Abstract
Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites. Here we show that a large-area, orientationally pure crystalline (OPC) methylammonium lead iodide (MAPbI3) hybrid perovskite film can be fabricated using a thermal-gradient-assisted directional crystallization method that relies on the sharp liquid-to-solid transition of MAPbI3 from ionic liquid solution. We find that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties. X-ray diffraction patterns reveal that the film is strongly oriented in the (112) and (200) planes parallel to the substrate. This film is structurally confined by directional crystal growth, inducing intense anisotropy in charge transport. In addition, the low trap-state density (7.9 × 1013 cm−3) leads to strong amplified stimulated emission. This ability to control crystal orientation and morphology could be widely adopted in optoelectronic devices.
Highlights
Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites
Homogeneous annealing of the substrate produces isotropically grown microarrays (Fig. 1e), and further lowering of the concentration B7 wt% with a small temperature gradient G, or homogeneous annealing leads to spherulitic dendrites (Fig. 1f)
In the case of MAPbBr3, 1–2 cm long micro-wires were generated by the thermal gradient method (Fig. 1g) compared with the randomly oriented microwires generated by homogeneous annealing (Supplementary Fig. 3)
Summary
Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites. Most investigations with respect to the film formation of perovskites have focused on optimizing grain size and crystallinity during the deposition of perovskite films, which is critical for reducing grain boundaries and defect density, suppressing charge recombination and increasing diffusion lengths of the charge carriers By virtue of these studies on materials and device engineering, the power conversion efficiencies (PCEs) of polycrystalline perovskite solar cells have surpassed 20% Recent reports of long charge carrier diffusion lengths and low trap-state densities from methylammonium lead halide (MAPbX3) based single crystal perovskites, which could lead to a new generation of highly efficient optoelectronic devices[22,23], have been a key motivator for researchers to bridge the performance and properties gap between perovskite single crystals and their polycrystalline film counterparts. Transistor devices constructed of OPCs show giant anisotropy (three orders of magnitude difference) in their field effect mobility depending on the orientation of perovskite films
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