Reduced Melting Temperature of Lead-Free 2D Perovskite Semiconductor to 142 °C

Hybrid metal halide perovskite (MHP) semiconductors are currently revolutionizing the research realm of electronic and photonic materials, with an overwhelming number of studies conducted on their synthesis and device fabrication and with particular emphasis on deposition of device quality films. The dominant film deposition techniques involve solution processing and vapor deposition, which have their advantages, but may be harmful to humans and the environment. Melt processing as an alternative route has been demonstrated before; however, due to the high <i>T_m</i> (close to <i>T_d</i>~ 200 °C, the degradation onset temperature), the approach tends to induce partial decomposition (loss of organic component and halides) in MHPs, which hinders their utility in certain applications. Furthermore, slight loss of organic components/halides creates defects that may significantly harm the optoelectronic properties of the MHP film, pointing to a need to develop MHPs with lower <i>T_m</i>. In this work, we therefore synthesized a lead-free 1-methylhexylammonium tin iodide (1-MeHa₂SnI₄) perovskite. Single crystal X-ray diffraction is used to resolve the crystal structure of the resulting compound, confirming its crystallization in a two-dimensional Ruddlesden-Popper phase. Synthetic design rules toward obtaining a low <i>T_m</i> value are summarized and adopted to achieve an exceptionally low <i>T_m</i> of 142 °C. The hydrogen bonding, cation penetration, inter- and intra-octahedral distortions, and electronegativities (ionic sizes) of the synthesized perovskite and its corresponding lead counterpart (1-MeHa₂PbI₄) are studied to understand the reduced <i>T_m</i> along with the comparison of the existing pairs of meltable Sn and Pb MHPs with same organic cations. Additionally, we developed a protocol to assess the stability of the MHP melt by performing iterative calorimetric scans, monitoring changes in the <i>T_m</i> and melt crystallization temperature (<i>T_c</i>) through multiple heat-cool cycles. This analysis confirmed the stable nature of the 1-MeHa₂SnI₄ melt, leading to its facile melt deposition into films on flexible substrates with visible range optoelectronic properties. Our present work serves as a display of the strength of chemical compositional engineering of perovskite structures for tuning the thermal properties, resulting in exceptionally low <i>T_m</i> and facile melt-processing of MHPs. Reduction in <i>T_m</i> not only lowers the thermal budget of the deposition process but also renders robustness to the melt for the development of cost-effective flexible optoelectronic devices on plastic/polymer substrates.