Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives

Organic–inorganic hybrid perovskites, which combine the superior optical and electronic properties and solution-processed manufacturing, have emerged as a new class of revolutionary optoelectronic devices with the potential for various practical applications. Encouraged by the advantages of longer carrier diffusion length, higher carrier mobility and lower trap densities as compared to the polycrystalline counterparts’, increasing research attention has focused on preparation and optimization of perovskite crystal candidates, via using various facile growth techniques, for the development of a wide range of optoelectronic applications. This chapter presents a comprehensive review of recent advances in the field of optoelectronic technologies based on different forms of single crystals, including bulk crystals and thin ones, with emphasis placed on the optimization of crystals and the relationship among the charge-carrier transport, operation mechanism, device architecture, and device performance. First, we introduce the main methods used to prepare bulk and thin single crystals, and analyze several aspects of their properties. Thereafter, the applications of single crystals into solar cells, photodetectors, light-emitting diodes, and lasers, are discussed in depth. Finally, we summarize the challenges of perovskite single crystals and propose further improvements in the synthesis approaches and device applications.


Introduction
Perovskite-structured materials have received increasing attention, since being discovered in the 1830s, because of their rich physical properties [1]. As shown in Figure 1a [2], the general chemical formula for such compounds is ABX 3 , in which A and B are different cations, and X is an anion that bonds to both the A and B cations. Owing to the flexibility of bond angles inherent in the perovskite structure, there are many different distortions that can occur from the ideal structure. Importantly, A can be organic cations, like methylammonium (MA + ) or formamidinium (FA + ) [4][5][6][7][8], B can be metal ions, such as Pb 2+ and Sn 2+ [9][10][11][12], and X is usually halide ions [13], and such a class of materials is known   [47], copyright 2015. 2c, MAPbBr 3 crystals from STL method. J. Cryst. Growth [48], copyright 2015. 2d, photographs of perovskite crystals with different halide ratio. Nature Photonics [49], copyright 2015. 2e, MAPbI 3 and MAPbBr 3 crystals growth at different time intervals. Nature Commun. [50], Copyright 2015. 2f, schematic of crystals growth. J. Mater. Chem. C [51], copyright 2016. 2 g, schematic of AVC method. Science [15], copyright 2015. studied the effect of molar ratio of MAX and PbX 2 in the precursor solutions on the crystal quality [52], e.g., perovskite crystals with different sizes and shapes were obtained after a 6-hour ITC crystallization process when changing the MAX: PbX 2 ratios from 1:1 to 2:1.
With an aim of growing a large-sized bulk perovskite crystal, such ITC method was further modified. Using such technique, the strategy of incorporation of seed crystal growth has been proven to be favorable for single crystals as large as convenient. Liu's group reported various large-sized perovskite crystals via using the modified ITC method, from which a number of larger-sized crystal (7 mm) were obtained through choosing good-quality seed crystals and repeating and carefully controlling the ITC process several times (Figure 2f). Moreover, Liu's group also successfully grew MAPb(Br x I 1 − x ) 3 single crystals with a finely-tuned bandgap [51]. The application of the different solubility of different perovskite single crystals at varying temperatures contributes to the time-saving feature of such ITC method.

Anti-solvent vapor-assisted crystallization (AVC) method
Another main method to grow perovskite crystals is the AVC method (Figure 2g), which was first introduced from Bakr's group [15]. In this method, the solvent plays a significant role because two or more solvents should be selected, of which one should be a good solvent that is less volatile, and the other is a bad solvent that is more volatile. The principle of this method can be described as follows: when the bad solvent slowly diffuses into the precursor solution, the proficiency of the crystal formation increases at the bottom of the sample vial owing to the insolubility of the material in the bad solvent. Other groups, like Loi's group and Cao's group, also applied this method to obtain the high-quality crystals [38,53]. Although the AVC method costs more time than the ITC method, its temperature-irrelevant characteristic is appealing to its widespread use.

