Pure crystal orientation and anisotropic charge transport in large-area hybrid perovskite films

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.

Figure 1. Growth of orientationally pure crystalline (OPC) films.

(a) Schematic representation of the thermal-gradient-assisted directional crystallization process of a MAPbI₃ perovskite. A solution is pulled at a constant velocity, V, through a thermal gradient, G, generated by local heating at T1. (b) Optical microscope image of the aligned OPC films of the perovskite and (c) the magnified image of the highlighted area in (b) (white square). (d) The intermediate stage of microarray growth in liquid films showing the secondary branches growing from the propagating main backbone. (e) Isotropically growing microarrays by homogeneous annealing of the substrate. Insets show the same sample at a lower magnification (upper right) and the direction of its growth (upper left). (f) Spherulitic dendrites generated by lowering concentration with small thermal gradient or homogeneous annealing. (g) Optical microscope image of directionally grown MAPbBr₃ wires using the thermal gradient method.

Figure 2. X-ray diffraction analysis.

Unit cell of the tetragonal MAPbI₃ perovskite showing (a) (110), (b) (002), (c) (112), (d) (200) planes indicated by blue lines. (e) X-ray diffraction of the OPC and (f) randomly oriented perovskite films. The aligned film only shows the (112) (200) and (224) (400) planes, whereas the randomly oriented film shows general X-ray diffraction patterns.

Figure 3. SEM and TEM analysis.

(a) Cross-sectional SEM image of the aligned OPC film. (b) Cross-sectional area where two branches meet and form grain boundaries (the area in red line in a). (c) Cross-sectional area with no grain boundaries observed in the main backbone area (the area in blue line in a). (d) Planar SEM image of the aligned film. (e) The electron diffraction corresponding to the entire TEM image of a thick specimen as shown in f. (f) The TEM image of a thick specimen. (g) The TEM image of a thin specimen and (h) its electron diffraction image. Inter-planer spacing of 0.33 nm can be indexed to the (004) plane.

Figure 4. Optical properties.

(a) Steady-state absorption and PL spectra of the OPC film. The absorption band edge and the PL maximum are located at 780 nm (1.59 eV) and 795 nm, respectively. (b) TA spectra of the aligned OPC film recorded following 650 nm excitation at various pump-probe delay times. (c) ns-TA decay profiles of 765 nm GSB recovery at various pump intensities. Higher excitation intensities resulted in an increased rate of the recombination. Traces are normalized to the maximum bleach signal at each excitation energy density. The data were fitted with a bi-exponential decay function (see Methods). (d) Kinetic profiles of 765 nm GSB recovery at low pump intensities. The data were fitted with a single-exponential decay function. (e) fs-TA decay profiles of 765 nm GSB recovery with various pump intensities. (f) Steady-state PL spectra from the aligned OPC film photoexcited using 650 nm (35 fs and 1 kHz pump pulses) light with increasing pump fluence approximately from 15 to 270 μJ cm⁻². Transition from SE to ASE was observed at 102 μJ cm⁻² (inset figure).

Figure 5. Electrical Properties.

Current–voltage characteristics of the OPC film showing Ohmic, TFL and Child region. The trap density was estimated from the TFL region. The onset voltage of the TFL region is ∼0.34 V.

Figure 6. Phototransistor characteristics of the OPC films.

(a,e) Illustrations of transistor structures showing two different geometries of source and drain electrodes deposited in the direction of the backbone and the branch, respectively. (b,f) Representative transfer characteristics of aligned OPC films measured along the direction of the main backbone (I_{DS //}, as illustrated in (a)) and in the direction normal to the backbone (I_DS ⊥, as illustrated in (e)), respectively, under both dark and white-light illumination (power density=0.5 mW cm⁻²) conditions at 78 K. Arrows in the graph show the sweep directions. (c,g) I_ds^1/2 and I_ds curves as a function of V_gs corresponding to b,f respectively. A linear fit (red line) was used to extract the mobility (μ) and threshold voltage (V_th) with the equation of FET devices: I_ds=(μWC₀/2L) (V_gs−V_th)². (d,h) The output characteristics of the devices under illumination. (i) Photoresponsivity characteristics measured along the direction of the backbone (black) and the branch (red), respectively.