, CsPbI₃ QDs synthesized by both hot injection and anion exchange method show decreased PL intensity, which is attributed to the thermal quenching of non-radiative traps (Figure 4A,B). Meanwhile, the broader FWHM is also observed with the increased temperature, which is related to the electron–phonon coupling,^44,45 more details can be seen in Note 1 (Supporting Information). In addition, the exciton binding energies of CsPbI₃ QDs synthesized by hot injection and anion exchange method are 28.40 and 45.29 meV, respectively (Figure 4C,D). Meanwhile, the gradually increased fluorescence lifetime of samples is observed (Figure 4E,F and Table S3), which is highly associated with the temperature increase from the thermal activation.^46,47

FIGURE 4. PL spectra of CsPbI3 QDs obtained by (A) hot injection and (B) anion exchange from 70 to 270 K. The integrated PL intensity (Arrhenius fit) of CsPbI3 QDs obtained by (C) hot injection and (D) anion exchange. The PL decay curves of CsPbI3 QDs obtained by (E) hot injection and (F) anion exchange from 70 to 270 K.

In order to compare the properties of CsPbI₃ QDs obtained by hot injection and anion exchange method, time-resolved PL decay spectra measurements were performed using a 375 nm laser as an excitation (Figure S8). As shown in Table S2, CsPbI₃ QDs synthesized by anion exchange show longer PL lifetime (τ_avg) with an increased PLQY (97%), which are the results of the increased radiative recombination rate (K_r) of CsPbI₃ QDs, indicating that CsPbI₃ QDs obtained by anion exchange strategy have fewer surface defects.⁴⁸ The reduced surface defects are highly associated with the fact that NAC can create more vacancies for the sufficient exchange of I⁻ ions in the process of ligand exchange. In parallel, the presence of DMII provides sufficient I⁻ ions for anion exchange and passivates the surface defects of QDs.

In addition to the photoluminescence, stability is also an important parameter to evaluate the quality of QDs.^49–51 Stability of CsPbI₃ QDs in polar solvents, thermal stability and photostability are performed. First, an equal amount of CsPbI₃ QD solution obtained by hot injection and anion exchange is treated by ethanol. After 30 min, the emission peaks of all samples show red shift behavior. Moreover, the PL intensity of CsPbI₃ QDs obtained by hot injection decreases to 17.31%, while the anion exchange case still maintains 38.5% of the initial performance, showing the improved anti-ethanol performance (Figure S9). Subsequently, an equal amount of CsPbI₃ QD solution is dropped onto a round glass and the temperature is raised from 25 to 160°C to evaluate the thermal stability. As seen in Figure S10, the CsPbI₃ QDs obtained by anion exchange method show improved thermal stability. Furthermore, the photostability of CsPbI₃ QD solution is conducted by continuous UV irradiation. It is found that the PL intensity of CsPbI₃ QDs synthesized by hot injection method is decreased by 30% after 60 min irradiation, while the anion exchanged case displays slow decrease behavior and keeps 94.55% of initial performance (Figure S11). These results further demonstrate that CsPbI₃ QDs obtained by anion exchange method show better stability than that of hot injection cases.

