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Observation of the growth of MAPbBr3 single-crystalline thin film based on space-limited method
Accepted Manuscript Observation of the Growth of MAPbBr3 Single-Crystalline Thin Film Based on Space-Limited Method Lipeng Han, Cai Liu, Lili Wu, Jingquan Zhang PII: DOI: Reference:
S0022-0248(18)30407-X https://doi.org/10.1016/j.jcrysgro.2018.08.027 CRYS 24718
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
13 April 2018 30 July 2018 26 August 2018
Please cite this article as: L. Han, C. Liu, L. Wu, J. Zhang, Observation of the Growth of MAPbBr3 Single-Crystalline Thin Film Based on Space-Limited Method, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/ j.jcrysgro.2018.08.027
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Observation of the Growth of MAPbBr3 Single-Crystalline Thin Film Based on Space-Limited Method Lipeng Hana, Cai Liua,*, Lili Wua,*, Jingquan Zhanga a
Institute of Solar Energy Materials and Devices, College of Materials Science and
Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, China, 610064 * Corresponding authors. Tel/Fax: +86 (0)28 85412542. E-mail addresses: [email protected] (C. Liu), [email protected] (L. Wu) Abstract: Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance. One key aspect is to be able to synthesize SCTFs with large surface areas. Here, we investigated the formation of MAPbBr3 SCTF by space-limited inverse temperature crystallization method. It was found that the weak wetting substrate could reduce friction force and promote precursor ion diffusion, which enabled a continuous growth of the thin perovskite single crystal along in-plane direction. SCTFs could be obtained at the initial stage of film growth and we also observed some signs of island growth model. Pyramid-shaped islands formed first and then converged to achieve top surface growth of SCTFs. After further growth, the precursor ion concentration near the SCTF gradually decreased. The lateral crystal growth was limited by the inefficient and anisotropic long range transportation of precursor ions along the micrometer-size or nanometer-size gap, which resulted in transformation of SCTFs into polycrystalline films. Low ion diffusion in the narrow gap is still the main constraint for the preparation of the larger and thinner SCTFs. Keywords: A1. MAPbBr3; A2. Single-crystalline thin film; B1. Space-limited method; B2. Lateral crystal growth; B3. Ion diffusion
Introduction Organic-inorganic halide perovskites have been attracting much attention due to their outstanding optoelectroinc properties, which are extremely suitable for high performance solar cells, light-emitting devices, photoelectric switches, highly sensitive detectors and so on [1,2]. Moreover, the easy solution-procedure and low cost attract a big number of researchers globally with great enthusiasm to this booming field. The present power conversion effciency of perovskite solar cells was certified to be more than 22% within a few years of its advent because of their superior properties such as high absorption coefficient, long carrier lifetime and highly balanced hole and electron mobility [3,4]. Despite these achievements, the majority of current perovskite-based devices still rely on polycrystalline thin films. In most applications, single-crystal materials can provide more remarkable properties than their polycrystalline counterpart [5,6] and devices based on a well-aligned single crystal always exhibit more excellent performance to those based on polycrystal [7,8]. For single crystal materials, they are important functional units that can be applied to various devices, whose performance can be seriously affected by their crystallinity and morphology. In order to further understand their physical and chemical properties of perovskite materials and find more applications, it is very important to investigate the characteristics of single crystal perovskite materials [9-11]. At present, many methods have been developed to grow bulk perovskite crystals [12] including adding PbI2 crystals to the solution of CH3NH3I (MAI) [13], anti-solvent vapor-assisted crystallization [14], inverse temperature crystallization [15], top-seed solution growth [16] and so on. Nevertheless, using bulk single crystals in photovoltaic devices may lead to severe degradation of device performance due to more carrier recombination with the increase of absorb layer thickness. As the mature preparation technology of bulk perovskite single crystals, the preparation of perovskite single-crystalline thin films (SCTFs) has gradually become a research hotspot for boosting device performance.
They can be prepared by space-limited inverse temperature crystallization (SLITC) method [17-21], surface tension-controlled inverse temperature crystallization method [22], cavitation-triggered asymmetrical crystallization strategy [23] and cast-capping method [24], with thicknesses varying from a few hundred nanometers to several hundred of micrometers. All above methods could generate a lateral driving force for the precursor ions to form the two-dimensional thin single-crystalline perovskites, which overcome the shortcomings of traditional single-crystal growth methods in their tendency to produce merely three-dimensional free-standing perovskite bulk single crystals. Among them, the SLITC method has been extensively implemented due to its simple apparatus configuration and easy film thickness adjustment by varying the thicknesses of spacers and the slit channel. Moreover, some pressure can be applied on substrates to decrease the distance between them, which can be tuned to dozens of nanometers to acquire much thinner SCTFs. Perovskite SCTFs exhibit extremely outstanding properties. Wan et al. revealed that monocrystalline perovskite films exhibited low defect density and good air stability [17]. Huang et al. prepared a solar cell based on monocrystalline perovskite active layer, which presented an efficiency of 17.8% [21]. Ma et al. reported a photodetector based on monocrystalline films perovskite active layer with a record performance of a 50 million gain, 70 GHz gain-bandwidth product, and a 100-photon level detection limit at 180 Hz modulation bandwidth [19]. From previous studies, it was found that large-sized perovskite SCTFs whose thicknesses are over 10 micrometers can be easily prepared, whereas films thinner than 10 micrometers are difficult to obtain for large areas. Meanwhile, very few studies reported the characteristics of the growth procedure of perovskite SCTFs. In the present study, we constructed a simple reactor consisting of two square substrates without spacer between them and used SLITC method to obtain perovskite SCTFs. Single-crystalline MAPbBr3 was chosen to be grown for its proper bandgap for potential application to intermediate band solar cells. We attempted to learn about
the micro growth process by analyzing experimental phenomena during the film preparation process and material characterization results to provide guidance for the subsequent preparation of large-area and high-quality perovskite SCTFs.