Thin single crystals
Bulk perovskite single crystals with thick sizes may cause the increase of charge recombination, which would lead to the degradation of their device performance and impede the practical applications. In this regard, growing thin perovskite crystals with a large area represents an effective approach to overcome the above obstacle and thus advances the further practical applications. Bakr et al. introduced a cavitation-triggered asymmetrical crystallization strategy, in which a very short ultrasonic pulse (≈1 s) was applied in the solution to reach a low supersaturation level with anti-solvent vapor diffusion and a thin crystal with several-micrometers grew on the substrates within hours (Figure 3a) [54]. Liu's group synthesized perovskite crystal wafers with a much thinner thickness using a dynamic flow micro-reactor system [55]. They put two thin glass slides in parallel into a container with a predefined separation to grow single crystals within the slit channel, as shown in Figure 3b. Su's group further used a space-limited ITC method and grew a 120-cm 2 single crystal on fluorine-doped tin oxide (FTO)-coated glass, of which the operation and the obtained 0.4-mm-thin single crystal are shown in Figure 3c [56]. Meanwhile, Wan et al. reported a space-confined solution-processed method to grow the perovskite single-crystalline films with adjustable thickness from nanometers to micrometers (Figure 3d) [57]. Benefitting from the capillary pressure, the perovskite precursor solution filled the whole space between two clean flat substrates, which were clipped together and dipped in the solution.
Currently, more promising approaches have been employed to grow thin single crystals with high quality and large scale. A one-step printing geometrically-confined lateral crystal growth method (Figure 3e) was introduced by Sung's group to obtain a large-scaled single crystal [58]. During the process, a cylindrical metal roller with a flexible poly-(dimethyl-siloxane) (PDMS) mold was wrapped and then rolled on a preheated SiO 2 substrate (180°C) with an ink supplier filled with the precursor solution. Alternatively, millimeter-sized single crystals were synthesized by Song's group by a facile seed-inkjet-printing approach (Figure 3f) [59]. Perovskite precursor solution was injected onto a silicon wafer, and then the ordered seeds were formed on the substrate with the evaporation of the droplets. Thereafter, the substrate with a saturated perovskite solution was covered and the single crystals can be grew as the solvent dried at room temperature. Seeds were used to inhibit the random nucleation and trigger the growth of single crystals.
As discussed above, some optimized space-limited approaches have been introduced and developed to synthesize perovskite thin crystals in recent years. Especially, size−/thickness-controlled thin crystals have also been widely used in various optoelectronic devices. With the aim to growing large-scaled and thicknesscontrolled thin crystals with longer carrier diffusion lengths, fewer defects, and higher efficiency, more promising strategies will be rewarding in the future.

Optical properties
There are two normal ways to study the optical properties of hybrid perovskite crystals: absorption and PL measurements. Bakr et al. characterized the steady-state  absorption and PL properties for MAPbBr 3 and MAPbI 3 crystals, as shown in Figure 4a and b [50]. Sharp band edges were observed in the absorption plots and the band gap values were determined to be 2.18 eV for MAPbBr 3 crystals and 1.51 eV for MAPbI 3 crystals; while the PL intensity peaks are located at 574 nm for MAPbBr 3 and 820 nm for MAPbI 3 . As for the MAPbCl 3 one, absorption measurement result revealed an edge at 435 nm (Figure 4c) [60]. Clearly, the optical absorption of perovskite crystals exhibited a clear-cut sharp band edge, which indicated that the single crystals are predominantly free from grain boundaries and have relatively low structural defects and trap densities.
More recently, there have been more broad publications on the apparent disparity in optical properties (i.e., absorption and PL) between perovskite single crystals and thin films, which can be attributed to the incorrect measurements as a result of reabsorption effects. Snaith's group performed a detailed investigation of the optical properties of MAPbBr 3 crystals as compared to those of the polycrystalline films by employing light transmission spectroscopy, ellipsometry, and spatially resolved and time-resolved PL spectroscopy [61]. They showed that the optical properties of the perovskite crystals were almost identical to those of polycrystalline films, and their observations indicated that the perovskite polycrystalline films were much closer to possessing 'single-crystal-like' optoelectronic properties than previously thought, and also highlighted the discrepancies in the estimation of trap densities from the electronic and optical methods (Figure 4d). For the further development of perovskite crystals, more detailed experimental investigations combined with theoretical calculations that focus on the optical features are required, which would assist in the preparation of the high-quality perovskite single crystals and the development of the high-performance device applications.

