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 CsPbI₃ NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI₃ exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI₃ and FA-doped CsPbI₃ 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 FAPbI₃ NCs had a cubic crystal structure, while the FA_0.1Cs_0.9PbI₃ NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr₃ NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA_0.1Cs_0.9PbI₃ NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI₃ 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_0.9PbI₃ and FAPbI₃ NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI₃ NCs.

Keywords: cesium; formamidinium; infrared; lead halides; nanocrystals; perovskites; photoluminescence.

Figure 1

Survey of the reported formabilities of the 3D and 1D polymorphs of nearly all known inorganic and hybrid ABX₃ compounds, where A is an alkali metal, organic cation (MA⁺ or FA⁺), or other single-charged metal ion (Ag⁺, Tl⁺, or Cu⁺); B = Pb, Sn, Mg, Ca, Sr Ba, Ti, V, Cd, Hg, Mn, Cu, Co, Zn, Tm, Dy, or Yb; and X = F, Cl, Br, of I. The tolerance and octahedral factors were mainly taken from the recent report of Travis etal. (a) Ideal 3D cubic interconnection of PbX₆ octahedra, as observed in α-FAPbI₃; (b) orthorhombically distorted 3D polymorph, which is commonly reported for CsPbBr₃ and was observed in FA-doped CsPbI₃ NCs in this study; (c) 1D hexagonal lattice found in the yellow FAPbI₃; and (d) 1D orthorhombic lattice found in the yellow CsPbI₃.

Figure 2

(a) Synchrotron XRD pattern (black) and best fit (purple, 2θ range of 3–30°; λ = 0.565 483 Å) for FAPbI₃ NCs using the cubic lattice, yielding a refined cell parameter of a = 6.3641 Å. The inset illustrates the cubic perovskite structure of FAPbI₃ and the off-axis disorder of the I^– anions. (b, c) High-resolution TEM images of FAPbI₃ NCs; (d) typical TEM image of FAPbI₃ NCs; (f) aspect ratio histogram for FAPbI₃ NCs.

Figure 3

(a) PL and absorbance spectra for FAPbI₃ nanosheets. (b and c) Corresponding TEM images showing 0.1–0.6 μm nanosheets.

Figure 4

(a) Synchrotron XRD pattern (black) and best fit (red, 2θ range of 3–30°; λ = 0.565 483 Å) for FA_0.1Cs_0.9PbI₃ NCs using the γ-orthorhombic phase of CsPbI₃. The inset illustrates the γ-orthorhombic phase of CsPbI₃. (b, c) HRTEM and (d) TEM images for FA_0.1Cs_0.9PbI₃ NCs, along with (e) a histogram of the aspect ratio.

Figure 5

(a) Optical absorption and PL spectra of FAPbI₃ NCs and FA_0.1Cs_0.9PbI₃ NCs before and after 6 months of storage. The insets contain photographs of the FAPbI₃ NCs and FA_0.1Cs_0.9PbI₃ NCs colloidal solutions in toluene under daylight (upper image) and under a UV lamp (λ = 365 nm; lower image). (b) PL decay traces for colloidal FAPbI₃ and FA_0.1Cs_0.9PbI₃ NCs.

Figure 6

(a) PL spectra before and after cation exchange within FAPbI₃ NCs (or CsPbI₃ NCs) using Cs-oleate (or FA-oleate). (b) PL spectra before and after anion exchange of FAPbI₃ NCs using OAm⁺Br^– (or OAm⁺I^–) showing the possibility of tuning the band gap from 570 to 780 nm.

Figure 7

(a) Schematic energy diagram of LED devices; the values for the energy levels for FAPbI₃ correspond to those reported in the literature for thin films. (b) EL spectra for FaPbI₃ NCs and FA_0.1Cs_0.9PbI₃ NCs. Inset: Photograph of LED using FA_0.1Cs_0.9PbI₃ NCs as the active layer. The use of the ETH logo as a pattern in the LED active layer is done with permission from ETH Zürich. (c) Current density versus voltage (J–V) and radiance versus voltage characteristics shown for FAPbI₃ NC-based devices, and the highest external quantum efficiency versus current density characteristics shown for the FAPbI₃ NC-based devices.

Figure 8

Amplified spontaneous emissions for films prepared from (a) FAPbI₃ NCs using dip-coating with heat treatment at 90 °C and (b) FA_0.1Cs_0.9PbI₃ NCs using simple drop-casting and heat treatment at 50 °C.