Cation Influence on Hot-Carrier Relaxation in Tin Triiodide Perovskite Thin Films.

Slow hot-carrier cooling may potentially allow overcoming the maximum achievable power conversion efficiency of single-junction solar cells. For formamidinium tin triiodide, an exceptional slow cooling time of a few nanoseconds was reported. However, a systematic study of the cation influence, as is present for lead compounds, is lacking. Here, we report the first comparative study on formamidinium, methylammonium, and cesium tin triiodide thin films. By investigating their photoluminescence, we observe a considerable shift of the emission peak to high energy with the increase of the excited-state population, which is more prominent in the case of the two hybrid organic-inorganic perovskites (∼45 meV vs ∼15 meV at 9 × 1017 cm-3 carrier density). The hot-carrier photoluminescence of the three tin compositions decays with a 0.6-2.8 ns time constant with slower cooling observed for the two hybrids, further indicating their importance.

The Origin of Broad Emission in ⟨100⟩ Two-Dimensional Perovskites: Extrinsic vs Intrinsic Processes.

2D metal halide perovskites can show narrow and broad emission bands (BEs), and the latter's origin is hotly debated. A widespread opinion assigns BEs to the recombination of intrinsic self-trapped excitons (STEs), whereas recent studies indicate they can have an extrinsic defect-related origin. Here, we carry out a combined experimental-computational study into the microscopic origin of BEs for a series of prototypical phenylethylammonium-based 2D perovskites, comprising different metals (Pb, Sn) and halides (I, Br, Cl). Photoluminescence spectroscopy reveals that all of the compounds exhibit BEs. Where not observable at room temperature, the BE signature emerges upon cooling. By means of DFT calculations, we demonstrate that emission from halide vacancies is compatible with the experimentally observed features. Emission from STEs may only contribute to the BE in the wide-band-gap Br- and Cl-based compounds. Our work paves the way toward a complete understanding of broad emission bands in halide perovskites that will facilitate the fabrication of efficient narrow and white light emitting devices.

Tin Halide Perovskites: From Fundamental Properties to Solar Cells.

Metal halide perovskites have unique optical and electrical properties, which make them an excellent class of materials for a broad spectrum of optoelectronic applications. However, it is with photovoltaic devices that this class of materials has reached the apotheosis of popularity. High power conversion efficiencies are achieved with lead-based compounds, which are toxic to the environment. Tin-based perovskites are the most promising alternative because of their bandgap close to the optimal value for photovoltaic applications, the strong optical absorption, and good charge carrier mobilities. Nevertheless, the low defect tolerance, the fast crystallization, and the oxidative instability of tin halide perovskites currently limit their efficiency. The aim of this review is to give a detailed overview of the crystallographic, photophysical, and optoelectronic properties of tin-based perovskite compounds in their multiple forms from 3D to low-dimensional structures. At the end, recent progress in tin-based perovskite solar cells are reviewed, mainly focusing on the detail of the strategies adopted to improve the device performances. For each subtopic, the current challenges and the outlook are discussed, with the aim to stimulate the community to address the most important issues in a concerted manner.