Experimental Section Materials Lead bromide (PbBr2, 99.999%), methylammonium bromide (MABr, 99.5%) and N,N-dimethylformamide (DMF, 99.9%) were purchased from Youxuan Advanced Election Tech Co. Ltd. (Yingkou, China). All materials were used without further purification. Substrates were fluorine-doped tin oxide conducting glass (FTO, Pilkington, thickness: 2.2 mm, sheet resistance 14Ω/square), Corning Eagle XG slim glass (glass type: alkaline earth boro-aluminosilicate, substrates for active-matrix flat panel displays) and common quartz glass. SCTFs preparation First, these substrates were cleaned with detergent, and then sequentially ultrasonic cleaned with deionized water, acetone and ethanol, finally dried with N 2. Then PbBr2 and MABr (1:1 by molar, 1.5 M) were dissolved in DMF. A piece of cleaned substrate (60 mm 45 mm) was put on the top of another without spacers to form a narrow gap. The perovskite precursor solution was added to the intersection of two substrate edges (Fig. 1). And it was absorbed into the gap by capillarity force. Finally, this system was placed on a heating plate with temperature of 80 °C to grow the film by inverse temperature crystallization until the growth of the MAPbBr3 thin film was completed. The temperature was slowly decreased to room temperature to ensure a good contact between the substrate and thin films. Then the substrate/perovskite/substrate sample was cleaved and the perovskite thin film left on either substrate was carried out the following measurements.
Perovskite solution
MAPbBr3 single crystal
Top substrate 80℃
Heater
Heater
Bottom substrate Fig. 1. The schematic diagram of growth process.
Characterization X-ray diffraction (XRD) measurements were performed using a DX-2600 X-ray diffractometer (Dandong Fangyuan Instrument Company, China) with Cu Kα radiation of the scan range from 10° to 70°. The transmittance spectra of thin films were recorded by using a PerkinElmer Lambda 950 UV−vis Spectrometer. A field emission scanning electron microscope (SEM) (S-4300, Hitachi) was used to acquire SEM images. The surface morphology was observed by atomic force microscopy (AFM, Multi-Mode 8 SPM, Brucker).The thin film thickness was measured by a step profiler (XP-2, Ambios Technology). A light and dark field metallographic microscope (Mshot, MJ33) was used to investigate crystal morphology of the perovskite thin film. The hydrophobicity of substrates was studied by dropping a deionized water droplet or perovskite precursor solution droplet onto their surface with a contact angle measuring unit (DSA25, Kruss). The photoluminescence (PL) spectra were recorded using an Edinburgh Instruments FLS980 spectrometer and a 473 nm laser was used as the excitation source. Results and discussion In order to grow perovskite SCTFs, we used the classical SLITC method. Crystal growth continuously consumes precursor ions. In turn, the inefficient long-range transportation of precursor ions along micrometer gap prevents further lateral crystal
growth. Since the perovskite precursor ions generally form complex with the solvent, the diffusion rate of precursor ions is partially determined by how fast the solvent molecules can transport on the substrate [25]. The diffusion rate of solvent molecules close to the substrate is determined by the surface tension [22,26,27]. On wetting substrates, the large surface tension imposes a friction force that pulls down the ion transportation [28]. As a result, the interaction between substrate surface and solvent molecules becomes an issue of great importance for precursor ion diffusion [21]. We have chosen three kinds of transparent substrates (FTO glass, Corning glass and quartz glass) for contact angle testing to analyze the interaction between different solvents and substrates. Contacting angles of deionized water on these substrates shown in Fig. 2a-c were 78°, 45° and 41° for FTO glass, Corning glass and quartz glass, respectively. Huang et al. hypothesized that the measured contact angle represented the wetting capability of substrate surfaces to water and it was also an indication of the wetting capability of substrate surfaces to perovskite precursor solution because of their strong polarity [28]. However, it differed significantly when using the perovskite precursor solution to conduct the contact angle test. The contact angle between FTO glass and the perovskite precursor solution reduced sharply from 78° to 0°, which was mainly caused by the difference of the surface tension between these solvents (σdeionized water: 72.8 mN/m, σperovskite precursor solution: about 39.3 mN/m) [22]. The contact angles between other substrates and the perovskite precursor solution only decreased slightly. It revealed that interactions between substrate and deionized water did not perfectly represent interactions between substrate and perovskite precursor solution. There was still some deviation among them. It can be seen that the contact angle between Corning glass and perovskite precursor solution was largest, which meant stronger hydrophobicity. After the contact angle test, precursor solution on different substrates began to crystallize slowly under the condition of natural volatilization of the solvent. The diffusion area of precursor solution on FTO glass was large because of small contact angle (Fig. 2g). During the
solvent evaporation process, crystal nucleuses first appeared in local areas. However, previously nucleuses were not able to grow continuously due to insufficient precursor ion transportation, which was caused by larger friction imposed by the FTO glass substrate surface. Finally, the entire precursor solution coverage area was covered with yellow materials, which is shown in Fig. 2g. For the other two substrates, the edge of the precursor solution droplet was preferentially nucleated, and the precursor ions in the droplet could replenish in time to promote the crystal growth. It's worth noting that there was still a small amount of yellow residue in the internal region of the previous droplet on the quartz glass substrate, which was marked with blue circles in Fig. 2i. However, it was not the case on the Corning glass substrate. Therefore, we chose Corning glass as the substrate for better precursor ion transportation during crystallization in the narrow gap due to weaker friction force.
