Ligand Engineering for Precise Control of Ultrathin CsPbI3 Nanoplatelet Superlattices for Efficient Light-Emitting Diodes

Technical Summary: Ligand Engineering for Precise Control of Ultrathin CsPbI3 Nanoplatelet Superlattices

Problem Statement
Colloidal lead halide perovskite nanoplatelets (PeNPLs) offer unique optoelectronic advantages over isotropic nanocubes, including the ability to tune emission via thickness and the potential for linearly polarized light emission due to exciton fine structure splitting. These properties are most pronounced in the ultrathin regime (three or fewer monolayers of PbI6 octahedra). However, synthesizing uniform ultrathin PeNPLs remains a significant challenge. The high surface-area-to-volume ratio of these nanostructures makes them highly sensitive to ligand dynamics and surface defects. Conventional synthesis using native ligands (oleate and oleylammonium) often results in poor colloidal stability, dynamic ligand desorption, and a broad distribution of thicknesses. This non-uniformity leads to mixed emission wavelengths, reduced color purity, and diminished polarization degrees. Furthermore, the insulating nature of native ligands hinders charge transport, limiting the performance of PeNPLs in light-emitting diodes (LEDs).

Methodology
The authors employed an ancillary ligand engineering strategy to regulate the nucleation and growth of CsPbI3 PeNPLs. The study involved:

1. Ligand Selection and Screening: Four candidate ancillary ligands were tested: benzoic acid (BAc), benzene sulfonic acid (BSAc), benzyl phosphonic acid (BPAc), and diphenyl phosphate (DPPAc). These were selected based on their functional groups (carboxylic, sulfonic, and phosphonic acids) and organic backbones.
2. Computational Modeling: Density functional theory (DFT) calculations were performed to determine surface adsorption energies, bond lengths, and Bader charge analysis to understand ligand-perovskite interactions at the atomic scale.
3. Synthesis and Characterization: CsPbI3 PeNPLs were synthesized using the ligand-assisted reprecipitation (LARP) method with the selected ancillary ligands added to the PbI2 precursor. Characterization included in-situ photoluminescence (PL) monitoring, liquid-phase 207Pb and 1H nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and transient absorption (TA) spectroscopy.
4. Film Assembly and Device Fabrication: PeNPLs were processed into thin films via spin-coating to form superlattices. The orientation of the nanoplatelets (edge-up vs. face-down) was controlled by the solvent evaporation rate. LEDs were fabricated with a specific device architecture (ITO/PEDOT:PSS:PFI/Poly-TPD/PeNPL/TPBi/ZADN/LiF/Al) to evaluate electroluminescence performance.

Key Contributions and Results

• Mechanistic Insight into Ligand Binding: DFT and NMR analyses revealed that ligands with phosphoryl functional groups (BPAc and DPPAc) exhibit the strongest binding to the perovskite surface, characterized by high adsorption energies and significant Pb-O hybridization. This strong coordination retards nucleation and regulates crystal growth kinetics.
• Achievement of Monodisperse Ultrathin PeNPLs: Among the candidates, BPAc proved most effective. While DPPAc also showed strong binding, its bulky organic backbone sterically hindered the attachment of native ligands, leading to reduced colloidal stability and polydispersity. In contrast, BPAc, with its less bulky backbone, allowed for high ligand density and effective passivation. This resulted in the synthesis of highly monodisperse 3-monolayer (3ML) CsPbI3 PeNPLs with a narrow thickness distribution (2.57 ± 0.06 nm) and a single sharp emission peak at 600 nm (FWHM ~21 nm).
• Enhanced Optical Properties and Film Uniformity: The BPAc-PeNPLs demonstrated a significantly higher photoluminescence quantum yield (PLQY) of ~65.7% in solution and ~47.4% in thin films, compared to ~36% and ~17% for pristine samples, respectively. The strong surface passivation suppressed non-radiative recombination and prevented the agglomeration and thickness broadening typically observed during film formation. Time-resolved PL and TA measurements confirmed prolonged carrier lifetimes and suppressed inter-thickness energy transfer in the BPAc films.
• Controlled Superlattice Assembly: The uniformity of BPAc-PeNPLs enabled the formation of well-ordered superlattices.
  • Edge-up Orientation: These films exhibited a significantly enhanced degree of linear polarization (DOP), reaching a median of 11.5% (compared to 3.4% for pristine), due to improved in-plane dipole alignment.
  • Face-down Orientation: These films showed a more Lambertian emission profile, indicating efficient light outcoupling.
• Record-Breaking LED Performance: LEDs utilizing face-down oriented BPAc-PeNPL superlattices achieved a maximum external quantum efficiency (EQE) of 13.1%. This represents the highest reported EQE for LEDs based on ultrathin (≤3 ML) PeNPLs. The devices also showed improved charge injection, lower turn-on voltages, and narrow, color-pure electroluminescence centered at 600 nm.

Significance
The paper establishes that ancillary ligand-induced synthesis is a decisive route to achieving uniform, ultrathin PeNPLs with robust orientation control. By balancing strong surface coordination (via phosphoryl groups) with minimal steric hindrance (via a benzyl backbone), the authors overcame the fundamental bottlenecks of thickness non-uniformity and surface defects. This work demonstrates that precise molecular engineering of ligands can fully utilize the multifunctionality of anisotropic PeNPLs, enabling both high-efficiency LEDs and linearly polarized light sources, which are critical for next-generation photonics and display technologies.