Pseudohalide engineering for crystallization kinetics and defect passivation in two-step fabricated Cs0.1FA0.9Pb0.9Sn0.1I3 perovskite solar cells with exceptional efficiency and stability

Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a leading research focus in photovoltaics due to their exceptional power conversion efficiency (PCE) and low fabrication costs. However, conventional perovskite materials face two critical challenges:

(1) poor thermal stability of organic cations (e.g., formamidinium FA⁺ and methylammonium MA⁺), which causes device degradation at elevated temperatures;

(2) toxicity of lead (Pb) and defect formation during crystallization, limiting both device performance and environmental sustainability.

Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃ films through thiocyanate (SCN⁻) anion-mediated crystallization and Cs⁺/Sn²⁺ co-doping:

• PCE = 21.34% (V_OC = 1.127 V, J_SC = 25.399 mA/cm², FF = 74.562%);
• 91.65% initial PCE retention after 1000 hours of operation at 85°C;
• SCN⁻ additives facilitated PbI₂-SCN⁻ coordination intermediates, reducing the energy barrier for crystallization;

Cs⁺ established efficient ionic transport channels, while Sn²⁺ narrowed the bandgap and improved film homogeneity.

This work underscores the immense potential of SCN⁻-mediated crystallization control and tailored compositional engineering as synergistic strategies for developing high-performance, thermally robust, and eco-friendly PSCs.

The team published their article in Nano Research on September 27, 2025.

1. Fabrication Process and Crystallization Mechanism:

Employing a two-step spin-coating process with SCN⁻-containing MASCN or FASCN as secondary crystallization agents, we successfully fabricated four perovskite films: MAPbI₃, FAPbI₃, Cs_0.1FA_0.9PbI₃, and Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃. These were integrated into n-i-p structured solar cells (ITO/SnO₂/perovskite/Spiro-OMeTAD/Au). SCN⁻ played a pivotal role in crystallization:

• Coordinating with PbI₂ via lone electron pairs on sulfur atoms to form PbI₂žSCN⁻ intermediates;
• Stabilizing the PbI₂ layer while creating active sites for formamidinium iodide (FAI) infiltration;
• Volatilizing as HSCN gas during annealing (via protonation reactions) without leaving residual impurities;
• Ultimately yielding high-quality perovskite phases.

2. Synergistic Cationic Engineering:

Simultaneous cationic engineering further enhanced film properties:

• Cs⁺ established ionic transport channels, facilitating SCN^- integration and improving crystallographic completeness;
• Sn²⁺ doping increased film smoothness, reduced the bandgap, and enhanced light-harvesting capabilities.

3. Multi-Strategy Synergistic Effects:

The synergistic interplay of SCN⁻ mediation, Cs⁺ incorporation, and Sn²⁺ substitution optimized the crystallographic quality, optoelectronic properties, and interfacial characteristics of the Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃ film.

• Morphology Modulation: The Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃ film exhibited maximized grain size (≈1.8 μm) with a water contact angle of 64.5°, indicating optimal hydrophobicity among all compositions.
• Crystallization Optimization: XRD analysis confirmed minimal PbI₂ residue at 60 mg/mL SCN⁻ concentration (attenuated peak at 12.7°). Excess SCN⁻ (>80 mg/mL) triggered 2D phase nucleation, evidenced by an emergent diffraction peak at 11.8°.
• Bandgap Engineering: Reduced bandgap to 1.50 eV (via Tauc plot analysis of UV-Vis spectra). Enhanced photoluminescence (PL) intensity with prolonged carrier lifetime of 168.06 ns (TRPL decay fitting).

4. Band Structure and Lattice Regulation Mechanisms:

Favorable band alignment facilitates efficient extraction of electrons and holes toward the SnO₂ electron transport layer (ETL) and Spiro-OMeTAD hole transport layer (HTL), respectively. Sn²⁺ doping optimizes the lattice structure by mitigating lattice mismatch at grain boundaries, resulting in enhanced film homogeneity and surface smoothness. Upon partial substitution of Pb²⁺ with Sn²⁺, the absorption edge redshifted to 830 nm (λ=830 nm), indicating effective bandgap narrowing.

5. Device Performance and Stability:

① Photovoltaic Performance:

The Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃-based solar cell delivered superior photovoltaic performance: V_OC = 1.127 V, J_SC = 25.399 mA/cm², FF = 74.562%, PCE = 21.34%， surpassing control devices (MAPbI₃/FAPbI₃/Cs_0.1FA_0.9PbI₃) by >15% relative efficiency.

② Exceptional Stability:

• 91.65% initial PCE retention after 1,000 hours in N₂ at 25°C;
• 83.19% efficiency retention after 660 hours of continuous operation at 85°C;

Conventional FAPbI₃/MAPbI₃ cells suffered significant degradation under identical conditions, validating the superiority of the new system.

③ Moisture Resistance & Process Reproducibility:

• Enhanced hydrophobicity (water contact angle: 64.5°) effectively mitigated moisture ingress;
• Statistical analysis of 50 independent devices: Average PCE = 18.92 ± 0.8% with tight distribution (coefficient of variation <4.2%), demonstrating excellent process reproducibility.

6. Scientific Impact:

This work establishes a novel design paradigm for high-performance, industrially viable PSCs, accelerating their lab-to-fab transition and advancing global clean energy initiatives.

This work was supported by by the National Natural Science Foundation of China (22479074 and 22475096), the General Project of the Joint Fund of Equipment Pre-research and the Ministry of Education (8091B02052407), the Natural Science Foundation of Jiangsu Province (BK20240400 and BK20241236), the Science and Technology Major Project of Jiangsu Province (BG2024013), the Scientific and Technological Achievements Transformation Special Fund of Jiangsu Province (BA2023037), the Academic Degree and Postgraduate Education Reform Project of Jiangsu Province (JGKT24_C001), the Key Core Technology Open Competition Project of Suzhou City (SYG2024122), the open research fund of Suzhou Laboratory (SZLAB-1308-2024-TS005), the Gusu Leading Talent Program of Scientific and Technological Innovation and Entrepreneurship of Wujiang District in Suzhou City (ZXL2021273), and the Chenzhou National Sustainable Development Agenda Innovation Demonstration Zone Provincial Special Project (2023sfq11).

About Nano Research

Nano Research is a peer-reviewed, open access, international and interdisciplinary research journal, sponsored by Tsinghua University and the Chinese Chemical Society, published by Tsinghua University Press on the platform SciOpen. It publishes original high-quality research and significant review articles on all aspects of nanoscience and nanotechnology, ranging from basic aspects of the science of nanoscale materials to practical applications of such materials. After 18 years of development, it has become one of the most influential academic journals in the nano field. Nano Research has published more than 1,000 papers every year from 2022, with its cumulative count surpassing 7,000 articles. In 2024 InCites Journal Citation Reports, its 2024 IF is 9.0 (8.7, 5 years), and it continues to be the Q1 area among the four subject classifications. Nano Research Award, established by Nano Research together with TUP and Springer Nature in 2013, and Nano Research Young Innovators (NR45) Awards, established by Nano Research in 2018, have become international academic awards with global influence.