Fig. 2. The contact angles between deionized water and varied substrates (a-c), the contact angles between MAPbBr3 precursor solution and varied substrates (d-f), MAPbBr3 crystal morphology images for FTO glass, Corning glass and quartz glass after natural volatilization of solvent (g-i).
If we use the precursor solution addition method of cast-capping method [24], it was easy to generate bubbles in the gap and influence crystallization during the heating process. As a result, we made some small improvements. The perovskite precursor solution was added to the edge of substrates and could be absorbed into the gap by capillarity force (Fig. 1). This liquid adding method can not only reduce the amount of precursor solution, but also effectively prevent bubbles inside the gap. It is noteworthy that the precursor solution diffused anisotropically in the gap. And the diffusion speed was higher at the edge of the substrate than that of interior zone. This observation was consistent with the results reported in the literature [21]. For the slow transportation of precursor ions, this anisotropic diffusion in the narrow gap may also have a great impact on the micro-zone crystallization. SLITC method enables MAPbBr3 perovskite thin film to grow in large planar domains between the top and bottom Corning glass substrates. At the initial stage of crystallization, the film on Coring substrates exhibited square shape (Fig. 3a). The lateral size of this thin film was over 1 mm, while its thicknesses determined by step profiler was about 6.38 micrometers (Fig. 3b). It may be the SCTF, which conformed to the characteristic of the MAPbBr3 SCTF [19-21]. Then SEM observation was carried out to study the crystal quality of this regular thin film. It can be seen that the semitransparent film was homogeneous. In addition, the crystal thickness was measured from the side facet by tilting the thin film. A representative crystal thickness shown in the image was estimated to be 6.0 μm (Fig. 3c). The top-view SEM image showed that the thin film was smooth and free of grain boundaries (Fig. 3d).
Fig. 3. Photograph (a), step profile (b), SEM images (c) and (d) of the MAPbBr3 regular sheet.
Some bulk single crystals made by anti-solvent crystallization method were ground to conduct the XRD measurement to obtain all diffraction peaks of MAPbBr3 crystal (Fig. 4a). The XRD pattern of the aforementioned thin film depicted in Fig. 4b showed four sharp diffraction peaks at 15.0°, 30.2°, 45.9° and 62.7°, which can be assigned to (001), (002), (003) and (004) planes, respectively, indicating an exposed face of (001) crystal plane [29,30]. The intensities of the (001) and (002) peak were quite strong, and the appearance of the secondary diffraction peak of (003) and (004) plane suggested acceptable crystalline perfection. It is important to note that the thin film only showed this unique family of reflections, which clearly indicated that its (001) plane grew two-dimensionally and was parallel to the substrate surface. To further study the crystalline quality of the film, the X-ray diffraction rocking curve of (001) plane was measured, as inserted in Fig. 4b. The full width at half-maximum (FWHM) of the (001) peak of the rocking curve was 0.26°. The small FWHM synchronously indicated good crystalline quality and small residual stress in the obtained regular crystal thin film [31]. Regular square shape, excellent crystal morphology and the XRD pattern coincided with the reported information of
MAPbBr3 SCTFs in the literatures [18,22,23]. Hence, the thin film with square shape was the MAPbBr3 SCTF.
Fig. 4. Powder XRD spectra of ground MAPbBr3 powders (a), MAPbBr3 SCTF (b) and inset: rocking curve of the (001) plane.
The optical properties of the MAPbBr3 SCTF were investigated using steady-state absorption and PL spectra. As shown in Fig. 5a, the SCTF showed a clear band edge cutoff with no excitonic signature, which suggested a minimal number of in-gap defect states. A narrow emission peak of PL spectroscopy located at 547 nm was in line with the previous reported value of the MAPbBr3 SCTF [18]. The smaller value of PL peak than that of the absorption onset (~560 nm) can be ascribed to the separation of excitons into free charges at room temperature [32]. The PL peak at longer wavelength (~600 nm) originated from surface defects of MAPbBr3 single crystal [33] was not identifed for the current SCTF, implying a high crystalline characteristic. Bandgap extracted from Tauc plot (Fig. 5b) was calculated to be 2.25 eV, which was very close to previously reported values (2.24 eV) for bulk single crystal and planar-integrated single crystalline film [17,34,35]. It should be noted that the bandgap value of the laminar MAPbBr3 SCTF was narrower than that of MAPbBr3 polycrystal film (2.31 eV) [36], which could improve photon harvesting and hence enhance photocurrent generation in solar cells.
