Layered methylhydrazinium lead halide perovskites: new crystal polymorphs with a tailored band gap and photoluminescence colour via halide substitution

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2024-05, Vol.12 (18), p.6653-6662
• Main Authors: Drozdowski, Dawid, Kabański, Adam, Stefańska, Dagmara, Ptak, Maciej, Mączka, Mirosław, Gągor, Anna
• Format: Article
• Language: English
• Subjects: Bromine; Chemical engineering; Color; Crystal structure; Energy bands; Energy gap; Freezing; High temperature; Iodine; Lead compounds; Low temperature; Metal halides; Optoelectronics; Perovskites; Photoluminescence; Polymorphism; Raman spectroscopy; Room temperature; Structural analysis; Temperature
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/D4TC00865K

An unrelenting focus on improving stability and optoelectronic performance of lead halide three-dimensional hybrid organic–inorganic perovskites (3D HOIPs, ABX 3 formula) leads to the usage of many chemical engineering techniques, including X-site alloying. Formerly we have shown the usefulness of this approach in 3D HOIPs comprising methylhydrazinium (MHy + ); this time we prove it again in the layered (A 2 BX 4 ) perovskites. Herein we report the synthesis, crystal structure and physicochemical characterization of MHy 2 PbBr 4− x I x in a wide concentration range ( x = 0.25, 0.3, 0.4, 1.1, 2.35, 2.5, 3.1). Reported compounds crystallise in the Ruddlesden–Popper phase with corner-sharing [PbX 6 ] 4− (X = Br, I) octahedra, forming (010) layers, separated by MHy + . An abundant temperature-driven polymorphism is observed, starting from the Br-rich compounds with a sequence of Pnmm , Pnma and P 2 1 / c phases on cooling, through the region with the P 1̄ low-temperature phase, and ending with the I-rich compounds with an incommensurately modulated Pnma (00 γ )0 s 0 superspace group. We also report, for the first time in the MHy-based perovskites, the 2D → 3D transition to Pm 3̄ m cubic symmetry and a 3D perovskite phase with a dominant presence of iodine. With the use of Raman spectroscopy, the dynamics of MHy + cations in particular phases is thoroughly analysed, revealing the positional disorder in room-temperature and high-temperature zones and its freezing at low temperatures. Linear optical studies demonstrate the decrease of the energy band gap (2.91 eV to 2.31 eV) and a strong change in photoluminescence colour (bluish-green to yellow-green) along with increasing iodine contribution.