Charge transport properties
As for hybrid perovskite crystals, in addition to the remarkable optical properties, their promising electrical properties have caught the great attention. In general, there are five common methods to measure the transport mobilities in perovskite crystals, including the space charge limited current (SCLC), time-offlight (TOF), Hall Effect, THz pulse and field-effect transistor (FET) measurement methods. Among these methods, the SCLC method is widely employed to determine the carrier mobility and trap density of perovskite crystals. The current-voltage (I-V) curve can be divided into three parts: the first region, where an Ohmic contact exists, hence the conductivity can be estimated; the second region is the trap-filling region, which is increased sharply at trap-filled limit voltage (V TFL ); and the third region, known as the child region. Trap density (n trap ) can be obtained by following the relation: n trap = (2V TFL εε 0 )/(eL 2 ), where ε 0 is the vacuum permittivity, ε is the relative dielectric constant, L is the crystal thickness, and e is the electron  charge. Moreover, the mobility (μ) is determined by fitting the I-V curve with Mott-Gurney's law: μ = (8JL 3 )/(9εε 0 V 2 ), where J is the current density. Liu's group designed the hole-only device (Figure 5a), and a large hole mobility of 67.27 cm 2 /Vs was estimated [62]. An SCLC method was also applied on MAPbBr 3 crystals, with an n trap of 5.8 × 10 9 cm −3 and a μ of 38 cm 2 /Vs [15]. I-V response of a MAPbCl 3 crystal was measured by Bakr's group with n trap = 3.1 × 10 10 cm −3 and μ = 42 cm 2 /Vs [60]. Another method to measure the μ is the TOF method. Bakr's group obtained the μ via using the TOF method (Figure 5b) [15], from which μ can be defined by the equation: μ = d 2 /(Vτ t ), where d is the sample thickness, V is the applied voltage, and τ t is the transit time that be provided by the transient current under different driving voltages [67,68]. The same method was also applied by Huang's group and the electron μ was verified to be 24.0 ± 6.8 cm 2 /Vs (Figure 5c) [63]. Apart from the above two methods, Bakr et al. also carried out the complementary Hall Effect measurements on perovskite crystals, confirming the μ ranging from 20 to 60 cm 2 /Vs [15]. Meanwhile, Huang's group applies the Hall Effect measurement [68], and they showed the crystals were slightly p-doped with a low free holes concentration. Thereafter, Podzorov's group increased the conductivity of MAPbBr 3 single crystals by sputtering Ti on the flat-faceted single crystal to form Hall bars (Figure 5d) [64], from which the Hall mobility was calculated to be 10 cm 2 /Vs.
Although the above measurement approaches have been widely used in the perovskite crystals, the obtained results from different groups are sometimes different. Sargent et al. demonstrated that one main challenge that may explain these order-of-magnitude discrepancies is that the Hall Effect, TOF, and SCLC methods all probe the mobilities near the respective Fermi levels during the experiments, and the (non-equilibrium, high-injection-level) Fermi level is widely different in each experiment [64]. In this regard, they developed a contactless method to measure the mobility of a perovskite crystal directly [64]. Plus, THz pulse measurement was also used to estimate μ. David et al. used a two-color laser plasma in dry air to generate multi-THz pulses and excited the large MAPbI 3 single crystals and detected the electric field by an air-biased coherent detection scheme with 1-30 THz ultrabandwidth after normal incidence reflection off the crystal facet (Figure 5e, f) [65]. Such spectra measurements indicate the ultrafast dynamics and efficiencies of free charge creation and remarkably high μ as high as 500-800 cm 2 /Vs. Furthermore, FETs are the fundamental components to realize digital integrated circuits, which are also often used as a platform to evaluate charge transport mechanism in the active materials. In this regard, bottom-gate, top-contact FETs were fabricated via using micrometer-thin MAPbX 3 (X = Cl, Br, and I) crystals as active layer (Figure 4g) [66], from which the field-effect μ values are up to 4.7 and 1.5 cm 2 /Vs in p-and n-channel devices, respectively (Figure 5h).
Carrier lifetime (τ) is an important parameter that should be considered when designing an optoelectronic device. Upon excitation by photons, the active materials will be in an excited state. After that, the photo-induced holes and electrons will recombine back to the ground state. Usually, if this recombination process, that is, the carrier lifetime of carriers, is sufficiently long, a high performance device will be expected. The τ of semiconductors strongly depends on the nature, dimension, and purity of the materials. Generally, τ can be obtained from the PL decay, transient absorption, as well as the transient photo-voltage decay and impedance methods [69]. Among these methods, the PL decay approach has been widely applied. The superposition of fast and slow components of carrier dynamics from the PL spectra measurement result yield τ ≈ 41 and 357 ns for MAPbBr 3 (Figure 5i) [15,70,71]. Transient absorption (TA) also suggests the recombination property of excitons which is used to determine the carrier lifetime through a bi-exponential fitting [60]. The carrier diffusion length L D can be further

