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      Dismantling the “Red Wall” of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium–Cesium Lead Iodide Nanocrystals

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          Colloidal nanocrystals (NCs) of APbX 3-type lead halide perovskites [A = Cs +, CH 3NH 3 + (methylammonium or MA +) or CH(NH 2) 2 + (formamidinium or FA +); X = Cl , Br , I ] have recently emerged as highly versatile photonic sources for applications ranging from simple photoluminescence down-conversion ( e.g., for display backlighting) to light-emitting diodes. From the perspective of spectral coverage, a formidable challenge facing the use of these materials is how to obtain stable emissions in the red and infrared spectral regions covered by the iodide-based compositions. So far, red-emissive CsPbI 3 NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI 3 exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI 3 and FA-doped CsPbI 3 NCs that are uniform in size (10–15 nm) and nearly cubic in shape and exhibit drastically higher robustness than their MA- or Cs-only cousins with similar sizes and morphologies. Detailed structural analysis indicated that the FAPbI 3 NCs had a cubic crystal structure, while the FA 0.1Cs 0.9PbI 3 NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr 3 NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA 0.1Cs 0.9PbI 3 NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI 3 NCs) regions was sustained for several months or more in both the colloidal state and in films. The peak PL wavelengths can be fine-tuned by using postsynthetic cation- and anion-exchange reactions. Amplified spontaneous emissions with low thresholds of 28 and 7.5 μJ cm –2 were obtained from the films deposited from FA 0.1Cs 0.9PbI 3 and FAPbI 3 NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI 3 NCs.

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          Most cited references 127

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          Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals

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            Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics.

            We show nanoscale phase stabilization of CsPbI3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI3 (α-CsPbI3)-the variant with desirable band gap-is only stable at high temperatures. We describe the formation of α-CsPbI3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.
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              Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

              Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410–700 nm) while maintaining high quantum yields of 20–80% and narrow emission line widths of 10–40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

                Author and article information

                ACS Nano
                ACS Nano
                ACS Nano
                American Chemical Society
                23 February 2017
                28 March 2017
                23 February 2018
                : 11
                : 3
                : 3119-3134
                Institute of Inorganic Chemistry and §Institute of Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich , Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland
                Laboratory for Thin Films and Photovoltaics and #Laboratory for Reliability Science and Technology, Empa−Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
                []Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell’Insubria , Via Valleggio 11, I-22100 Como, Italy
                []Istituto di Crystallografia and To.Sca.Lab, Consiglio Nazionale delle Ricerche , Valleggio 11, I-22100 Como, Italy
                Author notes
                Copyright © 2017 American Chemical Society

                This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

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                perovskites, lead halides, nanocrystals, photoluminescence, infrared, formamidinium, cesium


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