Study on the effect of chlorine on the growth of CH3NH3PbI3−xClx crystals

Organic-inorganic hybrid halide perovskites have attracted great interest for scientists and entrepreneurs due to their intrinsic properties and considerable business prospects in recent years. Perovskite single crystal can determine its properties well because of no grain boundaries for itself, so it is necessary to study the nucleation and growth process of perovskite crystals. In this work, CH3NH3PbI3−xClx single crystals with about 10 mm sized are prepared. It’s found that chlorine can restrict the nucleation and slow crystallization during the perovskite grown process. Moreover, the number of perovskite crystals can be controlled by tuning the content of chlorine in the precursor solutions, because chlorine can restrict the perovskite crystal nucleation as an inhibitor.


Introduction
Organic-inorganic hybrid perovskites (OIHPs) have attracted great interests because of their superior characteristics involving long carrier diffusion lengths [1] and other great properties [2]. The power conversion efficiency (PCE) of solar cells based on OIHPs has skyrocked from 3.8% [3] to 24.2% [4] in less than a decade. Moreover, OIHPs are also used in photo-sensors [5], transistors [6], light emitting diode, and so on. Polyhalogen mixed perovskites have attracted much attention because of their unique performance advantages [7]. CH 3 NH 3 PbI 3−x Br x [8] and CH 3 NH 3 PbBr 3−x Cl x [9] have been studied in detail as the typical polyhalogen mixed perovskites in previously. It has been confirmed that the introduction of chlorine into perovskite film can improve the performance of its solar cell devices [10][11][12][13][14][15][16][17]. Subsititution of chlorine in the perovskite can not only tune the band gap [18] of the material, but also improve the stability of the device. Consequently, scientists have paid much attention to CH 3 NH 3 PbI 3−x Cl x thin film and the solar cell devices based on it [10,19]. Multiple ways to fabricate perovskite thin films, such as one-step method and two-step method, the films made by these methods show that low or even negligible chlorine content in them but the quality of the films can be improved. Nevertheless, the role of chlorine in CH 3 NH 3 PbI 3−x Cl x crystals has been remained not clear until now. CH 3 NH 3 PbI 3−x Cl x single crystal has been successfully synthesized by literature reported [20][21][22], although the amount of chlorine element in the CH 3 NH 3 PbI 3−x Cl x single crystals is very little. It will be meaningful to study the perovskite single crystal, because single crystal can clearly show the key properties of their materials and it doesn't have grain boundries which also impact the performance of solar cells.
In this work, we successfully prepared CH 3 NH 3 PbI 3−x Cl x single crystals in large size. The impact of chlorine in the growth of CH 3 NH 3 PbI 3−x Cl x single crystals has been studied in detail. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Single crystal growth
In this work, CH 3 NH 3 PbI 3 (MAPbI 3 ) single crystal and all the CH 3 NH 3 PbI 3−x Cl x (MAPbI 3−x Cl x ) single crystals were grown via inverse temperature crystallization (ITC) method as the previous literature reported [23].
For obtaining CH 3 NH 3 PbI 3 single crystals, CH 3 NH 3 I and PbI 2 with the molar ratio of 1:1 were dissolved in 5 ml of ϒ-butyrolactone (GBL), and then the precursor (1 M, refer to as Precursor I) heated and stirred for 12 h at 70°C. Some small MAPbI 3 crystal seeds were appeared in the bottom of vial after the solution being heated for several hours at 120°C. Then, a selected seed was placed in the bottom of the vial with fresh precursor solution. The bulk MAPbI 3 single crystal (refer to as PI) was formed within several hours.
For obtaining MAPbI 3−x Cl x single crystals, the precursor solution was made by adding CH 3 NH 3 Cl and PbCl 2 in 1:1 molar ratio in Precursor I solution to replace a part of the solute, then the precursor solution with mole I:Cl ratio of 30:1, 22:1 and 14:1 were obtained (refer to as Precursor I 30 Cl 1 , Precursor I 22 Cl 1 and Precursor I 14 Cl 1 ). The precursor solution was heated and stirred for 12 h at 70°C. Followed, the solution was heated for several hours at 120°C. Some small MAPbI 3−x Cl x crystal seeds were appeared in the bottom of the vial. For preparing bulk MAPbI 3−x Cl x single crystal, a selected seed was placed in the bottom of the vial with fresh precursor solution. The bulk MAPbI 3−x Cl x single crystal was formed from the precursor with I: Cl=14:1, 22:1 and 30:1 (refer to as PICl 1, PICl 2, PICl 3, respectively) within several hours.
By changing a part of solvent of the precursor, the precursor (refer to as Precursor I 30 Cl 1 -D) with I: Cl=30:1 was replaced a part of GBL with 2.5 vol% DMSO to obtain CH 3 NH 3 PbI 3−x Cl x single crystal (refer to as PICl 4D) by using the same method.

Characterization
Energy dispersive spectroscopy (EDS) spectra of perovskite single crystal powders were measured by ZEISS EVO 18 scanning electron microscope (SEM). Powder x-ray diffraction (Powder XRD) patterns were measured by Rigaku/Smartlab diffractometer equipped with a Cu Kα radiation source in the range of 10°-60°with a step size of 0.01°. The Fourier transform infrared (FTIR) spectrum was measured by Nicolet/iS 50 spectrometer in the range of 400 cm −1 -4000 cm −1 . XPS spectra were measured by Thermo ESCALAB 250 with the excitation source of a monochromized Al Kα source.

