Letter pubs.acs.org/JPCL
Fabrication and Characterization of High-Quality Perovskite Films with Large Crystal Grains Teng Ma,*,†,‡ Qiwu Zhang,§ Daisuke Tadaki,∥,‡ Ayumi Hirano-Iwata,∥,‡ and Michio Niwano*,†,‡ †
Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 Japan ‡ CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China ∥ Graduate School of Biomedical Engineering, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *
ABSTRACT: Solution-processable organometal perovskite materials have been widely used in various kinds of devices. In these devices, the perovskite materials normally act as active layers. Grain boundaries and structural disorder in the perovskite layer would interfere the charge transport and increase recombination probability. Here we proposed a novel fabrication method to dramatically increase the crystal size by more than 20 times as compared with previously reported values. Exceptional structural order in the large crystals is illustrated by nanoscale surface morphology and a simple recrystallization method. Because of reduced grain boundaries and increased crystal order in perovskite layers, the lateral charge transport is significantly improved, as demonstrated by conductive atomic-force microscopy and performance of photodetectors. This deposition technology paves the way for future highperformance devices based on perovskite thin films.
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contact of two electrodes. In phototransistors, the channel should be formed by high-aspect-ratio single crystals to improve the charge transportation characteristic and the gate controllability. These demands urge us to find a method of forming uniform perovskite layers composed of large single crystals. In this report, we propose a novel fabrication method to dramatically increase the crystal size by more than 20 times as compared with previously reported values.6,14 We demonstrate that perovskite films we formed exhibit exceptional structural order in large single-crystal grains and that charge transport in the large crystals is largely improved due to reduced grain boundaries and improved crystal orders. In our previous report, we demonstrated that longer time of annealing helps lead tri-iodide perovskite crystals to grow, whereas longer annealing also leads to sublimation of methylammonium iodide (MAI) from perovskite crystals and formation of PbI2 crystals.16 To avoid the formation of PbI2, we have attempted to anneal the sample in saturated vapor of MAI after spin-coating MAI material on PbI2 layer. However, even though the sample was annealed at 150 °C for 6 h, the crystal size was still small, as shown in Figure S1. It seems that at this annealing temperature, the ion diffusion at the grain boundaries of crystals is not effective enough for fusion and growth of perovskite crystals. It has been reported that in perovskite
he organometal halide perovskite materials have demonstrated exceptional optical and electronic characteristics1−4 and have been widely used in various applications, such as solar cells, photo and X-ray detectors, phototransistors, light-emitting diode, and lasers.5−13 In these applications, the quality of perovskite active layers is directly related to the performance of the devices. In solar cell application, Huang’s group has reported that the crystal size of the perovskite layer is closely related to the charge lifetime and the power conversion efficiency (PCE).6,14 When the crystal size increased from 100 nm to 1 μm, the charge lifetime and PCE increased from 1.7 to 69 μs and from 9.9 to 18.1%, respectively. In other perovskite devices, such as photodetectors and phototransistors,8,15,11 because photogenerated charges migrate laterally in the perovskite layer through a long distance, about several tens of micrometers, the quality and the crystallinity of the perovskite have a greater impact on the device performance. Accordingly, it would be effective to improve the performance of perovskite devices by reducing the number of grain barriers on the chargetransporting path and by increasing the crystallinity of the perovskite films. Because of advantages such as fewer defects and longer charge lifetime, perovskite single crystals seem to be superior to multicrystalline perovskite film, in fabricating perovskite-based electronic and photonic devices.1,2,13 However, it is difficult to form a uniform single-crystal thin film with controllable thickness. In solar cell application, the perovskite layer should be pinhole-free to suppress the leak current caused by direct © 2017 American Chemical Society
Received: December 26, 2016 Accepted: January 27, 2017 Published: January 27, 2017 720
DOI: 10.1021/acs.jpclett.6b03037 J. Phys. Chem. Lett. 2017, 8, 720−726
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Schemes of the steps of proposed VAHT method. (b) SEM image of the perovskite layer formed by a modified VAHT method (without step 3). SEM images of the perovskite layer formed by VAHT method (c) at low-magnification (d) and from a cross-sectional view (e).