Photovoltaic cells
The widely studied hybrid perovskite solar cells with high performance are usually made from polycrystalline films; however, the current studies have also focused on the developments and optimization of single crystal perovskite solar cells, owing to their significant advantages. Huang et al. fabricated photovoltaic devices based on MAPbI 3 bulk crystals by depositing gold (Au) as anodes and gallium (Ga) as cathodes (Figure 6a) [63]. A red-shift of 50 nm of the EQE cutoff to 850 nm showed that MAPbI 3 crystals increased the upper limit of short-circuit current density (JSC) compared with the polycrystalline solar cells from 27.5 mA/cm 2 to 33.0 mA/cm 2 . Notably, as compared with the perovskite polycrystalline solar cells, the bulk crystal devices showed much lower efficiency, which was attributed to the fact that photogenerated carriers could not be fully collected in a thick active layer. Much thinner MAPbBr 3 monocrystalline films grown on indium tin oxide (ITO)-coated glass were applied into the solar cells, and the devices showed the best cell performance with a fill factor (FF) of 0.58, a JSC of 7.42 mA/cm 2 , an open-circuit voltage (VOC) of  1.24 V, and a PCE of 5.37% (Figure 6b) [54]. To enhance the device performance, Huang's group further fabricated crystal solar cells through interface engineering (Figure 6c), of which the best device showed a JSC of 20.5 mA/cm 2 , a VOC of 1.06 V, a FF of 74.1%, and a PCE of 16.1% [72]. The single crystal solar cell also displayed the better device stability of remaining nearly unchanged after storage in air for 30 days.
In addition to the vertical-structured solar cells, Huang's group also fabricated the lateral structure perovskite crystal device (Figure 6d) [73], which showed a VOC of 0.82 V and the highest PCE of 5.36% at 170 K. More recently, a 20-μm MAPbI 3 single crystal inverted p-i-n solar cell with a PCE as high as 21.09% and a FF up to 84.3% was fabricated [74], of which the cross-sectional SEM image and photovoltaic performance are shown in Figure 6e and f. To further realize the optimized performance of perovskite crystal solar cells, more efforts will be performed to enhance the sample quality and to design promising device structures.

Photodetectors
Photodetectors which can convert incident light into electrical signals are critical for various industrial and scientific applications. To evaluate the photodetector performance, several parameters are important, including responsivity (R), detectivity (D*), Gain (G), and linear dynamic range (LDR), which are listed and are explained in Table 1 briefly.

