Exploring Anisotropy on Oriented Wafers of MAPbBr3 Crystal Grown

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Exploring Anisotropy on Oriented Wafers of MAPbBr3 Crystal Grown by Controlled Antisolvent Diffusion Leilei Zhang, Yang Liu, Xin Ye, Quanxiang Han, Chao Ge, Shuangyue Cui, Qing Guo, Xiaoxin Zheng, Zhongjun Zhai, and Xutang Tao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00896 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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Crystal Growth & Design

Exploring Anisotropy on Oriented Wafers of MAPbBr3 Crystal Grown by Controlled Antisolvent Diffusion Leilei Zhang, Yang Liu,* Xin Ye, Quanxiang Han, Chao Ge, Shuangyue Cui, Qing Guo, Xiaoxin Zheng, Zhongjun Zhai, Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P.R. China Keywords: halide perovskites, single crystal, oriented wafers, anisotropy ABSTRACT: The emergence of organic–inorganic halide perovskites have reformed the research status of optoelectronics to a great extent. The bulk single crystals of halide perovskite, which in theory reflect the intrinsic physical properties of the material, are however hard to be integrated into functional devices. Just like the case that silicon wafers have revolutionized modern industries including electronics and solar cells, the availability of perovskite crystal wafers may pave their way to functional devices. Here firstly we designed a new settled temperature and controlled antisolvent diffusion system to precisely control over all key factors those affect supersaturation metastable zone during the crystal growth process, to grow MAPbBr3 single crystals with more than 50 mm in size. Secondly we fabricated MAPbBr3 single crystal wafers with different orientations, specifically, the (100), (010), (001), (110) and (111) wafers, with high crystalline quality (half peak width of rocking curve of 60-100 arc sec). Some key parameters were measured and compared on the wafers, where the results hint anisotropy of carrier transporting may exist for this pseudocubic structure. We hope the availability of oriented single crystal wafers can provide more scientists on materials and devices more probability to clarify the debatable physicochemical properties and to integrate the wafers as active layers or substrates in optoelectronic devices.

INTRODUCTION Organic–inorganic halide perovskites have been emerging as a new generation of photovoltaic material since the last eight years, although in the early 1990s, they have been included as active components in transistors and light emitting diodes. This field is now still under explosive growth of interest due to the intrinsic superior characteristics of halide perovskite materials including high absorption ability,1 high mobility,2-4 and the structural diversity and tenability.5-9 Seeing the rapid progress of various perovskite optoelectronic applications based on polycrystalline thin films, applications based on single crystals of halide perovskite have become intriguing because the single crystals reflect the intrinsic physical properties of the material, and would in theory perform much better than their polycrystalline counterparts. Halide perovskite single crystals have been proved to exhibit notably larger carrier mobility and longer diffusion length and carrier lifetime2 compared to polycrystalline thin films,10 thanks to the tremendously reduced bulk defect densities and the absence of grain boundaries.2 Thus new generation of optoelectronic devices based on perovskite single crystals come to the fore, to fully excavate the potential and to clarify the basic parameters of halide perovskite materials. Among them those based on MAPbBr3 (MA = CH3NH3+) single crystals are most representative ones. MAPbBr3, being with an energy band gap of ~ 2.14 eV, shows effective absorption

ability across the visible spectra band and better thermal/chemical stabilities compared with MAPbI3. Specialized for MAPbBr3 single crystals, photodetectors,1114 solar cells,15 lasers,16 nonlinear absorption,17 X-ray detectors,18,19 gamma-ray detectors,20 alpha-particle detectors21 and even detectors for environmental gases22 have been realized. It can be expected that experts on devices and material physics will develop more and higher-efficiency application potentials if mass and reproducible single crystalline wafers can be supplied. For the growth of bulk MAPbBr3 crystal, a traditional solution temperature-lowering technique was firstly employed.23,24 Millimeter-sized crystals could be attained during the slowly cooling down process of the precursor in hydrobromic acid aqueous solution.25,26 The temperature-lowering method is applicable for various MAPbX3 (X = Cl, Br, I, or their mixture) single crystals, however, is defective in time-consuming.27 A more popular method, called inverse temperature crystallization, was newly developed based on the abnormal retrograde solubility of organic-inorganic halide perovskite in specific organic solvents.28,29 By adopting this method, many groups have successfully grown bulk MAPbBr3 crystals for the studies of their properties and applications,11-14,18,22,28,30-35 among which a 44 × 49 × 17 mm3 sized MAPbBr3 single crystal, the biggest one to date, has been attained by Liu’s group.36 This method is quite convenient and rapid, thus being widely used; but for the same reason it generally resulted in inferior single

