Synthetic method of pure-red emissive CsPbI3 under ambient conditions for quantum-dot light-emitting diode application

Perovskite quantum dots (PeQDs) are expected to be used in ultra-high-definition television (UHDTV) due to their excellent optical properties. However, pure-red emissive (λ EL: 620–650 nm) PeQD-based LEDs (PeLEDs) are required for UHDTV with single-halogen PeQDs synthesized under ambient conditions, which is important for practical use. Hence, we established a novel synthetic method to prepare PeQDs under ambient conditions via a different reaction from conventional methods. We successfully prepared PeQDs with pure-red emissivity λ PL: 631 nm and near-unity photoluminescence quantum yield. The PeLEDs with the PeQDs showed pure-red emission of 647 nm.

][3][4][5] Therefore, they have attracted attention as next-generation light-emitting materials and are expected to be used in quantum-dot LEDs (PeLEDs). 6)To apply red emissive PeLEDs to ultra-high-definition television (UHDTV), they should satisfy Rec.2020, which defines the required color gamut for UHDTV as recommendation by the International Telecommunication Union.To fulfill this color gamut, pure-red emissive PeLEDs with an electroluminescence emission peak (λ EL ) of approximately 620-650 nm are suitable for Rec.2020. 7)n general, pure-red emissive PeLEDs are achieved by halogen-mixed PeQDs. 8)However, these PeQDs have two inherent issues: the EL spectrum changes due to ion migration owing to electronegativity differences between halide ions when the QLED is driven, 9) and the PLQY in the purered range is very low. 10)o solve these issues, it is essential to achieve pure-red PeQDs with only iodine halide ions such as CsPbI 3 or MAPbI 3 . 11,12)Comparing these pure-red PeQDs, MAPbI 3 are known to have relatively poor thermal stability due to organic components 13) but CsPbI 3 are ideally composed for these applications.CsPbI 3 is usually synthesized by the hot injection method 14) or the ligand-assisted reprecipitation (LARP) method. 15)Hot injection is impractical in this case because it requires high temperatures and an inert gas atmosphere, but LARP is suitable for practical applications. 16)In LARP, all precursors are dissolved in a high-polarity good solvent, and PeQDs are precipitated by mixing the solution with a low-polarity poor solvent, allowing them to be synthesized under ambient conditions, i.e., RT and atmospheric air.However, it has not been reported that single-halogen PeQDs synthesized by LARP have been applied to pure-red PeLEDs.
There are two reasons why pure-red PeLEDs could not be fabricated utilizing PeQDs prepared by LARP.First, high-polarity solvents, such as dimethyl sulfoxide and N,N-dimethylformamide, are used in the LARP collapsed crystal structure of PeQDs. 17)In LARP, lead (II) iodide (PbI 2 ) is used primarily as a source of Pb and halogen to synthesize red emissive PeQDs.The polar solvents replace iodine of PbI 2 in the precursor solution and form complexes with Pb.Therefore, in the precipitation process, the Pb-polar solvent complex and A site precursors coprecipitate to synthesize red emissive PeQDs.Due to the strong bonding between Pb and polar solvents, crystallized PeQDs have unstable structures that contain residual solvents in their structure.Eventually, phase transition of the crystal from luminescent to nonluminescent structures would be induced.The second reason is the decrease in PLQY due to halogen defects on the surface caused by suppression of crystal growth. 18)To reproduce purered, single-halogen PeQDs, it is essential to utilize the quantum size effect due to the suppression of crystal growth.However, PeQDs that have small particle sizes easily form halogen defects on their surfaces.Therefore, PLQY is decreased due to halogen defects.