Fig. 5. Steady-state absorption and PL spectra (a) Tauc plot displaying the extrapolated optical band-gap (b) of the MAPbBr3 SCTF.
During the observation of the obtained SCTF by using a microscope, we found some special surface morphology which might be helpful to derive the growth kinetics. The morphology of MAPbBr3 SCTF edge was shown in Fig. 6a. The figure showed that the right edge of the thin film was smooth, while there was a very obvious groove along the upper edge of the SCTF. The right end of this groove was in contact with the smooth edge area, which revealed the growth tendency of the edge. As the SCTF further expanded, its bottom preferentially crystallized and protruded outward to form thin rims, which then grew upwards and became smooth. It's worth noting that the thin rim did not grow upward at the same time, but a certain area grew first and then gradually spread to adjacent zone. As shown in Fig. 6b, there were some representative observations on the top surface of MAPbBr3 SCTF, which could provide some enlightenment about the longitudinal growth of the perovskite SCTF. Zone 1 resembled a pyramid-shaped island, which consisted of some smaller and smaller square slices (Fig. 6c). Zone 2 vividly depicted the initial stage of the convergence between islands. At the end of the convergence, islands have been very vague (zone 3). With further growth, islands formed a large smooth area. This growth situation coincided with the island growth model. Fig. 6d showed the 3D AFM image of the MAPbBr3 SCTF. In the 3D AFM image, the high and low regions were staggered. The humps looked like islands, and they also presented the pyramidal
shape, which was in accordance with the previously described characteristics of islands. Humps of different sizes also indicated that islands continued to grow and gradually converge.
Fig. 6. Optical micrograph of MAPbBr3 SCTF edge (a), MAPbBr3 SCTF top surface (b), a representative island (c) and 3D AFM image of 5 μm×5 μm area (d).
With continuous crystallization, the shape of SCTFs gradually became irregular and their growth tended to stop (Fig. 7a). A representative large crystal sheet was carried out the XRD measurement, and the results also changed significantly (Fig. 7b). The XRD pattern of the large MAPbBr3 crystal sheet showed other diffraction peaks at 21.2°, 33.8°, 43.2°, 55.9°, which could be assigned to (110), (210), (220) and (320) planes, indicating that the thin film growth exhibited other directions and the large crystal sheet was not the MAPbBr3 SCTF. It is noteworthy that there were obvious cracks in the rough edge of the large crystal sheet (Fig. 7c). Meanwhile, its surface was not smooth enough and cavities could also be found inside the crystal sheet (Fig. 7d).
Fig. 7. Photograph (a), XRD spectra (b) and optical micrographs (c, d) of large crystal sheet.
The MAPbBr3 SCTF growth along (001) crystallographic direction was more dominant and along other crystal planes was suppressed in the process of continuous film growth [32]. However, the precursor ion transportation can be greatly inhibited in the narrow slit. During the initial period of the crystal growth, adequate precursor ions can be obtained to maintain lateral growth. After further growth, the precursor ion concentration near the SCTF gradually decreased, and the ion diffusion rate also became slower and slower (Fig. 8a). Insufficient replenishment of precursor ions brought out local incomplete crystallization, which could lead to the appearance of cracks and cavities easily. Yanagi et al. prepared MAPbBr3 single crystals by using cast-capping method, which also appeared the similar phenomenon [24]. In addition, the diffusion of precursor ions in the confined gaps is anisotropic (Fig. 8b) [21]. Differences of precursor ion transportations in diverse regions caused the thin film to grow preferentially along the direction of sufficient ion replenishment, which may be also responsible for the non-single crystal thin film. Then initial SCTFs gradually grew into polycrystalline thin films. Therefore, it is difficult to obtain a large-area SCTF with thickness less than 10 μm by using SLITC method [19].
Fig. 8. Schematic illustration of correlation between ion diffusion and SCTF growth (a), schematic illustrations of anisotropic ion diffusion rates in the narrow gaps (b). Thick arrows represent large diffusion rates.
Conclusions We have prepared the MAPbBr3 SCTF by the simply SLITC method. Weak wetting Corning glasses were chosen as substrates to reduce the friction force between precursor solution and substrates and provide better transportation of precursor ions. In addition, we found the island growth model of the MAPbBr3 SCTF. Pyramid-shaped islands grew up first, then gradually converged to achieve the film growth process. However, the growth of large-area SCTFs still faces many problems, such as undesirable cracks and cavities inside the thin crystal sheet after continuous growth. Insufficient replenishment and the anisotropic transportation of precursor ions in narrow gaps constrain the growth of larger-area SCTFs. Based on these observed growth phenomena, promoting long-range transportation of precursor ions along micrometer-size or even nanometer-size gap to continuously replenish the depleted precursor ions by crystal growth is the key to obtain larger and thinner perovskite SCTFs.
Acknowledgement We would like to acknowledge Jumu Zhu for the XRD measurement. This work was supported by the National Natural Science Foundation of China (Grant No.
61704117).
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Highlights:
Weak wetting substrates could promote perovskite precursor ion diffusion.
Island growth mode of MAPbBr3 single-crystalline thin film (SCTF) was found.
