An Introduction to FAPbI3: Formamidinium Lead Iodide, CH(NH2)2PbI3

Formamidinum lead iodide (FAPbI₃) is a perovskite material often used in solar cells. This is made by combining formamidinium iodide (FAI) with PbI₂. FAPbI₃ was first used in 2014 as an alternative to MAPbI₃ . FAPbI₃ offers a narrower band gap, closer to the ideal band gap for solar cells, increasing potential device efficiencies. FAPbI₃ crystal structures combine formamidinium as the organic A cation. Formamidinium (FA⁺, CH(NH₂)₂⁺) with lead (Pb⁺) and three iodide anions (I^-).

There is some phase instability with pure FAPbI₃ crystals which limit its use in solar cells. At room-temperature, it struggles to remain in a black tetragonal perovskite phase (also known as the α-phase), instead converting into the yellow hexagonal non-perovskite phase (referred to as the δ-phase). They are therefore known as "phase unstable".

However with appropriate dopants and crystallization techniques, FAPbI₃-based solar cells have achieved PCEs of over >25% . Additionally, FAPbI₃ is often used in conjunction with other perovskite materials, such as MAPbBr₃ or CsPbI₃ . Mixing halides or A-cations means you can stabilize the perovskite α-phase while still maintaining the attractive properties of FAPbI₃.

FAPbI₃ Properties

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Property	FAPbI3	What Does This Mean?
Band Gap	1.45-1.51 eV	Closer than MAPbI3 to the ideal solar cell band gap (1.3 eV). The exact value with vary based on stoichiometry and dopants used.
Tolerance Factor	> 1	Related to crystal structure. This means FAPbI3 forms a hexagonal structure at room temperature. This is known as δ-FAPbI3 phase. δ-FAPbI is yellow, non-perovskite and not photoactive.
Carrier Diffusion Length	Up to 6.6 μm in single crystals	The distance electrons or holes can cover before undergoing radiative recombination. For many devices, this indicates the maximum distance an electron can move within the film and remain effective. The higher this number, the better.
Exciton Binding Energies	10 meV	Less than thermal energy at room temperature. This means at room temperature excitons will quickly separate and move through as free carriers through the perovskite material.

Benefits of FAPbI₃

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• FAPbI₃ has a narrow band gap, allowing it to absorb more light than MAPbI₃ films. This means that devices can be more efficient. FAPbI₃ can have band gaps of 1.45-1.51 eV , which is close to ideal solar cell band gap of 1.31 eV.
• FAPbI₃ has higher resistance to degradation at high temperatures. MAPbI₃ begins to exhibit thermal degradation over 85 °C , where as FAPbI₃ can withstand temperatures well over 150 °C.
• FA-based perovskites have a wider range of possible band gaps. By changing the ratio of halides in the perovskite you can acheive band gaps of up to 2.23 eV. This is ideal for using in perovskite tandem solar cells.

FAPbI₃ Issues

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• Conversion to α-FAPbI₃ phase requires require high temperatures (>150 °C) due to their high formation energy. This limits the types of substrates and transport layers you can use, and increases production costs.
• α-FAPbI₃ is metastable (or phase unstable) at room temperature, so can easily convert to the photo-inactive δ-phase. This means FAPbI₃ devices often degrade quickly.
• α-FAPbI₃ to δ-FAPbI₃ conversion is more likely in humid environments (Yi 2015). This can be stabilized by incorporating small amounts of the inorganic A cation, Caesium.
• Lead-based perovskites such as FAPbI₃ have environmental and toxicity issues. These issues will need to be addressed before mass production of these devices.

FAPbI₃ Deposition

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To create a FAPbI₃ film, dissolve formamidinium iodide and lead iodide in a polar solvent. Several solvents can dissolve FAPbI₃, including:

• Dimethylformamide (DMF)
• Dimethylsulphoxide (DMSO)

You can also grow and store FAPbI₃ as single crystals, then dissolve them before spin coating. This method assures equimolar rations of FAI and PbI₂ in the perovskite crystal.

