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

Formamidinum lead iodide (FAPbI₃) is a material used for perovskite solar cells. FAPbI₃ was introduced in 2014 as an alternative to MAPbI₃ (Eperon 2014). FAPbI₃ offers a narrower band gap, closer to the ideal band gap for solar cells, increasing potential device efficiencies. FAPbI₃ crystal structures combine formamadinium 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% (Park 2023). Additionally, FAPbI₃ is often used in conjunction with other perovskite materials, such as MAPbBr₃ (Jeon 2015) or CsPbI₃ (Lee 2015). Mixing halides or A-cations means you can stabilize the perovskite α-phase while still maintaining the attractive properties of FAPbI₃.

FAPbI₃ Properties
Benefits of FAPbI₃
FAPbI₃ Issues
FAPbI₃ Deposition
Important FAPbI₃ Papers
References

FAPbI₃ Properties

FAPbI<sub>3</sub> unit cell structure — FAPbI₃ unit cell structure.

Property	FAPbI₃	What Does This Mean?
Band Gap	1.45-1.51 eV (Eperon 2014, Min 2019, Tao 2019)	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 (Li 2016)	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 (Zhumekenov 2016)	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 (Galkowski 2016)	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

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 (Min 2019, Eperon 2014 ,Tao 2019), 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 (Conings 2015), where as FAPbI₃ can withstand temperatures well over 150 °C.
FA-based pervoskites 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

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.(Li 2015)
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 (Lee 2015).
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

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. The optimal spin coater parameters, such as spin speed and spin duration, will 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 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 Park 2023):

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 Xiong 2021):

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:

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. doi:10.1039/c3ee43822h
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. https://doi.org/10.1021/acs.chemmater.5b04107
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. https://doi.org/10.1126/science.aay7044
FAPbI₃ adn 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). https://doi.org/10.1002/aenm.201501310

Other References

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

Conings et al (2015). Intrinsic thermal instability of methylammonium lead trihalide perovskite. Advanced Energy Materials, 5(15). doi:10.1002/aenm.201500477

Gałkowski et al (2016). Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors. Energy and Environmental Science, 9(3), 962–970. https://doi.org/10.1039/c5ee03435c

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

Jeon et al (2015). Compositional engineering of perovskite materials for high-performance solar cells. Nature, 517(7535), 476–480. https://doi.org/10.1038/nature14133

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

Park et al (2023). Controlled growth of perovskite layers with volatile alkylammonium chlorides. Nature, 616(7958), 724–730. https://doi.org/10.1038/s41586-023-05825-y

Xiong, Z., Lan, L., Wang, Y., Lu, C., Qin, S., Chen, S., Zhou, L., Zhu, C., Li, S., Meng, L., Sun, K., & Li, Y. (2021). Multifunctional Polymer Framework Modified SnO2 Enabling a Photostable α-FAPbI3 Perovskite Solar Cell with Efficiency Exceeding 23%. ACS Energy Letters, 6(11), 3824–3830. https://doi.org/10.1021/acsenergylett.1c01763

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

Zhumekenov et al (2016). Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length. ACS Energy Letters, 1(1), 32–37. https://doi.org/10.1021/acsenergylett.6b00002

Contributors

Written by

Dr. Mary O'Kane

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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

Contents

FAPbI3 Properties

Benefits of FAPbI3

FAPbI3 Issues

FAPbI3 Deposition

Example Procedures: Spin Coating FAPbI3

Important Papers for FAPbI3 include: