Synthesis and Characterization of Multiple-Cation Rb(MAFA)PbI3 Perovskite Single Crystals

We synthesized multiple-cation Rb(MAFA)PbI3 perovskite single crystals for the first time. The effect of Rb+ substitution was systemically investigated, and the addition of 1.5 M 5% RbI was the optimum condition to obtain high-quality Rb(MAFA)PbI3 single crystals. Lattice shrinkage occurred in the Rb(MAFA)PbI3 single crystal because of the small ionic radius of Rb+, resulting in blue-shifted absorption and photoluminescence (PL) peaks. The 1.5 M 5% RbI-added (MAFA)PbI3 single crystal showed the longest carrier lifetime of 18.35 ns, exhibiting the highest photoresponse than other crystals. We believe that this work will provide a basic insight into the mixed-cation perovskite single crystals for the future optoelectronic applications.

Hybrid organic-inorganic perovskites with an ABX 3 structure (where A is a monovalent cation such as methylammonium (CH 3 NH 3 + ; MA) and formamidinium (CH 3 (NH 2 ) 2+ ; FA); B and X refer to divalent cations (Pb 2+ , Sn 2+ ) and halogen anions (I − , Br − , Cl − ), respectively) have been considered as promising optoelectronic materials because of their excellent optical and electrical properties [1][2][3][4][5] . However, these perovskites suffer from low stability in air because of the inherent instability of their organic cations 6,7 , which limits their practical applications. To overcome this limitation, compositional engineering of these perovskites has been intensively studied 6,[8][9][10] . For example, Zhang et al. obtained better crystal quality and enhanced device performance by adding 10% FA + to MAPbI 3 solar cells 11 . Wang et al. also obtained similar results when they mixed MA + and FA + cations in a ratio of 7:3 9 . Recently, Saliba et al. added Cs cations to MAFA perovskites and achieved great solar cell performance with improved thermal stability 12 . Zhang et al. also investigated the effect of incorporating Cs + in perovskites and demonstrated that triple-cation solar cells exhibit better performance than do single-cation solar cells in terms of thermal and humidity stabilities 13 . More recently, Rb has been introduced as the fourth candidate for multiple-cation perovskites 14,15 . Interestingly, based on the Goldschmidt tolerance factor, perovskite structures cannot be formed with Rb + because of its small ionic radius (152 pm) 14 . However, with the incorporation of small amounts Rb + , high photovoltaic efficiency and low hysteresis can be achieved because Rb + can significantly suppress the yellow phase in perovskite films 14,[16][17][18][19] . Despite these rapid improvements in device performance, the use of polycrystalline perovskites is still limited because of their grain boundary issues. Shao et al. reported that grain boundaries acts as ion migration channels, resulting in current hysteresis in perovskite optoelectronic devices 20 . Moreover, Wang et al. studied the morphology-dependent degradation of polycrystalline perovskite films and found that the grain boundaries in perovskites accelerate their degradation process because of the diffusion of moisture through them 21 . This limitation can be overcome using perovskite single crystals, which offer the following advantages: the absence of grain boundaries, low trap density, and excellent air stability 2,22,23 . However, the effect of mixed cations on single crystals remains nearly unexplored. To the best of our knowledge, there has been no report on triple-cation perovskite single crystals, especially those with Rb + incorporation.
In this study, we synthesized and characterized multiple-cation Rb(MAFA)PbI 3 perovskite single crystals for the first time. X-ray diffraction (XRD) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analyses confirmed the incorporation of Rb + ions into the perovskite single crystal. We found that an Rb concentration of 1.5 M (5%) was optimum to obtain Rb(MAFA)PbI 3 single crystals with high Rb ratios. The effect of the addition of Rb + on the perovskite single crystal was systemically studied via UV-Vis absorption and photoluminescence (PL) spectroscopy. Finally, based on the time-resolved PL (TRPL) and photocurrent data, we proved that the addition of Rb + increased the carrier lifetime and photoresponse of the perovskite single crystal. We believe that this work will provide a basic insight into the structure of mixed-cation perovskite single crystals for future applications.