Spin coating is a reliable deposition method for creating FAPbI₃ films. Optimal spin coater parameters, such as spin speed and spin duration, vary based on your chosen substrate, other transport layers, and device architecture. It may be necessary to experiment with these parameters to identify the most suitable spin coating procedure for your devices.

When using an inorganic HTL/ETL (for example, SnO₂), pre-treating substrates with a UV Ozone Cleaner before deposition can enhance wetting creating a more uniform film. If not, aim to deposit the perovskite layer promptly after depositing any previous layers.

Example Procedures: Spin Coating FAPbI₃

One example of FAPbI₃ deposition involves spin coating a 1.4 M eqimolar FAPbI₃ solution (doped with MDACl2 and MACl), dissolved in DMF:DMSO (8:1) within a glove box (process from ):

Step	Spin Speed	Duration	Comments
Spin 1	1000 rpm	10 s
Spin 2	5000 rpm	15 s	Quench with 1 ml of antisolvent
Anneal		20 min	120 °C

Alternatively, another deposition method involves spin coating a 1.8 M FAPbI₃ solution (doped with MACl), dissolved in DMF:DMSO (8:1) onto a modified SnO₂ ETL under atmospheric conditions (process from ):

Step	Spin Speed	Duration	Comments
Spin	5000 rpm	20 s	Quench with 800 µl of diethyl ether 10 s afer beginning spin cycle
Anneal 1		5 min	100 °C
Anneal		20 min	150 °C

Important Papers for FAPbI₃ include:

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• First use of Formamidinium in FAPbI₃ perovskites

  Eperon, G. E., Stranks, S. D., Menelaou, C., Johnston, M. B., Herz, L. M., & Snaith, H. J. (2014). Formamidinium lead trihalide: A broadly tunable perovskite for efficient planar heterojunction solar cells. Energy & Environmental Science, 7(3), 982.

• Incorporation of small amounts of Cs can reduce α-δ Phase Transition in FAPbI₃ films

  Li, Z., Yang, M., Park, J., Wei, S., Berry, J. J., & Zhu, K. (2015). Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide Solid-State alloys. Chemistry of Materials, 28(1), 284–292.

• Doping with MACl can help stabilize FAPbI₃

  Min, H., Kim, M., Lee, S., Kim, H., Kim, G., Choi, K., Lee, J. H., & Seok, S. I. (2019). Efficient, stable solar cells by using inherent bandgap of α-phase formamidinium lead iodide. Science, 366(6466), 749–753.

• FAPbI₃ and resistance to moisture and can be stabilized with small amounts of Cs

  Lee, J. W., Kim, D., Kim, H., Seo, S., Cho, S. M., & Park, N. (2015). Formamidinium and cesium hybridization for Photo‐ and Moisture‐Stable perovskite solar cell. Advanced Energy Materials, 5(20).

Formamidinium Iodide (FAI)

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Other References

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• Alsalloum et al (2020). Low-temperature crystallization enables 21.9% efficient single-crystal MAPbI₃ inverted perovskite solar cells. ACS Energy Letters, 5(2), 657–662. doi:10.1021/acsenergylett.9b02787
• Huang, Y., Lei, X., He, T., Jiang, Y., & Yuan, M. (2021). Recent progress on formamidinium‐dominated perovskite photovoltaics. Advanced Energy Materials, 12(4). https://doi.org/10.1002/aenm.202100690
• Liu et al (2020). Stabilization of highly efficient and stable Phase‐Pure FAPBI3 perovskite solar cells by molecularly tailored 2D‐Overlayers. Angewandte Chemie International Edition, 59(36), 15688–15694. https://doi.org/10.1002/anie.202005211
• Zheng et al (2016). Improved phase stability of formamidinium lead triiodide perovskite by strain relaxation. ACS Energy Letters, 1(5), 1014–1020. doi:10.1021/acsenergylett.6b00457