Perovskite has always been a star material in the field of optoelectronic research due to its excellent optoelectronic properties. In addition to the research progress in material systems, process methods, and interface engineering, the research on perovskite single crystals has also received much attention. Compared to ordinary perovskite polycrystalline films, the most obvious advantage of perovskite single crystals is that they have fewer grain boundaries, and the reduction of grain boundaries improves their defect density of states and carrier mobility. For example, the carrier migration length in CH3NH3PbI3 single crystal is as high as 175 microns, and the defect density is also as low as 109-1010 cm-3. The defect state density is 6 orders of magnitude lower than that of polycrystalline perovskite thin films, and also lower than commonly used photovoltaic materials such as polycrystalline silicon (1013-1014 cm-3), cadmium telluride (1011-1013 cm-3), and copper indium gallium selenium thin films (1013 cm-3). Therefore, perovskite single crystals have always been a hot topic in this research field, with a large amount of core research focused on the synthesis technology of single crystals. Generally speaking, the main techniques for single crystal synthesis include cooling crystallization, heating crystallization, solvent evaporation, and anti solvent assisted crystallization. Through the development and research of single crystal synthesis technology, various perovskite material systems (such as ABX3, A is MA, FA, Cs, B is Pb or Sn, X is halogen) single crystals have been obtained, and the single crystal size has also reached the centimeter level. The rapid development of single crystal synthesis technology is driving research in the field of perovskite crystal structure, crystal growth, and physical and chemical properties. However, at the same time, how to utilize the many advantages of single crystals to prepare single crystal perovskite devices has gradually become a new research direction in the field. Perovskite single crystal devices rely on single crystal thin films as the main photosensitive layer. Whether applied in fields such as photovoltaics, photodetectors, or light-emitting diodes, the integration of perovskite single crystal thin films and other device components needs to be considered. At the same time, single crystal thin films with too large thickness are not conducive to carrier transport, and the thickness control of single crystal thin films also needs to be considered. This article mainly summarizes the current preparation methods of single crystal devices, providing an overview of the problems and technical characteristics in the preparation process of perovskite single crystal devices for researchers. It is also hoped that through the summary of this article, everyone can have a general understanding of how to solve two problems in the preparation of perovskite single crystal devices, namely "integration of perovskite single crystal films in devices" and "thickness control of perovskite single crystal films".
Space limited single crystal thin film preparation technology
Through the introduction, we have learned that one of the difficulties in preparing single crystal devices is how to control the thickness of the single crystal and integrate it into the device. Currently, the use of clamps to compress the space for single crystal growth is a widely applicable method to prepare thin single crystal thin films. Compared with other single crystal thin films and thickness control methods, the spatial limitation method has simpler operating conditions and is suitable for the growth of thin films in various perovskite material systems. By restricting and designing the geometric space between the clamps, the thickness, crystallization, morphology, and size of single crystal thin films can be flexibly controlled. And as shown in Figure 1, the spatial confinement method can be combined with various single crystal synthesis techniques to prepare single crystal thin films, with excellent versatility and compatibility, and less substrate requirements, simplifying the overall preparation process of single crystal devices [2]. In 2020, the Bakr team from King Abdullah University has prepared MAPbI3 single crystal solar cell devices with an efficiency of 21.9% through space confinement method [3]. Summarizing the technical issues related to space confinement method, the following aspects need to be paid attention to: (1) Select appropriate single crystal synthesis and binding methods. Based on the previous introduction, there are mainly top seed solution growth method, cooling crystallization method, reverse heating crystallization method, and anti solvent steam assisted crystallization. The combination of different methods can affect the thickness control of single crystal preparation, The literature reports that MAPbBr3 single crystal films ranging from 13 nanometers to 4.5 microns can be prepared using the top seeding method. (2) The morphology of the single crystal thin film prepared by the spatial confinement method is also hexagonal, consistent with the morphology of large-sized single crystals, indicating that the spatial confinement method does not affect the growth mode of crystals, but only regulates the thickness of the thin film through geometric compression. (3) The thickness control of the spatial limitation method can first be directly controlled by adjusting the spatial distance of the clamp, which can generally be adjusted to the micrometer level. Subsequently, the thickness of the single crystal film can be further compressed and reduced by emphasizing the control of spatial pressure, and then adjusted to the nanometer level film. (4) Surface treatment of the substrate can regulate the energy barrier for nucleation of single crystal thin films. Hydrophilic treatment can lower the barrier, while hydrophobic treatment can increase it. Therefore, UV or ozone surface treatment of the sandwich can regulate the nucleation process and density of the intermediate layer thin film. (5) By increasing the fluidity of the precursor solution and the wettability of the substrate, the diffusion rate of precursor ions in a confined solution can be accelerated, thereby regulating the crystal size of single crystal thin films. In summary, the space limited growth method is a highly compatible and easy to operate method for preparing single crystal thin films. By combining the selection of single crystal synthesis methods, adjusting the geometric morphology and distance between the clips, the fluidity of the solution, and the hydrophilicity of the substrate can all play a role in controlling the thickness and quality of single crystal thin films. The common characterization method of spatial confinement method is generally XRD to observe the crystallization performance of single crystal thin films and calculate internal stress, and cross-sectional SEM to observe the thickness of single crystal thin films and the cross-sectional layering of devices. The preparation method is basically the solution method, and the applicable devices are generally conventional structure solar cells, junction photodetectors, and other devices.
