Dion-Jacobson phase

Dion-Jacobson phases are a type of perovskite structure represented by the general formula M^IA_(n-1)B_(n)O_(3n+1). The structure of this phase is considered a layered perovskite, meaning the structure consists of two-dimensional ABO₃ perovskite-like layers, where 'A' and 'B' are cations, 'M' is a monovalent cation, and "n" represents the thickness of each perovskite-like layer, specifically the number of BO₆ octahedra that corner-share in a stacking pattern. These perovskite-like layers of "n" thickness are separated by the monovalent cations. Ideally the 'A' cation has a 12-fold coordination, the 'B' cation has a 6-fold coordination, and the 'M' cation has 8-fold coordination.^[1] The first recognized Dion-Jacobson phases were reported by M. Dion in September 1981, in which Dion prepared and characterized a series of M^ICa₂Nb₃O₁₀ (M^I = Li, Na, K, Rb, Cs, NH₄, Tl) compounds.^[2]

Crystal structure of n=2 Dion-Jacobson phase CsLaNb₂O₇. Cs shown in orange, La in blue, Nb in teal, O in magenta.

Crystal structure

The general formula for the Dion-Jacobson phase is M^IA_(n-1)B_(n)O_(3n+1), where 'M' is a monovalent cation, 'A' and 'B' are cations, and oxygen represents the anions. The structure consists of corner-sharing BO₆ octahedra layers, that extend infinitely along two axes. Along the third axis, the BO₆ octahedra corner-share to a finite thickness, hence the variable "n". The 'A' cation is located the at the body-centered position of the eight BO₆ octahedra that form a cube. Depending on the identity of the monovalent cation, the perovskite-like layers could be displaced by 1/2 of the unit cell parameter, or not displaced at all.^[1]

Synthesis

Inorganic Dion-Jacobson perovskites, such as KLaNb₂O₇, have been synthesized via high temperature solid state reactions between the respective carbonate and oxide precursors.^[3] Two-dimensional Dion-Jacobson halide perovskites, such as (3AMPY)(MA)Pb₂I₇ (3AMPY = 3-(aminomethyl)pyridinium and MA= methylammonium) have been synthesized via precipitation reactions as well.^[4]

Applications

As perovskite solar cells have continued to grow in popularity, Dion-Jacobson (DJ) perovskites have begun to show up in photovoltaic applications, noted for their superior stability. At the structural level, they are a stronger candidate for PSCs than Ruddlesden-Popper phase (RP) perovskites, which is another phase commonly used in photovoltaics. DJ perovskites have a divalent organic cation which can bridge adjacent layers, eliminating the van der Waals gap with a stable structure formed by hydrogen bonding. The improved structure allows for more efficient interlayer charge transport and environmental stability, two common shortcomings of RP perovskites. ^[5] DJ perovskites can be used as the absorbing layer in 2D solar cells or as a layer for passivation on top of 3D perovskites to improve stability and power conversion efficiency (PCE). For example, a champion efficiency of 24.9% was achieved with a 2D DJ perovskite solar cell, which retained 97% of its initial PCE after 1000 hours in ambient air. ^[6] When used as a top layer in a 3D solar cell, a PCE of 19.5% was achieved with 83% of original PCE retained after 1260 hours under continuous illumination. ^[5] Through additive manufacturing, the performance of DJ perovskites can be improved even further. Incorporating chloride salts or thiocyanate salts into the precursor solution can coordinate ions from those compounds with undercoordinated ions often used in perovskites photovoltaics, such as Pb2+ and Sn2+, to reduce trap states and improve the PCE of DJ solar cells. Other additives, such as dimethyl sulfoxide (DMSO), can slow down crystallization to create larger grains and improve the structural properties of DJ perovskites. ^[7]