FAPbI₃ (formamidinium lead iodide): phases, band gap and solar cells

FAPbI₃ has the ABX₃ formula of a perovskite: A is the organic formamidinium cation, HC(NH₂)₂⁺, B is lead and X is iodide (Stoumpos et al. 2013). The black α-phase is the cubic perovskite (Jeong et al. 2021): corner-sharing PbI₆ octahedra with formamidinium in the cages. For the general picture of corner-sharing octahedra and the A-site cage, see the perovskite structure hub.

Formamidinium (2.79 Å) is larger than methylammonium (2.70 Å) and much larger than caesium (1.81 Å) (radii as given by Saliba et al. 2016). That size is why pure FAPbI₃ does not always form a perovskite: it can instead crystallise as a yellow δ-phase, a photoinactive, hexagonal, non-perovskite structure, and the black α-phase is sensitive to solvents and humidity (Saliba et al. 2016). In pure FAPbI₃ the yellow non-perovskite phase is the stable one at low temperature and the black phase forms only at high temperature (Zheng et al. 2022).

Replacing methylammonium with formamidinium narrows the band gap: FAPbI₃ has a gap of 1.48 eV against 1.55 eV for MAPbI₃ (Zheng et al. 2022), and component-pure α-FAPbI₃ films stabilised with a volatile ammonium acetate additive showed 1.48 eV and cells above 21% (Wang et al. 2023). The narrower gap extends absorption to about 840 nm (calculated from 1.48 eV). Yang et al. (2015) likewise note that the band gap of FAPbI₃ allows broader absorption of the solar spectrum than MAPbI₃, and Saliba et al. (2016) that it lies closer to the single-junction optimum.

• FA/MA cations. Mixing formamidinium and methylammonium on the A site extends the absorption onset further into the red and raised the short-circuit current over methylammonium-only cells (Pellet et al. 2014).
• Adding MAPbBr₃. Jeon et al. (2015) combined FAPbI₃ with MAPbBr₃; the bromide-containing mixture stabilised the perovskite phase of FAPbI₃ and gave cells above 18% under standard illumination.
• Triple cation (Cs/MA/FA). Saliba et al. (2016) added a small amount of caesium to FA/MA mixed-halide perovskites. The smaller Cs⁺ shifts the tolerance factor towards the cubic black phase, suppressing the yellow-phase impurities seen in Cs-free films; the films were more thermally stable, less sensitive to processing conditions, and reached a stabilised 21.1%, with about 18% after 250 hours under operation.
• Anion engineering of α-FAPbI₃. Jeong et al. (2021) used the pseudo-halide formate (HCOO⁻) to suppress anion-vacancy defects at grain boundaries and surfaces, reaching 25.6% (certified 25.2%).

Perovskite solar cells began with methylammonium lead halides as sensitisers at 3.8% (Kojima et al. 2009). By 2016, the best perovskite cells used a mixture of formamidinium and methylammonium as the A-site cations (Saliba et al. 2016). Yang et al. (2015) deposited dense FAPbI₃ films by intramolecular exchange of DMSO intercalated in PbI₂ with formamidinium iodide, and their FAPbI₃-based cells exceeded 20%. Jeong et al. (2021) describe the cubic α-phase of FAPbI₃ as the most promising semiconductor for highly efficient and stable perovskite solar cells, and their formate-treated α-FAPbI₃ cells reached 25.6%.

• Single crystals and powders. FAPbI₃ and related AMI₃ iodides have been prepared by several routes and their structures solved by single-crystal X-ray diffraction; their properties depend strongly on the preparation method (Stoumpos et al. 2013).
• Intramolecular exchange. Formamidinium iodide replaces DMSO intercalated in PbI₂, giving dense, large-grained (111)-oriented FAPbI₃ films without residual PbI₂ (Yang et al. 2015).

For a paper, slide or poster, draw the black α-phase as one cubic cell: a PbI₆ octahedron at each corner sharing iodides with its neighbours, and the formamidinium ion in the central cage. A side-by-side panel of the yellow δ-phase (hexagonal, non-perovskite) explains why phase stability matters. In the LabCanvas figure editor, the Library’s Perovskite crystal object draws the ABX₃ cell with its corner-sharing octahedra, which you can recolour, repeat over several cells and export as a high-resolution SVG, and the LabCanvas 3D Studio has a perovskite model you can turn and export as an image; the perovskite structure page shows the shared ABX₃ framework that FAPbI₃, MAPbI₃ and CsPbBr₃ all start from.