In visible region
Huang's group fabricated perovskite crystal photodetectors that exhibited a high sensitivity capacity, which led to a narrow-band photo-response with a full width at half maximum (FWHM) of less than 20 nm under V = −1 V (Figure 7a) [49]. EQE spectra of the single crystals showed a narrow peak near the absorption edge, which promised a detection application at a specific wavelength, with a peak D* over 2 × 10 10 Jones at 570 nm under V = −4 V (Figure 7b). Also, Huang et al. further fabricated vertical structured perovskite crystal photodetectors by using the non-wetting hole transport layer-coating substrates [75]. The noise currents are as low as 1.4 and 1.8 fA/Hz 1/2 at an 8-Hz frequency for the devices based on MAPbBr 3 and MAPbI 3 , respectively. Additionally, the photocurrent responses of both the MAPbBr 3 and MAPbI 3 devices were linear, and their LDRs are up to 256 and 222 dB, respectively. Sun's group introduced a planar-type photodetector on the MAPbI 3 crystal (001) facet with a highest R value of 953 A/W and EQE of 2.22 × 10 5 % at a light power density of 2.12 nW/cm 2 [76]. Wei's group used a two-step method to fabricate a self-powered photodetector based on a MAPbBr 3 crystal core-shell heterojunction [77]. The device showed a broad photo-response ranging from 350 to 800 nm and a peak R up to 11.5 mA/W. Hu's group fabricated photodetectors based on MAPbI 3 single crystal nanowires and nanoplates by transferring them to SiO 2 / Si slides [78]. The highest On/Off ratio approached 10 3 under a light illumination of 73.7 mW/cm 2 .  Although perovskite crystal photodetectors have shown better performance, macroscopic crystals cannot be grown on a planar substrate, restricting their potential for device integration. To overcome this shortcoming, Bakr et al. grew large-area planar-integrated crystal films onto the ITO-patterned substrates (Figure 7c) [42], and the fabricated photodetector possessed a high G (above 10 4 ) and a high gain-bandwidth product (above 10 8 Hz) relative to other perovskite devices. Furthermore, Liu's group fabricated a photodetector based on a thin perovskite crystal wafer by the space-limited crystallization method, which has about 100 pairs of interdigitated Au wire electrodes (Figure 7d) [55], and the R increased linearly as the radiance intensity decreased (Figure 7e). Moreover, Su's group sputtered the thin Au electrodes on a large-area MAPbBr 3 thin crystal to fabricate a narrowband photodetector [56]. Furthermore, Ma's group reported the superior-performance photodetectors based on MAPbBr 3 thin crystals [79], which displayed the R as high as 1.6 × 10 7 A/W and the highest G up to 5 × 10 7 .

In ultraviolet (UV) region
UV detection is a key technology in the fields of flame detection [80], remote security monitoring [81], environmental monitoring [82], and so forth. Researchers have endeavored to develop UV photodetectors based on perovskite crystals considering their excellent UV absorption properties. Visible-blind UV photodetectors based on MAPbCl 3 crystals a suitable bandgap of about 3.11 eV were fabricated (Figure 8a) [60], and the device showed the dark current as low as 4.15 × 10 −7 A at 15 V and a drastically high stability (Figure 8b). Planar-integrated MAPbCl 3 crystal UV photodetectors on ITO-deposited glass substrate were reported by Sargent et al. (Figure 8c) [83], which showed decreased R and G values as increased power density of a 385-nm laser (Figure 8d) [85].

In near-infrared (NIR) region
NIR photodetectors have widespread uses in telecommunications [86], as well as thermal and biological imaging [87][88][89][90]. Meredith's group demonstrated the perovskite crystal that overcame the large bandgap and presented photodetectors with performance metrics appropriate for NIR detection by using the trap-related linear sub-gap absorption (Figure 8e) [84]. A strong NIR photo-response was achieved in photodiodes based on MAPbI 3 crystals illuminated by a continuous 808-nm laser (∼10 mW/cm 2 ). The photodiodes could also respond to a laser with a wavelength as long as 1064 nm (Figure 8f).

In X-ray region
In addition to the common light detections from UV to IR, perovskite crystals have been employed for the detection of X-rays, which have important applications in medical diagnostics, clinical treatment, and the non-destructive testing of products [53]. Huang et al. fabricated a sensitive MAPbBr 3 crystal X-ray detector with the structure of Au/MAPbBr 3 /crystal/C 60 /BCP/Ag or Au (Figure 9a) [53]. Through reducing the bulk defects and passivating surface traps, the devices showed a detection efficiency of 16.4% at a near zero bias under irradiation with continuum X-ray energy up to 50 keV. The lowest detectable X-ray dose rate was 0.5 μGy air /s with a sensitivity of 80 μC/Gy air cm 2 , which is four times higher than the sensitivity achieved in α-Se-based X-ray detectors (Figure 9b). An X-ray detector based on p-i-n diode array made of a thick MAPbBr 3 single crystal was introduced by Chen's group [94], which displayed the highest sensitivity of 23.6 μC/mGy air cm 2 , indicating high potential for practical applications.

