Lead-Free Halide Double Perovskites—The Effects of Lanthanide Doping on Lattice Crystallinity and Related Photoluminescence Properties

Over the past decade, considerable efforts have been directed towards enhancing the efficiency of solar cells and optoelectronic materials as part of the green transition. Within the latter, Perovskite materials have made an impact due to their attractive characteristics and promising potential.<br/><br/>Despite their impressive performance in lighting applications, such as achieving photoluminescence cutting quantum yields exceeding 100% in some cases^[1], lead-halide materials still suffer from notable drawbacks including poor stability and lead toxicity^[1]. As an alternative, an analogous class of materials, identified as double perovskites, has been introduced, which are characterized by the substitution of the Pb²⁺ ion of the octahedral structure with both M⁺ and M³⁺metal ions in equal proportions (e.g. Cs₂AgInCl₆)^[2]. Indeed, double perovskites present remarkable thermal stabilities (up to 500°C) and attractive emission spectra spanning across the entire visible region^[3], ideal for a wide range of lighting applications.<br/>Introducing suitable additives through doping has shown promise in enhancing their performance, notably leading to a sharp increase in Photoluminescence Quantum Yield (PLQY). This enhancement is believed to stem from improved lattice crystallinity and the creation of new parity-allowed radiative recombination pathways^[4].<br/>Nevertheless, substantial further research is necessary to fully comprehend these materials.<br/>In this context, the current study aims to address these knowledge gaps by investigating various co-doped compositions of Cs₂Na_1-x[X_xY_yZ_1-y]Cl₆ halide double perovskites, by incorporating various elements such as Ag, Er, Yb, and Bi. The research employs Raman spectroscopy and photoluminescence measurements to evidence how changes in composition and crystallinity due to dopant addition correlate with variations in photoluminescence characteristics and QYs.<br/>In this regard, a comprehensive Raman analysis spanning from IR to UV, coupled with micro-luminescence, provides detailed insights into the structural attributes of these perovskites and their relationship to emission properties.<br/>This approach aims to advance our understanding of the underlying mechanisms governing the emission behavior of these innovative materials.<br/><br/>This study was developed in the framework of the research activities carried out within the Project “Network 4 Energy Sustainable Transition—NEST”, Spoke 1., Project code PE0000021, funded under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.3— Call for tender No. 1561 of 11.10.2022 of Ministero dell’Universita` e della Ricerca (MUR); funded by the European Union—NextGenerationEU.<br/><br/><br/>[1] Arfin, H., Angew. <i>Chem. Int. Ed.</i>, <b>2020</b>, 59, 11307-11311.<br/>[2] Zhao, J., <i>Journal of Alloys and Compounds</i>, <b>2022</b>, 895, 162601.<br/>[3] Marongiu, D., <i>APL Energy 1</i>, <b>2023</b>, 1, 021501.<br/>[4] Liu, F<i>., Mater. Chem. C</i>, <b>2022</b>, 10, 14232.