The MAPbBr3 SCTF with thickness of 6.38 μm was obtained.
Low ion diffusion in the narrow gap restricted the continuous growth of SCTFs.
S0022-0248(18)30407-X https://doi.org/10.1016/j.jcrysgro.2018.08.027 CRYS 24718
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
13 April 2018 30 July 2018 26 August 2018
Please cite this article as: L. Han, C. Liu, L. Wu, J. Zhang, Observation of the Growth of MAPbBr3 Single-Crystalline Thin Film Based on Space-Limited Method, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/ j.jcrysgro.2018.08.027
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Observation of the Growth of MAPbBr3 Single-Crystalline Thin Film Based on Space-Limited Method Lipeng Hana, Cai Liua,*, Lili Wua,*, Jingquan Zhanga a
Institute of Solar Energy Materials and Devices, College of Materials Science and
Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, China, 610064 * Corresponding authors. Tel/Fax: +86 (0)28 85412542. E-mail addresses: [email protected] (C. Liu), [email protected] (L. Wu) Abstract: Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance. One key aspect is to be able to synthesize SCTFs with large surface areas. Here, we investigated the formation of MAPbBr3 SCTF by space-limited inverse temperature crystallization method. It was found that the weak wetting substrate could reduce friction force and promote precursor ion diffusion, which enabled a continuous growth of the thin perovskite single crystal along in-plane direction. SCTFs could be obtained at the initial stage of film growth and we also observed some signs of island growth model. Pyramid-shaped islands formed first and then converged to achieve top surface growth of SCTFs. After further growth, the precursor ion concentration near the SCTF gradually decreased. The lateral crystal growth was limited by the inefficient and anisotropic long range transportation of precursor ions along the micrometer-size or nanometer-size gap, which resulted in transformation of SCTFs into polycrystalline films. Low ion diffusion in the narrow gap is still the main constraint for the preparation of the larger and thinner SCTFs. Keywords: A1. MAPbBr3; A2. Single-crystalline thin film; B1. Space-limited method; B2. Lateral crystal growth; B3. Ion diffusion
Introduction Organic-inorganic halide perovskites have been attracting much attention due to their outstanding optoelectroinc properties, which are extremely suitable for high performance solar cells, light-emitting devices, photoelectric switches, highly sensitive detectors and so on [1,2]. Moreover, the easy solution-procedure and low cost attract a big number of researchers globally with great enthusiasm to this booming field. The present power conversion effciency of perovskite solar cells was certified to be more than 22% within a few years of its advent because of their superior properties such as high absorption coefficient, long carrier lifetime and highly balanced hole and electron mobility [3,4]. Despite these achievements, the majority of current perovskite-based devices still rely on polycrystalline thin films. In most applications, single-crystal materials can provide more remarkable properties than their polycrystalline counterpart [5,6] and devices based on a well-aligned single crystal always exhibit more excellent performance to those based on polycrystal [7,8]. For single crystal materials, they are important functional units that can be applied to various devices, whose performance can be seriously affected by their crystallinity and morphology. In order to further understand their physical and chemical properties of perovskite materials and find more applications, it is very important to investigate the characteristics of single crystal perovskite materials [9-11]. At present, many methods have been developed to grow bulk perovskite crystals [12] including adding PbI2 crystals to the solution of CH3NH3I (MAI) [13], anti-solvent vapor-assisted crystallization [14], inverse temperature crystallization [15], top-seed solution growth [16] and so on. Nevertheless, using bulk single crystals in photovoltaic devices may lead to severe degradation of device performance due to more carrier recombination with the increase of absorb layer thickness. As the mature preparation technology of bulk perovskite single crystals, the preparation of perovskite single-crystalline thin films (SCTFs) has gradually become a research hotspot for boosting device performance.
They can be prepared by space-limited inverse temperature crystallization (SLITC) method [17-21], surface tension-controlled inverse temperature crystallization method [22], cavitation-triggered asymmetrical crystallization strategy [23] and cast-capping method [24], with thicknesses varying from a few hundred nanometers to several hundred of micrometers. All above methods could generate a lateral driving force for the precursor ions to form the two-dimensional thin single-crystalline perovskites, which overcome the shortcomings of traditional single-crystal growth methods in their tendency to produce merely three-dimensional free-standing perovskite bulk single crystals. Among them, the SLITC method has been extensively implemented due to its simple apparatus configuration and easy film thickness adjustment by varying the thicknesses of spacers and the slit channel. Moreover, some pressure can be applied on substrates to decrease the distance between them, which can be tuned to dozens of nanometers to acquire much thinner SCTFs. Perovskite SCTFs exhibit extremely outstanding properties. Wan et al. revealed that monocrystalline perovskite films exhibited low defect density and good air stability [17]. Huang et al. prepared a solar cell based on monocrystalline perovskite active layer, which presented an efficiency of 17.8% [21]. Ma et al. reported a photodetector based on monocrystalline films perovskite active layer with a record performance of a 50 million gain, 70 GHz gain-bandwidth product, and a 100-photon level detection limit at 180 Hz modulation bandwidth [19]. From previous studies, it was found that large-sized perovskite SCTFs whose thicknesses are over 10 micrometers can be easily prepared, whereas films thinner than 10 micrometers are difficult to obtain for large areas. Meanwhile, very few studies reported the characteristics of the growth procedure of perovskite SCTFs. In the present study, we constructed a simple reactor consisting of two square substrates without spacer between them and used SLITC method to obtain perovskite SCTFs. Single-crystalline MAPbBr3 was chosen to be grown for its proper bandgap for potential application to intermediate band solar cells. We attempted to learn about
the micro growth process by analyzing experimental phenomena during the film preparation process and material characterization results to provide guidance for the subsequent preparation of large-area and high-quality perovskite SCTFs.
