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What Are 2D Perovskites?

2D perovskites are perovskite materials with a layered crystal structure. They are made up of metal-halide sheets, separated by large organic cations called spacers. This layered 2D structure changes the optical and electronic properties of the material compared to the bulk (3D) perovskite. For this reason, 2D perovskites can be used in a wide range of interesting applications.

The main difference between 2D and 3D perovskites is their perovskite crystal structure. The layered and bulk perovskites have very different material properties and therefore are used for different applications.

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2D vs 3D Perovskites

At a glance, here are some of the differences between 2D and 3D perovskites.

Property 2D perovskites 3D perovskites
Structure Layers or sheets of metal halide octahedra seperated by large organic spacers. Has general chemical formula of RAn-1BnX3n+1. Bulk crystal with general chemical formula: ABX3
Stability Higher thermal and moisture stability - due to hydrophobic spacer cations Often vulnerable to ambient conditions including moisture, oxygen and high temperatures
Charge transport properties

High exciton binding energy so very stable excitons.

Poor charge transport properties between layers, due to quantum confinement effects.

Improved photolumiscent quantum yield (PLQY)

Weaker exciton binding energy so easily makes free charge carriers

Excellent charge transport in all directions, so much more conductive.

Lower PLQY
Band gap Higher (>2 eV) Lower (1.48-2 eV) - more suited for solar cells

2D Perovskite Structure

2D perovskites are made of metal halide ([BX6]) octahedral layers (where B is a divalent heavy metal, usually Pb, and X are halide ions) interspaced by large organic cations called spacers. They have the general chemical structure RAn-1BnX3n+1 where:

  • R is an organic spacer cation
  • A is a smaller, monovalent A-cation
  • M is a divalent metal cation (commonly lead (Pb) or tin (Sn))
  • X are halide ions
  • n is the number of inorganic perovskite layers between spacer layers
2D perovskites, quasi-2D perovskites and 3D perovskite crystal structure
2D perovskites, quasi-2D perovskites and 3D perovskite crystal structure

These spacer cations perform a similar role to organic A-cations in 3D perovskite structures. They balance out charge within the crystal structure. However, their larger molecular radius creates separation between their metal halide layers, which is instrumental in defining the 2D structure. You can tune the electrical and optical properties of the perovskite material by changing the type of spacer cation used, as well as the layers of dimensionality, n, and the metal halide composition. This separation significantly affects the electronic and optical properties of the material, leading to enhanced stability and tunable properties compared to their 3D counterparts.

The dimensionality of a perovskite is defined by the number of metal-halide layers between spacer layers, n:

  • For 2D perovskite materials, n=1 so there is only 1 layer of [BX6] octahedra between spacer layers. 
  • For a 3D perovskite crystal, n→∞. There are no spacer molecules at all.
  • Perovskites where n=2,3,4... are known as quasi-2D perovskites. These have a few [BX6] octahedral layers between spacer layers.

Quasi-2D Perovskites

Quasi-2D perovskites are interesting as they can combine some of the properties of 2D and 3D perovskites.

There isn't a strict n range that defines quasi-2D perovskites but one general guideline is if n<5 this perovskite is counted as low dimensional or quasi-2D.

Perovskites with a n>5 structure are sometimes referred to as quasi-3D perovskites. Generally once n>5, the perovskite crystal properties (e.g. band gap or binding energy) are more like the 3D perovskite than a 2D perovskite.

Types of 2D perovskite

There are three main types of 2D perovskite: ruddleston-popper (RP), dion-jacobson (DJ) and alternating cations in interlayer space (ACI). ACI perovskites are the least common so will not be discussed here.

Ruddleston-Popper (RP) perovskites

Ruddlesden-Popper (RP) perovskites are characterized by the chemical formula, C2An-1MnX3n+1, where:

  • C is a large, monovalent organic spacer cation
  • A is a smaller, monovalent A-cation
  • M is a divalent metal cation (commonly lead (Pb) or tin (Sn))
  • X are halide ions
  • n is the number of inorganic perovskite layers between spacer layers

One characteristic of RP perovskite is its "shifted" crystal structure is offset in both the a and b planes. They also contain a monobasic organic molecule as the spacer cation and have 2 spacer cations per unit cell.

