Organic–inorganic hybrid perovskites have electronic and optoelectronic properties that make them appealing in many device applications1,2,3,4. Although many approaches focus on polycrystalline materials5,6,7, single-crystal hybrid perovskites show improved carrier transport and enhanced stability over their polycrystalline counterparts, due to their orientation-dependent transport behaviour8,9,10 and lower defect concentrations11,12. However, the fabrication of single-crystal hybrid perovskites, and controlling their morphology and composition, are challenging12. Here we report a solution-based lithography-assisted epitaxial-growth-and-transfer method for fabricating single-crystal hybrid perovskites on arbitrary substrates, with precise control of their thickness (from about 600 nanometres to about 100 micrometres), area (continuous thin films up to about 5.5 centimetres by 5.5 centimetres), and composition gradient in the thickness direction (for example, from methylammonium lead iodide, MAPbI3, to MAPb0.5Sn0.5I3). The transferred single-crystal hybrid perovskites are of comparable quality to those directly grown on epitaxial substrates, and are mechanically flexible depending on the thickness. Lead–tin gradient alloying allows the formation of a graded electronic bandgap, which increases the carrier mobility and impedes carrier recombination. Devices based on these single-crystal hybrid perovskites show not only high stability against various degradation factors but also good performance (for example, solar cells based on lead–tin-gradient structures with an average efficiency of 18.77 per cent).
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
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We thank S. Shrestha for discussions on the polycrystalline device fabrication, S. Wang for analysis and discussions on the UPS, Y. Wang for discussions on the adhesion force measurement, J. Wu for analysis and discussion of the EBIC and S. Xiang for feedback on the manuscript preparation. This work was supported by the start-up fund by University of California San Diego. D.P.F. acknowledges California Energy Commission award no. EPC-16-050. The microfabrication involved in this work was in part performed at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, a member of the National Nanotechnology Coordinated Infrastructure, which was supported by the National Science Foundation (grant number ECCS-1542148). This characterization work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (contract 89233218CNA000001) and Sandia National Laboratories (contract DE-NA-0003525).
The authors declare no competing interests.
Peer review information Nature thanks Hyunhyub Ko and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 The mechanism of the lithography-assisted epitaxial-growth-and-transfer method.
a, Detailed schematic growth steps. Lp, pattern width; Ld, distance between two patterns; Lx–y, growth length in the x–y plane; Lz, growth length in the vertical z direction. b, Detailed epitaxial merging steps shown by SEM images (top view). First, individual single-crystals grow out of the mask. The lattice orientation of the epitaxial crystals is the same, which is controlled by the substrate. Then the individual crystals gradually expand and contact with each other. No lattice tilting or twisting can be found. Finally, completely merged single-crystal thin films are formed, where no grain boundaries can be seen. c, Titled SEM images of different growth behaviour under different growth temperatures and precursor concentrations. Low temperature and concentration can result in thin films (left), whereas high temperature and concentration lead to rods (right).
High-resolution TEM studies of transferred single-crystal MAPbI3 on different substrates (for example, gold for metals, glass for oxides and PDMS for polymers) using this growth-and-transfer method. The results show that there is no obvious lattice dislocation or polycrystalline structure formed at the interface, indicating that the re-adhesion/re-growth process maintains the single-crystal properties of the transferred materials.
a, Freestanding transferred single-crystal MAPbI3 thin films fabricated by soft polymer masks and corresponding bulk substrates. b, A large bulk substrate (left) that is used to epitaxially grow the single-crystal MAPbI3 thin film (left) and a transferred single-crystal MAPbI3 thin film using a rigid copper foil (20 μm thick) as the mask (right).
a–c, Carrier diffusion lengths (a) calculated from measured carrier mobilities (b) and carrier lifetimes (c) with different thicknesses of the single-crystal perovskite. Insufficient charge collection begins when the thickness goes beyond about 5 μm, which can result in a high recombination possibility in the absorber and thus a low device efficiency. Error bars come from three different measurements under the same condition.