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

crystalline quality, with the half peak width of X-ray diffraction rocking curve of several hundreds or even more than a thousand arc sec.27,37 Another method to grow MAPbBr3 crystal is through antisolvent diffusion. Utilizing the principle-different solubility of the precursors in different solvents, for example DMF as the good solvent and dichloromethane (DCM) as the antisolvent, as large as 6 mm-sized MAPbBr3 crystals were attained.38 Crystals grown by antisolvent diffusion have high quality,39 however the size is limited to millimeterscale due to lack of control on the growth process, which normally produce many small crystals in one batch. (Figure 1a) On the other hand, the processability of the organic-inorganic hybrid perovskite single crystals was always doubted because of their poor mechanical property, in contrast of the good solution processability of their polycrystalline films. There have almost never been comprehensive attempt about the orientation and fabrication of perovskite crystal wafers.37 Thus to fulfil the availability requirements of single crystal wafers of perovskite materials for both fundamental understanding of their debatable physicochemical properties and integrations as active layers or substrates in optoelectronic device, study on the manufacture of differently oriented single crystal wafers is of paramount value. Our group has successfully grew several bulk perovskite crystals, such as MAPbI3,40 MASnI3,41 CH(NH2)2SnI3,41 NH(CH3)3SnX3,42 and [NH2(CH2CH3)2]3BiX6,43,44 with the size of more than 10 mm in the past three years, basically relied on cooling of the precursor solutions in haloid acid. However an annoying issue, the severe corrosion from the haloid acid to the growth equipment, came out during the long crystallization process. Thus, we tried to improve the technique and setup of the existing antisolvent diffusion method, based on our experience and comprehension on controlling the crystallization process. We designed a new system called Settled Temperature and Controlled Antisolvent Diffusion (STCAD) method, to precisely control over all the influence factors during the crystal growth process. MAPbBr3 single crystals with more than 50 mm in size and high quality (half peak width of rocking curve of 60-100 arc sec) were grown by using the new method. Furthermore, crystalline wafers with different orientations were manufactured from the bulk crystals, and some key parameters were measured and compared on the wafers.

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Figure 1. (a) Schematic diagram of a traditional uncontrolled antisolvent diffusion crystallization process. (b) Schematic diagram of the settled temperature and controlled antisolvent diffusion setup. The temperature of the growth solution was controlled by a temperature controller, and the antisolvent vapor was controlled of its flow rate by the mass flowmeters.

EXPERIMENTAL SECTION

Equipment Design for Crystal Growth Figure 1a shows a representative scheme of the procedure of the existing antisolvent diffusion method. Conventionally a small vial containing MAPbBr3 precursor solution [e.g., in N, N-dimethylformamide (DMF)] was placed in a larger outer vial containing of antisolvent [e.g., dichloromethane (DCM), toluene, ethanol]. The outer vial was fully sealed and the inner vial was open or partially sealed to allow antisolvent to diffuse into the precursor solution. As shown in figure S1 and the references,2,38,45 a large number of millimeter-sized single crystals were obtained after standing for one to several days. The limitation in crystal size is mainly attributed to the spontaneous diffusion and crystallization process without control of the diffusion rate of antisolvent into the precursor solution. This seemingly slow diffusion is actually too much faster to realize a stable crystallization process. We must control the supersaturation of the growth solution to maintain a metastable zone where spontaneous formation of crystalline nuclei is impossible, but crystal growth on a seed occurs. Thus to realize precise control over all of the key factors those affect supersaturation metastable zone during the antisolvent diffusion process, an improved system, called STCAD, is designed to implement crystal growth in a settled temperature and controlled antisolvent diffusion mode. As shown in Figure 1b, the temperature of the growth solution was controlled through a set of resistive heater and a temperature controller, and was kept constant at 36 oC (when DCM was used as the antisovent) during the whole growth process. The antisolvent vapor was introduced into the growth solution by dry air as the carrier gas. The air carrier gas was generated by an air generator, and most importantly, was controlled of its flow rate by the mass flowmeters. The air pipeline connects to the upper part of a sealed antisolvent


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container, and then transports a certain amount of antisolvent vapor into the growth solution. Both the antisolvent and the growth solution are kept in constant temperatures. In such a way the degree of supersaturation of the growth solution can be accurately controlled by adjusting the flow rate of the carrier gas.

