Loredana Protesescu † , ‡ , Sergii Yakunin † , ‡ , Sudhir Kumar § , Janine Bär † , Federica Bertolotti ⊥ , Norberto Masciocchi ⊥ , Antonietta Guagliardi ⊥ , ∥ , Matthias Grotevent † , # , Ivan Shorubalko # , Maryna I. Bodnarchuk † , ‡ , Chih-Jen Shih § , Maksym V. Kovalenko , † , ‡
23 February 2017
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.