a, Schematics for calculating the position of the NMP. The SU-8/PDMS top layer is critical for minimizing the strain in the single-crystal perovskite layer. b, Optical (left) and SEM (right) images under different bending conditions. The single-crystal perovskite (about 2 μm thick) can be successfully bent to r ≈ 2.5 mm. All optical images share the same scale bar. All SEM images share the same scale bar.
a, Schematic growth processes with continuously exchanging the precursor solution, which allows the formation of the alloyed structure along the epitaxial growth direction. The perovskite substrate sits in a PDMS growth mould in precursor solution 1. A different precursor solution 2 is fed with designed rates (depending on the solution volume, a calculation example can be seen in Supplementary Discussion 1). b, Optical images showing two representative kinds of graded single-crystal perovskites. The alloyed region is at the interface (about 1 mm in width, depending on the alloying rate) between the different coloured crystals. Organic cations, inorganic atoms and halides can all be alloyed.
Extended Data Fig. 7 Density-functional theory simulations of the graded single-crystal perovskites.
a, Calculation results showing electronic band structures of the graded single-crystal MAPb0.5+xSn0.5−xI3. All structures show direct bandgaps at the Γ point. The Fermi level is normalized to the VBM, to show the shrinking tendency of the bandgap. b, Calculated effective masses for electrons and holes in the graded single-crystal MAPb0.5+xSn0.5−xI3 with an increasing tin concentration. The decreasing effective masses indicate increasing mobilities of both electrons and holes. The enhancement for holes is more pronounced than for electrons. c, Graded single-crystal MAPb0.5+xSn0.5−xI3 (left) showing a graded bandgap in comparison with the flat bandgap of conventional MAPbI3 (right).
Extended Data Fig. 8 Single-crystal perovskite thin-film light-emitting diodes and photodetectors fabricated using this growth-and-transfer method.
a, Transferred single-crystal MAPbBr3 arrays with each pixel about 100 μm by 100 μm. Inset: the transferred single-crystal MAPbI3 micro light-emitting diode arrays with each pixel about 1 μm by 1 μm. b, SEM images showing the textured single-crystal MAPbI3 thin film as a photodetector. Inset: a magnified SEM image of the cross-sectional structure of the device. PI, polyimide. c, Finite-difference time-domain optical simulation of the overall absorption by the textured structure (left) and the flat structure (right). The absorption by the textured thin film is much higher than that by the flat one because of the anti-reflective effect. d, EQE measurements of different device morphologies. The textured single-crystal film shows the highest quantum efficiency, which comes from the reduced surface reflections. e, Dark current measurements on both textured and flat single-crystal devices show that the current levels are similar, indicating the pinhole-free and high-quality thin films. The higher light current of the textured device reveals its higher absorption compared with the flat device. f, Responsivity results show that the textured devices are more sensitive to the input power. The inset shows that the textured devices exhibit a higher detectivity than the flat devices. The decreasing tendencies of the responsivity and detectivity at high input power may be due to the material degradation under strong light intensities.
In the single-crystal sample, only the surface areas are easy to be oxidized, indicating that the self-doping in deep areas away from the surface is relatively slow. In the polycrystalline sample, the oxidation is much faster in deep areas compared with the single-crystal samples, indicating that the grain boundaries facilitate the oxidation process.
a–d, Summarized tracking results of JSC (a), VOC (b), FF (c) and PCE (d). The differences between single-crystal and polycrystalline devices are not as large as for the shelf-stability tests in Fig. 4f, which may be because of the poor thermal stability of the hole transport layer (spiro) used in all of these devices. In any case, relatively speaking, single-crystal devices show a better stability than polycrystalline devices.
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Lei, Y., Chen, Y., Zhang, R. et al. A fabrication process for flexible single-crystal perovskite devices. Nature 583, 790–795 (2020). https://doi.org/10.1038/s41586-020-2526-z
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