Furthermore, to deeply evaluate the performance of QDs and demonstrate the potential applications of QDs in patterning and anti-counterfeiting labels, QDs solution with different emission were written on the paper. As shown in Figure 5A, CsPbCl₃ QDs, CsPbBr₃ QDs, and CsPbI₃ QDs emit blue, green and red light under 365 nm irradiation, respectively. Furthermore, a proof-of-concept anti-counterfeiting was demonstrated, in which a lotus pattern was printed on a polymethyl methacrylate (PMMA) substrate by using a designed seal coated with CsPbCl₃ QD solution. Although the pattern is  invisible under daylight, the anti-counterfeiting pattern of lotus can be easily identified under UV irradiation (Figure 5B and Supporting Video S1). To explore the potential applications of CsPbI₃ QDs in down-conversion WLEDs, CsPbI₃ QDs obtained by anion exchange method were mixed with YAG:Ce³⁺ yellow phosphors and coated onto InGaN blue light chips. The device shows an enhanced EL intensity with no discernible shift in spectral wavelength following a series of forward-bias currents (from 20 to 100 mA) (Figure 5C). As the forward current increases from 20 to 100 mA, the CIE coordinates change only over a small area, with Ra falling slightly from 92.3 to 86.4, color temperature (Tc) increasing from 5135 to 5619 K, and LE slightly decreasing from 106.83 to 98.79 lm/W (Figure 5D,E). Typically, the WLED emits bright white light with a CIE color coordinate of (0.3410, 0.3422) under 20 mA forward current (Figure 5F). As shown in Table S4, the performance of WLED in this work is improved compared with the literature based on red CsPbI₃ QDs as WLED color conversion materials. Stability of the prepared WLED device is evaluated after being placed for 1 week. The WLED shows the CIE color coordinate of (0.3411, 0.3462), Tc of 5142 K, LE of 116.82 lm/W, and Ra of 90.8 under 20 mA forward current (Figure S12). In addition, the stability of WLED during continuous operation was tested at 10 min intervals. After 60 min of continuous operation, the CIE color coordinate of the WLED is (0.3312, 0.3490), the Tc is 5555 K, the LE is slightly reduced to 115.59 lm/W, and the Ra is slightly reduced to 84.2, respectively. These results demonstrate that the WLED devices prepared by using ligand mediated anion exchanged CsPbI₃ QDs exhibit excellent performance.

FIGURE 5. (A) Pattern application of CsPbCl3 QDs, CsPbBr3 QDs, and CsPbI3 QDs. (B) Anti-counterfeiting application of CsPbCl3 QDs. (C) Electroluminescence spectra of the WLED operated with different forward-bias currents. (D) The enlarged CIE (x, y) upon forward current (inset shows the photo of one page illuminated by the WLED in dark condition). (E) LE, Ra, and Tc of the WLED at different forward currents. (F) The corresponding CIE color coordinates in a red star for the WLED in CIE diagram.

CONCLUSIONS

In summary, a ligand mediated anion exchange approach was developed to synthesize high-PLQY CsPbI₃ QDs by the introduction of NAC and DMII aqueous solution to CsPbBr₃ QDs solution. The presence of NAC serves as surface ligand which creates more halogen vacancies in CsPbBr₃ QDs, providing more adsorption sites for I⁻ ions. Moreover, the introduction of DMII provide sufficient I⁻ ions to promote the anion exchange. The ligand mediated anion exchange approach can not only effectively reduce the surface defects of QDs, but also avoid to destroy the structure of QDs, leading to the improved the performance of QDs. The resultant CsPbI₃ QDs yield PLQY of 97% and show remarkable stability. The as-fabricated WLED is constructed with a LE of 116.82 lm/W, which shows excellent stability in both environment and continuous operation.

AUTHOR CONTRIBUTION

Guoqing Tong and Yang Jiang supervised the project. Yajing Chang and Guoqing Tong conceived the ideas and designed the experiments. Yajing Chang conducted the synthesis of QDs and basic characterization. Liping Liu synthesis of perovskite QDs and characterization. Junchun Li helped with the analysis of TEM images and the fabrication of WLED. Jingting Yang helped with SEM measurements. Zongsheng Chen, Zhigang Li helped with the fabrication of anti-counterfeiting pattern. Shaobo Zhang and Ru Zhou provided valuable suggestions for the manuscript. All authors contributed to the writing of the paper.

ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of China (Grant No. U1632151); Natural Science Foundation of Anhui Province, China (Nos. 2108085ME149, 2108085ME147, 2308085QE137); Natural Science Foundation of Hefei City (No. 2022024); Key Research and Development Plan of Anhui Province (No. 2023t07020005); State Key Laboratory of Pulsed Power Laser Technology, China (No. SKL2021ZR03); China Postdoctoral Science Foundation (No. 2021M693968); Innovation Technology Platform Project Jointly Built by Yangzhou City and Yangzhou University (No. YZ2020268).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available in the supplementary material of this article.