Water-Assisted Perovskite Quantum Dots with High Optical Properties

Abstract

Lead halide perovskite quantum dots (PeQDs) have excellent optical properties, such as narrow emission spectra (FWHM: 18–30 nm), a tunable bandgap (λ_PL: 420–780 nm), and excellent photoluminescence quantum yields (PLQYs: >90%). PeQDs are known as a material that is easily decomposed when exposed to water in the atmosphere, resulting in causing PeQDs to lower performance. On the other hand, according to the recent reports, adding water after preparing the PeQD dispersion decomposed the PeQD surface defects, resulting in improving their PLQY. Namely, controlling the amount of assisting water during the preparation of the PeQDs is a significantly critical factor to determining their optical properties and device applications. In this paper, our research group discovered the novel effects of the small amount of water to their optical properties when preparing the PeQDs. According to the TEM Images, the PeQDs particle size was clearly increased after water-assisting. In addition, XPS measurement showed that the ratio of Br/Pb achieved to be close to three. Namely, by passivating the surface defect using Ostwald ripening, the prepared PeQDs achieved a high PLQY of over 95%.

1. Introduction

Lead halide perovskite quantum dots (PeQDs) have excellent optical properties, such as narrow emission spectra (FWHM: 18–30 nm) [1,2], a tunable bandgap (λ_PL: 420–780 nm) [3,4], and excellent photoluminescence quantum yields (PLQYs: >90%) [1,5,6]. Therefore, PeQDs have been investigated and applied in a variety of fields such as light emitting diodes (LEDs) [7,8], solar cells [9,10], and photodetectors [11,12]. In general, colloidal PeQDs are synthesized by two typical liquid-phase methods, the hot injection (HI) method [3,13] and the ligand-assisted reprecipitation (LARP) method [4,14]. The HI method is the most widely used method for preparing PeQDs because it is possible to prepare size-uniform PeQDs. According to the reports by Jun et al., by using the HI method, the higher optical properties of the prepared PeQDs were achieved by the ligand exchange of dilauryldimethylammonium bromide (DDAB) [15]. In detail, DDAB works as not only the supplying sources of Br, but also the ligand, which does not detach from the PeQD surface because of the quaternary ammonium. As a result, the narrow emission spectrum and high PLQY over 71% were achieved. In addition, the LED device with the PeQDs prepared by the HI method has achieved an external quantum efficiency of 23% in a red emission [6], 22% in a green emission [16], and 12.3% in a blue emission [16]. While the prepared PeQDs with the HI method achieves high optical properties, it is with requiring high temperatures over 100 °C and nitrogen during synthesis, resulting in its unsuitability for mass synthesis because of the high cost. On the other hand, the LARP method does not require a complicated environment to synthesize PeQDs. Therefore, not only conventional inorganic perovskites, but also organic-inorganic perovskites, could be synthesized [4,17,18]. In previous studies, various surface treatments have been applied to both inorganic and organic–inorganic perovskites by the LARP method, and the prepared PeQDs via the LARP method could achieve a high PLQY over 70%, similar to that prepared by the HI method [5,19].

The LARP method does not require a complicated environment; however, the water content of the solvent used by the synthesis of colloidal PeQDs was thoroughly controlled, with the value close to zero. In general, colloidal PeQDs are easily decomposed when they are exposed to the moisture in the atmosphere, resulting in PeQDs’ decreased performance. According to previous reports [20], the effect of moisture on the structure and properties of the perovskite compound have been revealed by the simulations combined ab initio molecular dynamics simulations and the first-principles density functional theory. The effect of a few water molecules on the structure of the perovskite surface was simulated, resulting in the formation of hydrated compounds in a humid environment. The electronic excitation of water-absorbed perovskite tended to weaken the Pb-halogen bonds around the water molecules. This indicated that the presence of water enhanced the decomposition of the perovskite. On the other hand, there are some reports of the following: the PLQY of PeQDs were drastically improved by assisting with a little water when synthesizing. In the reports by Ying et al., a little water was added to the hexane dispersion with PeQDs to decompose the PeQD surface defects, resulting in the improvement of PLQY [21]. Namely, controlling the amount of assisting water during the preparation of the PeQDs is a significantly critical factor to determining their optical properties and device applications.

