title: Dismantling the "Red Wall" of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium-Cesium Lead Iodide Nanocrystals.
abstract: Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs+, CH3NH3+(methylammonium or MA+) or CH(NH2)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 CsPbI3NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI3exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI3and FA-doped CsPbI3NCs 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 FAPbI3NCs had a cubic crystal structure, while the FA0.1Cs0.9PbI3NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr3NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA0.1Cs0.9PbI3NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI3NCs) 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-2were obtained from the films deposited from FA0.1Cs0.9PbI3and FAPbI3NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI3NCs.
authors.forename: Loredana
authors.initials: L
authors.lastname: Protesescu
authors.forename: Sergii
authors.identifier: 0000-0002-6409-0565
authors.initials: S
authors.lastname: Yakunin
authors.forename: Sudhir
authors.initials: S
authors.lastname: Kumar
authors.forename: Janine
authors.initials: J
authors.lastname: Bär
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell'Insubria , Via Valleggio 11, I-22100 Como, Italy.
authors.forename: Federica
authors.identifier: 0000-0002-6001-9040
authors.initials: F
authors.lastname: Bertolotti
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell'Insubria , Via Valleggio 11, I-22100 Como, Italy.
authors.forename: Norberto
authors.identifier: 0000-0001-9921-2350
authors.initials: N
authors.lastname: Masciocchi
authors.affiliation: 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.
authors.forename: Antonietta
authors.identifier: 0000-0001-6390-2114
authors.initials: A
authors.lastname: Guagliardi
authors.forename: Matthias
authors.initials: M
authors.lastname: Grotevent
authors.forename: Ivan
authors.initials: I
authors.lastname: Shorubalko
authors.forename: Maryna I
authors.initials: MI
authors.lastname: Bodnarchuk
authors.forename: Chih-Jen
authors.identifier: 0000-0002-5258-3485
authors.initials: CJ
authors.lastname: Shih
authors.forename: Maksym V
authors.identifier: 0000-0002-6396-8938
authors.initials: MV
authors.lastname: Kovalenko
keywords: cesium; formamidinium; infrared; lead halides; nanocrystals; perovskites; photoluminescence