Herein, we propose a strategy for the synthesis of highperformance pure-red emissive CsPbI 3 PeQDs via the synthesis method under ambient conditions without high-polarity solvents.In the LARP, precursors of PeQDs were dissolved by ionizing them in polar solvents.In contrast, in this study, the precursors were dissolved by complex, low-polarity solvent with a carboxylic acid or amine as ligands.This complex formation allows the precursor to dissolve in lowpolarity solvents without the use of high-polarity solvents.By mixing the solutions, PeQDs were successfully prepared under ambient conditions through ionic metathesis reactions, which are an exchange reaction of ions between complexes based on the HSAB principle. 19)Furthermore, the addition of octylammonium iodide (OAmI) provided an amine-type ligand and iodide ions to passivate crystal defects on PeQDs and improve their PLQY.In the previous methods, dodecyl benzene sulfonic acid was used as ligands to improve structural stability, 20) but it binds too strongly to Pb on the PeQD surface to get pure red PeQDs. 21)It seems that this phenomenon leads to Ostwald ripening due to Pb desorption, and small particles which exhibit quantum confinement effect could not be prepared. 22)In contrast, we have achieved pure-red emission and high stability without strong-binding ligands by combination of long-and shortchain ligands, such as oleylamine and octylamine, originating from OAmI to improve dispersibility. 23)In addition, treatment of the PeQD surface with short-chain ligands can improve the external quantum efficiency (EQE) of PeLEDs owing to their low electric insulation. 24)As a result, the synthesized PeQDs achieved 100% PLQY despite their emission wavelength in the pure-red region of 631 nm due to the quantum confinement effect.In addition, we successfully fabricated PeLEDs with the PeQDs.To the best of our knowledge, this is the only report of pure-red electroluminescence (λ EL = 647 nm) from PeQDs obtained by a RT synthesis method.
In this method, we selected cesium carbonate (Cs 2 CO 3 ), PbI 2 , and OAmI as precursors of CsPbI 3 .Two types of precursor solutions were prepared by dissolving them in a low-polarity solvent via formation of complexes with acid or amine ligands.PeQDs were prepared by mixing these precursor solutions.The details of the synthesis are shown in [Fig.1].
The concentration of iodide ions in the precursor solution has significant influence because halogen ions affect the optical properties of PeQDs and can suppress the formation of crystal defects that cause non-radiative recombination.Then, the optical properties of PeQDs with different molar ratios of iodine to Pb were evaluated by PL and UV-visible absorption (UV-vis) measurements.The supply of iodine was controlled by adjusting the amount of OAmI in a precursor solution.From the PL and UV-vis spectra, peaks were not observed in the ratios of 1:2.0 and 1:2.5, indicating that the iodide ion concentration in the system was too low and the crystal structure of PeQDs was not formed or bulk PeQDs formed.The dispersion is known to be Cs 4 PbI 6 and  .This emission wavelength range is similar to CsPbI 3 in a previous work, which suggests the contribution of quantum confinement effects. 25)A sufficiently high concentration of iodide ion allows the formation of PeQDs and further compensates for surface defects.As a result, the higher the Pb to iodine ratio, the higher the PLQY, which reaches nearly 100% of its PLQY when the ratio is 1:3.0 and 1:3.5 [Table I].
The PLQY obtained near unity indicates fewer defects in the PeQDs.Then, in order to evaluate this, composition ratios of the PeQDs were obtained from X-ray photoelectron spectroscopy (XPS).From the XPS measurements, the chemical compositions of Pb and iodine in the PeQDs at the ratios 1:3.0 and 1:3.5 were 1:2.7 and 1:3.4,respectively [Table II, Fig. S2].Frequently, under halogen-rich conditions, the ratio of halogen to Pb in XPS measurements exceeded 3.0. 26)This result indicates that the PeQDs at the ratio 1:3.5 passivate defects more adequately at 1:3.0.This result revealed that the iodine defects on the PeQD surface are well passivated, which contributes to the high PLQY.
