Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability

Abstract

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and optoelectronic applications are 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, herein, we propose a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules such as 3-(N,N-dimethyloctadecylammonio)propanesulfonate, resulting in much improved chemical durability. In particular, this class of ligands allows for the isolation of clean NCs with high photoluminescence quantum yields (PL QYs) of above 90% after four rounds of precipitation/redispersion along with much higher overall reaction yields of uniform and colloidal dispersible NCs. Densely packed films of these NCs exhibit high PL QY values and effective charge transport. Consequently, they exhibit photoconductivity and low thresholds for amplified spontaneous emission of 2 μJ cm^-2 under femtosecond optical excitation and are suited for efficient light-emitting diodes.

Scheme 1. (a) Depiction of Conventional Ligand Capping of Perovskite NCs Using Long-Chain Molecules with Single Head Groups, In the Ionized Form (OA^– or Br^–, OLAH⁺) and (b) a Novel Strategy wherein Cationic and Anionic Groups Are Combined in a Single Zwitterionic Molecule

The net effect of two possible sets of equilibria is facile ligand desorption during purification. Examples of long-chain sulfobetaines, phosphocholines, and γ-amino acids tested in this work are depicted left to right (n = 1): 3-(N,N-dimethyloctadecylammonio)propanesulfonate, N-hexadecylphosphocholine, and N,N-dimethyldodecylammoniumbutyrate.

Figure 1

Synthesis of zwitterionic-capped CsPbX₃ NCs, exemplified for CsPbBr₃: (a) reaction equation, (b) typical TEM images of CsPbBr₃ NCs, (c) absorbance and emission spectra, (d) QY of NCs covered with the 3-(N,N-dimethyloctadecylammonio)propanesulfonate and OA/OLA after two steps of purification on day 1 and after storage for 28 days.

Figure 2

Top and side views of a binding site in a model CsPbBr₃ NC (∼3 nm) computed at the DFT/PBE level of theory, using the CP2K software package. All structures have been fully relaxed. Cs atoms are drawn in gray, Pb in orange, Br in magenta, N in blue, C in light blue, O in red, S in yellow, and H in white. The binding site is circled in white for different ligands: (from left to right) conventional ligands OLAH⁺Br and OLAH⁺OA^– and the zwitterionic C₃-sulfobetaine. For computational advantage, the OLAH⁺ is replaced by methylammonium, the OA^– by acetate, and the side chain in the zwitterion by a butyl group. At the bottom, the electronic structure of each NC is shown by depicting the molecular orbitals (MOs) close to the valence and conduction bands. The contribution of each atom type to a given MO is represented with a different color (Cs in gray, Pb in orange, and Br in magenta). In this plot, the contribution from the ligands is negligible compared to the full NC due the large number of MOs of the latter. In Figure S17 we, however, illustrate the relative energy alignment of the NC versus the frontier orbitals of the ligands.

Figure 3

(a) Amplified spontaneous emission (ASE) spectra showing evolution of the ASE band and (b) the threshold behavior for the intensity of the ASE band. (c) Photoconductivity spectrum inset: photo of a colloidal solution and drop-casted film of standard OA/OLA NCs (left) and C₃-sulfobetaine-covered NCs (right). (d) Bias dependence of photoresponse, with the inset showing the scheme of a photodetector made from the substrate with an interdigitated electrode and a drop-casted film of NCs. (e) Corresponding work functions and HOMO–LUMO gaps and (f) current density and luminance vs applied voltage of a LED. Inset: electroluminescence spectrum measured at 3.5 V.