High-efficiency lead halide perovskite solar cells (PSCs) have been fabricated with α-phase formamidinium lead iodide (FAPbI3) stabilized with multiple cations. The alloyed cations greatly affect the bandgap, carrier dynamics, and stability, as well as lattice strain that creates unwanted carrier trap sites. We substituted cesium (Cs) and methylenediammonium (MDA) cations in FA sites of FAPbI3 and found that 0.03 mol fraction of both MDA and Cs cations lowered lattice strain, which increased carrier lifetime and reduced Urbach energy and defect concentration. The best-performing PSC exhibited power conversion efficiency >25% under 100 milliwatt per square centimeter AM 1.5G illumination (24.4% certified efficiency). Unencapsulated devices maintained >80% of their initial efficiency after 1300 hours in the dark at 85°C.

https://science.sciencemag.org/content/sci/370/6512/108.full.pdf

Impact of strain relaxation on performance of α-formamidinium lead iodide perovskite solar cells

Mixtures of cations or halides with FAPbI3 (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI3 perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN- anions promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

/pdf/vapor-assisted-deposition-of-highly-efficient-stable-black-1ww31j626n.pdf

Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells.

Organic–inorganic hybrid perovskites have electronic and optoelectronic properties that make them appealing in many device applications1–4. Although many approaches focus on polycrystalline materials5–7, single-crystal hybrid perovskites show improved carrier transport and enhanced stability over their polycrystalline counterparts, due to their orientation-dependent transport behaviour8–10 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). A solution-based lithography-assisted epitaxial-growth-and-transfer method is used to fabricate single-crystal hybrid perovskites on any surface, with precise control of the thickness, area and chemical composition gradient.

A fabrication process for flexible single-crystal perovskite devices

Thin Film Analysis By X Ray Scattering

FAPbI 3 ‐Based Perovskite Solar Cells Employing Hexyl‐Based Ionic Liquid with an Efficiency Over 20% and Excellent Long‐Term Stability

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 properties3–5. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization6–8, electrostriction9, annealing10–12, 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. A method of deposition of mixed-cation hybrid perovskite films as lattice-mismatched substrates for an α-FAPbI3 film is described, giving strains of up to 2.4 per cent while also stabilizing the metastable α-FAPbI3 phase for several hundred days.

Jingxin Zhang

Papers

Strain engineering and epitaxial stabilization of halide perovskites.