Ruddlesden-Popper (RP) lead halide perovskites are an emerging class of layered bulk semiconductors with general formula L2An-1PbnX3n+1. Here, L is a long-chain ammonium cation (e.g. butylammonium), A is a small cation (e.g. methylammonium), X are the halide anions (Cl, Br, I or a mixture), and n indicates the number of PbX6 octahedra in between two layers of L cations. The RP perovskites have attracted increasing attention as candidates to replace the so-called 3D perovskites (APbX3), which suffer from instability in operando conditions. In fact, the bandgap of both materials can be tuned by alloying halides in the structure, but the RP perovskites do not require nanostructuring to become photoluminescent. Moreover, pure-iodine RP perovskites are stable, while APbI3 compounds quickly decompose. However, it is still unclear if mixed-halide RP perovskites will suffer from the photoinduced segregation of halides to separate crystalline domains, as the 3D perovskites do. To address this question, we investigated butylammonium-methylammonium mixed-halide RP perovskites with n=1 and n=2 (BA2PbX4 and BA2MAPb2X7), where X was a mixture of Br and I in variable proportions. Powder diffraction analysis revealed that the periodicity of the layers is heavily dependent on the halide mixture, and reaches a maximum for compositions around Br/I ≈ 50/50. By solving the structure via single crystal X-ray diffraction, we demonstrated that Br and I occupy two different crystallographic positions, that are the equatorial (Br) and the axial (I) position of the PbX6 octahedra. This is in contrast with the 3D perovskites, where Br and I are alloyed in the same positions, and justifies the large periodicity expansion we observed. This finding pointed to two special stoichiometries allowing the complete separation of Br and I in the structure, namely Br/I = 1/1 for n=1 and Br/I = 4/3 for n=2. When investigated by means of micro-photoluminescence, we found that the spectral position stays constant under illumination, suggesting that mixed-halide RP do not undergo halide segregation. The same behavior is observed for other compositions as well, suggesting that the RP perovskites are intrinsically stable to halide segregation. We believe this is because the Br and I anions in the structure are naturally divided in two different crystallographic position, which eliminates the need of a demixing that is the driving force for halide segregation observed in 3D perovskites.

Investigation of optical and structural properties and stability in two-dimensional Ruddlesden-Popper halide perovskites

ACS Spring 2022

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