might, however, still affect the transport of charge carriers, thereby limiting the device efficiencies. A very long electron diffusion length, greater than 175 µm, has been reported for a CH 3 NH 3 PbI 3 single crystal (SC); [7] it decreases to ≈100 nm in polycrystalline films, due to these traps. [8] Therefore, higher efficiencies are expected for PeSCs incorporating perovskite SCs. Indeed, SC perovskitebased electronic devices have attracted much attention recently. [7,[9][10][11][12][13][14][15][16][17][18] In addition to solar cells, high-performance photodetectors [14][15][16][17] and X-ray detectors [18] featuring perovskite SCs have also been demonstrated. Perovskite SCs also exhibit greater thermostability and humidity stability when compared with their thin films. As a result, the challenge remains to develop relevant technologies for synthesizing and fabricating high-quality perovskite SCs for future applications.Many approaches have been proposed for growing perovskite SCs, [19] including temperature-lowering methods, [7] antisolvent vapor-assisted crystallization, [9] and inverse temperature crystallization. [20] The fabrication of efficient solar cells from perovskite SCs has, however, been mostly unsatisfactory, due to the imbalance between the absorption length and the carrier diffusion length; the thickness of the SCs is usually too great, resulting in poor charge carrier collection efficiencies. Laterally structured CH 3 NH 3 PbI 3 SC solar cells have been constructed through piezoelectric poling, but the PCEs obtained have remained low. [10] Several methods for growing planar/thin-film perovskite SCs effectively have been demonstrated recently. [11,12,16] For example, CH 3 NH 3 PbBr 3 SCs having thicknesses ranging from one to several tens of micrometers and lateral dimensions of up to several millimeters have been grown using cavitation-triggered asymmetric crystallization (CTAC), which provides additional energy to overcome the nucleation barrier; a PCE of 6.5% has been achieved for solar cells fabricated using this method. [11] Furthermore, space-limited inverse-temperature crystallization (SLITC) has been used to grow planar CH 3 NH 3 PbBr 3 SCs having a controllable thickness of 16 µm and lateral dimensions of 6 × 8 mm; a remarkable PCE of 7.11% was reported for such a device, which also displayed excellent stability. [12] Therefore, these novel approaches can allow the preparation of perovskite SC geometries having lateral crystal structures for applications in electronic devices.The synthesis of certain perovskite single crystals (SCs), including CH 3 NH 3 PbI 3 , through asymmetric crystallization is difficult, mainly because of the large difference in solubility of the precursors and/or the intrinsic nonpreference for growth in a direction along the substrate. Herein, an effective method is reported, seeded space-limited inverse-temperature crystallization (SSLITC), for growing CH 3 NH 3 PbI 3 SC plates having micrometer-scale diameters. In this process, a seed CH 3 NH 3 PbI 3 crystal ...

The synthesis of certain asymmetric perovskite single crystals (SCs)-including CH 3 NH 3 PbI 3 , which is used most commonly-for application in highperformance perovskite solar cells (PeSCs), remains very challenging. Herein, a promising but general method, differential space-limited crystallization (DSLC), is described for synthesizing high-quality perovskite single-crystal micro-plates. The thickness of the perovskite SCs is controlled by the difference between the thicknesses of two sets of polytetrafluoroethylene (PTFE) spacers. Because the DSLC method does not require very thin spacers, it simplifies the procedure of crystal growth. More importantly, the hydrophobicity of the PTFE spacers weakens the attraction between the surfaces of the confined space and the precursor complexes, thereby increasing the rate of diffusion of the precursor ions. Accordingly, the critical nucleation step is not limited by the low rate of diffusion of the ions. This approach is used to prepare mixed-cation lead iodide single-crystalline micro-plates for solar cell applications. The device performance of single-crystal PeSCs improves after introducing formamidinium ions. The stability of the single-crystal devices also improves relative to that of conventional thin-film counterparts. It is anticipated that this DSLC method can also be used to synthesize different types of asymmetrical perovskite SCs for other optoelectronic applications.