Growth of MAPbBr3 Single Crystal A detailed growth process of a large-sized MAPbBr3 single crystal is shown in Figure 2a. Firstly a small MAPbBr3 crystal (~ 1 mm in size) was introduced into a solution with 1.2 M equimolar mixtures of MABr and PbBr2 in DMF, as a seed. This seed crystal in practice is able to survive quite a long time in the 1.2 M concentration solution, during which period we controlled the antisovent diffusion rate by adjusting the flow of air carrier gas. At the beginning the air flow was very low to below 2 sccm (Standard Cubic Centimeters per Minute); after the seed crystal was observed to start to expand its size, the carrier gas flow rate was set at a value of 5-6 sccm. The crystal grew into ~ 10 mm after three days of antisovent diffusion. As the crystal grew larger, the carrier gas flow could be tuned faster to about 10 sccm to accelerate the growth process. In the whole process the temperatures of the growth solution and the antisolvent are kept constant at 36 °C and 30 °C, respectively. Thus the supersaturation of the growth solution can be exclusively tuned by the antisolvent diffusion, to maintain the growth in a metastable zone where spontaneous nucleation is avoided. A bulk single crystal with the size to several centimeters could be obtained in fifteen days. (Figure 2a) Using other solvents, such as toluene, ethanol and acetonitrile as the antisolvent was found to be also feasible for MAPbBr3 crystal growth, but with different optimum diffusion rate. (Figure S2) Figure 2b shows different size of as-grown MAPbBr3 single crystals, by changing the growth period and the total volume of the precursor solution. The crystal size reaches 50 × 50 × 21 mm3, which should be the largest MAPbBr3 single crystal ever reported as per we know. The upper face of the left crystal is curved because of its in touch with the round inner wall of the container, indicating that this method can be used for growth of ultrathin single crystals or other special shapes of crystals by the space-confined or geometry-defined techniques.46-49

the growth solution. (b) Photographs of the as-grown MAPbBr3 single crystals with different size. X-ray Diffraction. X-ray diffraction (XRD) patterns of powder and single crystal were collected using a BrukerAXS D8 ADVANCE X-ray diffractometer with Cu Kα irradiation (λ = 1.54056 Å) in range 9-60°(2θ), with each scan step of 0.02° and a step time of 0.4 s at room temperature. 1H and 13C NMR Spectroscopy. 1H and 13C NMR spectroscop were measured on a Bruker Advane 400 spectrometer in dimethyl (DMSO) with tetramethylsilane (TMS; δ = 0 ppm) sulfoxide as internal reference Coupling constants (J) are given in hertz (Hz).

TG-DSC Measurement. We use TGA/DSC1/1600HT analyzer (METTLER TOLEDO Instruments) to measure differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of MAPbBr3 with a heating rate of 10 °C /min. DSC Measurement. The differential scanning calorimetry (DSC) measurements were implemented on a NETZSCH DSC 200F3 analyzer. Raman spectrum were measured by using Thermo-Nicolet NEXUS 670. UV-vis Diffuse Reflectance Spectroscopy. UV-vis diffuse reflectance spectroscopy were measured using a spectrophotometer (Shimadzu UV-2550, japan.), BaSO4 was the reference sample. Transmission Spectrum Measurement. The optical transmission spectra was recorded at room temperature by using spectrometer (U-4001 spectrophotometer). PL and Lifetime Measurements. PL and lifetime measurements of bulk crystals were performed employing an FLS-980 Edinburgh Instruments. Thermal Diffusion and Thermal Conductivity Measurements. The measurement of the thermal diffusion and thermal conductivity of MAPbBr3 single crystal was used a thermal dilatometer (TMA/SDTA 840, Mettler-Toledo inc.). Specific Heating Measurement. Specific Heating was carried out using the differential scanning calorimetry method employing a simultaneous thermal analyzer (TGA/DSC1/1600HT, Mettler-Toledo Inc.), at a heating rate of 10 °C/min over the temperature range between room temperature and 300 °C, with sapphire as a reference. Thermal Diffusivity Measurement. Thermal diffusivity was measured employing a laser flash apparatus (NETZSCH LFA 457 Nanoflash) over the temperature range between room temperature and 250 °C. Raman spectra. Raman spectrum were measured by using Jobin Yvon HR 800.

Figure2. a) Photographs of the continuous growth of an MAPbBr3 crystal from 0 day (introduction of a seed crystal, which is too small to be seen in the picture) to 8th day. The picture on 15th day was taken after the extraction of

Quality Characterization. The quality assessment was done according to the measurements of a Bruker D8 Advance X-ray diffractometer and laue diffraction pattern (MWL 120,MULTIWIRE LABORATORIES, Ltd). I–V measurements. I–V measurements were made using a Keithley 4200 semiconductor parameter analyser.



RESULTS AND DISCUSSION Phase Characterization.The crystallinity and phase purity of the as-grown crystals were confirmed by X-ray diffraction (XRD) and Nuclear Magnetic Resonance (NMR) spectra, with the related results shown in Figure S3 and S4. The XRD patterns of single crystal of MAPbBr3 proved that at room temperature it belongs to the cubic structure (space group Pm-3m, a=b=c=5.961 Å), with the natural facets to be quadrilateral (100) planes, demonstrating its single crystalline nature. The XRD patterns of ground powers from a large crystal are in good agreement with the calculated ones of cubic MAPbBr3. Thermal Properties of MAPbBr3 Powders and Single Crystals. The thermal properties of the MAPbBr3 crystalline powders were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Figure S5 shows that MAPbBr3 is thermally stable up to 300 °C, distinctly higher than that of the polycrystalline films.50,51 The phase transition points appeared at ~ -50 °C and ~ -130 °C on the DSC curve are in accordance with the previous reported results.52 The asgrown MAPbBr3 single crystals can be stored stably in ambient environment for several months. More detailed thermal characteristics of MAPbBr3 crystal, e.g., specific heat, thermal diffusion, thermal expansion, and thermal conductivity were studied by the laser flash method on a polished 4 × 4 × 1 mm3 single crystal. As shown in figure S6, the specific heat of MAPbBr3 single crystal is 0.35 Jg-1K-1 at room temperature, and the value rises as temperature increasing. The relationship between thermal conductivity and temperature, derived from the density, specific heat and thermal diffusion of MAPbBr3 single crystal, is shown in Figure S6c, which also rises as temperature increasing, with a value of 0.292 Wm-1K-1 at room temperature. This value is much lower compared with those of common inorganic crystals, and even relative lower than the reported value of MAPbBr3 obtained from frequency domain thermoreflectance measurements (0.51 Wm-1K-1)53 and that from scanning near-field thermal microscopy measurements (0.44 Wm-1K-1).54 It should be noted that both the previous results are based on microscopic examinations in micron scale. The macroscopic properties may be more helpful for practical applications, because this ultralow thermal conductivity driven by strong optical–acoustic phonon scatterings is believed to hold promise for diverse applications such as thermoelectric devices.55-57 Absorption and Transmission Properties of MAPbBr3 Crystals. The absorbance of MAPbBr3 crystal in both UV-vis and infrared region were measured and the spectra are shown in figure S7. The UV-vis spectrum demonstrates an obvious “cut-off” edge at 580 nm, with no absorption tails or excitonic peaks, suggesting that MAPbBr3 is a direct bandgap semiconductor and it possesses high-quality with a minimal number of in-gap defects.35 The optical bandgap extracted from Tauc plots is determined to be 2.14 eV, which is smaller when

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compared to the MAPbBr3 polycrystalline thin films (~ 2.3 eV),58,59 and even slightly narrower than the values of other reported single crystals in literatures.28,32,48 In the infrared spectrum MAPbBr3 crystal has strong absorbance in most range, while becomes transmitted since 13 μm, (Figure S8) implying promising applications in the farinfrared and terahertz band. Steady-state and Transient Photoluminescence of MAPbBr3 Crystals. Steady-state and transient photoluminescence (PL) properties of MAPbBr3 crystal were studied and the spectra were shown in Figure S9 and S10. The PL peak position of single crystal is located at 529 nm, which is obviously blue-shifted compared with the values from other MAPbBr3 single crystals (e.g., 570 nm for the crystal grown by uncontrolled antisolvent diffusion,2 574 nm and 537 nm for the crystals grown by inverse temperature35) The PL emission is with of significant higher energy than the absorption onset, thus may be extinguished by themselves to benefit excitation energy utilization in the optoelectronic conversion process.31 It has been well established that the chemical and physical properties of the metal-halide perovskites are subjected to the preparation conditions, here both the relative smaller band gap and shorter PL emission should be correspond to the high quality of the crystal grown from totally controlled conditions. This inference is quite confident even we take into account the re-absorption of internally emitted light within the bulk crystal. Because when the same single crystal was ground into powders, its PL peak red-shifted 30 nm, and the full width at half maximum (FWHM) of the powdered MAPbBr3 crystal also broadened into 35 nm from 23 nm of the single crystal (Figure S9), due to more introduced surface defects upon grinding. Figure S10 shows the transient PL curve recorded at an excitation of 375 nm. The decay time fitted via tri-exponential showed three time components of a fast component (2.43 ns) and two slow dynamics (20.75 ns and 110.36 ns). The lifetimes are in agreement with some previous studies32,60,61 and likewise show discrepancies with some others,2 on account of different growth conditions. Fabrication of Differently Oriented Wafers. It must be admitted that the bulk metal-halide perovskite crystals, at least currently, cannot be directly integrated into functional devices. A key advance to bridge the crystals and devices is processing crystals into wafers, just like the case that silicon wafers have revolutionized modern industries including electronics and solar cells. For metalhalide perovskite crystals, only one group of Liu has fabricated MAPbI3 and FAPbI3 (FA = formamidinium) wafers with the natural crystal facet as the main surface. While as a well-known fact, the intrinsic anisotropism of single crystals renders the physical properties to be crystallographic orientation-dependent. E.g., the chemical, electrical and mechanical properties of Si wafers depend on different crystallographic orientations of the wafer surface, wherein Si (100), (110) and (111) wafers are commercial available.62 From this point, we devoted to systematic exploration about the orientation and


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fabrication of differently oriented monocrystalline wafers based on the as-grown large-sized perovskite single crystals. As shown in Figure 3a, the MAPbBr3 crystal was being cut into thin wafers by using a diamond wire sawing machine. A wire diameter of 0.25 mm, and a linear speed of 1.5 ms-1, as the key parameters, were adopted for the process. Before and during the cutting, the crystallography orientations of the wafers were determined on a desktop X-Ray orientation machine. Figure 3b represents the processed MAPbBr3 single crystal wafers with different orientations, specifically, the (100), (010), (001), (110) and (111) wafers with the labeled crystallographic planes being as the main surfaces.

Figure 3. (a) Photograph of the cutting process of MAPbBr3 crystal wafers on a diamond wire sawing machine. (b) Photographs of the processed MAPbBr3 single crystal wafers with different orientations, (100), (010), (001), (110) and (111) wafers with the labeled crystallographic planes being as the main surfaces.



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other processed perovskite wafers.37 Figure 5b shows the X-ray Laue diffraction patterns of the MAPbBr3 wafers, where the well-defined reflections confirm the orientation of the respective wafers, and also represent characteristic of high crystalline quality.

Figure 4. X-ray diffraction patterns of the MAPbBr3 single crystal wafers. Because at room temperature MAPbBr3 was assigned to cubic Pm-3m space group, the (100), (010) and (001) planes are crystallographically equivalent with the same diffractions. The bottom pattern shows the calculated powder XRD profiles of MAPbBr3. Characterization of Differently Oriented Wafers. The orientation of the MAPbBr3 single crystal wafers were verified by measuring their respective XRD patterns of the exposure surfaces. As shown in Figure 4, all the five kinds of different wafers show diffractions from only one family of parallel crystal planes, e.g., (100), (200) and (300) peaks for (100) wafers, with the peak locations at 14.849°, 29.698° and 45.665° respectively. Since the cubic space group of MAPbBr3 crystal at room temperature, the (010) and (001) planes are crystallographically equivalent, so the (010) and (001) wafers display the same diffractions as those of (100) wafer. For (110) and (111) wafers, diffraction peaks corresponding to the (110) and (220) planes [21.060° and 43.903°, for (110) wafer], and to the (111) and (222) planes [25.867° and 53.185°, for (111) wafer], clearly confirm the accurate orientation of the processed monocrystalline wafers. The crystalline quality of the processed MAPbBr3 wafers were checked by high resolution X-ray diffraction (HRXRD). In general a symmetrical rocking curve with smaller full width at half maximum (FWHM) represents higher crystalline quality. As shown in Figure 5a, all of the XRD rocking curves of (100), (110) and (111) faces show high intensity diffraction peaks with symmetrical profiles. The (100) wafer, which was fabricated utilizing the natural facet of as-grown MAPbBr3 crystals, demonstrates the smallest FWHM of 64.89 arc sec at 6.4539° in a θ scan mode. For the cut and polished (110) and (111) wafers, the rocking curves show strong peaks with the FWHM of 103.406 arc sec and 86.795 arc sec, respectively. These values are much smaller than the reported FWHM for MAPbBr3 crystals,63,64 and also smaller than those of the


