Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. 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–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

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Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted