Efficiency, stability, and cost are the three key parameters, or the golden triangle used for screening techno-commercial feasibility of a semiconducting material for solid state applications (Nat Commun 9, 5265 (2018)). While perovskites are rivalling the cost and efficiency of the well-established semiconductors, intrinsic stability of the material is a continuing challenge. Resolving the stability issue in perovskites has been one of the major thrusts of Prof. Letian Dou’s research group in Department of Chemical Engineering at Purdue University. I joined the research group in 2017, and since then I have gained increasing interest in two-dimensional (2D) halide perovskites because of their higher compositional flexibility and stability compared to their three-dimensional (3D) counterparts. My interest in understanding the role of 2D perovskites in improving the stability of the material was further piqued when a postdoctoral researcher in our group, Dr. Gao Yao, published his findings on the enhanced ambient stability of 2D perovskites incorporating newly synthesized π-conjugated thiophene organic ligands (Nat. Chem. 11, 1151–1157 (2019)). We became interested in exploring whether these ligands could potentially inhibit the undesired halide migration along with their superior ambient stability.

The first question we faced is the type of platform to use for analyzing halide diffusion in 2D perovskites. Based on previous diffusion studies performed on 3D perovskites, we realized that single crystal heterostructures offered the simplest system to visualize halide migration. The next challenge was synthesizing heterostructures comprised of phase pure 2D perovskites. To ensure phase purity of the perovskites, we employed a two-step approach for assembling the vertical heterostructures. The bottom layer of the heterostructure was synthesized using solvent evaporation method developed by Letian Dou et al. (Science 349, 6255, 1518-1521 (2015)) and Enzheng Shi et al. (Nature 580, 614–620 (2020)) and the top layer was then transferred using commonly utilized mechanical exfoliation technique. This two-step approach ensured a simple and reproducible assembly of phase pure atomically flat 2D perovskite vertical heterostructures.

A systematic study of different perovskite systems was then carried out to answer two fundamental questions – (1) What is the mechanism of halide inter-diffusion in 2D perovskites? and (2) How do the organic and inorganic components influence the halide diffusion?

By conducting a series of experiments, we found that the halide migration in 2D perovskites does not conform to the classical diffusion process mediated by a continuous concentration profile evolution. Instead, a unique layer-by-layer or “quantized” halide diffusion was observed which was dictated by the thermodynamically preferred halide compositions. Figure 1 shows a schematic representation of this unique mechanism.

We also analyzed the impact of inorganic octahedral layers and organic cations on halide inter-diffusion by studying various 2D perovskite vertical heterostructures. The organic cations played a governing role in influencing halide diffusion. Additionally, bulkier π-conjugated organic cations were found to successfully inhibit halide migrations across the Van der Waals heterostructure.

Our study provides a platform for qualitative and quantitative screening of 2D perovskites with the eventual goal of utilizing the discovered stable perovskites in commercial scale solar cells, light emitting diodes, and transistors.

Figure 1. Schematic representation of “quantized” halide inter-diffusion across a 2D perovskite vertical heterostructure.

Read our paper in Nature Nanotechnology: https://www.nature.com/articles/s41565-021-00848-w