Introduction to MAPbI3: Methylammonium Lead Iodide, CH3NH3PbI3

Methylammonium lead iodide (MAPbI₃) was one of the first perovskite materials used in perovskite solar cells. These crystal structures combine the organic A cation methylammonium (MA⁺, CH₃NH₃⁺) and divalent lead (Pb⁺) with three iodide anions (I^-). It is typically made from a solution of methylammonium iodide and lead iodide.

To this day, MAPbI₃ is one of the most popular perovskite solar cell materials. Using good doping, passivation and encapsulation techniques, devices using MAPbI₃ have achieved PCEs exceeding 20% (Li 2021, Alsalloum 2020).

MAPbI₃ Properties

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Property	MAPbI3	What Does This Mean?
Band Gap	1.55-1.64 eV (Frost 2014, Targhi 2018, Tao 2019)	Relatively large band gap, further away from the ideal solar cell band gap of 1.33 eV. Good, but could be better. The exact value with vary based on stoichiometry.
Tolerance Factor	0.81	Related to crystal structure. This means MAPbI3 can easily achieve and maintain tetragonal (absorbing) perovskite phase at room temperature.
Carrier Diffusion Length	>100 nm Up to 10 μm in single crystals (Shi 2015)	The distance that electrons or holes can travel before they radiatively recombine. For most devices, this represents how far the electron can travel in the film and still be useful. The higher this number, the better.
Exciton Binding Energies	10-24 meV	Less than thermal energy at room temperature. This means at room temperature excitons will quickly seperate and move through as free carriers through the peroskite material.

Benefits of MAPbI₃

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• MAPbI₃ perovskites have excellent optical and electronic properties.They have extremely long carrier diffusion lengths (Dong 2015, Shi 2015). They have naturally very low trap density and high carrier lifetime. All of these properties make them great candidates for solar cells.
• They can also be easily solution processed. They require relatively low temperatures (<100 °C) to convert to black perovskite film (Conings 2015) . This is much lower than the temperatures needed for FA-based and Cs-based perovskites.
• Also, you can create annealing-free MAPbI₃ perovskite films using low boiling point solvents (Noel 2017, Cassella 2023) and alternative post-deposition treatments such as air-knives or antisolvent quenching.
• MAPbI₃ is phase stable at room temperature after conversion, unlike FAPbI₃ and CsPbI₃.

MAPbI₃ Issues

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• Larger band gap limits achievable device efficiency. Band gap can be tuned by introducing bromide and chloride ions, but these mixed ion materials are not photostable (Noh 2013).
• Poor thermal stability due to volatile MA⁺. Exhibits phase transition well before 85°C (Conings 2015) .
• Degrade under exposure to moisture (Niu 2014 ,Yang 2015)
• The use of lead in these solar cells is still a big issue for many for environmental and toxicity reasons.
• MAPbI₃ perovskite solar cells tend to have limited lifetime under operational use.

MAPbI₃ Deposition

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To make a layer of MAPbI₃ film, you need to dissolve methylammonium iodide and lead iodide in a polar solvent. You can use multiple different solvents to dissolve MAPbI₃ including:

• Dimethylformamide (DMF)
• Dimethylsulphoxide (DMSO)
• 2-methoxyethanol (2-ME)
• Gamma-Butyrolactone (GBL)
• Acetonitrile

You can vary the molar ratio of these components to change film properties but an equimolar ratio (1:1) of MAI to PbI₂ is a good place to start. You can use a range of molar concentrations of MAPbI₃, often between 1-1.4 M. If you need a thicker perovskite layer, use a higher molar concentration.

One reliable way to deposit perovskite layers is through spin coating. Optimal spin coating variables will depend on your substrate, transport layers and device architecture. You will likely need to play around with speeds and timings to find the right spin coating recipe for you. If you have used an inorganic HTL/ETL (such as SnO₂), you can UV Ozone treat the device before deposition to improve wetting. Otherwise, try to deposit perovskite as soon as possible after first deposition.

To spin coat a MAPbI₃ perovskite layer, we recommend using a two-step deposition method. We have made perovskite devices in our glove box using the following spin program.

Here, we used a 1.1 M MAPbI₃ solution, dissolved in DMF was deposited on an SnO₂ ETL:

Step	Spin Speed	Duration	Comments
Spin 1	2000 rpm	10 s
Spin 2	5000 rpm	30 s	Quench with 300 µl of antisolvent 20 s in
Anneal		20 min	100 °C

Alternatively, one study deposited a 1.4 M MAPbI₃ solution, dissolved in GBL/DMSO on doped SnO₂ using the following spin recipe (Chen 2021):

Step	Spin Speed	Duration	Comments
Spin 1	1000 rpm	10 s
Spin 2	3000 rpm	30 s	Quench with 200 µl of chlorobenzene
Anneal		10 min	100 °C

Important Papers for MAPbI₃ include:

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• First use of perovskites in DSSCs.

  Kojima, A., Teshima, K., Shirai, Y., & Miyasaka, T. (2009). Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 131(17), 6050–6051. doi:10.1021/ja809598r

• First completely solid state perovskite solar cell.

  Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N., & Snaith, H. J. (2012). Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 338(6107), 643–647. doi:10.1126/science.1228604

• Demonstration that MAPbI₃ is thermally unstable.

  Conings, B., Drijkoningen, J., Gauquelin, N., Babayigit, A., D’Haen, J., D’Olieslaeger, L., … Boyen, H. (2015). Intrinsic thermal instability of methylammonium lead trihalide perovskite. Advanced Energy Materials, 5(15). doi:10.1002/aenm.201500477

• Excellent charge transport properties of MAPbI₃.

  Shi, D., Adinolfi et al (2015). Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science, 347(6221), 519–522. doi:10.1126/science.aaa2725

• Degradation of MAPbI₃ in humid environments.

  Yang, J., Siempelkamp, B. D., Liu, D., & Kelly, T. L. (2015). Investigation of CH₃NH3PBI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques. ACS Nano, 9(2), 1955–1963. doi:10.1021/nn506864k

Methylammonium Iodide (MAI)

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Other References

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1. Alsalloum, A. Y., Turedi, B., Zheng, X., Mitra, S., Zhumekenov, A. A., Lee, K. J., … Bakr, O. M. (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
2. Chen, Y., Zuo, X., He, Y., Qian, F., Zuo, S., Zhang, Y., Liu, L., Chen, Z., Zhao, K., Liu, Z., Gou, J., & Liu, S. (2021b). Dual passivation of perovskite and SNO 2 for High‐Efficiency MAPBI 3 perovskite solar cells. Advanced Science, 8(5). doi: 10.1002/advs.202001466
3. Cassella et al (2023). Binary solvent system used to fabricate fully Annealing‐Free perovskite solar cells. Advanced Energy Materials, 13(11). doi: 10.1002/aenm.202203468

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