Metal Halide Perovskite Single Crystals: From Growth Process to Application

crystals Article Metal Halide Perovskite Single Crystals: From Growth Process to Application Shuigen Li 1, *, Chen Zhang 2 , Jiao-Jiao Song 2 , Xiaohu Xie 1 , Jian-Qiao Meng 2,3, * ID and Shunjian Xu 1, * 1 2 3 * School of New Energy Science and Engineering, Xinyu University, Xinyu 338004, China; Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China; (C.Z.); (J.-J.S.) Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China Correspondence: (S.L.); (J.-Q.M.); (S.X.)



 Received: 21 April 2018; Accepted: 9 May 2018; Published: 17 May 2018 Abstract: As a strong competitor in the field of optoelectronic applications, organic-inorganic metal hybrid perovskites have been paid much attention because of their superior characteristics, which include broad absorption from visible to near-infrared region, tunable optical and electronic properties, high charge mobility, long exciton diffusion length and carrier recombination lifetime, etc. It is noted that perovskite single crystals show remarkably low trap-state densities and long carrier diffusion lengths, which are even comparable with the best photovoltaic-quality silicon, and thus are expected to provide better optoelectronic performance. This paper reviews the recent development of crystal growth in single-, mixed-organic-cation and fully inorganic halide perovskite single crystals, in particular the solution approach. Furthermore, the application of metal hybrid perovskite single crystals and future perspectives are also highlighted. Keywords: perovskite single crystals; growth process; application; solar cell; photodetector 1. Introduction Recently, organic-inorganic metal hybrid perovskites have shown great applied potential because of their impressive optical and electrical properties [1–5], which can be represented by the structure ABX3 , where A is CH3 NH3 + , CH(NH2 )2 + or Cs+ , B is Pb2+ or Sn2+ , and X is I– , Br– or Cl– [6–27]. The ideal ABX3 structure is cubic symmetry, where A and B ions are located at the eight corners and center of a cubic unit, respectively. The symmetry of ABX3 structures is based on the atomic species of the A and B sites. In a typical perovskite crystal structure, the A, B, and X ionic radii, e.g., RA , RB , and RX , should√correspond to a specific geometric relationship, known as the Tolerance factor [28–30]: t = (RA + RX )/ 2(RB + RX ). The ideal value of t should be 1 for cubic structures; otherwise, the structure tends to be distorted, or even destroyed [28,30,31]. For lead hybrid perovskite, the large organic cation at the A position, e.g., methylammonium (MA+ ) or formamidinium (FA+ ), is able to match the large radius of the Pb2+ ion at the B position and meet the tolerance factor t, while the halogen anions or their mixtures occupy the C positions, resulting in the formation of a 3D perovskite structure [32]. These perovskite-based materials, when used in the photovoltaic field, can provide remarkable properties, such as broad absorption from the visible to the near-infrared region, tunable optical and electronic properties [15,33–36], high charge mobility, and long exciton diffusion length and carrier recombination lifetime [32,37–48]. Within a few years, they have revolutionized the Crystals 2018, 8, 220; doi:10.3390/cryst8050220 www.mdpi.com/journal/crystals Crystals 2018, 8, 220 2 of 22 photovoltaic field; an efficiency of 22.1% from solution-processable perovskite-based solar cells has been reported [49]. In addition, lead hybrid perovskites have also been used in some other fields, such as laser [50], photodetector [51], light-emitting-diodes [52], thermoelectricity [53], and catalysis [54], demonstrating their potential application prospects. Until now, many intensive investigations have been based on polycrystalline thin films, one of the existing forms of perovskite, and most of the results have been focused on the perovskite polycrystalline film. With in-depth research, single crystals—another form of perovskite—have been found with low defect density. The carrier diffusion length of perovskite is sensitive to defects. When expanding the grain size, the carrier diffusion length of polycrystalline forms can increase to up to 1 µm, while large single crystals are able to provide even longer carrier diffusion lengths. Dong et al. prepared millimeter-sized MAPbI3 single crystals via a low-temperature solution approach, in which a carrier diffusion length of over 175 µm was obtained under 1 sun illumination, and a longer carrier diffusion length exceeding 3 mm could be produced under a weaker illumination with 0.003% sun illumination [55]. Shi et al. reported low trap-state density of states with an order of 109 –1010 cm−3 and carrier diffusion length > 10 mm in MAPbX3 single crystals [40]. The longer carrier diffusion length in single crystals with low trap-state density derives from their better extraction and transport of photogenerated charge carriers, resulting in a performance boost for optoelectronic devices. These meaningful findings will contribute to the development of perovskite-based materials, and will be extremely beneficial to further fundamentally investigate the intrinsic properties of perovskites single crystals. To date, single-organic-cation, mixed-organic-cation, and all-inorganic kinds of metal halide perovskite single crystals have been demonstrated. In this review, we will summarize the advances in the growth and application of the above perovskite single crystals. 2. Growth of Organic-Inorganic Hybrid Halide Perovskite Single Crystals Since organic-inorganic metal hybrid perovskite solar cells (PSCs) were studied for the first time [5], they have attracted particular attention due to their extraordinary performance. Since then, in-depth study on perovskite-based materials and devices has been carried out, and a series of research results have been obtained. Meanwhile, perovskite single crystals, which were reported about forty years ago [40,56,57], are studied again. 2.1. Growth of Single-Organic-Cation Halide Perovskite Single Crystals Solution temperature lowering (STL) is a traditional single crystal growth process. In 1987, Poglitsch et al. gained MA-based perovskite single crystals via a temperature-lowering method [58] in which they heated the mixed solution to 100 ◦ C, and perovskite single crystals were grown by cooling the solution to room temperature. In general, minimizing the number of nuclei is crucial to growing large single crystals. As an improved technology, seed-assisted growth is often adopted for the purpose of growing large-sized and high-quality single crystals, i.e., small crystals are firstly put into a single crystal precursor, followed the temperature-lowering process. Using a slow cooling rate of 0.1–0.2 ◦ C/h, Su et al. obtain (...truncated)