The Annealing Effect at Different Temperatures for Organic-Inorganic Perovskite Quantum Dots

Abstract

After the preparation of inorganic perovskite cesium lead iodide quantum dots (CsPbI₃ QD) by ligand-assisted reprecipitation (LARP), CsPbI₃ QD was added to the organic perovskite methylamine lead triiodide (CH₃NH₃PbI₃; MAPbI₃) to successfully form composite perovskite film. To obtain better perovskite quantum dot (PQD) crystal characteristics, this research used different annealing temperatures to discuss the crystallinity changes of perovskite quantum dots (PQD). Through X-ray diffraction (XRD) analysis, it was found that the preferred peak (110) of MAPbI₃ had maximum peak intensity when the annealing temperature increased to 120 °C. Based on the measurement results of PQD’s Ultraviolet-Visible spectrum, it was shown that the maximum absorption area was obtained at the wavelength of 350 nm~750 nm at the annealing temperature 120 °C. From the scanning electron microscope (SEM) measurement, it was found that the grain size was the largest at the annealing temperature 120 °C, and the grain size was 60.2 nm. The best crystallization characteristics of PQD were obtained at the annealing temperature 120 °C.

1. Introduction

Lead halide perovskite materials have attracted much attention in the past few years [1,2,3] because of their unique properties, such as low cost, the application of luminescent materials and their application in the field of optoelectronics [3,4]. Among the perovskite materials, methylamine lead triiodide (CH₃NH₃PbI₃; MAPbI₃) is suitable for solar cell absorber layers due to its wider excitation spectrum, better absorption capacity and higher carrier mobility [5,6,7]. However, there are also some shortages for MAPbI₃. For example, MAPbI₃ cannot be stored in the air for a long time because of its hygroscopicity of methylammonium (MA) cations and poor thermal stability [8]. This result will affect the efficiency of organic perovskite solar cells. Because of the above deficiencies, some literature has reported that the moisture/thermal stability and the optical properties of perovskite MAPbI₃ films would be greatly improved by doping inorganic quantum dots [9,10,11]. For example, J.H Han et al. doped plumbous sulfide (PbS) quantum dots (QDs) into MAPbI₃ film to enhance the photoelectric conversion efficiency of perovskite solar cell [12]. The treatment of thermal annealing is often used to remove the residual precursor solvent [13]. Besides, A. Natik et al. found that the grain size and energy band gap change with the annealing time [14]. At present, the synthesis of QDs is based on various methods, including hot injection, ligand-assisted reprecipitation (LARP), ultrasonication and solvothermal synthesis. In our work, CsPbI₃, the inorganic perovskite quantum dot (PQD), was prepared by the LARP because its reaction product is a colloidal dispersion which is easy to expand by spin coating. Besides, the method is simple, with high processing rate and low temperature, so it is widely used for preparing quantum dots [15]. However, the annealing temperature and time are the key factors affecting the crystallization, performance and morphology of the PQD film [16]. By adding CsPbI₃ PQD into the MAPbI₃ solution and controlling the annealing temperature, it can enhance the stability of the perovskite structure. In this article, a detailed study for the formation ratio of organic-inorganic perovskites at the optimal temperature is also presented.

2. Materials and Methods

2.1. Materials

All materials contain lead (II) iodide (ACROS organic, PbI₂, 99.9985%), cesium iodide (Alfa Aesar, Lancashire, UK, CsI, 99.9%), oleyl amine (ACROS organic, Geel, Belgium, C₁₈H₃₅NH₂, 90%), toluene ((J.T. Baker, Phillipsburg, NJ, USA, 99.8%), hexane (DUKSAN PURE CHEMICALS, Ansan, Korea, 97%), methylammonium iodide (UniRegion Bio-Tech, Hsinchu, Taiwan, CH₃NH₃I, 99%), dimethyl sulfoxide (DMSO, J.T. Baker, 99.5%, (CH₃)₂SO, 99%), gamma-butyrolactone (GBL, CHONEYE PURE CHEMICALS, Taipei, Taiwan, C₄H₆O₂, 99.9%) and dimethylformamide (DMF, J.T. Baker, Phillipsburg, NJ, USA, C₃H₇NO, 99.5%).

2.2. CsPbI₃ QDs Fabrication

OAm (90%, 2.4 mL), CsI (99.9%, 0.4 mmol), PbI₂ (99.9985%, 0.4 mmol) and DMF (99%, 10 mL) were mixed and stirred continuously for 10 s to prepare a quantum dots precursor solution, as shown in Figure 1a. The quantum dots precursor solution (1 mL) was added into stirring toluene (99.8%, 20 mL) for 10 s to obtain a crude quantum dots solution, as shown in Figure 1b. The resulting crude solution was centrifuged at 11,000 rpm for 15 min at 10 °C. The precipitate was collected and then successively dispersed in hexane. The above process was repeated several times.

