Effect of Reaction Time on Optical Properties of CsPbBr₃ Perovskite Nanocrystals

Abstract

1. Introduction

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have attracted significant attention because they demonstrate higher tolerance compared to organic-inorganic perovskites such as CH3NH3PbX3 NCs in applications such as light-emitting diodes and solar cells [1, 2]. CH3NH3PbX3 perovskites are also strongly associated with autodegradation because of their intrinsic instabilities [3]. So it is necessary to realize an efficient strategy for solving these problems. The thermal durability of organic-inorganic perovskites is regarded as one of the most critical issues for solar cell performance [4]. Perovskite materials are vulnerable to damage during thermal annealing, and heat dissipation is known to have an influence on perovskites, which results in thermal-induced mechanical stress and structural evolution within the NCs [5]. In organic-inorganic hybrid perovskites, the occurrence of a structural phase transition is correlated with its own characteristics and the contents of ambient air, temperature, and pressure, which have an effect on the durability of perovskite solar cells [6]. Replacement of the methylammonium (MA) cation with the less active cation of Cs can allow CsPbBr₃ NCs to exhibit superior thermal stability, chemical durability, and striking optical properties [7, 8]. The purpose of substituting MA cation is to form a more stable cubic phase, along with the appropriate modulation of the conduction band of the perovskite, which is the key feature to achieve stability [9]. Therefore, tailoring the optical properties of CsPbBr₃ NCs to make them more suitable for application is a promising approach [10]. CsPbBr₃ NCs can be fabricated employing a method in which oleylamine (OAm) and oleic acid (OA) are used as capping agents. The carboxylate and amine functional groups of OA and OAm are attached to the surface of the NCs during the reaction, resulting in the stability of the NCs. These long-chain hydrocarbon ligands promote the dispersion of CsPbBr₃ NCs in various organic media [11]. In particular, to fulfill the requirements for optoelectronic devices, the optical properties, growth stage, morphology, and structure of CsPbBr₃ NCs should be regulated during reproducible preparation, and various challenges in the synthetic strategy should be addressed [12].

To obtain more insight into this process, we here report the optical properties of CsPbBr₃ perovskite NCs fabricated with various reaction times. The UV-vis absorption and fluorescence (FL) measurements were conducted with a focus on the deterioration of the optical properties of the CsPbBr₃ NCs. Significant changes were also observed in the photoluminescence quantum yield (PLQY) of the CsPbBr₃ NCs with various reaction times. In addition, the single-crystalline cubic-phase CsPbBr₃ NCs and their size distributions were determined via field-emission transmission electron microscopy (FETEM).

2. Experimental details

For the preparation of Csoleate, Cs₂CO₃(0.391 g, 1.2 mmol), OA (1.27 mL, 4 mmol), and 1-octadecene (18.73 mL) were loaded into a 100 mL three-neck flask and vacuumed for 1 h at 120 °C. Then, the mixture was reacted at 160 °C under flowing N2 gas until the precur-sor solution was completely dissolved. Because Cs-oleate precipitates out of 1-octadecene at room temperature, it needs to be preheated to 100 °C before use. Next, for the synthesis of CsPbBr₃ NCs with vari-ous reaction times, PbBr₂ (0.149 g, 4 mmol) and 1-octadecene (18.73 mL) were placed in a 100 mL three-neck flask and vacuumed for 1 h at 120 °C, after which OA (1 mL) and OAm (3 mL) were injected at 120 °C. After PbBr₂ was completely solubilized, the mixture was heated up to 170 °C, and 2 mL of the Cs-oleate stock solution was injected swiftly into the reaction system. Subsequently, the sample was ob-tained after a set reaction time (5 s, 1, 5, 10, 20, 30, and 60 min). Then, the reaction mixture was cooled down with an ice-water bath. The resulting mixture was precipitated with tert-butanol by centrifugation at 7,000 rpm for 10 min. After centrifugation, the supernatant was eliminated, and the resulting precipitate was obtained. The resulting precipitate was re-dispersed in toluene for UV-vis and TEM measure-ments. UV-vis absorption and FL measurements of the CsPbBr₃ NCs were performed using a FLAME-S spectrophotometer (Ocean Optics Inc., Largo, FL, USA) at room temperature. The crystalline structure, morphology and size distributions of the CsPbBr₃ NCs were investi-gated by using a FETEM (Tecnai G², F30 S-Twin, 300 keV, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and discussion

The FL and absorption spectra of CsPbBr₃ NCs with various reac-tion times are shown in Fig. 1. The crystallite growth of CsPbBr₃ NCs in solution primarily depends on the LaMer mechanism [13]. The growth stage of the CsPbBr₃ NCs can be divided into three distinct phases, i.e., the nucleation, growth, and regrowth processes. The hot-injection in wet chemical route for colloidal synthesis of the CsPbBr₃ NCs has benefits with respect to controlling particle size and size dis-tribution [14]. At 5 s of reaction time, the CsPbBr₃ NCs showed a sharp absorption and a narrow emission peak. The full-width at half-maximum for the CsPbBr₃ NCs was 20.2, 18.6, 19.3, 18.8, 18.7, 19.9, and 20.3 nm for reaction times of 5 s, 1, 5, 10, 20, 30, and 60 min, respectively. As the reaction time increased, the FL peaks gradually decreased because of the loss of the quantum size effect induced by abnormal growth, as shown in Fig. 1(a) [15]. When the reaction time increased up to 5 min, the absorption peaks of the CsPbBr₃ NCs re-mained unchanged, as depicted in Fig. 1(b). After 5 min of reaction time, the absorption peaks for the CsPbBr₃ NCs became broader. The intensity of the absorption spectra gradually decreased with increas-ing reaction time, suggesting a decrease in the light absorption capac-ity [16].