Figure 1 Schematic diagram of spatial restriction method
Top Down Single Crystal Thin Film Preparation Technology
The development of large-sized single crystal synthesis technology has become relatively mature. If the growth of single crystal thin films is not carried out on the substrate, how can the preparation of single crystal blocks be integrated into the device? Researchers have proposed a top-down single crystal thin film preparation technique to address this issue. By physically or chemically cutting the prepared single crystal block crystals, the thickness of the single crystal can be controlled, while transferring the single crystal to the desired substrate thin film or preparing the device integration on the single crystal evaporation electrode. As shown in Figure 2, in 2016, Professor Liu Shengzhong's team used physical line cutting to directly cut FAPbI3 single crystals, resulting in thin perovskite single crystal wafers, which were applied in the preparation of photoconductive photodetectors. The best perovskite wafer thickness obtained through this physical line cutting method was around 100 microns [4]. In 2018, Peter M ü ller Buschbaum's team used sharp cutting tools and sandpaper to cut and grind single crystals, and fixed the single crystal thin film onto the carrier transport layer substrate of the device. Although the device efficiency was not high, it was also an attempt at single crystal top-down technology [5]. As shown in Figure 3, in 2018, the Yan team first prepared perovskite single crystal wafers with a thickness of 200 microns through physical cutting and polishing methods. Subsequently, the perovskite single crystal wafers were further thinned by chemical etching, resulting in the thinnest perovskite single crystal wafers with a thickness of 15 microns [6]. The top-down preparation method is a reliable method for thinning perovskite single crystals and preparing single crystal devices. Physical cutting and chemical etching can effectively control their thickness. The method process is simple and does not require excessive consideration of the conditions and factors of single crystal growth. However, it also has certain limitations in the accuracy of thickness thinning and the minimum thickness obtained. The general characterization method for top-down preparation of single crystal thin films in literature is to observe the preparation of single crystal thin films by examining the surface morphology of crystals under a microscope, observing the crystallization performance of single crystal thin films through XRD, and observing the thickness of single crystal thin films through cross-sectional SEM. The preparation method is generally physical cutting using sharp cutting tools such as diamond lines, or chemical etching using solution method, The applicable devices are generally conventional solar cells, junction photodetectors, and other devices.
Figure 2 Mechanical cutting of perovskite single crystal wafers
Figure 3 Mechanical cutting and chemical etching of perovskite single crystal wafers
Preparation of optoelectronic devices using perovskite single crystals as substrates
The preparation methods of the above two perovskite single crystal films mainly achieve the goal of integrating devices by placing or growing single crystal films on the carrier transport layer film. In addition, many studies have also used perovskite bulk single crystals as substrates to directly deposit thin films and electrodes of other layers to prepare perovskite single crystal optoelectronic devices. The most common device structure is a horizontally structured solar cell, as shown in Figure 4. A layer of metal electrode is first deposited on one side of the single crystal, followed by a carrier transport layer and another electrode, to finally prepare a single crystal photovoltaic cell [7-8]. At the same time, this structure is often used in the field of perovskite single crystal detectors. For photoconductive detectors, it is generally only necessary to evaporate the interdigital electrode on the single crystal to conduct photodetector testing; For photovoltaic or detector devices with junction structures, the deposition of each layer of thin film can also be carried out through the process of front and back evaporation, as shown in Figure 5. In 2020, Haotong Wei's team prepared X-ray detectors by depositing Au electrodes on the upper layer of perovskite single crystals and evaporating C60 layers, BCP, and Cr electrodes on the other side of the single crystals [9]. In summary, for the transverse structure, charge carriers propagate along the plane, so there is no need to consider the thinning of perovskite single crystals. Therefore, using perovskite single crystals as a substrate can directly prepare solar cells and photodetectors with transverse structures. Meanwhile, by depositing thin films on both sides of the single crystal, conventional structured photovoltaic devices and perovskite single crystal X-ray detection devices can be prepared. Compared with the first two methods, directly using perovskite single crystals as substrates to prepare devices does not harm the single crystals. At the same time, reasonable structural selection and design can also avoid difficulties such as controlling the thickness of perovskite single crystals. However, due to the single structure and process, the efficiency of photovoltaic devices is not high. This method is more suitable for the preparation of single crystal devices for visible light detectors and ray detectors.