Experimental Section Materials Lead bromide (PbBr2, 99.999%), methylammonium bromide (MABr, 99.5%) and N,N-dimethylformamide (DMF, 99.9%) were purchased from Youxuan Advanced Election Tech Co. Ltd. (Yingkou, China). All materials were used without further purification. Substrates were fluorine-doped tin oxide conducting glass (FTO, Pilkington, thickness: 2.2 mm, sheet resistance 14Ω/square), Corning Eagle XG slim glass (glass type: alkaline earth boro-aluminosilicate, substrates for active-matrix flat panel displays) and common quartz glass. SCTFs preparation First, these substrates were cleaned with detergent, and then sequentially ultrasonic cleaned with deionized water, acetone and ethanol, finally dried with N 2. Then PbBr2 and MABr (1:1 by molar, 1.5 M) were dissolved in DMF. A piece of cleaned substrate (60 mm 45 mm) was put on the top of another without spacers to form a narrow gap. The perovskite precursor solution was added to the intersection of two substrate edges (Fig. 1). And it was absorbed into the gap by capillarity force. Finally, this system was placed on a heating plate with temperature of 80 °C to grow the film by inverse temperature crystallization until the growth of the MAPbBr3 thin film was completed. The temperature was slowly decreased to room temperature to ensure a good contact between the substrate and thin films. Then the substrate/perovskite/substrate sample was cleaved and the perovskite thin film left on either substrate was carried out the following measurements.
Perovskite solution
MAPbBr3 single crystal
Top substrate 80℃
Heater
Heater
Bottom substrate Fig. 1. The schematic diagram of growth process.
Characterization X-ray diffraction (XRD) measurements were performed using a DX-2600 X-ray diffractometer (Dandong Fangyuan Instrument Company, China) with Cu Kα radiation of the scan range from 10° to 70°. The transmittance spectra of thin films were recorded by using a PerkinElmer Lambda 950 UV−vis Spectrometer. A field emission scanning electron microscope (SEM) (S-4300, Hitachi) was used to acquire SEM images. The surface morphology was observed by atomic force microscopy (AFM, Multi-Mode 8 SPM, Brucker).The thin film thickness was measured by a step profiler (XP-2, Ambios Technology). A light and dark field metallographic microscope (Mshot, MJ33) was used to investigate crystal morphology of the perovskite thin film. The hydrophobicity of substrates was studied by dropping a deionized water droplet or perovskite precursor solution droplet onto their surface with a contact angle measuring unit (DSA25, Kruss). The photoluminescence (PL) spectra were recorded using an Edinburgh Instruments FLS980 spectrometer and a 473 nm laser was used as the excitation source. Results and discussion In order to grow perovskite SCTFs, we used the classical SLITC method. Crystal growth continuously consumes precursor ions. In turn, the inefficient long-range transportation of precursor ions along micrometer gap prevents further lateral crystal
growth. Since the perovskite precursor ions generally form complex with the solvent, the diffusion rate of precursor ions is partially determined by how fast the solvent molecules can transport on the substrate [25]. The diffusion rate of solvent molecules close to the substrate is determined by the surface tension [22,26,27]. On wetting substrates, the large surface tension imposes a friction force that pulls down the ion transportation [28]. As a result, the interaction between substrate surface and solvent molecules becomes an issue of great importance for precursor ion diffusion [21]. We have chosen three kinds of transparent substrates (FTO glass, Corning glass and quartz glass) for contact angle testing to analyze the interaction between different solvents and substrates. Contacting angles of deionized water on these substrates shown in Fig. 2a-c were 78°, 45° and 41° for FTO glass, Corning glass and quartz glass, respectively. Huang et al. hypothesized that the measured contact angle represented the wetting capability of substrate surfaces to water and it was also an indication of the wetting capability of substrate surfaces to perovskite precursor solution because of their strong polarity [28]. However, it differed significantly when using the perovskite precursor solution to conduct the contact angle test. The contact angle between FTO glass and the perovskite precursor solution reduced sharply from 78° to 0°, which was mainly caused by the difference of the surface tension between these solvents (σdeionized water: 72.8 mN/m, σperovskite precursor solution: about 39.3 mN/m) [22]. The contact angles between other substrates and the perovskite precursor solution only decreased slightly. It revealed that interactions between substrate and deionized water did not perfectly represent interactions between substrate and perovskite precursor solution. There was still some deviation among them. It can be seen that the contact angle between Corning glass and perovskite precursor solution was largest, which meant stronger hydrophobicity. After the contact angle test, precursor solution on different substrates began to crystallize slowly under the condition of natural volatilization of the solvent. The diffusion area of precursor solution on FTO glass was large because of small contact angle (Fig. 2g). During the
solvent evaporation process, crystal nucleuses first appeared in local areas. However, previously nucleuses were not able to grow continuously due to insufficient precursor ion transportation, which was caused by larger friction imposed by the FTO glass substrate surface. Finally, the entire precursor solution coverage area was covered with yellow materials, which is shown in Fig. 2g. For the other two substrates, the edge of the precursor solution droplet was preferentially nucleated, and the precursor ions in the droplet could replenish in time to promote the crystal growth. It's worth noting that there was still a small amount of yellow residue in the internal region of the previous droplet on the quartz glass substrate, which was marked with blue circles in Fig. 2i. However, it was not the case on the Corning glass substrate. Therefore, we chose Corning glass as the substrate for better precursor ion transportation during crystallization in the narrow gap due to weaker friction force.
