'Green' synthesis of highly luminescent lead-free Cs 2 Ag x Na 1− x Bi y In 1− y Cl 6 perovskites

A new “green” and mild synthesis of highly stable microcrystalline Cs2AgxNa1−xBiyIn1−yCl6 (CANBIC) perovskites under ambient conditions was developed that is scalable to the multi-gram production. Under UV illumination, the CANBIC perovskites emit intense broadband photoluminescence (PL) with a quantum yield (QY) of 92% observed for x = 0.35 and y = 0.01–0.02. The combination of strong UV absorbance and broadband visible emission, high PL QY, and long PL lifetimes of up to 1.4 μs, along with an outstanding stability makes these CANBICs a promising material class for many optical applications.Graphical abstract: “Green” synthesis of highly luminescent lead-free Cs2AgxNa1−xBiyIn1−yCl6 perovskitesLead halide perovskites have gone through an unprecedented rapid development from one of many promising photovoltaic materials to a real competitor for established photovoltaic absorbers such as silicon, copper–indium–gallium chalcogenides, and CdTe.1–4 This triggered the search for novel lead-based perovskite compounds and particularly for “greener” lead-free structural analogs with comparable optical and photovoltaic characteristics. Due to the many possible compositions of lead-free perovskites (LFPs), especially of the so-called double LFPs with a couple of Pb2+ being substituted by M+M3+ combinations,1,3 there are hundreds of structures which could be identified as promising candidates for photovoltaics, light emission, and light management (down-shifting, up-conversion, etc.).1,3–6 Given the broad variability of alkali metal and halide positions in LFPs, these structures translate into thousands of compounds to be synthesized and tested for light-conversion applications which presents a huge experimental challenge.LFP tunability can be exemplified by In-based double compounds, A2MIInX6 (A – alkali metals, X – halides), where all four positions can be independently varied. In particular, MI can be occupied by Na+, K+, Ag+ or their mixtures, A – by Cs+, Rb+, or organic cations, and X – by individual and mixed halides. At the same time, InIII can be substituted by a number of MIII cations, including BiIII and SbIII.1,4 The interest in such compounds, particularly in Bi-doped Cs2AgxNa1−xInCl6 (referred to as CANIC) or Cs2AgxNa1−xBiyIn1−yCl6 (referred to as CANBIC), is constantly growing due to their intense broadband photoluminescence (PL) combined with a unique tunability of the photophysical properties and an outstanding chemical stability.5–21In view of this compositional variability, the progress in the material science of LFP requires a transition from conventional single-compound trials to high-throughput studies based on robot-assisted synthesis and characterization, ideally supported by machine-learning-based data analysis.22–24 The feasibility and efficiency of high-throughput experiments in the field of lead perovskite photovoltaics were recently demonstrated.25–30However, the majority of the reported syntheses of double LFPs require high energy input by hydrothermal treatment or calcination. Furthermore, a considerable amount of manual intermediate steps is needed which cannot be automated, as well as the usage of hazardous chemicals such as concentrated HCl in the case of chloride LFPs1,3,6,7,9,14,17,21 or unstable and aggressive substances for bromide and iodide LFPs.1,3,21 More favorable milder temperature-, saturation-, or antisolvent-driven precipitation approaches, conventionally employed for the fabrication of lead-based perovskites, often yield poorly controlled mixtures of perovskite and non-perovskite phases when applied to double LFPs.1,13,31 As a result, the development of simple and mild procedures for the reproducible and phase-controlled synthesis of double LFPs under ambient conditions with robotized equipment still remains challenging.In this communication, we introduce such an approach to the fabrication of microcrystalline lead-free CANIC and CANBIC perovskites, yielding compositionally controlled single-phase products under ambient conditions in a single step without any additional thermally activated steps or the need for hazardous chemicals. By focusing on the optical properties of the most strongly emissive CANBIC perovskite composition, we show that our approach results in a very stable and structurally perfect material emitting broadband PL in the visible spectral range with a PL quantum yield (QY) close to unity.