Phase-pure two-dimensional layered perovskite thin films

Two-dimensional layered metal-halide perovskites (2D-LMHPs) are a promising family of organic–inorganic hybrid semiconductor materials because of their superior electronic and optical properties and high stability. To date, solution-processed 2D-LMHP thin films have a multiple quantum wells (QWs) structure, which has seriously impeded further progress in optoelectronics. Compared with 2D-LMHPs with multiple QWs, 2D-LMHPs with phase-pure QWs have a flattened energy landscape, resulting in less energy or charge-transfer losses and making them less susceptible to degradation. They would thus be attractive to promote the development of perovskite-based devices. In this Perspective article, we first elucidate the structure and optoelectronic properties of phase-pure 2D-LMHP films. Second, we systematically discuss their precursor engineering, focusing on stoichiometry, ligand design, chemical compositional engineering and formation energy aspects. Third, we comprehensively summarize the intermediate phase growth mechanism, in situ dynamic transformation observation and methodologies for the formation of quasi-2D perovskites with phase-pure structures. Finally, we deliberate the prospects and challenges of phase-pure 2D perovskites as a new family of semiconductors.