The variations of the FL peak position and intensity of the CsPbBr₃ NCs as a function of reaction time are displayed in Fig. 2. The FL peaks of as-obtained CsPbBr₃ perovskite NCs produced at reaction times of 5 s, 1, 5 , 10, 20, 30, and 60 min are located at 516.8, 518.2, 517.5, 517.9, 517.9, 519.3, and 526.8 nm, respectively. The redshift in the FL peaks of the CsPbBr₃ NCs with increasing reaction time is due to the ripening of CsPbBr₃ NCs [12,17]. The FL intensities decreased steadily, and the FL peaks nearly disappeared with increasing reaction time, as shown in Fig. 2, which can be attributed to the increased size of the CsPbBr₃ crystal. The CsPbBr₃ NCs were diluted with toluene solutions to give the same band edge optical density (0.05 at the respective band edge absorption peaks). The FL intensity of the CsPbBr₃ NC sample with a reaction time of 60 min decreased by nearly 20 times compared with that of the CsPbBr₃ NC sample with a reaction time of 5 s. These results are related to the induced charge trap states resulting from the surface decomposition reaction or the elimination of surface ligands [3].

Figure 3(a) shows the photoluminescence quantum yields (PLQYs) of the CsPbBr₃ NCs synthesized at a set reaction temperature (170 °C) and varying reaction times (5 s – 60 min). The relative PLQY of the CsPbBr₃ NCs were obtained by comparing the integrated emis-sion of the CsPbBr₃ NC samples with a standard fluorescence dye (Coumarin 500 in ethanol, QY = 47%) [18]. The absorbance of the CsPbBr₃ NC samples was kept at less than 0.1 at 390 nm to prevent re-absorption [19]. The CsPbBr₃ NCs exhibited relative PLQYs (20–35%). The highest PLQY (35%) was attained for CsPbBr₃ NCs pre-pared at a reaction time of 5 s, and the overall PLQY decreased as the reaction time increased because of the change in size and mor-phology of CsPbBr₃ NCs [20]. Figures 3(b) and 3(c) show the digital photographs of the as-obtained CsPbBr₃ NCs prepared at various re-action times under white light and single 365 nm UV light source. PL quenching of the CsPbBr₃ NCs can be also observed with the naked eye under a UV light, as seen in Fig. 3(b). As the reaction time in-creased, the solution containing CsPbBr₃ NCs, with a yellow precip-itate and colorless transparent supernatant, turned dark and became turbid. The initial light green color of CsPbBr₃ NCs turned to yellow phases with increasing reaction time, suggesting extensive precipita-tion of the CsPbBr₃ NCs prepared at high temperature as shown Fig. 3(c). The yellow phases are caused by large CsPbBr₃ crystal growth, and this phenomenon revealed a broader size distribution [3, 16].

Figures 4(a) and 4(b) represent the low-magnification bright-field TEM image and selected-area diffraction (SAED) pattern of CsPbBr₃ NCs on the specific high temperature (≥130 °C) and reaction time (5 s). As shown in Fig. 4(a), almost all of the square-shaped CsPbBr₃ NCs uniformly formed at 170 °C, which is well-matched with already reported morphological characteristics of conventional colloidal CsPbBr₃ NCs [16,21]. The SAED ring pattern in Fig. 4(b) indicates that the lat-tice spacing of 0.41, 0.29, and 0.21 nm corresponds to the (110), (200), and (220) lattice planes, which agree well with the interplanar spacings of cubic-phase CsPbBr₃, respectively [22]. The nanostructural characteristics of the CsPbBr₃ NCs are also confirmed via the high-resolution TEM image depicted in Fig. 4(c) and fast Fourier transform pattern obtained from [001] beam direction (Z, zone axis). The lattice spacing of 0.41 and 0.58 nm corresponds to the (110) and (100) interplanar spacings of cubic-phase CsPbBr₃, respectively [22, 23]. The number of CsPbBr₃ NCs and size distributions is plotted with Gaussian distributions, as shown in Fig. 4(d), and the average size of the CsPbBr₃ NCs is 13.4 nm. The histogram of the particle size distribution for the CsPbBr₃ NCs was obtained by using ImageJ software [17, 24].

4. Conclusions

We investigated the influence of reaction time on the structural and optical properties of assynthesized CsPbBr₃ perovskite NCs. The reaction conditions of CsPbBr₃ NCs can significantly affect the structure and crystal form of the NCs. The redshift of the FL wavelength and resultant decrease in the total absorption intensity with increasing reaction time was attributed to large CsPbBr₃ crystal growth by the ripening process. With increasing reaction time, the FL intensities and PLQYs gradually decreased because of the loss of quantum confinement and formation of charge trap states. TEM observations demonstrated that the CsPbBr₃ NCs with an initial reaction time of 5 s were well-crystallized in a cubic lattice. Our observations provide clear evidence that the CsPbBr₃ perovskite NCs are significantly affected by the reaction conditions, which can lead to significant changes in the particle morphology and associated optical properties. The results of this study suggest the importance of investigation of the optical properties in CsPbBr₃ NCs with various synthetic factors affecting the reliability and durability of optoelectronic devices.

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