We review the important differences that exist in the chemistry and physics of colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) as compared to conventional semiconductor NCs made of metal pnictides and chalcogenides. We survey the the synthesis methods, optical properties and prospects of these NCs for optoelectronic applications [1, 2, 3].

  The absorption spectral, sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 400-800 nm by composition as well as by the NC size and shape [4-5]. Post-synthestic chemical transformations of colloidal NCs, such as ion-exchange reactions, provide an avenue to compositional fine tuning or to otherwise inaccessible materials and morphologies [6]. The photoluminescence of these NCs is characterized by narrow emission line-widths of <100 meV (12-45 nm from blue-to-near-infrared), wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 100%. Cs- and FA-based perovskite NCs are highly promising for luminescence downconversion (bright and narrow emission at 530 and 640 nm; backlighting for displays), for light-emitting diodes and as precursors/inks for perovskite solar cells. In particular, high purity colloids are ideal for further engineering as needed for photochemical/photocatalytic applications. Towards these applications, a unique feature is that perovskite NCs appear to be trap-free without any electronic surface passivaiton, making photogenerated electrons and holes readily availably for surface chemical reactions.

  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 [7].

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