Microwave-Assisted Preparation of Organo-Lead Halide Perovskite Single Crystals.

The efficiency of organo-lead halide perovskite-based optoelectronic devices is dramatically lower for amorphous materials compared to highly crystalline ones. Therefore, it is challenging to optimize and scale up the production of large-sized single crystals of perovskite materials. Here, we describe a novel and original approach to preparing lead halide perovskite single crystals by applying microwave radiation during the crystallization. The microwave radiation primarily causes precise heating control in the whole volume and avoids temperature fluctuations. Moreover, this facile microwave-assisted method of preparation is highly reproducible and fully automated, it and can be applied for various different perovskite structures. In addition, this cost-effective method is expected to be easily scalable because of its versatility and low energy consumption. The crystallization process has low heat losses; therefore, only a low microwave reactor power of 8-15 W during the temperature changes and of less than 1 W during the temperature holding is needed.

Organo-lead halide perovskites of the general formula APbX3, where A stands for a monovalent organic cation (for example, the commonly used methylammonium = MA, or formamidinium = FA) and X represents a halide anion or halide anions mixtures (Cl–, Br–, I–),[1] are promising materials for the development of photovoltaic cells,[1−3] light emitting diodes,[4,5] lasers,[6−8] ultraviolet-to-infrared photodetectors[9−11] and X-ray and γ-ray detectors.[12,13] The success comes from their unique properties such as a suitable bandgap, high absorption coefficient, tunable optical properties, long and balanced electron–hole diffusion lengths, and low exciton binding energy.[14−17]

Perovskites can easily be deposited from their precursor solutions into a thin film by solution-based processes.[18−24] These processes are low-cost, convenient, and efficient. Nevertheless, the prepared films possess defects that cause nonradiative recombination, current–voltage hysteresis, and rapid degradation due to the low stability of the resulting devices.[1,16,17,25−28] The limitations can be reduced through preparation of single crystals, which possess low trap densities, higher crystallinity, and fewer ionic defects and grain boundaries, and thus the efficiency of the resulting functional devices can increase dramatically.[1,17] There are several approaches for how a single perovskite crystal can be grown: slow evaporation,[29] inversion temperature crystallization (ITC),[30] modified inversion temperature crystallization,[31] antisolvent vapor assisted,[32] top seeded solution crystal growth,[33] bottom seeded solution crystal growth,[34] the temperature lowering method,[35] and the Bridgman growth method.[36] The ITC method uses the deterioration of the precursors’ solubility with increasing temperature. Crystal growth is provided at higher temperatures, and temperature graduation must be regulated very precisely.[30] In general, the crystal growth is dependent on the speed at which temperature rises, the number of heating steps, the heating medium used, and many other external factors, and therefore, the crystallization cannot be completely controlled and monitored. The effects of external factors mostly lead to a production of fusion crystals, crystal clusters, or crystal fields completely inappropriate for application in electronic devices.[15]

Microwave-assisted synthesis has been successfully applied in the preparation of various types of inorganic[37] and organic compounds,[38] complexes and nanoparticles[37] including technological processes.[39] The major advantage of microwave irradiation consists in the conversion of electric energy into kinetic energy of particles by dielectric heating. Kinetic energy is ultimately converted into heat, and therefore, homogeneous and fast heating of the reaction mixture is very precisely controllable, and this leads to higher reaction yields, improved reproducibility, and very easy upscaling for applications in industrial fields.[39]

Here, we have developed efficient and fully reproducible microwave-assisted synthesis of organo-lead halide perovskite single crystals as a means to eliminate all of the above-mentioned problems. Our research provides a novel and original approach for growth of large-sized perovskite crystals with a highly organized crystal lattice by applying microwave radiation. We have found that lead-halide perovskite crystallization is not as time-consuming and energy-intensive as conventional methods due to effective heating. Moreover, the production of only one mostly bigger single crystal was followed. This new approach is highly efficient, reproducible, and easily upscalable because of the precise heating control.

As precursors MABr (methylamonium bromide, 98%, Sigma-Aldrich), MAI (methylamonium iodide, 98%, Dyesol), FABr (formamidinium bromide, > 98%, Ossila), FAI (formamidinium iodide, > 98%, Ossila), PbBr2 (lead(II) bromide, 99.999%, Sigma-Aldrich), and PbI2 (lead(II) iodide, perovskite grade, Sigma-Aldrich) were used without any further purification. The solvents used were DMF (N,N-dimethylformamide, anhydrous synthesis grade, Fisher Scientific) and GBL (γ-butyrolactone, reagent plus >99%, Sigma-Aldrich).