In our research, we focused on the LARP method and clarified the novel effects of the small amount of water to their optical properties when preparing the PeQDs. This method could be achieved with a high performance of the PeQDs in a one-pot preparation by assistance with water. In detail, this method, combining the phenomenon of water decomposing PeQDs and Ostwald ripening, was proposed [22]. It could be predicted that a little water could enhance the Ostwald ripening. According to the TEM Images, the PeQDs particle size was clearly increased after water-assisting. In addition, XPS measurement showed that the ratio of Br/Pb achieved to be close to three. Namely, by passivating the surface defect using Ostwald ripening, the prepared PeQDs achieved a high PLQY of over 95%.

2. Materials and Methods

2.1. Materials

PbBr₂ (99%) was purchased from Tokyo Chemical Industry. Hydrobromic acid (HBr 47–49%), methylamine methanol solution (MA/MeOH 40%), toluene (99.5%), 1-methyl-2-pyrrolidone (NMP 99.5%), and acetonitrile (99.5%) were purchased from Wako Pure Chemical Industries. Oleic acid (90%) and octylamine (99%) were purchased from Sigma-Aldrich. Methanol (MeOH 99.5%) and ethyl acetate (AcOEt 99.5%) were purchased from Kanto Chemical. All the materials were used without purification.

2.2. Synthesis of CH₃NH₃Br (MABr)

First, 30 mL of MA/MeOH solution was maintained at 0 °C in an ice bath and stirred for 2 h after the addition of 7 mL of HBr. After the reaction, the solvent was evaporated at a pressure of 0.1 MPa at 45 °C. The obtained white crystals were recrystallized and washed by MeOH and AcOEt and dried in a vacuum at 40 °C for 24 h.

2.3. Synthesis of Methylammonium Lead Halide (MAPbBr₃) PeQDs

MAPbBr₃ PeQDs were synthesized by the LARP method: 0.20 mmol MABr, 0.20 mmol PbBr₂, 15 µL of octylamine, and 18 µL of oleic acid were dissolved in 1.0 mL of NMP as a polar solvent. Then, 250 µL of precursor solution was dropped into 4.5 mL of toluene with 0, 2.5 (0.056 vol%), 5.0 (0.11 vol%), and 7.5 (0.16 vol%) µL of water as a non-polar solvent under vigorous stirring for 10 min. To precipitate the obtained colloidal solution, acetonitrile was added at a volume ratio of 1:1, and then the mixed solutions were centrifuged at 16,500 rpm for 5 min, and the supernatants were discarded. The precipitate was dispersed in 1 mL toluene, and the solutions were centrifuged at 8000 rpm for 2 min. The dispersed supernatant was obtained and filtered through a polytetrafluoroethylene filter with a pore size 0.2 µm.

2.4. Characterization

X-ray diffraction (XRD) patterns of the samples were obtained from in-plane diffraction and were measured on a Rigaku Smart Lab (using Cu Kα radiation at 45 kV and 200 mA). The samples were observed by a JEOL JEM- 2100F transmission electron microscope (TEM) (accelerating voltage of 200 kV). The visible absorption spectra of the samples were obtained on a JASCO V-670 spectrophotometer (detecting wavelength range of 400 to 600 nm). The photoluminescence (PL) spectra and photoluminescence quantum yield (PLQY) of samples were obtained with a JASCO FP- 8600 luminescence spectrometer (exciting wavelength of 400 nm). The PL lifetime was obtained using a Hamamatsu C11367 Quantaurus-Tau. The result of the XPS measurement was obtained by using a Thermo Fisher Scientific Theta probe.