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Figure 5. (a) High-resolution X-ray diffraction rocking curves of the (100), (110) and (111) wafers on their respective main planes. (b) Laue diffractions of MAPbBr3 wafers on their respective (100), (010) and (001) planes.

One may wonder why we make so much effort to fabricate differently oriented wafers of a crystallographically isotropic (cubic) crystal. A fact here we should point out is the precondition for identification of MAPbBr3 as cubic structure resting on identification of the organic MA+ cation as of random orientations in the X-ray crystallography statistics.65 While this cognition was challenged by a number of research results, e.g., highresolution TEM has directly observed that the MA+ cations in MAPbBr3 crystals have ordered orientations and static off-centered configurations in different domains at room temperature.66 Comparative study also revealed the (110) crystallographic plane displaying 150% enhancement of optoelectronic performance than the (100) plane of an MAPbBr3 single crystal.14 Thus a systematic study based on different orientated single crystalline wafers is imperative for elucidating whether there is anisotropy in this pseudocubic structure. We then measured the Raman spectra of different wafers, because Raman scattering is polarization sensitive and can reveal information on the orientation of molecules in single crystals. As shown in Figure S11, the peak at 970 cm1 for (100) and (111) wafers, corresponding to the C–N

stretching, red-shifts to 967 cm-1 for (110) wafer; the vibration along C-N axis, appears at 1256 cm-1 for (100) wafer, red-shifts to 1255 cm-1 for (110) wafer and blue-shifts to 1258 cm-1 for (111) wafer; the same changing trend occurs for the NH3+ asymmetric bending peak at 1478 cm-1 for (100) wafer, which red-shifts to 1477 cm-1 for (110) wafer and blue-shifts to 1480 cm-1 for (111) wafer.32,67-69 When we changed the excitation wavelength from 633 nm to 1064 nm, the Raman resonance signals became weaker, while similar orientation-dependent vibrational modes were still observed. (Figure S12) Red shift of C–N stretching peak means a relative weaker bond strength, and vice versa. All these scattering shifts are related to the vibrational modes of the MA+ cations, which indicate that regarding the organic MA+ cation in MAPbBr3 crystals as of random orientations may be not the true case. There should be orientation anisotropy for different domains with static off-centered MA+ in different crystal facet due to the interactions of the cations with PbBr3- anions. These results imply that the ferroelectric order70 and charge transport anisotropy is possible in this material.



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Figure 6. (a) Typical dark current–voltage curves of the space charge-limited current (SCLC) devices on (100), (110) and (111) MAPbBr3 crystal wafers. Insets in respective panels show charge transport directions along different crystallographic orientations assuming the MA+ cations possessing defined and ordered orientations. (b) Diagrams of the distribution of derived carrier mobility and trap density depending on different wafer orientations.

Carrier Transport Anisotropy of Differently Oriented Wafers. The carrier transport properties for different MAPbBr3 wafers were evaluated by space charge limited current method. As shown in the insets of Figure 6a, the two Au electrodes of the device were evaporated on the two opposite main surfaces for different wafers, thus leading to the carrier transport to be along different crystallographic orientations. The typical dark currentvoltage (I-V) curves for (100), (110) and (111) wafers are shown in Figure 6a, which showed the Mott–Gurney’s power law dependence. For the devices on all the three kinds of wafers, with the increasing bias voltage, an Ohmic region at lower bias and a quadratic dependence following a space charge-limited current model at higher bias were clearly separated by a kink point which is defined as the trap-filled limit voltage (VTFL). The typical trap density (ntraps) and carrier mobility (μ) derived from the I–V traces are in the orders of 1010 cm-3 and tens of cm2 V-1 S-1, respectively. As shown in Table S3, we compared the carrier mobilities and trap densities of MAPbBr3 crystals with the literature values. Both μ and ntraps of our crystal wafers are comparable to the values of bulk single crystals reported by other researchers,71 and are much superior than those of polycrystalline films.72 When compared among the different oriented wafers, we found the carrier mobility and trap densities indeed vary with different orientations. As shown in Figure 6b, the histogram statistics of μ and ntraps plotted from ten devices for each wafers, the carrier mobilities are 80.54 ± 31.40 cm2 V-1 S-1, 72.34 ± 17.19 cm2 V-1 S-1 and 57.68 ± 14.53 cm2 V-1 S-1, for the (100), (110) and (111) wafers, respectively;