In gamma-ray (γ-ray) region
Similar to X-ray detectors, the γ-ray detectors are also widely used in many fields, owing to the non-invasive detections. However, γ-ray detectors need to work in a weak radiation field pulse mode and perform event-by-event detections to sort out the intensity vs. the energy of the radiation quanta. Large and balanced μ and τ are needed for high-energy detection. Huang et al. reported high-quality MAPbI 3 crystals that were applied to γ-ray detection with a 4% efficiency when operating in the γ-voltaic mode [63]. Kovalenko et al. demonstrated MAPbI 3 crystals for γ-ray detection (Figure 9c), and a 59.6 keV 241 Am energy spectrum was acquired [91]. A fit of bias dependence of photocurrent with Hecht model indicated a high μτ product of ∼10 −2 cm 2 /V (Figure 9d) [95,96].  Huang's group further reported a Cl − dopant compensation of MAPbBr 3 single crystal process to fabricate a low-cost γ-ray detector [92]. MAPbBr 2.94 Cl 0.06 crystals with a larger μτ product were equipped with a guard ring electrode to mitigate their leakage current (Figure 9e). The 137 Cs energy spectrum obtained by such crystals with a full-energy peak resolution of 6.50% is compared with the spectrum obtained by CZT and NaI(Tl) detectors (Figure 9f). A high-performance MAPbI 3 crystal γ-ray spectrometer was designed by Kanatzidis et al. [93], and the asymmetrical electrodes (Schottky-type) were applied to prohibit the hole injection from the anode or to reduce the leakage current (Figure 9g). The best energy resolution of the device for 241 Am 59.5 keV γ-rays was ∼12%; while the best energy resolution achieved for 57 Co 122 keV was 6.8% (Figure 9h and i).

Light-emitting diodes (LEDs) and lasers
With the exceptional PL efficiency and high color purity, perovskite crystals can also perform as high-performance LEDs [97]. Most of the existing perovskite LEDs employ a polycrystalline film with sizes of nanometers to micrometers, and coherent light emission is a challenge [98]. In Yu's work, the LEDs with the structure of ITO/MAPbBr 3 micro-platelet/Au cathode had the turn-on voltage of about 1.8 V and could last for at least 54 h with a luminance of ∼5000 cd/m 2 (Figure 10a) [99].
The excellent properties, including a small trap density, long lifetime and electron-hole diffusion length, and large carrier mobility, also make perovskite crystals suitable for laser devices with low lasing thresholds and high qualities. Xiong's group grew typical MAPbI 3 triangular nano-platelets and optically pumped them by a femtosecond-pulsed laser (Figure 10b) [100], and the peaks centered at λ = 776.7, 779.2, 781.9, 784.3, and 786.8 nm appeared over the spontaneous emission band with a FWHM of ∼1.2 nm (Figure 10c), when the pump fluence was increased to 40.6 μJ/ cm 2 . Zhu et al. demonstrated room-temperature lasing via using MAPbI 3 crystal nanowire, which had a broad tenability covering the NIR to visible region [101].  From Figure 10d, a sharp peak appeared at 787 nm in the representative emission spectra and grew rapidly with increasing the pump laser fluence (P) with the lasing threshold P Th of ∼595 nJ/cm 2 . Additionally, MAPbBr 3 crystal square micro-disks were synthesized into a 557-nm single-mode laser based on a built-in whispering gallery mode micro-resonator by Fu's group [102], from which a P Th = 3.6 μJ/cm 2 was observed, and a sublinear regime was observed below the threshold (Figure 10e). Uniform-sized MAPbBr 3 microplates were also created by Jiang et al. by using " liquid knife" and were made into lasers [103]. A 400-nm pulsed laser beam was used as a pump source to excite microplates, and a spontaneous emission peak centered at 550 nm with a FWHM of 20 nm was observed (Figure 10f).

Challenges and open issues
Hybrid perovskite single crystals have shown great potential in high-performance optoelectronic devices; however, several challenges and issues still remain in terms of their practical applications. They mainly include (1) the effects of surface defects, (2) the large-area fabrication, as well as (3) the stability of the perovskite single crystal devices and (4) the health and environmental concerns.

