MAPbI₃ (methylammonium lead iodide): structure, band gap and solar cells

MAPbI₃ has the perovskite ABX₃ structure with an organic A cation: lead sits in octahedra of six iodide ions, the PbI₆ octahedra share corners in three dimensions, and the methylammonium cation CH₃NH₃⁺ sits in the cavities between them. The general ABX₃ lattice, and how halide perovskites relate to oxides such as SrTiO₃ and BaTiO₃, is explained on the perovskite structure page.

MAPbI₃ has three phases. Above about 327 K it is cubic; between about 165 K and 327 K, which includes room temperature, it is tetragonal; below about 165 K it is orthorhombic (Deretzis & La Magna 2017). The space groups are Pm3̄m, I4/mcm and Pnma (Ong et al. 2015). Neutron and synchrotron X-ray diffraction place the cubic–tetragonal transition near 330 K and the tetragonal–orthorhombic one around 160 K, both first order, the upper one very close to tricritical (Whitfield et al. 2016).

330 K is about 57 °C (calculated), and Whitfield et al. discuss how these structural transitions affect the performance of MAPbI₃ solar cells (Whitfield et al. 2016). The methylammonium cation tumbles almost isotropically in the cubic phase, orders subtly in the tetragonal phase and is essentially frozen below the tetragonal–orthorhombic transition (Bernard et al. 2018).

1. 2009: a sensitizer in a liquid cell. Kojima et al. found that nanocrystals of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ self-assembled on mesoporous TiO₂ sensitize it to visible light. The CH₃NH₃PbI₃ cell responded up to 800 nm and converted sunlight with 3.8% efficiency; the bromide cell gave a high photovoltage of 0.96 V (Kojima et al. 2009). In such liquid-junction cells the perovskite dissolves in the electrolyte, so performance degraded quickly (Kim et al. 2012).
2. 2012: solid state. Replacing the liquid electrolyte with the solid hole conductor spiro-MeOTAD, Kim et al. made MAPbI₃ cells on a 0.6 μm mesoporous TiO₂ film with a short-circuit current of 17.6 mA cm⁻², an open-circuit voltage of 0.888 V, a fill factor of 0.62 and an efficiency of 9.7%, and stability greatly improved: unencapsulated cells stored in air were stable for more than 500 h (Kim et al. 2012).
3. 2012: an inert scaffold. Lee et al. replaced TiO₂ with mesoporous alumina as an inert scaffold, so electrons are transported through the perovskite itself. Their meso-superstructured cell reached 10.9% and open-circuit voltages above 1.1 V with a 1.55 eV absorber (Lee et al. 2012).
4. 2013: about 15%. Sequential deposition, in which PbI₂ in nanoporous TiO₂ is converted to the perovskite by a CH₃NH₃I solution, gave about 15% (Burschka et al. 2013), and a planar cell with a vapour-deposited perovskite absorber gave over 15% without any nanostructure (Liu et al. 2013).

• A band gap suited to sunlight. The optical gap of MAPbI₃ is about 1.5 eV and direct, with the valence band at −5.43 eV and the conduction band at −3.93 eV, slightly above that of TiO₂ (Kim et al. 2012). Powder MAPbI₃ emits at the band edge around 780 nm (Kim et al. 2012).
• Strong absorption. MAPbI₃ nanocrystals absorb about an order of magnitude more strongly than the dye N719, so submicron-thick films are enough (Kim et al. 2012).
• Long diffusion lengths. Electrons and holes in solution-processed MAPbI₃ films travel about 100 nm before recombining, and the two lengths are balanced (Stranks et al. 2013; Xing et al. 2013); in the mixed halide CH₃NH₃PbI₃₋ₓClₓ they exceed 1 μm (Stranks et al. 2013). Comparable absorption and diffusion lengths explain why such cells are efficient (Xing et al. 2013).
• Few traps in single crystals. MAPbX₃ single crystals have trap densities of 10⁹–10¹⁰ cm⁻³, comparable to the best photovoltaic silicon, and diffusion lengths over 10 μm (Shi et al. 2015); in solution-grown MAPbI₃ crystals diffusion lengths exceed 175 μm under 1 sun and 3 mm under weak light (Dong et al. 2015).

• Mesoscopic, sensitized. MAPbI₃ on mesoporous TiO₂, first with a liquid electrolyte (Kojima et al. 2009), then with a solid hole conductor (Kim et al. 2012).
• Meso-superstructured. The perovskite coats an inert alumina scaffold and carries the electrons itself (Lee et al. 2012).
• Sequential (two-step) deposition. PbI₂ is deposited first and converted into MAPbI₃ by CH₃NH₃I, giving better control of the morphology and reproducibility (Burschka et al. 2013).
• Planar heterojunction. A flat perovskite layer between two charge-selective contacts, no nanostructure needed (Liu et al. 2013; Stranks et al. 2013).
• Radiation sensing. Thick MAPbI₃ single crystals also convert gamma rays to electricity (the gammavoltaic effect), with 3.9% efficiency from a caesium-137 source (Dong et al. 2015).

MAPbI₃ decomposes rapidly in moist air. Under controlled humidity, the first step of the degradation is a hydrated intermediate containing isolated PbI₆⁴⁻ octahedra, and the choice of hole transport layer strongly affects how long the film underneath survives (Yang et al. 2015). Thermal instability and moisture sensitivity are the main obstacles to commercial use (Ciccioli et al. 2019). Heating MAPbI₃ under high vacuum in an electron microscope precipitates PbI₂; in single-crystal nanorods the decomposition reversed on cooling and was fully reversible up to 100 °C (Jo et al. 2020).

The most efficient perovskite solar cells all contain lead, which raises serious environmental concerns for large-scale use. Tin is the obvious substitute, but tin perovskites are unstable, and in tests on zebrafish a tin perovskite proved harmful too, through acidification, so tin may not be the ideal replacement for lead (Babayigit et al. 2016).

For a paper, slide or poster, draw MAPbI₃ as corner-sharing PbI₆ octahedra with a CH₃NH₃⁺ cation in each cavity; tilt the octahedra for the tetragonal room-temperature phase, or keep them straight for the cubic phase. For a device figure, show the layer stack: glass and transparent electrode, an electron transport layer such as TiO₂, the perovskite, a hole conductor such as spiro-MeOTAD and a gold contact. In the LabCanvas figure editor, the Library’s “Crystal” object draws the perovskite lattice in 3D with or without octahedra, and the device objects include perovskite solar cell stacks; all export as high-resolution SVG and can be recoloured and resized.