The exfoliation of RP crystals is relatively easy so individual layers of RP 2D materials are easily accessible. These materials will have larger interlayer spacing compared to DJ materials which will affect their charge transfer properties and overall stability.

Dion-Jacobson (DJ) perovskites

Dion-Jacobson (DJ) perovskites are characterized by the chemical formula, DAn-1MnX3n+1, where:

  • D is a divalent large, organic spacer cation.
  • A is a smaller, monovalent cation located within the perovskite layer.
  • M is a divalent metal cation (commonly lead (Pb) or tin (Sn))
  • X are halide ions
  • n is the number of inorganic perovskite layers between spacer layers

For DJ perovskites, the layers are stacked directly on top of each other, without the offset seen in RP structures. They also use divalent cations (meaning they contain 2 valence electrons per molecule) in the spacer layer, so only require one molecule per unit cell. 

These materials are known for their more compact layering compared to RP perovskites. This may offer advantages for charge mobility and efficiency in solar cell applications. They are also reportedly more stable than RP perovskites, although there is some debate surrounding this.

Ruddleston-popper and dion-jacobson 2D perovskite crystal structures.
Ruddleston-popper and dion-jacobson 2D perovskite crystal structures

2D Perovskite Applications

2D perovskites and quasi-2D perovskites can be used for many different applications:

  • Quasi-2D perovskites are often employed in light-emitting diodes (LEDs) due to their high PLQY and strong exciton binding properties.
  • Although 2D perovskites have limited functionality as solar cells, they can enhance the performance of 3D perovskite solar cells by acting as passivating layers or dopants.
  • Additionally, using both 3D and 2D materials leads to the creation of 2D/3D perovskite solar cells, combining the benefits of both material systems.
  • 2D perovskites can form colloidal nanoplatelets or sheets, which can be incredibly useful for controlling charge properties within devices (LEDs, PV, photodetectors,etc) (Weidman 2017).
  • 2D materials can also be used in phosphorescent and TADF molecules (Zhou 2019). 

Issues with 2D Perovskites

Although 2D perovskites are more stable than 3D perovskites, they still have some of the same vulnerabilities. These include:

  • Moisture Instability: Hydrophobic spacer cations can help create a barrier against moisture degradation. However, excessive moisture exposure can still degrade 2D perovskites, especially for high values of n.
  • 2D Structural Instability: Higher n quasi-2D perovskites can degrade into other quasi-2D structure inducing degradation. For example, an n=5 quasi-2D perovskites can degrade into various n=2 and n=3 phases in high moisture environments which allows moisture or oxygen into the system.
  • Halide Instability: The stability of all perovskites depend on their halide composition. Some evidence suggests that 2D-bromides are less stable than MAPbBr3, while the opposite has been found for iodide-based perovskites. Additionally, halide degradation can occur in mixed-halide 2D systems, just as it does in 3D systems.
  • Illumination: 2D and quasi-2D perovskites can suffer from photo-oxidation effects.
  • Ion migration: Illumination and high temperatures can lead to the migration of spacers through the perovskite. This can be an issue in 2D/3D perovskites or other structured systems.

Some Examples of Spacer Cations Used in 2D Perovskites

References

Chen, P., Bai, Y., Wang, S., Lyu, M., Yun, J., & Wang, L. (2018). Perovskite solar cells: In situ growth of 2D perovskite capping layer for stable and efficient perovskite solar cells. Advanced Functional Materials, 28(17). https://doi.org/10.1002/adfm.201870113

Cao, D. H., Stoumpos, C. C., Farha, O. K., Hupp, J. T., & Kanatzidis, M. G. (2015). 2D homologous perovskites as light-absorbing materials for solar cell applications. Journal of the American Chemical Society, 137(24), 7843–7850. https://doi.org/10.1021/jacs.5b03796

Contributors

Written by

Dr. Mary O'Kane

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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