2D (layered) halide perovskites: Ruddlesden–Popper, Dion–Jacobson and the n value

A 3D halide perovskite such as MAPbI₃ is an endless framework of corner-sharing PbI₆ octahedra with a small cation in every cage (see the perovskite structure hub). Cut that framework into slabs n octahedra thick and fill the gaps with cations too large to fit in the cages, and you have a 2D (layered) perovskite. The formula of the butylammonium series, (BA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁, shows the parts: two bulky butylammonium (BA) spacers, n − 1 methylammonium (MA) ions inside the slab, and n layers of PbI₆ octahedra (Cao et al. 2015). Phase-pure members from n = 1 to n = 5 have slabs 0.641 to 3.139 nm thick (Blancon et al. 2018), and n = ∞ is MAPbI₃ itself (Cao et al. 2015). These layered compounds belong to the wider family of organic–inorganic perovskites, which spans 3D and lower-dimensional structures (Saparov & Mitzi 2016).

The integer n sets the thickness of the inorganic layer, and phase-pure crystals with a single n value up to n = 5 have been synthesised (Blancon et al. 2018).

• Ruddlesden–Popper (RP). General formula A′₂Aₙ₋₁BₙX₃ₙ₊₁: two spacer cations A′ per formula unit, such as butylammonium (Cao et al. 2015; Blancon et al. 2018) or phenethylammonium (Smith et al. 2014).
• Dion–Jacobson (DJ). General formula A′Aₙ₋₁PbₙI₃ₙ₊₁: one spacer A′ per formula unit, so the spacer must carry a 2+ charge (from the charge balance of the formula). Mao et al. (2018) reported the first complete homologous DJ series in halide perovskites, with 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP) as A′; their inorganic layers lie exactly on top of one another. Moving the aminomethyl group from the 4- to the 3-position gives smaller band gaps.

Each inorganic slab is a natural quantum well: the electron and hole wavefunctions stay in the perovskite layer with little leakage into the organic spacers (Blancon et al. 2018). Two effects follow.

• Quantum confinement widens the band gap as the slab thins: in the BA series it rises from 1.52 eV for MAPbI₃ to 2.24 eV for n = 1 (Cao et al. 2015).
• Dielectric confinement strengthens the electron–hole attraction, because the organic spacers (dielectric constant about 2.2) screen it less than the perovskite (about 4–5.2). Together the two give exciton binding energies of 470 meV at n = 1 falling to 125 meV at n = 5, with dielectric confinement dominating in the thinnest slabs (Blancon et al. 2018). Excitons are predicted to stay bound above room-temperature thermal energy up to n ≈ 20, a slab about 12.6 nm thick (Blancon et al. 2018).

3D MAPbI₃ is unstable to moisture and needs anhydrous processing (Smith et al. 2014). Smith et al. (2014) used the n = 3 phenethylammonium phase (PEA)₂(MA)₂Pb₃I₁₀ as the absorber: its films were more moisture resistant than MAPbI₃, devices could be made at ambient humidity, and the cells gave an open-circuit voltage of 1.18 V and 4.73%. Cao et al. (2015) made cells from the (BA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁ series; the n = 3 compound gave 4.02% and kept its performance after long exposure to high humidity, and the films self-assembled with the inorganic layers perpendicular to the substrate.

The low efficiency came from the organic spacers acting as insulating layers that block out-of-plane charge transport. Tsai et al. (2016) overcame this with near-single-crystalline films whose inorganic planes are aligned out-of-plane, reaching 12.52% with no hysteresis; unencapsulated cells kept over 60% of their efficiency for over 2,250 hours under constant illumination and tolerated 65% relative humidity better than 3D cells, and encapsulated cells showed no degradation. Dion–Jacobson phases have also been used as absorbers, with 7.32% for the best 3AMP-based compound (Mao et al. 2018).

Confinement also helps light emission. Yuan et al. (2016) made a mixed material of grains with different quantum-size-tuned gaps that funnels excitations to the lowest-gap emitter, so radiative recombination outcompetes trapping; their near-infrared LEDs reached an external quantum efficiency of 8.8% and a radiance of 80 W sr⁻¹ m⁻².

For a paper, slide or poster, draw the slabs side on: n layers of corner-sharing octahedra, then a layer of spacer cations, repeated. Showing n = 1, 2 and 3 next to the 3D (n = ∞) framework makes the n value obvious, and a schematic band diagram of wide-gap organic barriers and narrow-gap perovskite wells explains the confinement. For a Dion–Jacobson phase, draw a single doubly charged spacer layer between adjacent slabs, with the slabs directly above one another. In the LabCanvas figure editor, the Library’s Perovskite crystal object draws corner-sharing octahedra that you can stack into slabs, add spacer molecules to, recolour and export as a high-resolution SVG; the perovskite structure page shows the 3D framework the slabs are cut from.