Surface defects
The absence of grain boundaries makes perovskite crystals acquire better optical and charge transport properties than their polycrystalline counterparts. However, the surface of crystals usually possesses lots of chemical impurities, dangling bonds, surface dislocations, and under-coordinated atoms, and becomes disordered owing to hydration, thus decreasing the carrier mobility and carrier diffusion length and promote the recombination of carriers near the crystal surface [76,[104][105][106]. Thus, the further decrease of defects, especially the surface defects, is required, aiming to gain high-quality perovskite crystals. To realize high-performance optoelectronic devices based on perovskite crystals with low-level surface defects, more research should be carried out on the surface passivation.

Large-area fabrication
Hybrid perovskite thin crystals are freer of grain boundaries and exhibit better transport properties than those of the polycrystalline candidates, so their large-area fabrication will ensure a promising future. However, the embedding of volatile and vulnerable organic components on fragile inorganic framework makes them difficult to be fabricated with a large area by deposition techniques or solution-based methods [42,54]. Furthermore, thin crystals were grown directly on conductive substrates like FTO-or ITO-glass [42,56], and tailored substrates, such as SiO 2 / Si [97], which provide in-situ growth for thin crystals and be directly made into devices. Nevertheless, these large-area thin crystals have rough surfaces and a great number of surface defects, and thus their optoelectronic properties remain inferior to the bulk counterparts. Further optimization of growth methods for large-area thin crystals is needed for industry productions in future.

Long-term stability
Low stability of the current hybrid perovskite crystal devices hinders their broad practical application. Several factors that affect the device stability, like ion migration [107,108], can cause hysteresis and photo-induced phase separation, and the interaction DOI: http://dx.doi.org /10.5772/intechopen.95046 between single crystals and their surroundings lies in the degradation of perovskite by humidity and light [109][110][111]. Therefore, to further enhance the stability of single crystal devices, optimized device structures should be designed to control the ion migrations. Meanwhile, various compositions and interface engineering approaches are also intensively investigated to confront this critical issue. In addition, encapsulation has been demonstrated to be a valid method to protect hybrid perovskite devices.

Health and environmental concerns
The growth of hybrid perovskite crystals adopt heavy metal ions, like lead (Pb) or tin (Sn), and organic functional groups, which can impact both the environment and human health. This critical issue needs to be overcome, aiming for further commercialization. As for the common MAPbI 3 perovskite crystal, the Pb-ion is toxic to both the human health and natural environment; while the organic solvents used during the growth process of crystals are also toxic and easily penetrate into the human body [112]. To solve these problems, capsulation and recycling are needed in the use of crystal materials and organic solvents. Furthermore, other alternative metals to Pb, with a lower toxicity, are also being studied, such as bismuth and antimony [113,114], and thus, the optoelectronic properties of these Pb-free perovskite crystals need to be explored further for device applications.

Conclusions
More recently, hybrid perovskite crystals, having different dimensional forms: bulk and thin crystals, and micro−/nano-plates, have been widely explored as functional layers for optoelectronic devices owing to their excellent physical properties combined with the advantage of ease of processing. Although these types of devices are still in the early stages of development, a strong potential for a variety of technological and commercial applications clearly remains. Here, we presented a comprehensive overview of the recent advances in hybrid perovskite crystals with respect to the background knowledge on the optoelectronic properties and charge transport dynamics of crystals, and their applications in the area of optoelectronic devices, and a fundamental understanding of the device performance. We summarized the main growth methods for the bulk crystals and also some modified and optimized approaches to synthesize thin crystals. The detailed discussions are focused on charge transport characteristics, operation mechanisms, and challenges, aiming to provide a critical understanding of further advance in materials design and device engineering in a variety of optoelectronic technologies.
In conclusion, the research progress achieved to date in the area of perovskite crystal optoelectronic devices, with the emphasis placed on challenges faced by the research community, has been summarized systematically, and finally perspective on the opportunities offered by this emerging family to photoactive materials in practical and commercial technologies is also proposed. Further exploration of high-quality perovskite crystals, combined with an in-depth understandings of working mechanism of devices, indicates a promising future for wide applications with markedly-enhanced performance.