Hong Kong Polytechnic University (PolyU) researchers have made a groundbreaking advancement in materials science by developing lead-free tin halide perovskite nanocrystals with unprecedented photoluminescence efficiency. This achievement, detailed in a recent Nature Synthesis publication, marks a significant step forward for sustainable optoelectronic technologies. Led by Assistant Professor Jun Yin from PolyU's Department of Applied Physics, the team has overcome longstanding challenges in tin-based perovskites, achieving a record photoluminescence quantum yield (PLQY) of 42.4 percent for formamidinium tin triiodide (FASnI3) nanocrystals. This near-infrared emitting material now rivals lead-based counterparts in performance while eliminating toxicity concerns.
Perovskite nanocrystals, tiny crystals structured in the ABX3 perovskite lattice where A is a large cation like formamidinium (FA), B is a metal cation such as tin (Sn), and X is a halide like iodide (I), have revolutionized fields like displays and solar cells due to their tunable light emission and high efficiency. However, traditional lead halide perovskites pose environmental and health risks because of lead's toxicity. Tin halide variants offer a promising lead-free alternative, but they have historically suffered from rapid oxidation of Sn2+ to Sn4+ and defect-induced non-radiative recombination, resulting in PLQYs below 1 percent—severely limiting practical use.
Understanding the Challenges in Tin Halide Perovskites
Tin halide perovskite nanocrystals face two primary defect issues: bulk defects like tin vacancies (VSn) and iodide vacancies (VI), and surface defects that trap excitons, quenching light emission. Previous efforts adjusted precursor ratios or applied post-treatments, but these yielded PLQYs rarely exceeding 20 percent, often much lower for hybrid organic-inorganic systems like FASnI3.
The PolyU team's insight came from density functional theory (DFT) simulations, revealing a trade-off: tin-rich conditions suppress bulk defects but exacerbate surface ones, while tin-poor conditions do the opposite. This fundamental limitation demanded a novel dual-suppression strategy.
The Computational Roadmap to Success
Prof. Yin's group began with high-throughput DFT calculations to map defect formation energies across chemical potential landscapes. They simulated three regimes—tin-rich (C1), moderate (C2), and iodide-rich (C3)—identifying dominant defects in each. Surface analysis focused on SnI2- and FAI-terminated facets, common in nanocrystals.
Next, they screened monovalent cations for passivation: 2-thiopheneethylammonium (TEA+) for soft Lewis basic sites binding Sn, and sodium (Na+) for ionic stabilization. Binding energy computations and density of states analysis confirmed these ligands eliminate mid-gap trap states without compromising band structure.
Step-by-Step Synthesis Breakthrough
- Tin-rich precursors: Used Sn(II) 2-ethylhexanoate and SnI2-trioctylphosphine (TOP) complex to maintain Sn2+ and favor bulk defect suppression.
- Cation incorporation: Added TEAI and sodium carboxylate for surface passivation, forming defect-tolerant TEA+-anion and Na+-I- layers.
- Halide tuning: Trace chloride (Cl0.03) further boosted octahedral integrity.
- Hot-injection: Rapid nucleation at high temperature yielded uniform rod-like nanocrystals averaging 10-20 nm.
Transmission electron microscopy (TEM) confirmed single-crystalline cubic structure with 6.4 Å (100) spacing, while high-angle annular dark-field scanning TEM (HAADF-STEM) mapped uniform Sn/I distribution.
Record-Breaking Performance Metrics
The optimized FASnI3 nanocrystals exhibited:
- PLQY: 42.4% ± 1.0% at 860 nm (NIR), vs. 0.5% prior.
- Excited-state lifetime: 158 ns (vs. 0.38 ns control).
- Zeta potential: +27 mV, indicating stable colloidal dispersion.
- Stability: Retained 90% PLQY after 30 days in air.
Alloyed FA/Cs variants shifted emission to red (700 nm) with 30%+ PLQY, demonstrating versatility. Ab initio molecular dynamics validated surface reconstruction, where TEA+ anchors via thiophene-Sn bonds, preventing vacancy formation.
This surpasses global benchmarks; for context, top lead-free PeNCs rarely exceed 30% PLQY. See the full study in Nature Synthesis.
PolyU's Role in China's Materials Science Leadership
PolyU, a leading technological university in Hong Kong, exemplifies China's push in advanced materials. Prof. Yin, cited in Stanford's top 2% scientists (nanoscience), heads a dynamic group blending theory and experiment. Collaborators include international experts like Edward H. Sargent (Northwestern) and Hong-Tao Sun (University of Science and Technology of China), underscoring cross-institutional synergy.
In mainland China, institutions like Tsinghua University and Peking University advance mixed Sn/Pb perovskites (efficiencies >25% for solar cells), but pure tin NCs lag. PolyU's pure-tin record positions Hong Kong as a hub, supported by Hong Kong Research Grants Council funding.
Applications in Displays, LEDs, and Beyond
High-PLQY tin PeNCs enable vibrant, flexible displays without cadmium or lead. In LEDs, NIR emission suits bioimaging and night vision. For solar cells, multiple exciton generation (MEG) in tin PeNCs boosts photocurrent efficiency, as prior PolyU work showed 87% MEG.
Recent 2026 advances include tin PSCs at 15-20% efficiency (Jinan University), but NCs like PolyU's promise tandem cells exceeding 30%. Photodetectors benefit from tunable bandgap (1.2-2.0 eV). For more on perovskite solar progress, visit PolyU's research highlights.
Overcoming Oxidation: A Paradigm Shift
Sn2+ oxidation forms Sn4+ vacancies, p-doping the material and creating traps. PolyU's tin-rich + passivation duo stabilizes Sn2+, with molecular dynamics showing rigid surface octahedra. This generalizable method applies to nanowires and alloys, expanding the toolkit.
Implications for Chinese Higher Education and Industry
This PolyU milestone bolsters China's dominance in perovskites, with >50% global publications. Universities like USTC and Fudan lead in tin PSCs, but PolyU excels in NCs. It attracts global talent, fostering PhD/postdoc programs in quantum materials.
Government initiatives like the 14th Five-Year Plan prioritize lead-free tech, linking academia to firms like BOE for displays. Expect commercialization by 2028, creating jobs in synthesis, characterization, and device engineering.
Future Outlook: Scalable Production and Hybrids
Challenges remain: gram-scale synthesis and thin-film integration. PolyU envisions ligand exchange for devices. Combined with tandem architectures, efficiencies could hit 30%+. Environmentally, lifecycle assessments show 90% less toxicity than lead PeNCs.
In China, collaborations with CAS institutes accelerate transfer. Prof. Yin's group seeks partners for LED prototypes.
Photo by Kanchanara on Unsplash
Career Opportunities in Perovskite Research
China's boom demands experts. PolyU and peers offer postdoc/PhD roles in nanomaterials. Skills in DFT, colloidal synthesis, and spectroscopy are prized. Salaries: postdocs HKD 400k+/year; faculty HKD 1M+. Explore openings at AcademicJobs research positions.
- PhD in Applied Physics: Quantum materials focus.
- Postdoc: Defect engineering in PeNCs.
- Faculty: Optoelectronics track.