Results and discussion
As shown in figure 1, high quality MAPbI 3−x Cl x single crystals have been successfully grown in their precursor solution, which size can be reach to nearly 10 mm length.
During the MAPbI 3−x Cl x single crystal growing process, it can be observed that the number of small crystals on the bottom of the vial decreases with increasing molar ratio of Cl: (Cl+I) in the precursor. The number of crystals on the bottom of a vial after heating for 5 h is shown in figures S1(a)-(d). Figure 2 shows the crystal quantity statistics with different molar ratio of Cl:(Cl+I) after heating for 5 h. It can be seen that 25 smaller crystals were precipitated from Precursor I. However, 22, 10 and 7 crystals were precipitated from Precursor I 30 Cl 1 , Precursor I 22 Cl 1 and Precursor I 14 Cl 1 , respectively. Obviously, it can be indicated that Cl element in the precursor can restrict the formation of perovskite nucleus. Furthermore, the more time is needed for growing crystals with the more Cl element in the precursor. It is about 3 h that small crystals appeared for Precursor I, Precursor I 30 Cl 1 and Precursor I 22 Cl 1 . However, more than 5 h is needed for Precursor I 14 Cl 1 . The molar ratio of Cl: (Cl+I) in Precursor I 14 Cl 1 is 6.67% which is over than 5%. Comparing with Precursor I 30 Cl 1 and Precursor I 22 Cl 1 , higher Cl content in Precursor I 14 Cl 1 , which slow down the CH 3 NH 3 PbI 3−x Cl x crystal growing speed significantly. According to previous research [24], it is known that Cl atom can be coordinated to Pb atom, as a result, it is more difficult to form Pb-I-Pb. As shown from scheme 1, consequently, it therefore caused the slower nucleation and crystal growth. Cl can retard the formation of CH 3 NH 3 PbI 3−x Cl x during perovskite film fabrication [17,25], both of MACl and PbCl 2 indicate that chlorine can slow down the rate of crystallization. In our work, we found that Cl can also slow down the rate of perovskite single crystal growth. Although we call the perovskite CH 3 NH 3 PbI 3−x Cl x , the Cl content in perovskite crystal is very little in this work and the previous literatures. So we think that the formation of Pb-I-Pb is crucial to the crystal growth. The crystal growth prepared with Cl contained precursor is slower than without Cl. In addition, it takes more time to obtain the same size crystal with increasing Cl content in precursor.
In addition, as shown in figure S1e is available online at stacks.iop.org/MRX/7/015522/mmedia only 3 MAPbI x Cl 3−x crystals precipitated from the I 30 Cl 1 -D precursor solution, after heating for 5 h, which contained 2.5 vol% DMSO in GBL solvent. Only due to 2.5 vol% DMSO added in the precursor, the number of MAPbI x Cl 3−x crystals was decreased significantly from over 20 to about 4, which may be attributed to the formation of complexes between DMSO and Pb [26]. With the volume of DMSO raised to 3.3 vol% and 5 vol% in the precursor replaced GBL, respectively, even no crystal can be obtained after heating for 5 h. Moreover, even the precursor was heated for 36 h, there was still no crystals appeared in the precursor with 3.3 vol% or 5 vol% DMSO, respectively.
The Cl content is calculated by using XPS datas ( figure 3(a)) and EDS datas ( figure 3(b)), respectively. Both of them show the same trend, the molar ratio of Cl : I increases in single crystal with increasing Cl content in precursor. Although the Cl content of the same sample is different by using two different measurements, it may due to under different test condition. Figure S2 shows XPS datas. Figures S3-S6 show EDS datas.  XRD is used to characterize the structure of MAPbI 3 and MAPbI 3−x Cl x single crystals, which tested after grinding into powder. As shown in figure 4(a), the peaks at 2θ of 14.1°, 28.4°and 43.2°are attributed to MAPbI 3 [27], moreover, they are also the main peaks of MAPbI 3−x Cl x assigned to the (110), (220) and (330) planes, respectively [28]. In fact, the difference of the XRD patterns between MAPbI 3−x Cl x and MAPbI 3 crystals is not significant, because the content of chlorine in MAPbI 3−x Cl x crystals is very little. However, figure 4(b) shows the peaks of MAPbI 3−x Cl x move to large angles compared to MAPbI 3 . It verified that Cl incorporates into the perovskite crystals successfully.
The freshly crushed MAPbI 3−x Cl x single crystal powders were examined by EDS. It is verified that chlorine incorporates into the MAPbI 3−x Cl x single crystal. The signals of chlorine have been detected for all the MAPbI 3−x Cl x single crystal samples. As shown in figure 5, it is focused on one big particle, the homogeneous dispersion of chlorine elemental can be seen in the EDS spectrum.

Conclusions
In this work, high quality MAPbI 3−x Cl x single crystals with more than 10 mm have been successfully obtained by inversion temperature crystallization method. Chlorine element was incorporated into the MAPbI 3−x Cl x crystals by adding different amount of PbCl 2 and MACl in their precursor solution. It can be inferred that chlorine in the precursor should restrict the nucleation and growth of perovskite crystals, which is the inhibitor for the MAPbI 3−x Cl x single crystals growth.