Fig. 2. The contact angles between deionized water and varied substrates (a-c), the contact angles between MAPbBr3 precursor solution and varied substrates (d-f), MAPbBr3 crystal morphology images for FTO glass, Corning glass and quartz glass after natural volatilization of solvent (g-i).
If we use the precursor solution addition method of cast-capping method [24], it was easy to generate bubbles in the gap and influence crystallization during the heating process. As a result, we made some small improvements. The perovskite precursor solution was added to the edge of substrates and could be absorbed into the gap by capillarity force (Fig. 1). This liquid adding method can not only reduce the amount of precursor solution, but also effectively prevent bubbles inside the gap. It is noteworthy that the precursor solution diffused anisotropically in the gap. And the diffusion speed was higher at the edge of the substrate than that of interior zone. This observation was consistent with the results reported in the literature [21]. For the slow transportation of precursor ions, this anisotropic diffusion in the narrow gap may also have a great impact on the micro-zone crystallization. SLITC method enables MAPbBr3 perovskite thin film to grow in large planar domains between the top and bottom Corning glass substrates. At the initial stage of crystallization, the film on Coring substrates exhibited square shape (Fig. 3a). The lateral size of this thin film was over 1 mm, while its thicknesses determined by step profiler was about 6.38 micrometers (Fig. 3b). It may be the SCTF, which conformed to the characteristic of the MAPbBr3 SCTF [19-21]. Then SEM observation was carried out to study the crystal quality of this regular thin film. It can be seen that the semitransparent film was homogeneous. In addition, the crystal thickness was measured from the side facet by tilting the thin film. A representative crystal thickness shown in the image was estimated to be 6.0 μm (Fig. 3c). The top-view SEM image showed that the thin film was smooth and free of grain boundaries (Fig. 3d).
Fig. 3. Photograph (a), step profile (b), SEM images (c) and (d) of the MAPbBr3 regular sheet.
Some bulk single crystals made by anti-solvent crystallization method were ground to conduct the XRD measurement to obtain all diffraction peaks of MAPbBr3 crystal (Fig. 4a). The XRD pattern of the aforementioned thin film depicted in Fig. 4b showed four sharp diffraction peaks at 15.0°, 30.2°, 45.9° and 62.7°, which can be assigned to (001), (002), (003) and (004) planes, respectively, indicating an exposed face of (001) crystal plane [29,30]. The intensities of the (001) and (002) peak were quite strong, and the appearance of the secondary diffraction peak of (003) and (004) plane suggested acceptable crystalline perfection. It is important to note that the thin film only showed this unique family of reflections, which clearly indicated that its (001) plane grew two-dimensionally and was parallel to the substrate surface. To further study the crystalline quality of the film, the X-ray diffraction rocking curve of (001) plane was measured, as inserted in Fig. 4b. The full width at half-maximum (FWHM) of the (001) peak of the rocking curve was 0.26°. The small FWHM synchronously indicated good crystalline quality and small residual stress in the obtained regular crystal thin film [31]. Regular square shape, excellent crystal morphology and the XRD pattern coincided with the reported information of
MAPbBr3 SCTFs in the literatures [18,22,23]. Hence, the thin film with square shape was the MAPbBr3 SCTF.
Fig. 4. Powder XRD spectra of ground MAPbBr3 powders (a), MAPbBr3 SCTF (b) and inset: rocking curve of the (001) plane.
The optical properties of the MAPbBr3 SCTF were investigated using steady-state absorption and PL spectra. As shown in Fig. 5a, the SCTF showed a clear band edge cutoff with no excitonic signature, which suggested a minimal number of in-gap defect states. A narrow emission peak of PL spectroscopy located at 547 nm was in line with the previous reported value of the MAPbBr3 SCTF [18]. The smaller value of PL peak than that of the absorption onset (~560 nm) can be ascribed to the separation of excitons into free charges at room temperature [32]. The PL peak at longer wavelength (~600 nm) originated from surface defects of MAPbBr3 single crystal [33] was not identifed for the current SCTF, implying a high crystalline characteristic. Bandgap extracted from Tauc plot (Fig. 5b) was calculated to be 2.25 eV, which was very close to previously reported values (2.24 eV) for bulk single crystal and planar-integrated single crystalline film [17,34,35]. It should be noted that the bandgap value of the laminar MAPbBr3 SCTF was narrower than that of MAPbBr3 polycrystal film (2.31 eV) [36], which could improve photon harvesting and hence enhance photocurrent generation in solar cells.
