Exploration of Near-Infrared-Emissive Colloidal Multinary Lead Halide Perovskite Nanocrystals Using an Automated Microfluidic Platform

Abstract

Hybrid organic-inorganic and fully inorganic lead halide perovskite nanocrystals (NCs) have recently emerged as versatile solution-processable light-emitting and light-harvesting optoelectronic materials. A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. In this context, all three archetypal A-site monocationic perovskites-CH₃NH₃PbI₃, CH(NH₂)₂PbI₃, and CsPbI₃-suffer from either chemical or thermodynamic instabilities in their bulk form. A promising approach toward the mitigation of these challenges lies in the formation of multinary compositions (mixed cation and mixed anion). In the case of multinary colloidal NCs, such as quinary Cs _xFA_{1- x}Pb(Br_{1- y}I _y)₃ NCs, the outcome of the synthesis is defined by a complex interplay between the bulk thermodynamics of the solid solutions, crystal surface energies, energetics, dynamics of capping ligands, and the multiple effects of the reagents in solution. Accordingly, the rational synthesis of such NCs is a formidable challenge. Herein, we show that droplet-based microfluidics can successfully tackle this problem and synthesize Cs _xFA_{1- x}PbI₃ and Cs _xFA_{1- x}Pb(Br_{1- y}I _y)₃ NCs in both a time- and cost-efficient manner. Rapid in situ photoluminescence and absorption measurements allow for thorough parametric screening, thereby permitting precise optical engineering of these NCs. In this showcase study, we fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission line widths (to below 40 nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Detailed structural analysis revealed that the Cs _xFA_{1- x}Pb(Br_{1- y}I _y)₃ NCs adopt a cubic perovskite structure of FAPbI₃, with iodide anions partially substituted by bromide ions. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs _xFA_{1- x}Pb(Br_{1- y}I _y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting a high external quantum efficiency of 5.9% and a very narrow electroluminescence spectral bandwidth of 27 nm.

Keywords: formamidinium; halides; microfluidics; nanocrystals; perovskites; quantum dots.

Figure 1

Formabilities of the 3D and 1D polymorphs of CsPbI₃ and FAPbI₃ compounds and the goal of this study: near-infrared emissive LHP NCs. The PbI₆ octahedra of α-FAPbI₃ NCs are assembled in a 3D cubic metastable lattice, which spontaneously converts into a 1D hexagonal version (nonluminescent) at room temperature. In the case of CsPbI₃, the PbI₆ octahedra of FAPbX₃ NCs are assembled in a 3D orthorhombic metastable lattice (γ-phase), which eventually converts at room temperature into a 1D orthorhombic δ-phase (nonluminescent). The goal of this study is highlighted with a question: can high-throughput microfluidic screening identify the existence of stable multinary Cs_xFA_1–xPb(Br_1–yI_y)₃ phases in the form of colloidal NCs, which cover the PL region of 700–800 nm, i.e., in-between ternary 3D phases (CsPbI₃ and FAPbI₃)? We note that bulk α-FAPbI₃ emits at 840 nm and γ-CsPbI₃ emits at 710 nm, whereas their NC counterparts are commonly reported to emit at ≤700 and ≤780 nm, respectively.^,−, It is also noted that the space groups reported for the γ- and δ-phases of CsPbI₃ do not differ (while their structures manifestly do), as they can easily be interconverted by simple axis permutations. We used the original Pbnm and Pnma for the γ- and δ-forms, respectively, to maintain consistency with past literature.

Figure 2

(Left) Illustration of the segmented-flow reaction platform equipped with online PL and absorbance modules for the synthesis and real-time monitoring of Cs_xFA_1–xPbX₃ perovskite NCs. The microfluidic platform allows for a systematic and independent variation of precursor molar ratios, such as Cs/Pb, FA/Pb, Cs/FA, and Br/I, growth times (determined by the flow rate and tube lengths), and temperature. Droplets are generated by adjusting the flow rates of the carrier phase (50–200 μL/min) and that of the dispersed phase (1.2–50 μL/min). (Right) Illustration of a typical flask-based hot-injection synthesis of Cs_xFA_1–xPbX₃ NCs. Overall, synthesis optimization was performed by mutual information exchange between flask-based experimentation (identification of suitable precursors, solvents, and capping ligands) and microfluidics (optimization of the reaction parameters). The optimized reaction parameters were successfully transferred from microfluidics back into flask reactors, followed by up-scaling and additional postsynthetic characterization (XRD, electron microscopy, and stability tests).

Figure 3

Microfluidic synthesis of Cs_xFA_1–xPbI₃ NCs. Variation in the (a) PL spectra, (b) fwhm, and (c) PL maximum as a function of temperature for Cs_0.03FA_0.97PbI₃ NCs (with the variation in the Cs/FA molar ratio indicated). Other parameters were as follows: FA/Pb = 9.3, Cs/Pb = 0.3, and reaction time = 10 s. (d–f) Temporal evolution of the normalized online PL spectra, PL maxima, and fwhm of Cs_0.02FA_0.98PbI₃ NCs at 80 °C.

Figure 4

(a) PL spectra of colloidal Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs synthesized using the microfluidic platform and exhibiting composition-tunable band-gap energies between 690 and 780 nm with fwhm values of 40–65 nm and (b) representative online PL and in online absorption spectra at different quantities of Cs⁺ and Br^– in the reaction mixture.

Figure 5

Optical absorption and PL spectra of Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs synthesized in conventional flask reactors, exhibiting a fwhm of 40 nm. (b) Bright-field scanning TEM (STEM) image of Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. (c) Synchrotron XRD pattern (black) and best fit (red, 2θ range of 0.5–130°; λ = 0.563 729 Å) for Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs, yielding a refined lattice parameter (a = 6.3296 Å) and the anionic composition. The inset illustrates the cubic perovskite structure of Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs (space group Pm3̅m, with y = 0.87 and x = 0), in which the perovskite framework consists of PbX₆ units sharing the octahedral corners; the X^– anions are disordered in four equivalent positions.

Figure 6

(a) Energy diagram of LED devices with Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs as emissive layers. (b) Current density and radiance versus voltage characteristics of device 1. (c) External quantum efficiency versus current density characteristics shown for devices 1 and 2. (d) Narrowest EL spectra of device 1 and device 2.