Results and Discussion
The Rb(MAFA)PbI 3 perovskite single crystals were grown using an inverse temperature crystallization (ITC) method 24 . The structure of Rb(MAFA)PbI 3 and the detailed growth process are illustrated in Fig. 1a. (MAFA)PbI 3 has a cubic structure at room temperature 23,25 , and smaller Rb + ions randomly replace the organic cations 26 , MA + , and FA + , reducing its cubic volume. Generally, the solubility of common precursor salts increases with an increase in temperature. Perovskite salts, on the other hand, show reduced solubilities in solvents such as γ-butyrolactone (GBL) at elevated temperatures 24 . Hence, we synthesized Rb(MAFA)PbI 3 single crystals with different Rb ratios using a GBL-based ITC method. The filtrates of 1 M MAPbI 3 and FAPbI 3 solutions were mixed in a 1:1 ratio. To this mixture, 1 and 1.5 M RbI solutions (5, 10, and 15%) were added (see Supplementary Information for details).
After storing the mixed solution in a convection oven at 130 °C for 6 h, the dodecahedral Rb(MAFA)PbI 3 single crystals were grown, as shown in Fig. 1b. The shape of single crystal highly depends on the growth solution; both dodecahedral-and cubic-shaped single crystal of the same composition can be grown by varying the growth solution condition 27 . The size of the crystals depended on the RbI content. The average crystal size was found to be 3-5 mm. We further proved the absence of grain boundaries in the Rb(MAFA)PbI 3 single crystals via scanning electron microscopy (SEM) ( Figure S1). As mentioned earlier, the absence of grain boundaries is one of the most important advantages of single crystalline perovskites over polycrystalline perovskites. In order to investigate the effect of the Rb ratio on the structure of the Rb(MAFA)PbI 3 single crystals, we carried out a powder XRD analysis on them. The diffraction peak at 13.9° in Fig. 2a corresponds to the (001) plane of the cubic perovskite structure 25 , which means that the (MAFA)PbI 3 single crystal had a cubic phase before the addition of Rb + . We analyzed the changes in the XRD patterns of the single crystal as a function of the RbI concentration. However, no additional peak was observed after the addition of Rb + . This indicates that the Rb(MAFA)PbI 3 single crystal retained the original cubic phase of (MAFA)PbI 3 even after the addition of Rb + . For more details, we zoomed the (001) plane in Fig. 2b, and it was found that the diffraction peak shifted to higher angles depending on the Rb ratio. We fitted the diffraction peak at 13.9° to Lorentzian function, and the average FWHM value of Rb(MAFA)PbI 3 perovskite crystals was about 0.11°, which is sharper than that of the perovskite film 26 . Generally, FWHM of XRD profiles gives information about crystal quality of materials 28 ; thus, we assumed that the single crystalline perovskites showed sharper FWHM than that of polycrystalline perovskites as a result of no grain boundaries. Further, we compared the FWHM values with the Rb + addition, and the 1.5 M 5% RbI-added single crystal showed slightly broader FWHM of 0.13°. Considering the small size of Rb + , the broader FWHM can also be understood by the effect of crystal distortion in Rb(MAFA)PbI 3 single crystals. We plotted the shifted peak values as a function of the RbI content (Fig. 2c) and observed a significant peak shift when 5% RbI was added (blue arrow). Recently, Shi et al. have reported similar XRD peak shifts for Rb + -incorporated FAPbBr 3 films. They reported that the diffraction peak shifted toward wider angles because of the Rb + -induced crystal distortion due to the small ionic radius (152 pm) of Rb + (compared to that of FA + (253 pm)) 29 . In other words, the diffraction peak shift increased with an increase in the Rb + content. Thus, by monitoring the shifted peak values in Fig. 2c, we assumed that the addition of 5% RbI was optimum. Further, we compared the other diffraction peaks and observed the remaining PbI 2 peak at 25.9° from the 1 M 5% RbI-added crystal, as shown in Fig. 2d. Although the addition of 5% RbI was optimum the addition of 1.5 M RbI resulted in better crystal quality than that obtained when 1 M RbI was added. Hence, it can be stated that the addition of 1.5 M 5% RbI was optimum to synthesize high-quality Rb(MAFA)PbI 3 single crystals with a high Rb + concentration.
To demonstrate the Rb + substitution, we carried out the ToF-SIMS analysis of the single crystals with various RbI concentrations. We selected I − (126.9) and Pb + (207.9) for comparison and monitored the emitted Rb + (84.9) signals from the Rb(MAFA)PbI 3 single crystals as a function of the RbI content. Figure 3a shows the ToF-SIMS depth profiles of the secondary ions emitted from the Rb(MAFA)PbI 3 single crystals. As shown in Fig. 3a, the intensity of the Rb + signal changed significantly with an increase in the Rb ratio. We also plotted the changes in the intensity of the emitted I − , Pb + , and Rb + ions as a function of the RbI content at a sputtering depth of 300 nm (Fig. 3b). We found that the intensity of both I − and Pb + did not change; however, that of Rb + increased gradually with an increase in the RbI content. In particular, the 1.5 M 5% RbI-added single crystal showed the largest number of Rb cations. However, at higher RbI contents of 10% or 15%, the Rb cation substitution ratio decreased significantly (Fig. S2). This indicates that the addition of 1.5 M 5% RbI is the optimum condition to obtain Rb(MAFA)PbI 3 single crystals with high Rb + content. This is consistent with the XRD results shown in Fig. 2.
We further investigated the effect of the RbI content on the optical properties of the Rb(MAFA)PbI 3 single crystals. Linear absorption and PL spectra of the single crystals with various Rb contents are shown in Fig. 4a,b. Here, we have not shown the data for 15% RbI addition because of the small size and low yield of the corresponding single crystal (Fig. S3). Basically, in the absence of Rb, the (MAFA)PbI 3 perovskite single crystal showed an optical band edge and a PL peak at around 870 and 800 nm, respectively. The above-band gap PL characteristics of perovskite single crystals have been reported 2,30 ; however, the origin of this effect is still under debate. After the addition of Rb + into the (MAFA)PbI 3 single crystal, the absorption and PL peaks slightly blue-shifted by about 5.17 and 6.29 nm, respectively. According to previous reports, metal-halide-metal bonds, which are related to the electronic band structure of perovskites, are indirectly affected by the lattice shrinkage due to Rb + substitution, leading to a down-shifted valence band maximum 29 . As a result, the Rb(MAFA)PbI 3 perovskite single crystals showed blue-shifted absorption and PL peaks. The expanded absorption spectrum to the near-infrared region is needed to be addressed because the solar cell efficiency would be improved owing to a significant amount of solar energy in this wavelength range. The average thickness of Rb(MAFA)PbI 3 crystals was 1.48 mm, and the optical band edges were located at approximately 865 nm for all crystals.
The TRPL measurements of the single crystals with various Rb ratios were also carried out. Figure 4c shows the normalized PL decay curves of the Rb(MAFA)PbI 3 single crystals with various Rb ratios. We extracted two  (Table 1). In contrast, the 5% RbI-added single crystal showed a longer carrier lifetime than the other crystals. The 1.5 M 5% RbI-added (MAFA)PbI 3 single crystal showed the longest carrier lifetime of 18.35 ns. We expect that Rb + -based perovskite single crystals have low trap-state densities and long carrier diffusion lengths through the remarkably increased lifetime in the TRPL results. From these results, it can be stated that the addition of Rb + increases the carrier lifetime of perovskite single crystals, which is beneficial for future applications. Lastly, we have carried out additional experiments to disclose the practical effect with Rb + addition as shown in Fig. S4. We studied photocurrent properties of the Rb(MAFA)PbI 3 perovskite single crystals. For

Conclusions
We successfully synthesized multiple-cation Rb(MAFA)PbI 3 perovskite single crystals for the first time. The effect of Rb + substitution on the properties of the perovskite single crystals was systemically investigated using XRD and ToF-SIMS measurements. It was found that the addition of 1.5 M 5% RbI was the optimum condition for the formation of high-quality Rb(MAFA)PbI 3 single crystals. Lattice shrinkage occurred in the Rb(MAFA)PbI 3 single crystal because of the small ionic radius of Rb + . As a result, blue-shifted absorption and PL peaks were observed upon Rb + substitution. The 1.5 M 5% RbI-added (MAFA)PbI 3 perovskite single crystal showed the longest carrier lifetime of 18.35 ns, with the highest photoresponsivity compared to other crystals. We believe that the fundamental understanding and optimization of the Rb + -incorporated perovskite single crystals will introduce a new platform for the development of multiple cation single crystals for future optoelectronic applications.