the trap densities are 2.52 ± 1.64 × 1010 cm-3, 2.86 ± 2.38 × 1010 cm-3 and 6.97 ± 1.64 × 1010 cm-3, for the (100), (110) and (111) wafers, respectively. We can see the (100) wafers show the highest carrier mobility and the lowest trap density; while the (111) wafers show the lowest carrier mobility and the highest trap density. Different oriented dipole may exist on different wafers because of the static orientation of MA+ cations. (Inset of Figure 6a) Thus the defect migration behaviors along different crystallographic directions in MAPbBr3 single crystal maybe orientation-dependent,14 and the difference in carrier transport should correlate to the orientation of the wafers. Whereas because people now do not substantially understand the carrier transport mechanism in these hybrid perovskite materials, and our results still have large deviations, it is yet far to draw a definite conclusion about the anisotropy of this pseudocubic structure. To explore whether the elemental compositions of different faceted wafers affect the carrier transporting properties, we measured the elemental composition of each different faceted wafers by Energy Dispersive Spectrometer (EDS) and X-Ray Fluorescence (XRF) techniques. As shown in Figure S13 and S14, the molar ratio of Br and Pb for (100) wafer is a little less than 3, that for (110) wafer is a little more than 3, and that for (111) wafer is equal to 3. The EDS Mapping (Figure S15) indicates that all of the C, N, Br and Pb elements distribute evenly on the (100), (110) and (111) planes. We believe the presence of defects may cause variation of the trap densities, while the defect tolerance of perovskite materials may make the effect on carrier transporting to be not significant.73,74 Thus currently we still cannot confirm whether the charge transporting


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divergence in different wafers correlates to the elemental composition.

CONCLUSION In conclusion, considering the fact that the emergence of organic–inorganic halide perovskites have reformed essentially the research status of optoelectronic fields, however, the bulk single crystals of halide perovskite, which in theory reflect the intrinsic physical properties of the material, can still be hardly integrated into functional devices. We have mainly contributed two advances, one is designation of a settled temperature and controlled antisolvent diffusion system to precisely control over all the key factors those affect supersaturation metastable zone during the crystal growth process; MAPbBr3 single crystals with more than 50 mm in size were grown by using the new method. The other is fabrication of crystalline wafers with different orientations and high quality (half peak width of rocking curve of 60-100 arc sec) from the bulk crystals, and some key parameters were measured and compared on the wafers. By fulfilling the availability requirements for oriented single crystal wafers of perovskite materials, we hope to collaborate with more materials and devices scientists on both fundamental understanding of their debatable physicochemical properties and integration of the wafers as active layers or substrates in optoelectronic devices.

ASSOCIATED CONTENT

Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details and additional characterization data.

AUTHOR INFORMATION

Corresponding Author * [email protected] * [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors acknowledge support from the National Natural Science Foundation of China (Grant Nos. 21772115, 51227002, and 51272129), National Key Research and Development Program of China (Grant No. 2016YFB1102201), and the Program of Introducing Talents of Disciplines to Universities in China (111program No. b06015). Y.L. is grateful for the support from Qilu Young Scholars and Tang Scholars.

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For Table of Contents Use Only

Exploring Anisotropy on Oriented Wafers of MAPbBr3 Crystal Grown by Controlled Antisolvent Diffusion Leilei Zhang, Yang Liu,* Xin Ye, Quanxiang Han, Chao Ge, Shuangyue Cui, Qing Guo, Xiaoxin Zheng, Zhongjun Zhai, Xutang Tao*

Different oriented MAPbBr3 single crystal wafers [(100), (010), (001), (110) and (111)] were fabricated from bulk crystals which were grown by a new designed settled temperature and controlled antisolvent diffusion system.


Exploring Anisotropy on Oriented Wafers of MAPbBr3 Crystal Grown

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