Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs 2 AgBiBr 6 , in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs 2 AgBiBr 6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO 2 . Femtosecond UV−vis−NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron−phonon coupling via Froḧlich interactions is observed in terms of pronounced coherent oscillation of a strong A 1g optical phonon mode of the double perovskite at 177 cm −1 . Similar behavior is also found for the related vacancy-ordered perovskite Cs 3 Bi 2 Br 9 and the parent compound BiBr 3 . Excitonic effects and electron−phonon coupling are known to induce unwanted electron−hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.

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Double perovskites are promising candidates for less toxic and highly stable metal halide perovskites, but their optoelectronic performances still lag behind those of the lead halide counterpart, due to the indirect nature of the bandgap and the strong electron–phonon coupling. Reducing the dimensionality of Cs2AgBiBr6 down to a 2D layered form is strategic in order to tune the band gap from indirect to direct and provides new insights into the structure–property relationships of double perovskites. Herein, we report on a series of monolayer 2D hybrid double perovskites of formula (RA)4AgBiBr8, where RA represents different primary ammonium large cations with alkyl- and aryl-based functionalities. An in-depth experimental characterization of structure, film morphology, and optical properties of these perovskites is carried out. Interestingly, the variation of the ammonium cation and the interplanar distance between adjacent inorganic monolayers has peculiar effects on the film-forming ability and light emission properties of the perovskites. Experiments have been combined with DFT calculations in order to understand the possible origin of the different emissive features. Our study provides a toolbox for future rational developments of 2D double perovskites, with the aim of narrowing the gap with lead halide perovskite optoelectronic properties.

Emphasis was recently placed on the Cs2AgBiBr6 double perovskite as a possible candidate to substitute toxic lead in metal halide perovskites. However, its poor light-emissive features currently make it unsuitable for solid-state lighting. Lanthanides doping is an established strategy to implement luminescence in poorly emissive materials, with the additional advantage of fine-tuning the emission wavelength. We discuss here the impact of Eu-and Yb-doping on the optical properties of Cs2AgBiBr6 thin films, obtained from solution-processing of hydrothermally synthesized bulk crystalline powders, by combining experiments and density functional theory calculations. Eu(III) incorporation does not lead to the characteristic 5 D0→ 7 F2 emission feature at 2 eV, while only a weak trap-assisted sub band-gap radiative emission is reported. Oppositely, we demonstrate that incorporated Yb(III) leads to an intense and exclusive photoluminescence emission in the nearinfrared as a result of the efficient sensitization of the lanthanide 2 F5/2→ 2 F7/2 transition.

The preparation of absorber layers composed of methylammonium tin iodide (CH 3 NH 3 SnI 3 ) in a two-step process was investigated. This material is designed as a less toxic alternative to CH 3 NH 3 PbI 3 which is commonly used as active material in perovskite solar cells. Tin(II) iodide (SnI 2 ) layers prepared by physical vapor deposition were converted to CH 3 NH 3 SnI 3 by reaction with a spin-coated solution of methylammonium iodide (MAI). The perovskite particles formed in this process were over 200 nm in size and reached full surface coverage. A band gap of 1.23 eV was determined and the material, thus, absorbs over a broad part of the solar spectrum, broader even than CH 3 NH 3 PbI 3 . The chemical composition and solid state structure of the prepared films were analyzed by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. The films turned out to be remarkably stable, another key prerequisite for applications as absorber layers in perovskite solar cells.