3. Results

3.1. Water-Assisted LARP Method

Figure 1 shows a schematic illustration of the water-assisted LARP method. MAPbBr₃ PeQDs are prepared as follows: the MAPbBr₃ PeQD precursor solution was injected into the mixed solvent as a non-polar solvent, which was composed of toluene and water (Figure S1). Water was completely dispersed in toluene by irradiating the ultrasound. Water is known as a solvent that decomposes MAPbBr₃ PeQDs. By assisting with quite a small amount of water, however, it is possible to induce Ostwald ripening, which is a phenomenon by which small sized MAPbBr₃ PeQDs are rapidly decomposed and redeposited on large ones [22]; finally, the surface defect of the large sized MAPbBr₃ PeQDs was passivated, resulting in improving their optical properties. Therefore, the difference in optical properties between the prepared MAPbBr₃ PeQDs without or with water are evidenced.

3.2. Ostwald Ripening of MAPbBr₃ PeQDs by Assisting Water

To explore the effects of water for the prepared MAPbBr₃ PeQDs, their morphologies were evaluated. As shown in Figure 2 and Figure S2, the prepared MAPbBr₃ PeQDs with the assisted water showed larger-sized PeQDs compared to those prepared without water. Considering the MAPbBr₃ PeQD sizes and that their distribution value was 12.7 ± 2.8 nm (Figure 2), the mono-dispersed size and uniformity of MAPbBr₃ PeQDs was successfully prepared. This indicated that under assisting with a small amount of water, the small-sized MAPbBr₃ PeQDs were redeposited on a large one by Ostwald ripening. This phenomenon implied that the surface defect of the large-sized MAPbBr₃ PeQDs was passivated by a small one, resulting in proceeding the upsizing of their particle size (Figure 3). From the distribution of the obtained particle size, it is clear that the assisting water promoted Ostwald ripening. In detail, small-sized MAPbBr₃ PeQDs are decomposed and redeposited on the larger MAPbBr₃ PeQDs. As a result, surface defects on the large sized ones were predicted to passivate by this phenomenon.

3.3. The Effect of the Assisting Water on the Prepared MAPbBr₃ PeQDs

To investigate the effects of the water-assisted procedure on a perovskite crystal structure on the prepared MAPbBr₃ PeQDs, their XRD patterns were measured. From XRD patterns, the prepared samples with between 0 vol% and 0.16 vol% of assisting water obviously show the strong peaks attributed to the (100), (110), (200), (210), (220), and (300) diffraction planes (Figure 4 and Figure S3), which indicated a cubic perovskite crystal structure [23,24]. The XRD patterns results showed that the optimal water-assisted content will not have a negative effect on the crystal structure.

Figure 5a and Figure S4a show the UV-vis absorption spectra of MAPbBr₃ PeQDs without and with water. As shown in the UV-vis absorption spectra, the characteristic absorption peaks of MAPbBr₃ PeQDs were observed. Furthermore, the absorbance of MAPbBr₃ PeQDs with assisted water was lower than that of MAPbBr₃ PeQDs without water (Figure S5). Their decrease in absorbance indicated that the number of the prepared MAPbBr₃ PeQDs decreased with increasing the water amount [25]. Decreasing their absorbance of the MAPbBr₃ PeQDs with assisted water was reasonable because the small sized MAPbBr₃ PeQDs was redeposited on the large one, and the total number of MAPbBr₃ PeQD particles was decreased. Figure 5b and Figure S4b showed the normalized PL spectra of the MAPbBr₃ PeQDs. The PL spectra of the prepared MAPbBr₃ PeQDs corresponded to the previously reported ones with cubic structures [26]. The PL peak of MAPbBr₃ PeQDs was red shifted from 522 nm to 526 nm by assisting with water and had a narrower FWHM compared with that of the MAPbBr₃ without water (

Table 1. Summary of optical properties of MAPbBr₃ PeQDs prepared without water (0 µL) and with water (2.5–7.5 µL).

