Synthetic approaches for thin-film halide double perovskites

Inspired by the fascinating class of hybrid organic-inorganic halide perovskite materials, halide double perovskites have emerged as non-toxic Pb-free contenders for application as active layers in optoelectronic devices. Heterovalent substitution of Pb²⁺ by non-toxic metal cations yields the double-perovskite structure, which allows for compositional flexibility. In fact, the compositional space is large, given that multiple cations and halides can be combined, resulting in >10⁶ perovskite combinations. A starkly increasing number of stable halide double-perovskite compositions are theoretically predicted. The synthesis, however, lags behind, and many double perovskites are primarily synthesized as powder samples instead of thin films. The latter, however, are needed for thin-film devices such as solar cells, light-emitting devices, and thin-film transistors. Comparing the synthetic approaches successfully applied to hybrid perovskites to methods used for the fabrication of double perovskites, the latter are clearly in their very infancy. The question is whether solution engineering and compositional modification strategies can be exploited to match the exceptional optoelectronic properties of hybrid perovskites. This review is motivated by a text mining effort that illustrates not only the prevalence of powder over thin-film synthesis but also the discrepancy between the number of compositions experimentally realized and studied and the many predicted compositions. Here we summarize the synthesis aspects of halide double perovskites, and in particular of thin films, including deposition techniques and synthetic modifications to alter film properties.