My interests in halide perovskites originated from 7 years ago when I read two halide perovskite solar cell literatures published on Nature (Nature 2013, 501, 395–398; Nature 2013, 499, 316–319). I was amazed by the rapid development, high performance and simple processing of perovskite solar cells. At that time, I was a PhD student working on silicon solar cells and two-dimensional materials. I realized that the perovskite solar cell was very promising to be commercialized and became competitive with silicon solar cells.

Fortunately, I got the chance to join Prof. Letian Dou’s group (Davidson School of Chemical Engineering, Purdue University) as a postdoc research associate in 2017 and began to work on two-dimensional halide perovskites. Till now, halide perovskites have gained great success in optoelectronic devices including solar cells and light-emitting diodes. Considering halide perovskites are a class of semiconductors with high photoluminescence quantum efficiency, great tunability in structure and properties, high defect tolerance, and solution processability, we believed halide perovskites would provide us a great material platform and plenty of opportunities to explore additional applications beyond solar cells and light-emitting diodes, such as transistors and memory devices.

As we know, semiconductor heterostructures are essential building blocks of modern electronics and optoelectronics. Therefore, constructing heterostructures between different halide perovskites represents a promising route to expand the applications of halide perovskites. It was reported that three-dimensional CsPbBr₃-CsPbCl₃ perovskite heterostructures could be realized through ion diffusion method (Proc. Natl Acad. Sci. 2017, 114, 7216-7221), but the fast intrinsic ion mobility within CsPbX₃ (X=Cl, Br) leads to ion interdiffusion and large junction widths, and finally the heterostructure disappeared completely and became a CsPbBr_xCl_3-x alloy (Proc. Natl Acad. Sci. 2018, 115, 11929–11934; Nano Lett. 2018, 18, 1807-1813). So far, atomically sharp heterostructures of halide perovskites with high stability has not yet been achieved.

Before I began to work on halide perovskite heterostructures, our group found the conjugated organic ligands could substantially enhance the environmental and thermal stability of two-dimensional halide perovskites (Nat. Chem. 2019, 11, 1151-1157). This inspired us to incorporate the π-conjugated organic ligands into halide perovskite heterostructures. From then on, my journal with two-dimensional halide perovskite heterostructures began.

As expected, we found two-dimensional halide perovskite lateral heterostructures with π-conjugated organic ligands (such as 2T, bithiophenylethylammonium) possessed better thermal stability over those with alkyl ligands (such as butylammonium, BA). In addition, it is surprising that the π- conjugated organic ligands also substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites. In this Nature paper, we demonstrate an epitaxial growth strategy to achieve highly stable and widely tunable lateral epitaxial heterostructures, multi-heterostructures, and superlattices of 2D halide perovskites using several types of π-conjugated organic ligands, which bridges the gap between layered halide perovskites with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides. At the same time, the band alignments of two-dimensional halide perovskite heterostructures can be modulated either by varying the inorganic composition in the lateral in-plane direction or by modifying the molecular structure in the out-of-plane direction (Figure 1). To the best of our knowledge, such multicomplex integrated systems have not previously been realized in other nanoscale heterostructures. The vertical organic semiconducting ligands will provide additional benefits to discover new physics and promote the device performance.

Overall, these findings represent critical fundamental insights into the stabilization of halide perovskite semiconductor materials and provide a new materials platform for future applications such as complex and molecularly thin superlattices, devices, and integrated circuits.

Read our paper in Nature: https://www.nature.com/articles/s41586-020-2219-7

Figure 1. Schematic illustration and band alignment of (2T)₂PbI₄-(2T)₂PbBr₄ lateral heterostructure.  (Nature 2020). The pairs of blue and green lines in the band diagrams represent the conduction band minimum and valence band maximum of inorganic [PbBr₄]²⁻ and [PbI₄]²⁻ octahedral layers, respectively. The broad, semi-transparent pairs of grey lines correspond to the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the 2T⁺ organic layers, respectively.