Figure 2. (a−f) Surface morphologies of perovskite layer after annealing for different time periods using VAHT method. (g) Schemes of morphology changes during annealing. (h) Evolution of average grain size with annealing time.
hot plate kept at 200 °C to form a uniform PbI2 layer (shown in Figure S2). To form a uniform MAI layer on the PbI2 layer, an MAI solution with a high concentration (100 mg mL−1 in 2propanol at 70 °C) is spin-coated at a fast spin speed (8000 rpm). Subsequently, the sample is transferred to an oven (kept at 200 °C) that is filled with nitrogen and saturated MAI vapor. At 200 °C, if samples are annealed on a hot plate, then MAI would immediately sublime from perovskite layers, even if we cover the sample with a Petri dish to generate an MAI-vaporsaturated environment. This is caused by a temperature
oxides, ions diffused more effectively at a higher temperature,17,18 and higher temperature were needed to eliminate internal defects and to form larger crystals in a polycrystalline thin film.19,20 We therefore increased the annealing temperature to promote ion diffusion between adjacent grains and proposed a vapor-assisted high-temperature (VAHT) method, as schematically shown in Figure 1a. The procedure for formation of perovskite crystals is as follows: First, 1 M PbI2 solution in dimethylformamide (DMF) is spin-coated on a SiO2 substrate. Before the PbI2 layer dries up, the sample is quickly put on a 721
DOI: 10.1021/acs.jpclett.6b03037 J. Phys. Chem. Lett. 2017, 8, 720−726
Letter
The Journal of Physical Chemistry Letters
isolated islands to lower free energy. Because the critical ratio partially depends on the wetting angle of thin-film materials on substrates,24 it is possible to form high-aspect-ratio perovskite crystals with larger size by using substrates with higher affinity to perovskite crystals. Long-term annealing induced a shrinkage of grains and consequently enlarged the area where the substrate surface is not covered with the perovskite layer. After 5 h of annealing, isolated single crystals with well-defined straight grain boundaries were formed, as shown in Figure 2f. The shrinkage of isolated large crystals may be mainly driven by a kinetic process. Because molecules at the surface are normally energetically less stable those packed in the crystal, high-aspectratio crystals tend to transform to crystals with high volume to surface area ratio and to form smaller but thicker crystals. The reorientation of crystals and mass transportation observed during the annealing process supports our assumption that high temperature facilitates ion migration in perovskite films. On the basis of the experimental results we presented above, we summarize the crystal growth process when using VAHT method, as shown in Figure 2g. First, small crystals are created due to interdiffusion and reaction between the MAI and PbI2 stacked layers. As reported by Kawamura et al., the equivalent lattice constant of perovskite crystal increases linearly with the temperature.25 At high temperature, mainly due to wider moving path of mobile ions as well as higher thermal energy, ion migration in the crystals and through grain boundaries is largely promoted. This helps the small crystals to fuse with each other and rearrange their orientation, leading to the formation of larger crystals. At the same time, the formation of PbI2 is prevented by the saturated MAI vapor surrounding the small crystals. However, at the same time, grain size to film thickness ratio increases as the crystal grows. When the ratio reaches a critical value, the high-aspect-ratio perovskite crystals tend to form crystals with smaller aspect ratio; the crystals prefer to shrink in the lateral direction and grow in the vertical direction. This trend becomes more noticeable when the perovskite layer is annealed for a longer time, as we have shown in Figure 2e,f. We have derived the average grain size by dividing the area of a given portion of the surface by the number of grains existing on the portion. In Figure 2h, we plot the average grain size as a function of annealing time. We can see that small crystals,