2.3. Synthesis of CH₃NH₃PbI₃

CH₃NH₃I (198.75 mg) and PbI₂ (576.25 mg) were mixed into the sample bottle; then DMSO (0.5 mL) and GBL (0.5 mL) were mixed into the mixture powder. The solution was stirred at 300 rpm for 24 h, as shown in Figure 1c.

2.4. Fabrication of Thin Films

CH₃NH₃I (50 µL) and CsPbI₃ (1 mg) were mixed and then spin-coated on a glass substrate in two steps, at 1000 rpm for 10 s and 5000 rpm for 20 s, respectively. The main purpose of 1000 rpm for 10 s was to make the film thickness uniform. By using high-speed rotation at 5000 rpm for 20 s, the purpose was to make the film thickness more uniform. Toluene was dropped on the spinning film at 15 s during the second step. Hereafter, the sample was heated at 80 °C for 15 min to form a film.

2.5. Rapid Thermal Annealing (RTA) Process

Finally, the annealing method of the samples was performed by rapid thermal annealing (RTA) with different temperatures (100–160 °C) in ambient air for 10 min.

2.6. Characteristic Measurements

The absorption spectrum of the thin films was measured by ultraviolet/visible (UV/vis) absorption spectroscopy (HITACHI, U-3900, Tokyo, Japan). X-ray diffraction (XRD) data of films were recorded by the Bruker D8 Discover X-ray diffractometer (Bruker AXS Gmbh, Karlsruhe, Germany) with grazing incidence X-ray diffraction (XRD). The top-view surface morphologies of the films were determined by field-emission scanning electron microscopy (FESEM, JEOL6330 Cryo, Peabody, MA, USA). The peak intensity photoluminescence (PL) of the thin film was measured by iHR320 (HORIBA, Kyoto, Japan).

3. Results

Figure 2a shows the transmission electron microscope (TEM) image of CsPbI₃ QDs. The size of CsPbI₃ QDs is 16 nm, as shown in Figure 2a, and the d-space of CsPbI₃ QDs is 0.30 nm, as shown in Figure 2b.

To analyze the PQD film morphology after annealing at different temperatures, scanning electron microscope (SEM) was used to measure each sample to obtain top-view images, and the results are shown in Figure 3a–e. Figure 3a shows the PQD film without annealing. Then, grain clusters were obviously discovered. It can be observed that the morphology of PQD film annealed at 100 °C was similar to the pristine PQD film, as shown in Figure 3b. Under this temperature, even though there was crystallization on the surface, the compactness of the film is poor. As the annealing temperature continuously increased to 120 °C, it can be observed that the compactness of the film increases and the grains are evenly distributed, as shown in Figure 3c. This was because the annealing treatment caused accumulation of the atoms in the crystal lattice, and the atoms can be rearranged and recrystallized owing to the disappearance of the vacancy. In Figure 3d, the grain size of the PQD film began to decrease and the vacancy between grains was more obvious when the annealing temperature increased to 140 °C. As the annealing temperature continuously increased to 160 °C, the change of the surface on PQD film was even more severe. In other words, PQD film was degraded if the annealing temperature was above 120 °C. Besides, the morphology of the acicular crystal was observed and the vacancy between the grains was larger because of the decrement of the grain size, as shown in Figure 3e.

In Figure 4, the grain content of the PQD film formed by MAPbI₃ and CsPbI₃ under different annealing temperatures (100–160 °C) was analyzed by XRD. It can be observed that the content of the main peak was similar between pure MAPbI₃ and PQD films without annealing, as shown in Figure 4a,b. The main peaks were, respectively, at 2θ = 14° and 2θ = 28° where the intensity of peak was weak. As the annealing temperature of PQD film increases to 100 °C, it can be observed that the intensity of both PQD (110) and MAPbI₃ (220) was increased. At the annealing temperature of 120 °C for PQD film, the intensity of both PQD (110) and MAPbI₃ (220) was increased even more. At the peak location of 12.7°, it was discovered that a small peak belonging to PbI₂ (001) was also increased gradually. When the annealing temperature of PQD film continuously increased to 140 °C, the intensity of PQD (110) and MAPbI₃ (220) was decreased because the PQD film began to degrade. With the annealing temperature of PQD film continuously increasing to 160 °C, both PQD (110) and MAPbI₃ (220) nearly disappear. Besides, the intensity of PbI₂ (001) was increased at annealing temperature above 120 °C. This is because PQD film was reduced into PbI₂ due to over temperature. According to the literature, a small amount of PbI₂ played a pivotal role in terms of the efficiency of the perovskite solar cells [17].

According to the literature, PQD (110) was primarily composed of CsPbI₃ (100) and MAPbI₃ (110) [18,19]. Therefore, peak fitting imitating software (Origin85) was used to estimate the change in area of CsPbI₃ (100) (2θ = 14.1°) and MAPbI₃ (110) (2θ = 14.15°) under different annealing temperatures. Figure 5a–c indicated that as the annealing temperature increased, the area of MAPbI₃ (110) gradually increased and that of CsPbI₃ (100) gradually decreased. The minimum full width at half maximum (FWHM) values of CsPbI₃ (100) and MAPbI₃ (110) were, respectively, 0.11° and 0.08° at annealing temperature 120 °C. This meant that there was a favorable condition to form PQD (110) at 120 °C. As the annealing temperature was up to 160 °C, it was discovered that the area of MAPbI₃ (110) was decreasing (FWHM = 0.09) and that the area of CsPbI₃ (100) was gradually increasing (FWHM = 0.14). The FWHM value of both materials was gradually increasing, as shown in Figure 5d,e.

