Despite the successful application of polycrystalline halide perovskites in photovoltaics and optoelectronics, single-crystal halide perovskite devices are lacking systematic studies due to the scalability and thickness control issues. Single-crystal halide perovskites exhibit reduced defect and grain boundaries, enhanced carrier dynamics, long-term stability, and more importantly, the ability to be strained by heteroepitaxial growth for functionality manipulation. Although frequently attempted, the strain engineering of halide perovskites remains uncontrollable and unstable under room temperature due to the lack of suitable substrates and appropriate growth methods.
To address this issue, we report a unique heteroepitaxial growth technique to effectively strain α-FAPbI3) based on our pioneering homoepitaxial growth method of halide perovskites. Halide perovskite substrates provide opportunities for the chemical epitaxial growth of halide perovskite thin films. On the one hand, the chemical and structural similarity allows the uniform chemical epitaxial growth of halide perovskites. On the other hand, the lattice parameters and crystal structures can be effectively tuned by varying compositions for strain engineering of the epitaxial films.
In our study, liquid phase epitaxial growth is adopted to grow strained α-FAPbI3 film on a series of high quality mixed methylammonium lead chloride/bromide substrates with different lattice mismatch. We find that the compressive strain applied by the substrates can effectively manipulate the electronic band structure and properties of the epitaxial α-FAPbI3. Valence band maximum can be lifted to reduce the bandgap of α-FAPbI3, which can potentially allow more light absorption. Hole mobility is also increased due to the reduced hole effective mass manipulated by compressive strain. Finally, a high-performance photodetector based on the strained α-FAPbI3 is demonstrated.
Surprisingly, we also find that the epitaxial growth can be utilized to structurally stabilize the pure α-FAPbI3. Although considered as one of the most promising perovskites for photovoltaics, α-FAPbI3 suffers from structural stability issues under room temperature. Epitaxial stabilization provides an alternative for the α-FAPbI3 stabilization other than the existing methods. Chemical bonds at the epitaxial interface tightly lock the epitaxial α-FAPbI3 lattices and present its phase transition into photo-inactive δ-FAPbI3. We anticipate this epitaxial stabilization method can have a profound impact on the synthesis of new perovskites predicted by machine learning for various applications.
Figure. Work summary. a, Schematic structure of the epitaxial α-FAPbI3 on mixed methylammonium lead chloride/bromide single-crystal substrates. b, Scanning electron microscopy images of the epitaxial α-FAPbI3. Scale bar, 2 µm. Inset, a magnified image showing a clear interface. Scale bar, 200 nm. c, Photoluminescence spectra of the α-FAPbI3 with different strains. d, Hole mobilities by Hall effect measurements showing a peak hole mobility at -1.2% strain. Inset, schematic device setup for hole mobility measurement. e, Phase stability revealed by photoluminescence spectroscopy. A thin, strained epitaxial α-FAPbI3 shows a phase stability up to 360 days while a thick, strain-free epitaxial α-FAPbI3 phase changes to yellow δ-FAPbI3 within 1 day. Inset, optical images of the two samples, showing a clear difference of the phase stability. Scale bars, 2 mm.
Nevertheless, further studies are needed to polish the growth and fabrication protocol and extend the platform technology to different application aspects. Questions remain in how to accommodate the epitaxial film with substrate into the existing photovoltaic/optoelectronic fabrication protocol, exploring scalable deposition methods which can reduce the film thickness down to several atomic layers, determining the new functionality generated by the epitaxial strain, and resolving the resistance of halide perovskites to the environmental conditions (e.g., moisture, oxygen, and temperature).