Stable and Highly Emissive Infrared Yb-Doped Perovskite Quantum Cutters Engineered by Machine Learning

Quantum cutting (QC) is an appealing energy conversion process wherein a single high-energy photon is transformed into two lower-energy photons. This process holds great promise in enabling applications in low-energy illumination and quantum communications.^{1, 2}
Yb doping is an effective possibility to achieve QC in perovskite nanocrystals (NCs) due to the favored Yb³⁺ cooperation and the ionic nature of perovskites.^3-5 However, currently, all Yb-doped perovskite quantum cutters are capped with conventional highly-dynamic binding oelylamine (OAm) and oleic acid (OA) ligands pair, which suffer from severe stability problems and hinder their potential applications in optoelectronics and communications.⁶
Recently, zwitterionic molecules, that carry both cation and anion functional groups, have shown great potential in preserving structural integrity as well as excellent quantum efficiency of perovskite NCs due to the multiple-functional groups chelating effect and eliminated proton exchange process.^7-9 However, few works have been reported on the Yb doped perovskite NCs with strong binding zwitterionic type ligands. Developing an in-situ synthesis method with stronger binding ligands is pressing for perovskite quantum cutters.
Here, we present the first demonstration of achieving a high NIR PLQY close to the QC limit alongside exceptional stability in Yb-doped perovskite NCs. We developed an in-situ phosphine oxide synthesis route to prepare robust and highly efficient Yb doped perovskite CsPbCl₃ NCs through zwitterionic type 3-(N,N-Dimethylpalmitylammonio)propanesulfonate (ASC-16) ligands. Tri-n-octylphosphine oxide (TOPO) was employed to assist the dissolution of Ytterbium(III) chloride hexahydrate precursor, which plays a critical role for effective Yb doping.
Moreover, we replaced the traditional trial-and-error approach involving time- and labor-intensive experimentation using Bayesian Optimization (BO) to optimize for NIR photoluminescence quantum yield (PLQY) through QC process. Across only 2 iterations of the active learning process, we were able to rapidly discover NCs with ultrahigh NIR PLQY close to QC limit (~192±5%), which is one of the highest for Yb doped perovskite NCs ever reported. Due to the strong coordination between zwitterionic ligands and NCs’ surface revealed by density functional theory (DFT) calculations, the prepared Yb doped CsPbCl₃ perovskite NCs exhibits greatly improved stability under high UV laser flux, continuous annealing, and also long-term colloidal storage stability even at Singapore’s humid environment (at least three months). Our work not only provides an in-depth understanding of the surface modification of doped NCs, but also facilitates efforts to study the fundamental physics behind QC and NIR quantum communications.
1. Near infrared quantum cutting for photovoltaics. Advanced Materials 21, 3073 (2009).
2. Recent progress in quantum cutting phosphors. Progress in materials Science 55, 353 (2010).
3. Picosecond quantum cutting generates photoluminescence quantum yields over 100% in ytterbium-doped CsPbCl₃ nanocrystals. Nano letters 18, 3792 (2018).
4. Anion Exchange and the Quantum-Cutting Energy Threshold in Ytterbium-Doped CsPb (Cl_1–xBr_x)₃ Perovskite Nanocrystals. Nano letters 19, 1931 (2019).
5. Negative Thermal Quenching in Quantum-Cutting Yb³⁺-Doped CsPb (Cl_1–xBr_x)₃ Perovskite Nanocrystals. ACS nano 17, 17190 (2023).
6. Highly dynamic ligand binding and light absorption coefficient of cesium lead bromide perovskite nanocrystals. ACS nano 10, 2071 (2016).
7. Colloidal CsPbX₃ (X= Cl, Br, I) nanocrystals 2.0: Zwitterionic capping ligands for improved durability and stability. ACS energy letters 3, 641 (2018).
8. Highly Concentrated, Zwitterionic Ligand-Capped Mn²⁺:CsPb(Br_xCl_1–x)₃ Nanocrystals as Bright Scintillators for Fast Neutron Imaging. ACS Energy Letters 6, 4365 (2021).
9. Designer phospholipid capping ligands for soft metal halide nanocrystals. Nature 626, 542 (2024).