COMMUNICATION
(1 of 7)and CsSnI 3 has, to date, been less encouraging, with solar cell efficiencies of <5% for solution-processed thin film devices [10] that are likely limited by the low degree of crystalline ordering. [11] Indeed, structural ordering has been linked in traditional semiconductors to (a) carrier transport, where mobilities increase from amorphous-Si (1 cm 2 V −1 s −1 ) [12] to single crystalline Si (1400 cm 2 V −1 s −1 ), [13] (b) recombination rates, where unpassivated grain boundaries act as quenching sites for charge carriers and excited states, and (c) quantum confinement, which can make even Si an excellent NIR emitter with luminescent efficiency >60%. [14] These factors, among others, have motivated the recent interest in halide perovskite single crystal growth. [15] Thus, one of the main challenges for enhancing the properties of halide perovskites for high end optoelectronic applications is to obtain epitaxial crystalline films that can also be integrated into heteroepitaxial and quantum well structures. The epitaxial growth is a key step