Strain engineering is a powerful tool with which to enhance semiconductor device performance1,2. Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties3,4,5. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization6,7,8, electrostriction9, annealing10,11,12, van der Waals force13, thermal expansion mismatch14, and heat-induced substrate phase transition15, the controllable and device-compatible strain engineering of halide perovskites by chemical epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here we report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. We investigated strain engineering of α-formamidinium lead iodide (α-FAPbI3) using both experimental techniques and theoretical calculations. By tailoring the substrate composition—and therefore its lattice parameter—a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI3 thin film. We demonstrate that this strain effectively changes the crystal structure, reduces the bandgap and increases the hole mobility of α-FAPbI3. Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI3-based photodetector.
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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 T. N. Ng and Z. Wu for guidance on the transient photocurrent measurement; P. Liu and S. Yu for sharing the Rikagu Smartlab diffractometer; D. P. Fenning and X. Li for discussions; Q. Lin for guidance on the reciprocal space mapping measurements; S. Wang for analysis and discussions of the UPS; Y. Zeng for training on the Renishaw inVia Raman spectrometer; Y. Li, Y. Yin and M. Chen for guidance on the finite element analysis simulations; and S. Xiang for constructive feedback on manuscript preparation. This work was supported by the startup fund by the University of California San Diego. The microfabrication involved in this work was performed at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, a member of the National Nanotechnology Coordinated Infrastructure, which was supported by the the National Science Foundation (grant number ECCS-1542148). K.Y. acknowledges the National Science Foundation under award number ACI-1550404 and computational resources from Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562.
The authors declare no competing interests.
Peer review information Nature thanks Jian Shi, Lijun Zhang 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 Characterization of substrate quality with different growth methods and its impact on the epitaxial strain.
a–f, Rocking curve measurements of substrates grown by the inverse temperature crystallization (ITC) and slow solvent evaporation (SSE) methods. Lower full-width at half-maximum (FWHM) values by the SSE indicate better crystal quality. g, XRD patterns of strained α-FAPbI3 on a substrate with higher crystal quality (red curve) and relaxed α-FAPbI3 on a substrate with lower crystal quality (grey curve). Dislocations in the substrates can propagate into and relax the strain in the epitaxial α-FAPbI3. The vertical dash line labels the (001) peak position of strain-free α-FAPbI3. The peak position from the strain-relaxed FAPbI3 (grey curve) shifts back to that of strain-free α-FAPbI3.
Extended Data Fig. 2 Atomic force microscopy morphology characterization of strained and strain-relaxed epitaxial α-FAPbI3 films.
a, b, A topography image (a) and the corresponding height scanning curve (b) of the red line in a of a strained epitaxial α-FAPbI3 thin film. c, d, A topography image (c) and the corresponding height scanning curve (d) of the black line in c of a strain-relaxed epitaxial α-FAPbI3 thick film. Results show that the strained thin film adopts a layer-by-layer growth model. No cracks or holes can be detected. As the film thickness increases, the total strain energy builds up and generates dislocations that propagate throughout the film and relax the strain, leading to the formation of cracks and holes. These cracks and holes are typically regarded as a signature of strain relaxation.
Extended Data Fig. 3 First-principles calculations of the strained α-FAPbI3 unit cell and experimental absorption spectra of the strained α-FAPbI3 under different strains.
a, Evolution of lattice volume and bandgap as a function of strain for three α-FAPbI3 lattices with different FA+ organic cation orientations. For the bandgap calculations, spin–orbit coupling is incorporated owing to the heavy element Pb, and the hybrid functionals within Heyd–Scuseria–Ernzerhof formalism with 25% Hartree–Fock exchange are employed. b, The absorption spectra of the strained α-FAPbI3 thin films. The absorption onset redshifts will the increasing strain, which agrees with the photoluminescence spectra and prove that the strain can alter the bandgap of the α-FAPbI3. c, The c axis length of the unit cell when biaxially straining the a/b axes. The slope of the fitted line shows a Poisson’s ratio of about 0.3. d, C–N and C=N bond lengths at different strain levels. Simulation results show that the deformation of the FA+ skeleton is very small under the applied biaxial strain.