Organo-lead halide perovskite single crystals of MAPbBr3, MAPbI3, FAPbBr3, and FAPbI3 were prepared by the ITC method described in our previous work[40−42] with a modification. Perovskite precursor solutions were prepared from a 1 M solution of N,N-dimethlyformamide (DMF) for MAPbBr3 and γ-butyrolactone (GBL) in the case of MAPbI3 and FAPbI3, whereas the lead halides and corresponding organohalides were kept in a molar ratio of 1:1. In order to prepare a homogeneous MAPbI3 perovskite precursor solution, it was necessary to heat the suspension to 60 °C and to sonicate it. The FAPbBr3 perovskite precursor solution was prepared from a 1 M solution of PbBr2 and FABr dissolved in a mixture of solvents DMF and GBL in a molar ratio of 1:1. In all cases, the precursor solution was filtrated over a PTFE syringe filter (0.45 μm), and the filtrate was placed into a microwave reactor. For the preparation of all the crystals, the total volume of the resulting perovskite precursor solution was 2 mL, and the preparation of the precursor solution was performed under ambient conditions. The crystallization was performed in a microwave reactor (Anton Paar Monowave 300) in a G10 glass reactor with a Ruby thermometer in an immersion tube. The temperature program was set according to our previous experiments,[40−42] and this is shown in detail in Figure A,B for bromide-based and iodide-based perovskite crystals, respectively. The temperature profiles for MAPbBr3, MAPbI3, FAPbBr3, and FAPbI3 were set in intervals from 35 to 80 °C, from 70 to 119 °C, from 35 to 60 °C, and from 70 to 115 °C, respectively. The optimized crystallization programs were saved in the instrument’s memory, and high reproducibility was observed.

Figure 1

Temperature profiles for preparation of bromide-based (A) and iodide-based (B) perovskite single crystals.

During the optimization experiments, we observed that faster temperature increases led to polycrystalline material formation. For example, with regard to the preparation of bromide-based perovskite crystals, when the temperature was being increased in 10 °C steps with 30–60 min intervals, the polycrystals consisting of tiny (less than 0.5 mm) blended crystals were formed. On the other hand, when the temperature was being increased gradually with more steps (5 °C steps) and at longer intervals (60–120 min), as shown in Figure A, single crystals were grown in all cases, which is in the line with the previous work.[40−42] Furthermore, the longer the highest temperature value was set, the larger the resulting single crystals were, which has also been described previously.[40−42] For instance, when a temperature over 75 °C was set for MAPbBr3 perovskite for 1 h, the single crystal obtained was only 1 mm in size; nevertheless, when a temperature over 75 °C was set almost for 8 h, then the resulting single crystal was 5 mm in size as shown in Figure A. Finally, we have optimized programs for each perovskite crystal type by changing the speed of heating and the time at the set temperature according to Figure , and similar trends were observed in the preparation of iodide-based single crystals.

Figure 2

Photos of discussed perovskite single crystals (A) and powder XRD spectra of bromide-based (B) and iodide-based (C) perovskite single crystals.

The photos of crystal samples prepared from 2 mL precursor solution by the above-mentioned methods with their dimensions are shown in Figure . As expected, the orange bromide-based perovskite single crystals were cubic. The microwave reactor’s software limited the length of the crystallization program, and thus the maximal obtained size was 5 ± 1 and 3 ± 0.5 mm for MAPbBr3 and FAPbBr3 crystals, respectively. With regard to the iodide-based perovskite crystals, the black “hexagonal based” single crystals were regular and 2.5 ± 0.3 or 1.5 ± 0.3 mm in size in the case of MAPbI3 and FAPbI3 perovskites, respectively.

The mechanism of crystallization is considered to be the same as in the conventional ITC method. Used solvents (DMF and GBL) are suitable for microwave heating because of the presence of a dipole moment in the molecules, and thus, they can interact with the microwave irradiation which can be homogeneously absorbed through the whole volume of the sample, and as gained kinetic energy can be transferred into heat.[39] Compared to conventional ITC methods, the microwave-assisted method for perovskite single crystals preparation is not influenced by many different parameters, such as temperature profile, pressure, external mechanical factors, etc.[2] Microwave-assisted crystallization enhanced the reproducibility by means of precise temperature and environment monitoring. Simultaneously, the homogeneous heating of the sample was supposed to help to create one nucleation center. Finally, the crystallization was less time- and energy-consuming because of effective heating across the whole volume and because of the low 8–15 W power of the microwave reactor during the temperature changes and of less than 1 W during the temperature holding, the power profile is given in detail in Figure S1. This is a significantly lower energy when compared to a classic magnetic stirrer with a power of 800 W needed during the whole procedure.