Water-Assisted	λPL/nm	FWHM/nm	PLQY/%
0 µL	522	24.9	82.7
2.5 µL	524	23.3	91.6
5.0 µL	526	21.8	95.6
7.5 µL	524	23.0	90.4

). The slight red shift of the MAPbBr₃ PeQDs with assisting water could be implied to promote the Ostwald ripening by the addition of water [27]. As a result, their particle sizes of the prepared MAPbBr₃ PeQDs were uniformity, and it achieved the narrow FWHM emission [28]. In short, the PL spectra of MAPbBr₃ PeQDs were red shifted after the assisting of water, which clarified that their crystal growth occurred due to Ostwald ripening.

The PLQYs of the prepared MAPbBr₃ PeQDs without water showed 82.7%. On the other hand, their PLQY values were increased up to 95.6% with only 5 µL assisted water (Table 1). This result implied that the optimal assisted water to prepare MAPbBr₃ PeQDs effectively proceeded to decompose the small-sized MAPbBr₃ PeQDs, resulting in passivating on the surface defects of the large sized ones by Ostwald ripening. The prepared MAPbBr₃ PeQDs were further investigated by time-resolved photoluminescence (Figure 6a and Figure S6a,

Table 2. PL decay time constant, a short-lifetime, and a long lifetime of MAPbBr₃ PeQDs prepared without water (0 µL) and with water (2.5–7.5 µL).

	A1/%	A2/%	A3/%	τ1/ns	τ2/ns	τ3/ns	τave./ns
0 µL	50.2	44.8	5.0	3.3	10.9	29.1	12.6
2.5 µL	32.3	49.3	18.4	1.8	9.6	22.5	14.8
5.0 µL	50.2	44.1	5.7	4.1	13.3	33.4	15.4
7.5 µL	36.5	54.1	9.4	3.1	10.5	25.7	13.6

) to determine the charge carrier lifetime. The PL decay curve indicated that the PL lifetime of the MAPbBr₃ PeQDs increased to 15.4 ns with 5 µL of assisted water compared to 12.6 ns without water. From these results, the longer τ_ave of the prepared MAPbBr₃ PeQDs with assisted water of 5 µL indicates that the surface defects on the large MAPbBr₃ PeQDs were passivated by small MAPbBr₃ PeQDs as an Ostwald ripening, resulting in efficiently suppressing non-radioactive recombination by small PeQD passivation on their surface [29,30]. On the other hand, in the prepared MAPbBr₃ PeQDs with assisted water over 5 µL, the excess water worked to collapse MAPbBr₃ PeQDs because of their polarity, and it caused the increasing of MAPbBr₃ PeQD surface defects, resulting in a shorter τ_ave. From the XPS analysis, the ratio of Br/Pb showed the near unity ideal values of 2.90 on the MAPbBr₃ PeQDs with water compared with the value of 2.75 for that without water [31]. In addition, the Br 3d_5/2 and Br 3d_3/2 peaks on the Br 3d spectra shifted to higher binding energies by 0.1 eV, which means that Pb-Br interactions on the MAPbBr₃ PeQDs were stronger after assisting water (Figure 6b and Figure S6b) [31]. It was confirmed that the surface defects of MAPbBr₃ PeQDs were passivated due to the longer τ_ave in the PL lifetime and the improvement ratio of Br/Pb by XPS measurement, which resulted in a significant improvement in PLQY. These results indicated that the water-assisted LARP method could promote the Ostwald ripening and passivate the Br defects on MAPbBr₃ PeQDs.

4. Conclusions

In summary, we have proposed a method to improve the performance of MAPbBr₃ PeQDs in one pot by a water-assisted LARP method. It was confirmed that the water-assisted LARP method promoted the Ostwald ripening process, in which small particles are dissolved and re-deposited onto larger ones. In detail, it showed that the optimal water-assisted content will not have a negative effect on their crystal structure. In addition, the PL spectra of MAPbBr₃ PeQDs were red shifted after being water-assisted, which clarified that their crystal growth occurred due to Ostwald ripening. Therefore, the ratio of Br/Pb was close to the ideal ratio of three after being water-assisted, resulting in a high PLQY value of over 95%. Our study provides that a one-pot method with a little water achieves high performance and elucidates the effect of water on the preparation process of MAPbBr₃ PeQDs.