To consider optimal synthesis conditions, the crystal morphology and structure of the PeQDs at the ratios 1:3.0 and 1:3.5 were evaluated by Transmission Electron Microscope (TEM) and X-ray diffraction (XRD).From 200 particles observed by TEM, we determined average particle size of the PeQDs and their size distribution in each condition.The PeQDs at the ratio 1:3.0 have a slightly heterogeneous particle size distribution and an average particle size of 11.Experimentally, we chose ethyl acetate (EtOAc) as a solvent for synthesis of the PeQDs because it is a widely used solvent for the synthesis and purification of PeQDs and exhibits high yields of PeQDs. 27,28)It can be considered that the solvent caused desorption of the amine ligands from PeQD surface in the preparation system, resulting in variations in crystal growth. 29)n contrast, the PeQDs at the ratio 1:3.5 has a homogeneous particle size distribution and average particle size of 8.1 ± 1.1 nm [Figs.3(c), 3(d)].This difference in the diameter was attributed to the higher supply of amine ligands by OAmI, which was less affected by the desorption of the ligands by EtOAc.
From the XRD as shown in [Fig.4], the PeQDs at ratios 1:3.0 and 1:3.5 showed diffraction patterns (100) and (200) planes attributed to the CsPbI 3 perovskite structure.Based on the TEM and XRD results, we successfully prepared PeQDs with a homogeneous particle size distribution using a novel synthesis method under ambient conditions.The relationship between particle size and emission wavelength of PeQDs due to the quantum confinement effects has often been reported. 30)In comparison, the PeQDs obtained in this study have shorter wavelengths than the particle size. 31)onsidering the aforementioned high PLQY, as well as XPS and XRD results, this might be due to the phase transition of a part of the CsPbI 3 surface to Cs 4 PbI 6 .Further investigation is required for a detailed structural analysis and discussion.In this work, the formation of purered emissive CsPbI 3 is clearly revealed, and the synthesis method proposed in this study is effective and versatile comparable to the hot injection method.
In view of practical applications, the stability of the PeQDs is a very important aspect.Thus, these PeQDs were evaluated for optical properties and structural stability for 7 days under ambient conditions.From the PL measurement, the PeQDs showed almost no change in the PL spectra, and PL intensity and PLQY were maintained at more than 97% of the initial values for 7 days [Fig.S3].In addition, XRD measurements show almost no change in the diffraction pattern attributed to PeQDs for 7 days, revealing a high structural stability [Fig.S4].This high structural stability was also attributed to fewer defects, as discussed previously based on the XPS results.5].Although this result does not exceed the top record of 25.8%, 32) it is worth mentioning that this is the highest value for pure-red LEDs fabricated utilizing RT synthesized CsPbI 3 .From these results, we report successful fabrication of pure-red emissive PeLEDs using ambientcondition synthesized CsPbI 3 for the first time.
In conclusion, PeQDs were successfully prepared by ionic metathesis reaction under ambient conditions.OAmI was used as a defect compensator to suppress crystal growth and passivate for iodine defects on the PeQD surface.As a result, the PeQDs showed pure-red emissivity and PLQY≒100%.We also reported successful fabrication of pure-red emissive PeLEDs using PeQDs prepared under ambient conditions for the first time.This synthesis method is promising as one of the strategies to solve issues with the LARP method and enable the fabrication of pure-red PeLEDs suitable for Rec.2020.025002-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .
Fig. 1.Schematic illustration of the synthetic method in this study.All processes were carried out under ambient atmosphere and at RT.

Fig. 3 .
Fig. 3. TEM images and particle size distributions of the PeQDs synthesized under the ratios of Pb: I = 1:3.0(a), (b) and 1:3.5 (c), (d).The distributions were calculated by counting more than 200 particles from TEM images.
Finally, we built PeLED devices with CsPbI 3 as emissive layer and evaluated their performance.PeLEDs (structure: ITO/PEDOT:PSS/CsPbI 3 PeQDs/TPBi/Liq/Al) were fabricated, and additional information such as an energy diagram and detailed evaluation results are shown in [Fig.S5].The PeLEDs exhibited a single EL peak at 647 nm and 7.16% of EQE [Fig.

Table I .
Summary of optical properties of the PeQDs.

Table II .
Summary of compositions and PLQY of the PeQDs.