In order to confirm the perovskite structure, we have performed a powder XRD analysis (see Figure B,C). Powder XRD measurements were performed on an Empyrean (PANalytical) diffractometer using Cu Kα (1.540598 Å) radiation. The measurement parameters were as follows: tube current 30 mA and voltage 40 kV; scan axis gonio; step size 0.013°2θ; time per step 96 s. The results were evaluated using the software HighScore Plus. With regard to the MAPbBr3 perovskite structure, the cubic Pm3̅m space group was confirmed with a unit cell parameter a = 5.9355 Å. The peaks positioned at 14.91°, 21.19°, 25.92°, 30.16°, 33.75°, 37.06°, 43.13°, and 45.89°2θ correspond to the reflections (100), (110), (111), (200), (210), (211), (220), and (300), respectively.[43] The FAPbBr3 perovskite structure was cubic-like as well with the space group Pm3̅m and unit cell parameter a = 5.9993 Å. The found peaks at 14.84°, 20.98°, 25.78°, 29.73°, 33.40°, 36.71°, 42.64°, 45.39°, and 48.01°2θ correspond to the reflections (001), (011), (111), (002), (012), (112), (022), (003), and (013), respectively.[44] The MAPbI3 perovskite crystal was in the tetragonal I4/mcm space group with unit cell parameters a(001) = 12.6569 Å; b(110) = 6.2741 Å; c(101) = 7.2708 Å. Peaks found at 13.98°, 14.10°, 19.91°, 23.46°, 24.47°, 28.15°, 28.42°, 30.89°, 31.62°, 31.83°, 34.78°, 34.94°, 37.19°, 40.44°, 40.63°, 42.58°, 42.80°, 43.03°, 43.20°, 45.69°, and 47.37°2θ correspond to the reflections (002), (110), (112), (211), (202), (004), (220), (213), (114), (222), (204), (312), (321), (224), (400), (215), (006), (411), (314), (420), and (413), respectively.[45] The FAPbI3 perovskite crystal’s phase was confirmed to be in an α-phase whose space group is Pm3̅m with unit cell parameter a = 6.3626 Å. Observed peaks at 13.99°, 20.49°, 24.22°, 28.04°, 31.42°, 34.59°, 40.04°, 42.64°, and 49.70°2θ correspond to the reflections (001), (011), (111), (002), (012), (112), (022), (003), and (222), respectively.[46]

Optical characterization of the perovskite single crystals was performed using a UV/vis/NIR Spectrometer Lambda 1050 equipped with a 150 mm InGaAs Int. Sphere (PerkinElmer). The UV/vis/NIR spectroscopy was applied in a transmission mode in order to estimate the onset of absorption. As expected, incorporation of iodide anions instead of bromide into the perovskite structure caused a shift of onsets of absorption closer to the NIR region. The transmission spectra in Figure A,B show clearly apparent onsets of absorption, which have been recalculated into an energy of 2.15, 2.12, 1.44, and 1.36 eV for MAPbBr3, FAPbBr3, MAPbI3 and FAPbI3 crystals, respectively. The results are in agreement with the reported band gaps for the mentioned crystals, which are 2.28, 2.15,[47] 1.47,[48] and 1.41 eV,[44] respectively.

Figure 3

Optical characterization of perovskite single crystals, UV–vis spectra of bromide-based (A) and iodide-based (B) perovskites and PL spectra of bromide-based (C) and iodide-based (D) perovskite; the excitation wavelength was 405 nm.

PL spectroscopy was performed on the perovskite crystals as well. PL spectra were recorded in a cryostat OptistatDry (Oxford Instruments), and a 405 nm laser for the excitation of the sample was used with a 420 nm LP filter. The emission maxima (see Figure C,D) of the recorded spectra were 546, 553, 782, and 829 nm for the MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3 crystals, respectively. The excitation wavelength was set the same for all the measurement, 405 nm, and the excitation power was 4 mW. The previously reported values of emission maxima are 544, 587, 784, and 843 nm, respectively.[44] The shift of emission maxima as well as the broadening of the spectra can be attributed to the influence of autoabsorption[49] and of crystal defects and surface states.[47] Optical properties are summarized in Table .

Table 1

Optical Properties of Prepared Perovskite Crystals

perovskite crystal	emission maximum (nm)	onset of absorption (eV)
MAPbBr3	546	2.15
FAPbBr3	553	2.12
MAPbI3	782	1.44
FAPbI3	829	1.36

In order to evaluate the difference between MAPbBr3 single crystals prepared by the microwave-assisted and by the conventional method described previously by our group,[40−42] we compared the UV–vis and PL spectra of both crystals; see in Figure S2. As expected, the onset of absorption was identical for both samples; it was 2.15 eV. Interestingly, the shoulder in the higher wavelengths in emission spectrum did not appear in the case of the conventionally prepared crystal. The shoulder in the region of approximately 560 nm corresponds to the low-energy transition state of the perovskite that is usually surpassed at room temperature.[50] The reason that the shoulder is observed could be fewer amounts of defects in the crystal grown by the microwave-assisted method.

In conclusion, this communication presents a novel, cost-effective microwave-assisted synthesis of perovskite single crystals. We have introduced a new method for preparation of MAPbBr3, FAPbBr3, MAPbI2, and FAPbI2 perovskite bulk crystals in a microwave reactor. The prepared perovskite crystals exhibit the same properties as when prepared by traditional ITC methods. This microwave-assisted method of perovskite crystallization is highly reproducible due to the precise heating control in the whole volume of the solution and the possibility of a fully automated program setting. This technique is convenient, versatile, and easily scalable, and it can be applied not only for the already mentioned types of perovskite materials, but also for various perovskite structure modifications. Moreover, our new method represents an environmentally friendly approach due to the very high energy efficiency and low energy consumption. We strongly believe that the microwave-assisted preparation of perovskite single crystals will make the fabrication of ultimate functional devices (such as solar cells, LEDs, photodetectors, etc.) more convenient and efficient.