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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

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

FAPbI<sub>3</sub> unit cell structure
α-FAPbI3 (left) is perovskite and photoactive. δ-FAPbI3 (right) polytypes are non-perovskite and not photoactive.
  • 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.

  • 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 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).

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


Written by

Dr. Mary O'Kane

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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