(11f) Microfluidic Studies of Room-Temperature Synthesized Perovskite Nanocrystals

Emergence of organic/inorganic metal-halide perovskites as a new class of photovoltaic material with excellent electron conducting and light harvesting properties, have enabled breakthrough applications at device scales in light emitting diodes (LEDs) and solar cells.^{1, 2} Over the past 7 years, efficiency of perovskite-based solar cells has increased significantly from 3.8% ³ to 21.1%,⁴ illustrating their potential as a cost and energy-effective substitute to conventional silicon-based solar cells. Recent advancements in solution-phase processing of perovskite nanocrystals (i.e., quantum dots)^{5, 6} have stimulated research efforts towards continuous nano-manufacturing of high-quality perovskite nanocrystals. Precise band-gap engineering of perovskite nanocrystals, similar to well-studied II-VI and IV-VI semiconductor nanocrystals⁷ would facilitate their utilization in photovoltaics and LEDs. However, the lack of fundamental understanding of different mechanisms involved during the colloidal synthesis of perovskite nanoparticles has limited their large-scale nanomanufacturing. Further integration of perovskite nanocrystals in solar cells and LED sectors would strongly benefit from a better understanding of different mechanisms involved during the early stage formation and growth kinetics of these nanocrystals.

In this work, we designed and developed a modular microfluidic platform for fundamental studies of the synthesis of colloidal cesium lead halide perovskite nanocrystals using in-situ absorption/fluorescence spectroscopy. The developed microfluidic platform comprised of a tubular microreactor (fluorinated ethylene propylene with an inner diameter of 750 μm), 3 computer-controlled syringe pumps, and a custom-designed 3-port flowcell, enabling real-time spectral characterization of in-flow synthesized perovskite nanocrystals. The separation of the spectral characterization tool (i.e., the portable flowcell) and the temperature-controlled microreactor enabled a systematic study of the effect of mixing timescale on the nucleation and growth of perovskite nanocrystals. The developed microfluidic strategy enabled studies of 4 orders of magnitude change in the reaction timescales (15 ms to 5 min) on the quantum yield and size distribution of the synthesized perovskite nanocrystals. Furthermore, the automated microfluidic setup was utilized for the high-throughput screening of the experimental parameter space associated with the synthesis of colloidal perovskite nanocrystals (2000 spectral data were obtained within 20 min). Furthermore, utilizing the developed microreactor, the effects of the solvent and surface ligands on the intermediate growth pathways, final size, quantum yield, and size distribution of the resulting nanocrystals were studied. The obtained characteristic kinetic information of different mechanisms involved in the nucleation and growth stages of perovskite nanocrystals will be utilized to develop the design principles and demonstrate the technology for continuous large-scale manufacturing of engineered perovskite nanocrystals for applications in photovoltaics and LEDs.

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