Figure 4 Schematic diagram of perovskite single crystal X-ray detector structure
Layered perovskite single crystal exfoliation and transfer technology
Compared to three-dimensional perovskite, the formation of two-dimensional perovskite is achieved by cutting the three-dimensional perovskite lattice through a large amine chain cation isolation layer. The weak van der Waals interaction between the isolation layer and three-dimensional perovskite allows researchers to easily obtain ultra-thin perovskite single crystal wafers through mechanical exfoliation, thereby completing transfer and device preparation. The delamination method of layered two-dimensional perovskite single crystals is mainly achieved by pasting transparent tape, which is the same as the delamination technology of two-dimensional semiconductors such as graphene. Because the principles are similar, their transfer techniques also draw inspiration from two-dimensional semiconductor transfer techniques, including wet transfer such as etching and dry transfer such as roll to roll. A relatively simple and applicable method is PDMS assisted dry transfer, which is based on the viscoelastic adhesion of PDMS to perovskite single crystal materials. After reversing and moving to the desired position, the PDMS can be peeled off [10]. Layered perovskite exfoliation technology can continuously thin two-dimensional perovskite single crystals and prepare corresponding single crystal devices. By exfoliating the single crystal, the purity and good crystallinity of the crystalline phase of the single crystal thin film can be maintained. As shown in Figure 6, Peidong Yang's research group successfully prepared pure phase two-dimensional perovskite light-emitting diodes using single crystal exfoliation transfer technology to achieve blue light emission [11]. In summary, the peeling and transfer technology of perovskite single crystals is mainly applied in two-dimensional layered perovskite, and the related technologies are based on the research of two-dimensional semiconductors. Therefore, they are relatively mature. The peeled two-dimensional perovskite has good crystalline phase purity and is also suitable for preparing two-dimensional perovskite single crystal devices in terms of thickness. Compared to the above three technologies, they have certain material limitations and are not applicable in three-dimensional perovskites.
Figure 5 Schematic diagram of two-dimensional perovskite single crystal blue light LED structure and single crystal thin film
In addition to the above-mentioned single crystal film preparation techniques, there are also techniques such as ultrasonic triggered asymmetric crystallization and vapor phase epitaxy growth that have been applied to the preparation of perovskite single crystal films. We will not go into detail here, and the research and innovation of different methods are all driving the progress of perovskite single crystal device preparation technology. Regardless of the development of preparation technology for perovskite single crystal devices, two core issues need to be addressed: thickness control of single crystal thin films and integration with other parts of the device's thin films. Perovskite single crystal devices retain the excellent optoelectronic properties and extremely low defect density of single crystals, which is of great significance for the preparation of efficient, stable, and high-purity crystalline perovskite optoelectronic devices in the future. The references cited in this article are also listed below, hoping to provide some help and inspiration for the research of researchers.
Reference:
[1] Li, Junyu, et al. "Perovskite Single Crystals: Synthesis, Optoelectronic Properties, and Application." Advanced Functional Materials (2020): 2008684.
[2] Li, Linyi, et al. "Space-confined growth of metal halide perovskite crystal films." Nano Research (2020): 1-16.
[3] Alsalloum, Abdullah Y., et al. "Low-temperature crystallization enables 21.9% efficient single-crystal MAPbI3 inverted perovskite solar cells." ACS Energy Letters 5.2 (2020): 657-662.
[4] Liu, Yucheng, et al. "20‐mm‐Large single‐crystalline formamidinium‐perovskite wafer for mass production of integrated photodetectors." Advanced Optical Materials 4.11 (2016): 1829-1837.
[5] Schlipf, Johannes, et al. "Top-Down approaches towards single crystal perovskite solar cells." Scientific reports 8.1 (2018): 1-8.
[6] Lv, Qianrui, et al. "A universal top-down approach toward thickness-controllable perovskite single-crystalline thin films." Journal of Materials Chemistry C 6.16 (2018): 4464-4470.
[7] Liu, Ye, et al. "Fast growth of thin MAPbI3 crystal wafers on aqueous solution surface for efficient lateral‐structure perovskite solar cells." Advanced Functional Materials 29.47 (2019): 1807707.
[8] Song, Yilong, et al. "Efficient lateral-structure perovskite single crystal solar cells with high operational stability." Nature communications 11.1 (2020): 1-8.
[9] Song, Jinmei, et al. "Facile strategy for Facet competition management to improve the performance of perovskite single-crystal X-ray detectors." The journal of physical chemistry letters 11.9 (2020): 3529-3535.
[10] Kang, Junmo, et al. "Graphene transfer: key for applications." Nanoscale 4.18 (2012): 5527-5537.
[11]Chen H, Lin J, Kang J, et al. Structural and spectral dynamics of single-crystalline Ruddlesden-Popper phase halide perovskite blue light-emitting diodes[J]. Science advances, 2020, 6(4): eaay4045.
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