**1. 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 ABX3, 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<sup>+</sup> ) or formamidinium (FA<sup>+</sup> ) [4–8], B can be metal ions, such as Pb2+ and Sn2+ [9–12], and X is usually halide ions [13], and such a class of materials is known

**Figure 1.**

*1a, perovskite crystal structure.* Nature Photonics *[2], copyright 2014. 2b, CTF-corrected high-revolution TEM image.* Science *[3], copyright 2018.*

as organic–inorganic hybrid perovskites. It was reported that a stable structure of hybrid perovskites can form where 0.81 < *T.F.* (tolerance factor) < 1.1 and 0.44 < *O.F.* (octahedral factor) < 0.90 [14]. X-ray diffraction (XRD) measurements were widely used to characterize their structures. As for MAPbBr3 and MAPbI3 crystals, XRD measurements displayed the excellent single crystal properties [15]. Transmission electron microscopy (TEM) measurements were performed to provide a more intuitive picture of perovskite crystals structures (**Figure 1b**), via using contrast-transfer-function corrected method to overcome their electron beam-sensitive property [3]. After the first attempt to employ hybrid perovskite films as active sensitizers into photovoltaic devices [16], hybrid perovskite solar cells have continued to set new efficiency benchmarks [17–23], due to the excellent properties, such as ease of processing, tunable optical band gaps [24, 25], long carrier diffusion length [26], and low trap density [15], as well as large absorption coefficients and high photoluminescence (PL) efficiency [27, 28], and their relatively high power conversion efficiency (PCE) has been increased to as high as 25.2% [29]. Moreover, Leveraging their promising features, hybrid perovskites also have the potential for employment in other optoelectronic applications, including photodetectors [30], transistors [31], phototransistors [32], light-emitting diodes (LEDs) [33], and lasers [34].

However, a vast array of prior research on perovskite optoelectronic devices has been centered on polycrystalline films. The polycrystalline samples usually suffer from grain boundaries, relatively higher trap densities and defects, and low stability, which would obviously obscure their potential in applications [35–37]. More recently, researchers have paid more attention to perovskite single crystals, which possess promising characteristics of no grain boundaries [15], relatively low trap density [38], large charge carrier mobility, and long carrier diffusion length [39–41]. In this regard, extensive efforts are being devoted to developing effective methods to improve the perovskite crystal quality and optimize the device performance. Existing in the forms of bulk or thin crystals, perovskite crystal samples have been widely applied in various optoelectronic applications [39, 42], and have made rapid and great strides in research progress [43–46].

In this chapter, we aim to summarize the recent achievements, ongoing progress, and the challenges to date in the area of hybrid perovskite single crystals, practically MA-based ones (MAPbX3, X = Cl, Br, and I), from the perspective of both materials and devices with an emphasis placed on the optimization of crystal quality, and provide an outlook on the opportunities offered by this emerging family of materials in field of optoelectronic applications.

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**Figure 2.**

*copyright 2015.*

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives*

According to the lower solubility of MAPbX3 in HX (X = Cl, Br, and I) solution as the temperature decreases, Tao's group introduced the STL method to synthesize a MAPbI3 bulk single crystal (**Figure 2a**) [47]. After the reaction between methylamine (CH3NH2) and hydro-iodic acid (HI) in a cold atmosphere, the obtained white microcrystal MAI was reacted with Pb(CH3COOH)2∙3H2O in aqueous HI, and the solution was then cooled to 40°C. A 10 mm × 10 mm × 8 mm black MAPbI3 single crystal was grown in about one month (**Figure 2b**). Lin's group discovered a more efficient way, and they synthesized the single crystals with a size of 5 mm in just around 10 days [48]. Lin et al. selected high-quality seeds and dropped them back into fresh solution and obtained single crystals sized up to 1 cm (**Figure 2c**). Furthermore, MAPbBr3 − xClx and MAPbI3 − xBrx mixed-halide perovskite crystals were studied using such method [49]. Hydro-bromic acid with hydrochloric acid or hydro-iodic acid were mixed in different molar ratios into methylamine and lead (II) acetate solution to fabricate single-halide and mixed-halide perovskite crystals (**Figure 2d**). The time-consuming factor is the biggest drawback of this method, which has indirectly led to the domination of other crystallization methods.

As a radically faster perovskite crystal synthesis approach, the ITC method has widely been applied in recent years. It was observed that the exhibited crystals from such method can be shape-controlled, higher quality, and obtained quicker compared with other growth techniques. Bakr et al. introduced this method to rapidly grow high-quality bulk crystals [50]. As shown in **Figure 2e**, an orange MAPbBr3 crystal and a black MAPbI3 crystal were grown within 3 hours. Chen's group further

*2a, schematic of STL method. 2b, image of MAPbI3 with {100} and {112} facets.* CrystEngComm *[47], copyright 2015. 2c, MAPbBr3 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, MAPbI3 and MAPbBr3 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],* 

**2. Growth of hybrid perovskite single crystals**

*2.1.1 Solution temperature-lowering (STL) method*

*2.1.2 Inverse temperature crystallization (ITC) method*

*DOI: http://dx.doi.org/10.5772/intechopen.95046*

**2.1 Bulk single crystals**

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives DOI: http://dx.doi.org/10.5772/intechopen.95046*