a, b, Temperature-dependent photoluminescence of strained (a) and strain-free (b) α-FAPbI3 before normalization. c, d, Temperature-dependent photoluminescence of strained (c) and strain-free (d) α-FAPbI3 after normalization. Both samples exhibited uniform bandgap narrowing and FWHM narrowing with decreasing the temperature. e, f, Temperature-dependent photoluminescence (PL) FWHM of strained (e) α-FAPbI3 and strain-free (f) α-FAPbI3 with fitting. Results show that the strained α-FAPbI3 has a higher Γ0, γLO and ELO than that of strain-free α-FAPbI3 owing to the strain-induced crystalline quality reduction and the strain-enhanced carrier-phonon scattering. Γ0, temperature-independent emission linewidth term associated with the structural disorder scattering. γLO, charged-carrier-optical-phonon coupling constant. ELO, optical phonon energy.
a, b, Thickness-dependent in-plane XRD of −1.2% strained (a) and −2.4% strained (b) α-FAPbI3 thin films. Vertical lines label the peak position of the fully strained films. Plastic strain relaxation at relatively high thickness can be evident by the peak shifting to lower angle and peak broadening. c, Thickness-dependent relaxation constant R of the epitaxial α-FAPbI3 thin films with different strains. Results show that the critical thickness decreases with increasing strain. d, Fitting of the experimental critical thicknesses with the People and Bean and the Matthew and Blakeslee models (see Supplementary Information refs 69 and 70). Experimental results agree well with the People and Bean model, indicating that the plastic strain relaxation due to the dislocations generated during the epitaxial growth is the dominating relaxation mechanism.
a, The intersects of the curves with the baseline in the high-binding-energy region give the Fermi level position of corresponding strained α-FAPbI3 films. There is a clear shift of the intersects to higher-binding-energy levels when the compressive strain becomes larger. b, The intersects of the curves with the baseline in the low-binding-energy region give the energy difference between the Fermi level and the VBM. All α-FAPbI3 films have p-type character according to the calculated Fermi level position in the bandgap. Meanwhile, the VBM is pushed up more than the CBM with increasing strain. Inset, a schematic band diagram of the α-FAPbI3 under different strains.
Extended Data Fig. 7 Space-charge-limited-current measurement of the epitaxial α-FAPbI3 with different strains.
a–d, I–V characteristic curves for the space-charge-limited-current measurement of the epitaxial α-FAPbI3 film with different strains. While the forward scans indicate a typical trap-filling process with increasing the applied voltage, the reverse scan doesn’t show a detrapping process. Number of experiments, n = 5 for each strain value. ntrap, calculated trap density.
XPS spectra of: a, I 3d; b, Pb 4f; c, Br 4p; and d, Cl 2p photoelectrons from a strained α-FAPbI3 film. Results show that Br and Cl are absent in the strained α-FAPbI3.
a, Responsivity as a function of strain level in α-FAPbI3. Devices under −0.8%, −1.2% and −1.4% compressive strain give better responsivity compared to the strain-free devices. Further increasing the compressive strain can lead to a higher density of dislocations, which reduces the responsivity. Number of experiments, n = 5 for each strain value. b, c, Detectivity (b) and gain G (c) of the photodetector based on α-FAPbI3 under −1.2% strain, indicating enhanced performance. q, element charge. Jd, dark current density. d, Normalized external quantum efficiency (EQE) of the photodetector based on α-FAPbI3 under −1.2% strain, showing an extended infrared absorption range.
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Chen, Y., Lei, Y., Li, Y. et al. Strain engineering and epitaxial stabilization of halide perovskites. Nature 577, 209–215 (2020). https://doi.org/10.1038/s41586-019-1868-x