Fig. 5. Steady-state absorption and PL spectra (a) Tauc plot displaying the extrapolated optical band-gap (b) of the MAPbBr3 SCTF.
During the observation of the obtained SCTF by using a microscope, we found some special surface morphology which might be helpful to derive the growth kinetics. The morphology of MAPbBr3 SCTF edge was shown in Fig. 6a. The figure showed that the right edge of the thin film was smooth, while there was a very obvious groove along the upper edge of the SCTF. The right end of this groove was in contact with the smooth edge area, which revealed the growth tendency of the edge. As the SCTF further expanded, its bottom preferentially crystallized and protruded outward to form thin rims, which then grew upwards and became smooth. It's worth noting that the thin rim did not grow upward at the same time, but a certain area grew first and then gradually spread to adjacent zone. As shown in Fig. 6b, there were some representative observations on the top surface of MAPbBr3 SCTF, which could provide some enlightenment about the longitudinal growth of the perovskite SCTF. Zone 1 resembled a pyramid-shaped island, which consisted of some smaller and smaller square slices (Fig. 6c). Zone 2 vividly depicted the initial stage of the convergence between islands. At the end of the convergence, islands have been very vague (zone 3). With further growth, islands formed a large smooth area. This growth situation coincided with the island growth model. Fig. 6d showed the 3D AFM image of the MAPbBr3 SCTF. In the 3D AFM image, the high and low regions were staggered. The humps looked like islands, and they also presented the pyramidal
shape, which was in accordance with the previously described characteristics of islands. Humps of different sizes also indicated that islands continued to grow and gradually converge.
Fig. 6. Optical micrograph of MAPbBr3 SCTF edge (a), MAPbBr3 SCTF top surface (b), a representative island (c) and 3D AFM image of 5 μm×5 μm area (d).
With continuous crystallization, the shape of SCTFs gradually became irregular and their growth tended to stop (Fig. 7a). A representative large crystal sheet was carried out the XRD measurement, and the results also changed significantly (Fig. 7b). The XRD pattern of the large MAPbBr3 crystal sheet showed other diffraction peaks at 21.2°, 33.8°, 43.2°, 55.9°, which could be assigned to (110), (210), (220) and (320) planes, indicating that the thin film growth exhibited other directions and the large crystal sheet was not the MAPbBr3 SCTF. It is noteworthy that there were obvious cracks in the rough edge of the large crystal sheet (Fig. 7c). Meanwhile, its surface was not smooth enough and cavities could also be found inside the crystal sheet (Fig. 7d).
Fig. 7. Photograph (a), XRD spectra (b) and optical micrographs (c, d) of large crystal sheet.
The MAPbBr3 SCTF growth along (001) crystallographic direction was more dominant and along other crystal planes was suppressed in the process of continuous film growth [32]. However, the precursor ion transportation can be greatly inhibited in the narrow slit. During the initial period of the crystal growth, adequate precursor ions can be obtained to maintain lateral growth. After further growth, the precursor ion concentration near the SCTF gradually decreased, and the ion diffusion rate also became slower and slower (Fig. 8a). Insufficient replenishment of precursor ions brought out local incomplete crystallization, which could lead to the appearance of cracks and cavities easily. Yanagi et al. prepared MAPbBr3 single crystals by using cast-capping method, which also appeared the similar phenomenon [24]. In addition, the diffusion of precursor ions in the confined gaps is anisotropic (Fig. 8b) [21]. Differences of precursor ion transportations in diverse regions caused the thin film to grow preferentially along the direction of sufficient ion replenishment, which may be also responsible for the non-single crystal thin film. Then initial SCTFs gradually grew into polycrystalline thin films. Therefore, it is difficult to obtain a large-area SCTF with thickness less than 10 μm by using SLITC method [19].
Fig. 8. Schematic illustration of correlation between ion diffusion and SCTF growth (a), schematic illustrations of anisotropic ion diffusion rates in the narrow gaps (b). Thick arrows represent large diffusion rates.
Conclusions We have prepared the MAPbBr3 SCTF by the simply SLITC method. Weak wetting Corning glasses were chosen as substrates to reduce the friction force between precursor solution and substrates and provide better transportation of precursor ions. In addition, we found the island growth model of the MAPbBr3 SCTF. Pyramid-shaped islands grew up first, then gradually converged to achieve the film growth process. However, the growth of large-area SCTFs still faces many problems, such as undesirable cracks and cavities inside the thin crystal sheet after continuous growth. Insufficient replenishment and the anisotropic transportation of precursor ions in narrow gaps constrain the growth of larger-area SCTFs. Based on these observed growth phenomena, promoting long-range transportation of precursor ions along micrometer-size or even nanometer-size gap to continuously replenish the depleted precursor ions by crystal growth is the key to obtain larger and thinner perovskite SCTFs.
Acknowledgement We would like to acknowledge Jumu Zhu for the XRD measurement. This work was supported by the National Natural Science Foundation of China (Grant No.
61704117).
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Highlights:
Weak wetting substrates could promote perovskite precursor ion diffusion.
Island growth mode of MAPbBr3 single-crystalline thin film (SCTF) was found.
The MAPbBr3 SCTF with thickness of 6.38 μm was obtained.
Low ion diffusion in the narrow gap restricted the continuous growth of SCTFs.