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

Abstract

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.