Because the sunlight was resulting in generating more carriers, the grain size is larger and the efficiency of perovskite absorbing was higher [20]. Therefore, the grain size at different annealing temperatures was calculated from XRD results and Scherrer Equation (1) [21].

$$D=\frac{k\lambda }{\beta cos \theta }\left(\mathrm{Scherrer} \mathrm{equation}\right)$$

(1)

where k refers to the shape factor, the value of which was approximately 0.9. D refers to the grain size (nm). λ refers to the wavelength of the X-ray. β refers to the FWHM value of the diffraction peak. θ refers to the diffraction angle. Calculation results indicated that the grain size of PQD film was sequentially (1) 33.7 nm, (2) 54.6 nm, (3) 60.2 nm, (4) 55.6 nm and (5) 54.9 nm for (1) the untreated film, (2) under the annealing temperature of 100 °C, (3) under that of 120 °C, (4) under that of 140 °C and (5) under that of 160 °C. Then, when the annealing temperature was at 120 °C, the maximum average grain size was 60.2 nm, the phenomenon of which was consistent with the result, as shown in Figure 5c.

Figure 6 shows the area ratio of CsPbI₃ (100) and MAPbI₃ (110) under different annealing temperatures. It was revealed that the area ratio of CsPbI₃ (100) and MAPbI₃ (110) was approximately 1:9 at the annealing temperature from 120 °C to 140 °C. Besides, the area ratio of CsPbI₃ (100) was decreased with the annealing temperature until 140 °C. Then, most of MAPbI₃ (110) were degraded into PbI₂ at the annealing temperature of 160 °C. Furthermore, the degradation of MAPbI₃ was more susceptible to temperature than that of CsPbI₃, so the ratio of CsPbI₃ will be higher than that of MAPbI₃ at annealing temperature of 160 °C. The maximum absorbance and the condenser film can be obtained by comparison with optical properties and SEM analysis of PQD film at annealing temperature of 120 °C. Thus, this ratio is favorable for the PQD film.

According to our previous research, the doping of a small amount of CsPbI₃ QDs can obtain better thermal stability to reduce the density of surface trap states [22], with the result that perovskite grain size continuously increases and arranges closer. The phenomenon was consistent with the result, as shown in Figure 3c.

Figure 7a shows the absorption spectrum of the PQD film without annealing and at the annealing temperature of 100–160 °C. By annealing the PQD film, it was revealed that the absorbance from the whole spectrum range was enhanced, and the maximum absorption area occurred at 120 °C. When the annealing temperature was further increased to 160 °C, absorbance begins to decrease.

Then all the above samples were stored in ambient air for 168 h to test stability of the PQD films, as shown in Figure 7b. It can be found that the PQD films still exhibit typical perovskite absorption spectrum after 168 h, indicating that doping QD into perovskite can effectively resist the change of moisture and oxygen. However, at annealing temperature of 160 °C, the appearance of PbI₂ peak in absorption spectrum indicated that the surface of the PQD film was destroyed, with the result that the stability of PQD film became poor and part of the PQD film was degraded to PbI₂. The phenomenon is consistent with the above results.

The image of the films annealed at temperature of 100–160 °C is shown as Figure 7c. Morphology of thin film for all samples presented the perovskite black phase (named α phase), but partial area of surface revealed yellow δ phase at annealing temperature of 160 °C, as shown in the insert image of Figure 7c.

The PL of the film at different annealing temperatures under consideration is shown in Figure 8a. It is found that the peak intensity of PL increases with temperature, but it drops after 120 °C, and it is shown that the best crystallinity of PQD will be obtained when the annealing temperature reaches 120 °C.

Figure 8b shows PL measurement for pristine PQD with annealing temperature of 120 °C (black line), which was stored in ambient air for 168 h (red line). The difference of PL intensity between black line and red line is about 15%, i.e., less variation. Therefore, the stability of PQD can be enhanced by annealing treatment.

4. Conclusions

In our result, organic-inorganic doped quantum dot films were prepared to understand the reaction of perovskite films at different annealing temperatures. From the SEM images, it was found that thermal annealing treatment can cause the rearrangement of atoms in the perovskite, resulting in reduction of defects and obtaining a compactness film at 120 °C. In the XRD pattern, it was revealed that the perovskite film will obviously degrade into PbI₂ when the annealing temperature reaches 160 °C. This will affect the absorbance of the film. Through peak fitting imitating software to further understand the ratio of MAPbI₃ to CsPbI₃ at different temperatures, it was shown that a small amount of CsPbI₃ can cause grain arrangement growth at 120 °C and the grain size was 60.2 nm.